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Possible mechanism for explaining the origin and size distribution of Martian hematite spherules
Planetary and Space Science 92 (2014) 16–23
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Possible mechanism for explaining the origin and size distribution of Martian hematite spherules Anupam K. Misra n, Tayro E. Acosta-Maeda, Edward R.D. Scott, Shiv K. Sharma Hawaii Institute of Geophysics and Planetology, SOEST, University of Hawaii Honolulu, HI 96822, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2013 Received in revised form 18 January 2014 Accepted 24 January 2014 Available online 5 February 2014
Mysterious hematite spherules, also known as “blueberries”, observed at Meridiani Planum on Mars have been widely accepted as concretions which are formed by precipitation of aqueous fluids. One of the biggest mysteries is that all observed Martian blueberries are limited in size with maximum diameter of 6.2 mm. In contrast, terrestrial concretions are not size limited. In this article, we discuss significant differences between Martian blueberries and Earth concretion analogs. Puzzling observations from Mars Exploration Rovers Opportunity and Spirit suggest that the spherules may not be concretions but are cosmic spherules formed by ablation of meteorites. The perfect spherical shape of spherules, their observed size limit, and all other physical properties are easily explained by a meteorite ablation model. Evidence that some of these spherules are only few years old strongly constrains concretion and other growth mechanisms related to aqueous processes that require the existence of water on Mars in its recent history. The large number of hematite spherules in Meridiani Planum may be due to a big rare iron meteorite impact event in this region sometime in the past. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Mars spherules Mars blueberries Hematite spherules Meteorite Concretions Cosmic spherules
1. Introduction On January 24, 2004, the Mars Exploration Rover Opportunity landed in Eagle crater on Meridiani Planum. This site was chosen because earlier MARS Global Surveyor TES data showed it to contain large amounts of the mineral hematite (Christensen et al., 2000). Because on Earth, hematite is formed through aqueous processes, the discovery of hematite in Martian soil suggests the past presence of water. One of the surprising discoveries of the Rover was the presence of large numbers of hematite spherules (Klingelhöfer et al., 2004; Squyres et al., 2004a; Squyres et al., 2004b), now commonly known as “blueberries”. Initial scientific investigation (McLennan et al., 2005; Squyres et al., 2004a; Squyres et al., 2004b) suggested that the blueberries and their fragments are the primary carriers of hematite at the site. The hematite in the Martian soil is mostly observed on the surface, and the subsurface soil in the excavated trenches is dominated by basaltic sand. The blueberries are typically 4 mm in diameter, mostly perfect hard spheres with very fine grain and no observed internal structure. The entire inventory of the observed spherules is suggested to be within the upper 1 cm thickness of the surface soil (Calvin et al., 2008; Christensen et al., 2004; McLennan et al., 2005; Squyres et al., 2004a; Squyres et al., 2006a; Squyres et al., 2004b).
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Corresponding author. Tel.: +1 808 9565095. E-mail address: [email protected] (A.K. Misra).
http://dx.doi.org/10.1016/j.pss.2014.01.020 0032-0633 & 2014 Elsevier Ltd. All rights reserved.
For the past 9 years, most of the scientific community has widely accepted that the Mars blueberries are concretions (Calvin et al., 2008; Christensen et al., 2004; Glotch and Bandfield, 2006; Glotch et al., 2006; Kula and Baldwin, 2012; McLennan et al., 2005; SeftonNash and Catling, 2008; Squyres et al., 2004a; Squyres et al., 2006a; Squyres et al., 2004b; Squyres et al., 2009; Squyres et al., 2006b). Possible concretion terrestrial analogs have been found in southern Utah, in the Jurassic Navajo Sandstone (Chan et al., 2004; Chan et al., 2005), and in Lake Brown, Australia (Bowen et al., 2008). However, a comparison of the iron oxide concretions from the Navajo Sandstone and images and scientific data made available by NASA reveals remarkable differences between the terrestrial concretions and Martian blueberries. Here we present evidence that indicates that the blueberries are not concretions and suggest that instead they are of meteoritic origin.
2. Physical properties of Earth concretions and Martian spherules By definition, a concretion is a compact mass of mineral matter, usually spherical or disk-shaped, embedded in a host rock of a different composition. Concretions are formed by precipitation of mineral matter from the liquid phase, usually about a nucleus. The physical mechanism of nucleation and growth is required for the formation of concretions. This growth mechanism puts no limit on
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the size of the concretion. Concretions on Earth are large (centimeters to meters) (Abdel-Wahab and McBride, 2001; Chan et al., 2004) and seem to have no size limit. In contrast, all blueberries found on Mars are limited in size (diameter less than 6.2 mm) (Calvin et al., 2008; McLennan et al., 2005; Sefton-Nash and Catling, 2008). Apart from the size difference, concretions come in variety of shapes and are rarely perfect spheres (Bowen et al., 2008; Chan et al., 2004). The interior of a Mars spherule is hard, uniform, and extremely fine grained (Arvidson et al., 2004; Herkenhoff et al., 2004). Terrestrial concretions are not spherical but metallic looking shells, which enclose the grains of the host matrix as part of the concretion mechanism (Bowen et al., 2008). Fig. 1 shows the picture of few terrestrial concretions from Jurassic Navajo Sandstone, southern Utah. The concretions have been cut in half to reveal the interior structure. The figure illustrates that these concretions are not perfect spheres and do not show a size limit. The sizes of these Earth spherules are also much larger than the Martian spherules which have the upper size limit of 6.2 mm . The interior structure of the concretions is different than the outer layer. Micro-Raman spectroscopy was used to identify the chemical compositions of terrestrial concretions. Raman spectra were measured at multiple locations on all the samples. Fig. 2 shows representative Raman spectra of terrestrial concretions showing
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Black grain outside Earth Spherules
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B
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Brown grain inside Earth Spherules 465
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Rose Quartz 61 13
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C 413
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Intensity
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Goethite
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the spectral differences between the interior brown grains and outer black grains. It is shown that the brown grains located inside the concretions are quartz. For comparison, Raman spectra of rose quartz, hematite and goethite minerals are also shown. The Raman spectrum of rose quartz is identical to the Raman spectra of brown grains from inside of the concretions which confirm that the interior structure is mostly quartz, with grain size of the order of 150 μm. The black grains located at the outer layer of concretion also show dominating quartz peaks along with additional Raman peaks associated with goethite. The Raman analysis indicates that the exterior shell layer is mainly quartz with a very thin coating of goethite. Our Raman analysis of iron oxide concretions is consistent with that of Knauth et al. (2005). Concretion cements around the grains of the host matrix, and hence grains of host matrix are always included in the terrestrial concretions (Bowen et al., 2008). The concretion mechanism does not explain or account for the perfectly size-limited, homogeneous hematite spherules excluding the grains of host soil found on Mars. Another important fact is that on Earth erosion plays a critical role in the formation of concretions by removing the surrounding matrix and releasing the concretions. Therefore, the Earth concretion analogs are dull metallic objects that show signs of erosion and evidence of pitting. Often the outer surface also shows flow patterns, indicating the presence of aqueous media (Bowen et al., 2008; Chan et al., 2004). The erosion process also dictates that the Earth concretions are several thousand years old (Bowen et al., 2008). Another puzzle is that Martian blueberries are mostly concentrated within 1 cm of the surface (Arvidson et al., 2004; Bowen et al., 2008; Squyres et al., 2004a; Squyres et al., 2004b). This observation is not consistent with the formation of mineral precipitation from ground-water. As the level of ground-water goes down, the concentrations of dissolved minerals are expected to increase in the subsurface soil. For the terrestrial concretions, relatively larger numbers are observed in the deeper soil (Bowen et al., 2008) below the 1 cm top layer. However, no blueberries were observed in the deeper soil when trenches were excavated on Mars (Christensen et al., 2004; Squyres et al., 2004a; Squyres et al., 2006a). This observation suggests that a physical mechanism capable of depositing spherules from the top is more favorable than mechanisms involving ground-water. Previous research suggests (Squyres et al., 2004a; Squyres et al., 2004b) that hematite-rich spherules are embedded in the rock and erod from them. One of the images (Fig. 3) taken on sol 251
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Fig. 1. Earth concretions analog. Picture of Earth concretion analog samples from Jurassic Navajo Sandstone, southern Utah (courtesy William Mahaney). Samples have been cut in half to show the interior structure.
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Hematite
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Raman Shift ((cm-1) Fig. 2. Raman analysis of Earth concretions. Micro-Raman spectra of inner and outer grains of Earth concretions measured with 785 nm laser excitation. The Raman spectra of interior brown grains are identical to that of reference Raman spectrum of rose quartz. The black grains in the outer layer are goethite coated quartz.
Fig. 3. Blueberries on Wopmay Rock suggesting they landed from the top. Wopmay rock observed on sol 251 showing hundreds of fully exposed blueberries on it. Concentration of blueberries in region A is higher than that away from the rock (region B), and no blueberries are observed in the shadow of the rock as shown in region C; suggesting that these blueberries fell from the top. Image courtesy of NASA/JPL.
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from the Opportunity Rover is that of the unusual 1 m lumpy rock “Wopmay” found on the lower slopes of Endurance Crater (MER mission image a, Wopmay, 2004). The image displays hundreds of blueberries on its surface. A closer look confirms that all the spherules on the rock are fully formed spheres and are fully exposed, i.e., none of the spheres are seen as partially emerged. If the spherules are concretions, we would expect to see some partially exposed blueberries protruding from the rock. An erosion process as a part of concretion mechanism would produce both completely and partially exposed spherules on all the surfaces of the rock. The distribution of the blueberries around the rock strongly suggests that they fell from above. This is indicated by comparing the high concentration of the blueberries on the left side of the rock (region A) with the lower concentration of blueberries in region B away from the rock. Another supporting observation is that no blueberries can be seen in region C, which is in the shadow of the rock. The absence of blueberries in region C is not consistent with a concretion mechanism. We would also like to point out that the likelihood of dust devils transporting the blueberries is also ruled out here. A dust devil event would fill region C with blueberries and would also remove blueberries from the slopes of the Wopmay rock.
3. Proposed meteorite ablation mechanism and evidences supporting the cosmic spherule model We propose that the observed spherules originate from the ablation of meteorites. Meteoroids, of various sizes, enter the Mars atmosphere at high speed and low temperature. Upon entering the atmosphere, they are heated by ram pressure (defined as a pressure exerted on a body moving through a fluid medium) and friction. The smaller meteoroids are completely melted like micrometeorites. Bigger meteoroids, because of their large mass and thermal conductivity, need more time to melt completely. Melting begins at the meteoroid's surface and as soon as the surface liquid reaches a critical depth, the liquid falls away and immediately breaks down into smaller spherical drops because of atmospheric drag and surface tension. This liquid drop formation also limits the thickness of the fusion crust of the meteorite, which is only a few mm thick. The maximum size of droplets is controlled by surface tension of liquid and the atmospheric drag force. Because the terminal velocity of the small drops is lower than that of the bigger meteoroid, the liquid drops decrease in speed, which results in lower ram pressure, in turn causing the temperature of the drops to decrease and become solid spherules. The melting mechanism also produces imprints on the meteorite surface from the falling liquid drops. Strong evidence in support of this theory is the observed iron meteorite named “Heat Shield Rock” on sol 349 (Fig. 4), which was found on Mars (MER mission image b, Heat Shield Rock. R. D. G. Frederick, 2005). The MER science team confirmed that the composition of this meteorite is mainly iron, with kamacite as the primary Fe-bearing mineral and around 7 wt% nickel (Fairén et al., 2011; Schröder et al., 2008). A careful examination of the surface of the fusion crust shows regmaglypts and several circular imprints. This suggests an estimation of the approximate size of the molten drops removed from the meteorite, which is consistent with the size of the blueberries lying on the ground. The impact of the meteorite on the ground is expected to form several micro-berries and mini-berries from the liquid which was still attached to the meteorite before the impact. Fig. 4 clearly shows a larger concentration of smaller blueberries in the immediate surroundings of the meteorite forming a halo. There are also several blueberries shown on top of the meteorite. The presence of these blueberries on the meteorite is not
Fig. 4. Blueberries and imprints on the fusion crust of an iron meteorite found on Mars. Iron meteorite named “heat shield rock” observed on sol 339 by the Opportunity Rover showing a few blueberries on it and several imprints formed by spherules removed from the fusion crust. Image 1P159435226EFF40DPP taken from Sol 352. Image courtesy of NASA/JPL/Cornell.
consistent with the concretion phenomenon. First, a meteorite has to contain enough liquid water to initiate concretion. Second, the host matrix must erode significantly to expose the spherules. After such a concretion–erosion process, one would not expect to see any imprint left on the fusion crust. In addition to the cosmic spherules formed by the meteorite ablation mechanism, the impact of a significantly large meteorite on the ground can also produce impact spherules by melting the ground. The cosmic spherules are not expected to make any impact spherules due to their low mass. The impact spherules are possible on Mars but are expected to be formed in the localized impact regions associated with large meteorite landing sites and not distributed over a large area on Meridiani Planum. One strong support for the origin of spherules from meteorites is that it puts an upper limit on the size of the spherules. By assuming that the spherules will break up if the atmospheric drag force is greater than the surface tension force, and at terminal velocity the drag force is equal to the gravitational force, one can estimate (Clift et al., 1978) the diameter of the drop (D) to be proportional to rffiffiffiffiffiffi s Dp gρ Where s is surface tension of the liquid, g is acceleration due to gravity and ρ is the density of the liquid. Simplifying the above equation rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 s2 g 1 ρ 1 ¼ D1 s1 g 2 ρ 2 where subscripts 1 and 2 represent two different types of liquids. Raindrops are usually 1 mm to 2 mm in size. Drops as large as 3 mm that are not spherical (Beard and Chuang, 1987, Perlman 2005) are sometimes observed. When the raindrop reaches 4.5 mm, it immediately breaks up into small drops and microdrops by opening up like a parachute (Perlman 2005, Fox 2009). Using s1 ¼0.073 N/m surface tension of water, g1 ¼9.8 m/s2 as Earth gravity, ρ1 ¼ 1000 kg/m3 as density of water, s2 ¼ 1.46 N/m surface tension of molten iron (Kim et al. 2006), g2 ¼ 3.675 m/s2 as Mars gravity, and ρ2 ¼7860 kg/m3 as the density of iron one can estimate the diameter (D2) of blueberries, assuming that they are formed from the melting of iron meteorites. Substituting these values gives D2 ¼2.6 D1, which estimates the size of blueberry D2 ¼3.9 mm if D1 ¼1.0035 mm. The blueberry of size 7.8 mm corresponding to a 3 mm raindrop size is not expected to be spherical at all. Any
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blueberry reaching 12 mm in size (corresponding to D1 ¼4.5 mm) would immediately break up into smaller drops and microdrops by opening up as a parachute. This theory can be easily disproved if even one blueberry out of thousands observed so far on Mars violates the size limit. However, so far no blueberry is shown to be larger than 6.2 mm (Calvin et al., 2008; McLennan et al., 2005; Sefton-Nash and Catling, 2008; Squyres et al., 2004a; Squyres et al., 2004b) and the proposed theory holds. The theory also explains the presence of large numbers of micro-berries observed only on the top surface layer of Mars. The observation of a large number of spherules and microspherules is not an indication of a large number of meteoritic events on Mars. This is because a small meteorite of size 4 cm in diameter entering the Martian atmosphere at an angle is expected to make 1000 spherules of 4 mm in size which will be distributed over a large area along the meteorite trajectory. The vast majority of spherules are isolated spheres and very rarely doublets and triplets have been observed (McLennan et al., 2005; Squyres et al., 2004b; Squyres et al., 2006b, Calvin et al., 2008). Rare occurrences of doublets and triplets have been suggested to be a strong evidence of the concretion mechanism by McLennan et al. (2005) because they are statistically unlikely in volcanic and impact spherules. With the meteorite ablation mechanism it is possible to form doublets and triplets. The meteorite ablation mechanism also explains why the occurrence of doublets and triplets will be very rare in comparison to isolated spherules. As two liquid drops come in contact with each other, doublets can be formed. The mechanism, however, suggests that the probability of the formation of doublets and triplets would be very small as this would require a recombination of drops near the liquid–solid phase transition. A recombination well above the melting point in the liquid phase would form a big liquid drop which will break up again into smaller drops. An interesting prediction is that it is more likely to observe doublets and triplets consisting of individual drops of different diameters rather than the same diameter because the recombination is more likely to happen if one drop is moving faster than the other drop. Another prediction is that one should also observe berries other than blueberries representing meteorites of different compositions. All these predictions are in agreement with observations on Mars (Calvin et al., 2008; McLennan et al., 2005; Squyres et al., 2004b; Squyres et al., 2006b). Another issue that is not consistent with the concretion mechanism is the age of the spherules. Because erosion is an important step in formation of the concretion, the spherules are expected to be thousands of years old, like those found on Earth. Fig. 5 shows mostly eroded spherules from sol 369 (MER mission image c, sol
Fig. 5. Evidence supporting spherules of different ages. Image 1M160940487 EFF42D9P2956M2M1-BR (sol 369) showing coexistence of old and new berries indicating large age difference. Courtesy: NASA/JPL & M Lyle.
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Fig. 6. Evidence supporting spherules of different compositions. Image from sol 319 showing yellow berries along with blue berries indicating chemical difference. Image courtesy of NASA/JPL.
369 (2005)), however, a careful look at the photograph reveals a very shiny (and hence younger) spherule in the figure (indicated by arrow). The shine of the spherule indicates that this spherule is much younger than the other dull looking spherules because most polished metallic surfaces starts looking dull after few years due to weathering. An Earth analog would be comparing metallic parts of a new car to an older car. The self-portrait pictures of Opportunity Rover from 2004, 2007 and 2011 show weathering effect on the Rover due to Martian atmosphere. The coexistence of old and new spherules on Meridiani Planum is compatible with the meteorite ablation model, but not with the concretion model. The concretion model would require the surface water to come back after thousands of years and form new concretions on the surface of Meridiani Planum. Later in this article we show that some of the spherules are only a few years old, which would require an extremely fast concretion phenomenon. In Fig. 6 (MER mission image d, sol 319, 2005) we observe the co-existence of both blue and yellow berries, indicating that other types of meteorites do fall on Mars and form size-limited spherules. Although this is a false color image, the false color scheme does indicate a change in optical reflectance and hence a different chemical compositions for blue and yellow berries. And finally, we look at the evidence that strongly supports the cosmic spherules mechanism but is impossible to explain the concretion mechanism. If spherules do originate from meteorites then there is a small possibility that they will fall on top of the Rovers and other introduced man-made objects. For the meteorite model to hold, any spherules observed on the Rovers must hold the size limit and also be shiny with no sign of erosion. The ability to collect only the shiny spherules over the eroded spherules would rule out possibility of collecting spherules from the ground due to dust devil events. The evidence of spherules on man-made objects will indicate that they fell after January 24, 2004. One of the places to look for evidence is on the heat shield. However, the heat shield landed with a high speed, which may have disturbed the spherules lying on the ground and hence it is possible that it picked up spherules from the ground during the impact. Therefore, the presence of spherules on the heat shield may not be a convincing case. Other man-made objects are the Rovers themselves and unlike the heat shield they landed smoothly and traveled at a low speed. Fig. 7 (sol 322) shows few spherules collected by the Opportunity Rover (MER mission image, Opportunity Self-Portrait (2005)). The size of these spherules matches with the size of the blueberries shown on the ground in Fig. 7 at right bottom corners of the photograph. Fig. 8 shows what seem to be micro-spherules at the two locations labeled “A” on the solar panel of the Opportunity Rover. The image (1M156679326EFF3981P2979M2M1) was taken by the microscopic imager on Opportunity Rover on sol 321. The image also shows a burned
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Fig. 7. Evidence of blueberries landed on the Opportunity Rover. Image taken from sol 322 showing blueberries on the Opportunity Rover.
Fig. 8. Evidence of micro-blueberries landed on the Opportunity Rover. Image (sol 321) shows microberries on the solar panel of the Opportunity Rover at locations labeled “A” and burned mark at location “B”.
spherical object has appeared on the bottom of the solar panel marked “E” and the effects of dust devils can be seen in the area marked “F”, which shows a collection of dust only. The ability of these round objects to leave marks and roll off suggests that they fell on the Rover from above with enough speed to imprint solar panels. We propose that these objects are indeed cosmic spherules and that their presence on the Rovers is evidence that these spherules are only few years old. This strongly constrains concretion and other growth mechanisms (Fan et al., 2010) related to aqueous processes because it would require the existence of water on Mars in its recent history and a much faster growth process for spherules formation. According to Mössbauer data (Klingelhöfer et al., 2004), blueberries at Meridiani Planum are made of hematite. A recent publication by Minitti et al. (2005) showed that it is possible to form hematite in a CO2 atmosphere at temperatures above 900 1C from high-FeO glass-rich basalts. Cooper et al. (1996) determined that with FeO concentration 4 1.9 wt%, and temperature of 700 1C, the Fe migration is favored over Ca and Mg migration. Hence it is possible for hematite (and a hematite coating) to form from meteorites that have some iron in them under the present or past conditions existing on Mars. Aqueous alteration after being deposited in the ground could also account for the formation of hematite. The formation of hematite from iron is possible in the atmosphere and has been suggested by Bohren and Olivero (1984) in an article on the evidence for formation of hematite particles at 60 km altitude on Earth. The Mössbauer instrument on the Opportunity Rover is mostly a surface technique due to the limited penetration depth of gamma rays used. Hence both pure hematite spherules and hematite-coated spherules will appear the same. A combined analysis of Mössbauer with Mini-TES indicates that in some cases spherules are composed of a mixture of hematite and basaltic material (McLennan et al., 2005). Both Mössbauer and Mini-TES instruments provide measurements on a large area and represent average composition of several spherules along with background soil. Mini-TES, similar to Mössbauer is also a surface technique. The instruments on the Opportunity Rover are not capable of analyzing chemical composition of individual spherules and would not be able to separate out different types of spherules in a mixed population. Such analytical capability is available to the ChemCam LIBS instrument on the Curiosity Rover.
4. Discussion and summary impact mark at location “B”. The circular burn marks on the solar panels of the Rover are in agreement with the meteorite ablation model as the cosmic spherules and micro-spherules would be hot. Other more convincing evidence is the observation of spherical shiny objects on the solar panels of the Spirit Rover, which landed near Gusev Crater. While investigating the Gusev Crater area the Spirit Rover rarely observed spherules on the ground and, therefore, it is highly unlikely that it selectively picked spherules from the ground during dust devil events. One can further argue that these spherules are not airborne from the ground. We would expect to see more dust and older and smaller spherules because they would be aerodynamically responsive to air lifting due to larger surface area-to-mass ratio. Any spherule collected by the Spirit Rover is strong a evidence of cosmic spherule. The images from the Spirit Rover taken on sol 330 show few spherical objects on the solar panels marked as “C” and “D” (Fig. 9a) (MER mission image, Spirit Self-Portrait, 2005). An image (Fig. 9b) of the same solar panels taken almost a year later on sol 586 (MER mission image, Spirit Self-Portrait, 2006) shows that two of the objects have rolled off and only the impact prints can be seen. A new
The evidence presented in previous sections strongly suggests that Martian spherules are cosmic spherules formed from the ablation of meteorites. If the Martian spherules originate from meteorites, they would be expected to be hard, extremely fine grained, perfect spheres, polished, size-limited, and show no sign of core nucleation. In addition they will be observed only on the top soil and will be missing in deeper soil. All these properties are consistent with observed data on the Martian blueberries. Table 1 lists the observed data on Martian spherules and shows that all of the observations are easily explained with the meteorite ablation model. Same tests are also shown for a concretion model for comparison. The thin Mars atmosphere creates a unique environment for spherules forming from meteorites. On the Earth's Moon, the lack of atmosphere deprives the meteorites of a heating mechanism; hence, no cosmic spherules are expected to be on the Moon. Meteorites falling on the Moon are expected to make the impact glass spherules. On Earth, cosmic spherules are of mm-size and are found in abundance (Brownlee, 1985; Brownlee et al., 1997; Taylor and Brownlee, 1991; Taylor et al., 2000) and roughly 10,000 t of
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Sol 330
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Sol 586 F C D
E Fig. 9. Evidence of blueberries landed on the Spirit Rover. Images (a) and (b) comparing same solar panels of Spirit Rover from sol 330 and sol 586 showing blueberries that have rolled off a year later from solar panels “C” and “D”. A new blueberry is seen in solar panel “E”. Area “F” shows a collection of dust only due to dust devil events.
Table 1 Observations and physical properties of Martian spherules and their comparison with concretion and meteorite ablation models. Observations
Agreement with concretion model
Agreement with meteorite model
1. All blueberries are less than 6.2 mm in diameter (size limitation) 2. Blueberries show no grain structure 3. Blueberries show no nucleation 4. Coexistence of old and fresh spherules (age difference) 5. Coexistence of blue and yellow berries (chemical difference) 6. Existence of blueberries limited to top soil and missing in deeper soil 7. Spherules found on manmade objects (Opportunity and Spirit Rovers) 8. Interiors of spherules missing grains of host soil 9. Hematite (composition) for blueberries 10. Predominantly perfect spheres (shape) 11. Large amount of micro-berries observed only on top surface 12. Blueberries appear embedded in soil 13. Hard (mechanical strength) 14. Circular burn spot on solar panel
No No No No No (may be) No No No Noa No No Yes Yes No
Yes Yes Yes Yes Yes Yes Yes Yes Yesb Yes Yes Yes Yes Yes
a b
Hematite-coated host soil grains (quartz, gypsum, etc.). Other chemical compositions (e.g. yellow berries) are also possible.
interplanetary dust impact the atmosphere each year (Brownlee, 1985). Mars has unique, ideal conditions (low gravity and thin CO2 atmosphere) for forming larger spherules. Mars is also located just next to the asteroid belt and therefore would be a suitable place to find large amounts of cosmic spherules (Fairén et al., 2011). The large number of blueberries in Meridiani Planum might be due to a big rare iron meteorite breaking up and partially melting over this region. A big iron meteorite entering the Martian atmosphere at low angle and exploding in the atmosphere can form the large amount of blueberries and microberries distributed over a several kilometer area on Meridiani Planum. This possibility is consistent with the fact that most of the blueberries on Meridiani Planum are likely to be very old, in excess of 3.5 Ga as suggested by McLennan et al. (2005). The global mapping of hematite coverage in Sinus Meridiani shows an elliptical distribution (Plate 2, Christensen et al. 2001). The hematite-rich unit is sharply defined and has not been mobilized. A relatively fresh, unnamed 22 km crater located at 31S, 6.61W and its continuous ejecta blanket showing no detectable evidence of hematite (Christensen et al. 2001) suggests that
hematite deposit is older than the crater and has not been redistributed. The elliptical distribution and sharply defined hematite boundary is consistent with a meteoroid origin. An iron meteorite of 40 m in diameter is equivalent to 1012 blueberries of 4 mm in diameter and can cover a large area. Several large iron meteorites have been discovered in Meridiani Planum (Ashley et al. (2011); Fairén et al. (2011)). Chappelow and Golombek (2010) argue that six iron meteorites are the result of a single large rare event. We suggest that the hematite spherules and the iron meteorites are the result of the same large rare impact. We infer that the Martian impact event resembled the Canyon Diablo impact in that most of the projectile was converted into melt droplets and multi-kg fragments (Artemieva and Pierazzo, 2011). However, in the Martian impact, a very low impact angle to the horizontal may have ensured that most of the impact energy was deposited in the atmosphere, as in the recent Chelyabinsk event (Borovička et al., 2013). The new shiny spherule (shown in Fig. 5), yellow berries (shown in Fig. 6) and cosmic spherules on the rovers are not part of the same event and may be due to stony or stony iron meteorites.
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5. Conclusion The evidence presented in this paper strongly suggests that the Martian berries (yellow and blue) originate from meteorites and are not concretions. In the cosmic spherule model, a meteoroid entering the Martian atmosphere melts and forms berries, microberries and nano phase materials. The maximum size of the spherules is limited by the surface tension of molten material and the atmospheric drag force. The estimated maximum size of blueberries, assuming they are formed from ablation of iron meteorites, is consistent with the observed data on Mars. In addition, the cosmic spherule model explains all observed properties of Martian hematite spherules including the mostly perfect spherical shape, largely isolated spheres, fine grain structure and no internal structure and nucleation. The observation of spherules and micro-spherules on Rovers is also in agreement with the cosmic spherule model. The circular burn spot on solar panel of rover is a strong evidence of hot microberry impact and this could be of concern to future Mars Rovers and other exploration missions and should be taken into account when designing equipments. The instruments should be able to withstand the impact of hot berries falling from sky. Finally, the cosmic spherule model also explains why the blueberries, yellow berries and micro-berries are only observed on the top surface layer and missing in the deeper soil.
Acknowledgments The authors would like to thank Nancy Hulbirt and May Izumi for their valuable help with figures and editing. We would also like to thank William Mahaney for providing the Earth concretion analog samples from Jurassic Navajo Sandstone, southern Utah. We are also thankful to Dr. Christopher McKay, Gary R. Huss and another anonymous reviewer for their critical review and constructive comments, which greatly helped to improve the manuscript. All the data and images from Mars are courtesy of NASA and JPL. JPL image policy allows use of images without prior permission as cited on their website 〈http://www.jpl.nasa.gov/images/ policy/〉 References Abdel-Wahab, A., McBride, E.F., 2001. Origin of giant calcite-cemented concretions, temple member, Qasr El sagha formation (Eocene), faiyum depression, Egypt. J. Sediment. Res. 71, 70–81. Artemieva, N., Pierazzo, E., 2011. The canyon diablo impact event: 2. Projectile fate and target melting upon impact. Meteorit. Planet. Sci. 46 (6), 805–829. Arvidson, R.E., Anderson, R.C., Bartlett, P., Bell, J.F., Christensen, P.R., Chu, P., Davis, K., Ehlmann, B.L., Golombek, M.P., Gorevan, S., Guinness, E.A., Haldemann, A.F.C., Herkenhoff, K.E., Landis, G., Li, R., Lindemann, R., Ming, D.W., Myrick, T., Parker, T., Richter, L., Seelos, F.P., Soderblom, L.A., Squyres, S.W., Sullivan, R.J., Wilson, J., 2004. Localization and physical property experiments conducted by Opportunity at Meridiani Planum. Science 306, 1730–1733. Ashley, J.W., Golombek, M.P., Christensen, P.R., Squyres, S.W., McCoy, T.J., Schröder, C., Fleischer, I., Johnson, J.R., Herkenhoff, K.E., Parker, T.J., 2011. Evidence for mechanical and chemical alteration of iron‐nickel meteorites on Mars: process insights for Meridiani Planum. J. Geophys. Res. 116, http://dx.doi.org/10.1029/ 2010JE003672 (E00F20). Beard, K.V., Chuang, C., 1987. A new model for the equilibrium shape of raindrops. J. Atmos. Sci. 44, 1509–1524. Bohren, C.F., Olivero, J.J., 1984. Evidence for haematite particles at 60 km altitude. Nature 310, 216–218. Borovička, J., Spurný, P., Brown, P., Wiegert, P., Kalenda, P., Clark, D., Shrbený, L., 2013. The trajectory, structure and origin of the Chelyabinsk asteroidal impactor. Nature 503, 235–237. Bowen, B.B., Benison, K.C., Oboh-Ikuenobe, F.E., Story, S., Mormile, M.R., 2008. Active hematite concretion formation in modern acid saline lake sediments, Lake Brown, Western Australia. Earth Planet. Sci. Lett. 268, 52–63. Brownlee, D.E., 1985. Cosmic dust: collection and research. Annu. Rev. Earth Planet. Sci. 13, 147–173.
Brownlee, D.E., Bates, B., Schramm, L., 1997. The Leonard Award Address Presented 1996 July 25, Berlin, Germany: the elemental composition of stony cosmic spherules. Meteorit. Planet. Sci. 32, 157–175. Calvin, W.M., Shoffner, J.D., Johnson, J.R., Knoll, A.H., Pocock, J.M., Squyres, S.W., Weitz, C.M., Arvidson, R.E., Bell, J.F., Christensen, P.R., de Souza, P.A., Farrand, W.H., Glotch, T.D., Herkenhoff, K.E., Jolliff, B.L., Knudson, A.T., McLennan, S.M., Rogers, A. D., Thompson, S.D., 2008. Hematite spherules at Meridiani: results from MI, MiniTES, and Pancam. J. Geophys. Res.: Planets, 113; (E12S37). Chan, M.A., Beitler, B., Parry, W.T., Ormo, J., Komatsu, G., 2004. A possible terrestrial analog for haematite concretions on Mars. Nature 429, 731–734. Chan, M.A., Bowen, B.B., Parry, W.T., 2005. Red rock and red planet diagenesis: comparisons of Earth and Mars concretions. GSA Today 15, 4–10. Chappelow, J.E., Golombek, M.P., 2010. Event and conditions that produced the iron meteorite Block Island on Mars. J. Geophys. Res. 115, http://dx.doi.org/10.1029/ 2010JE003666 (E00F07). Christensen, P.R., Bandfield, J.L., Clark, R.N., Edgett, K.S., Hamilton, V.E., Hoefen, T., Kieffer, H.H., Kuzmin, R.O., Lane, M.D., Malin, M.C., Morris, R.V., Pearl, J.C., Pearson, R., Roush, T.L., Ruff, S.W., Smith, M.D., 2000. Detection of crystalline hematite mineralization on Mars by the thermal emission spectrometer: evidence for near-surface water. J. Geophys. Res.: Planets 105, 9623–9642. Christensen, P.R., Morris, R.V., Lane, M.D., Bandfield, J.L., Malin, M.C., 2001. Global mapping of Martian hematite mineral deposits: remnants of water-driven processes on early Mars. J. Geophys. Res.: Planets 106, 23873–23885. Christensen, P.R., Wyatt, M.B., Glotch, T.D., Rogers, A.D., Anwar, S., Arvidson, R.E., Bandfield, J.L., Blaney, D.L., Budney, C., Calvin, W.M., Fallacaro, A., Fergason, R.L., Gorelick, N., Graff, T.G., Hamilton, V.E., Hayes, A.G., Johnson, J.R., Knudson, A.T., McSween, H.Y., Mehall, G.L., Mehall, L.K., Moersch, J.E., Morris, R.V., Smith, M.D., Squyres, S.W., Ruff, S.W., Wolff, M.J., 2004. Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity Rover. Science 306, 1733–1739. Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, Drops, and Particles. Academic Press, New York Cooper, R.F., Fanselow, J.B., Poker, D.B., 1996. The mechanism of oxidation of a basaltic glass: chemical diffusion of network-modifying cations. Geochim. et Cosmochim. Acta 60, 3253–3265. Fairén, A.G., Dohm, J.M., Baker, V.R., Thompson, S.D., Mahaney, W.C., Herkenhoff, K.E., RodrÍGuez, J.A.P., Davila, A.F., Schulze-Makuch, D., El Maarry, M.R., Uceda, E.R., Amils, R., Miyamoto, H., Kim, K.J., Anderson, R.C., McKay, C.P., 2011. Meteorites at Meridiani Planum provide evidence for significant amounts of surface and nearsurface water on early Mars. Meteorit. Planet. Sci. 46, 1832–1841. Fan, C., Xie, H., Schulze-Makuch, D., Ackley, S., 2010. A formation mechanism for hematite-rich spherules on Mars. Planet. Space Sci. 58, 401–410. K. Fox, How a raindrop is like an exploding parachute, ScienceNOW, 20 July 2009. 〈http://news.sciencemag.org/sciencenow/2009/07/20-02.html〉. Glotch, T.D., Bandfield, J.L., 2006. Determination and interpretation of surface and atmospheric miniature thermal emission spectrometer spectral end-members at the Meridiani Planum landing site. J. Geophys. Res.: Planets 111 (E12S06). Glotch, T.D., Christensen, P.R., Sharp, T.G., 2006. Fresnel modeling of hematite crystal surfaces and application to martian hematite spherules. Icarus 181, 408–418. Herkenhoff, K.E., Squyres, S.W., Arvidson, R., Bass, D.S., Bell, J.F., Bertelsen, P., Ehlmann, B.L., Farrand, W., Gaddis, L., Greeley, R., Grotzinger, J., Hayes, A.G., Hviid, S.F., Johnson, J.R., Jolliff, B., Kinch, K.M., Knoll, A.H., Madsen, M.B., Maki, J.N., McLennan, S.M., McSween, H.Y., Ming, D.W., Rice, J.W., Richter, L., Sims, M., Smith, P.H., Soderblom, L.A., Spanovich, N., Sullivan, R., Thompson, S., Wdowiak, T., Weitz, C., Whelley, P., 2004. Evidence from Opportunity's microscopic imager for water on Meridiani Planum. Science 306, 1727–1730. Kim, H.S., Kobayashi, Y., Nagai, K., 2006. Modeling of the surface tension of liquid Fe–P alloy by calculation of liquidus line in Fe–P binary system. J. Mater. Res. 21, 1399–1408. Klingelhöfer, G., Morris, R.V., Bernhardt, B., Schröder, C., Rodionov, D.S., de Souza, P.A., Yen, A., Gellert, R., Evlanov, E.N., Zubkov, B., Foh, J., Bonnes, U., Kankeleit, E., Gütlich, P., Ming, D.W., Renz, F., Wdowiak, T., Squyres, S.W., Arvidson, R.E., 2004. Jarosite and Hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer. Science 306, 1740–1745. Knauth, L.P., Burt, D.M., Wohletz, K.H., 2005. Impact origin of sediments at the opportunity landing site on Mars. Nature 438, 1123–1128. Kula, J., Baldwin, S.L., 2012. On hematite as a target for dating aqueous conditions on Mars. Planet. Space Sci. 67, 101–108. McLennan, S.M., Bell Iii, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., de Souza, P.A., Farmer, J., Farrand, W.H., Fike, D.A., Gellert, R., Ghosh, A., Glotch, T.D., Grotzinger, J.P., Hahn, B., Herkenhoff, K.E., Hurowitz, J.A., Johnson, J.R., Johnson, S.S., Jolliff, B., Klingelhöfer, G., Knoll, A.H., Learner, Z., Malin, M.C., McSween, H.Y., Pocock, J., Ruff, S.W., Soderblom, L.A., Squyres, S.W., Tosca, N.J., Watters, W.A., Wyatt, M.B., Yen, A., 2005. Provenance and diagenesis of the evaporite-bearing burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 95–121. MER mission image a, Wopmay, 2004. Mars Exploration Rover Mission, JPL/NASA. Press Release Images: Opportunity “Wopmay in False Color”, available at: 〈http://marsrover.nasa.gov/gallery/press/opportunity/20041104a/08-SS-01 -Wopmay-B278R1.jpg〉. MER mission image b, Heat Shield Rock. R. D. G. Frederick, 2005. The lying colors of Mars Opportunity's eye-candy, online image courtesy NASA/JPL/Cornell available at: 〈http://www.norwebster.com/mars/mer/images/meteorite.jpg〉. MER mission image c, sol 369, 2005. M. Lyle, Mars Exploration rover imagery, available at: 〈http://mars.lyleresearch.com/imagery/1M160940487EFF42D9P2956M2M1.JPG.html〉.
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MER mission image d, sol 319, 2005. R. D. G. Frederick, The lying colors of Mars Opportunity's eye-candy, online image courtesy NASA/JPL/Cornell available at: 〈http://www.norwebster.com/mars/mer/images/bbxroad.jpg〉. MER mission image, Opportunity Self-Portrait, 2005. Mars Exploration Rover Mission, JPL/NASA. Press Release Images: Opportunity Self-Portrait, Sols 322-323, 〈http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20050218a. html〉. MER mission image, Spirit Self-Portrait, 2005. Mars Exploration Rover Mission, JPL/ NASA. Press Release Images: Spirit Self-Portrait, Sols 329-330〈http://marsro vers.jpl.nasa.gov/gallery/press/spirit/20050218a.html〉. MER mission image, Spirit Self-Portrait, 2006. Mars Exploration Rover Mission, JPL/ NASA. Press Release Images: Spirit, Still Shining After All This Time (Vertical), 〈http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20060104a.html〉. Minitti, M.E., Lane, M.D., Bishop, J.L., 2005. A new hematite formation mechanism for Mars. Meteorit. Planet. Sci. 40, 55–69. H. Perlman. Are raindrops shaped like teardrops? USGS website (2005), 〈http://ga. water.usgs.gov/edu/raindropshape.html〉. Schröder, C., Rodionov, D.S., McCoy, T.J., Jolliff, B.L., Gellert, R., Nittler, L.R., Farrand, W.H., Johnson, J.R., Ruff, S.W., Ashley, J.W., Mittlefehldt, D.W., Herkenhoff, K.E., Fleischer, I., Haldemann, A.F.C., Klingelhöfer, G., Ming, D.W., Morris, R.V., de Souza, P.A., Squyres, S.W., Weitz, C., Yen, A.S., Zipfel, J., Economou, T., 2008. Meteorites on Mars observed with the Mars Exploration Rovers. J. Geophys. Res.: Planets 113 (E06S22). Sefton-Nash, E., Catling, D.C., 2008. Hematitic concretions at Meridiani Planum, Mars: their growth timescale and possible relationship with iron sulfates. Earth Planet. Sci. Lett. 269, 366–376. Squyres, S.W., Arvidson, R.E., Bell, J.F., Brückner, J., Cabrol, N.A., Calvin, W., Carr, M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Marais, D.J.D., d’Uston, C., Economou, T., Farmer, J., Farrand, W., Folkner, W., Golombek, M., Gorevan, S., Grant, J.A., Greeley, R., Grotzinger, J., Haskin, L., Herkenhoff, K.E., Hviid, S., Johnson, J., Klingelhöfer, G., Knoll, A.H., Landis, G., Lemmon, M., Li, R., Madsen, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., Morris, R.V., Parker, T., Rice, J.W., Richter, L., Rieder, R., Sims, M., Smith, M., Smith, P., Soderblom, L.A., Sullivan, R., Wänke, H., Wdowiak, T., Wolff, M., Yen, A., 2004a. The Opportunity Rover’s Athena science investigation at Meridiani Planum, Mars. Science 306, 1698–1703.
23
Squyres, S.W., Grotzinger, J.P., Arvidson, R.E., Bell, J.F., Calvin, W., Christensen, P.R., Clark, B.C., Crisp, J.A., Farrand, W.H., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., Knoll, A.H., McLennan, S.M., McSween, H.Y., Morris, R.V., Rice, J.W., Rieder, R., Soderblom, L.A., 2004b. In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars. Science 306, 1709–1714. Squyres, S.W., Arvidson, R.E., Bollen, D., Bell, J.F., Brückner, J., Cabrol, N.A., Calvin, W.M., Carr, M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Des Marais, D.J., d’Uston, C., Economou, T., Farmer, J., Farrand, W.H., Folkner, W., Gellert, R., Glotch, T.D., Golombek, M., Gorevan, S., Grant, J.A., Greeley, R., Grotzinger, J., Herkenhoff, K.E., Hviid, S., Johnson, J.R., Klingelhöfer, G., Knoll, A.H., Landis, G., Lemmon, M., Li, R., Madsen, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., Morris, R.V., Parker, T., Rice, J.W., Richter, L., Rieder, R., Schröder, C., Sims, M., Smith, M., Smith, P., Soderblom, L.A., Sullivan, R., Tosca, N.J., Wänke, H., Wdowiak, T., Wolff, M., Yen, A., 2006a. Overview of the Opportunity Mars Exploration Rover mission to Meridiani Planum: Eagle Crater to Purgatory Ripple. J. Geophys. Res.: Planets, 111; a (E12S12). Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, B.C., Grotzinger, J.P., Jolliff, B.L., McLennan, S.M., Tosca, N., Bell, J.F., Calvin, W.M., Farrand, W.H., Glotch, T.D., Golombek, M.P., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., McSween, H.Y., Yen, A.S., 2006b. Two years at Meridiani Planum: results from the Opportunity Rover. Science 313, 1403–1407. Squyres, S.W., Knoll, A.H., Arvidson, R.E., Ashley, J.W., Bell, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., Cohen, B.A., de Souza, P.A., Edgar, L., Farrand, W.H., Fleischer, I., Gellert, R., Golombek, M.P., Grant, J., Grotzinger, J., Hayes, A., Herkenhoff, K.E., Johnson, J.R., Jolliff, B., Klingelhöfer, G., Knudson, A., Li, R., McCoy, T.J., McLennan, S. M., Ming, D.W., Mittlefehldt, D.W., Morris, R.V., Rice, J.W., Schröder, C., Sullivan, R.J., Yen, A., Yingst, R.A., 2009. Exploration of Victoria crater by the Mars Rover Opportunity. Science 324, 1058–1061. Taylor, S., Brownlee, D.E., 1991. Cosmic spherules in the geologic record. Meteoritics 26, 203–211. Taylor, S., Lever, J.H., Harvey, R.P., 2000. Numbers, types, and compositions of an unbiased collection of cosmic spherules. Meteorit. Planet. Sci. 35, 651–666.
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Possible mechanism for explaining the origin and size distribution of Martian hematite spherules Anupam K. Misra n, Tayro E. Acosta-Maeda, Edward R.D. Scott, Shiv K. Sharma Hawaii Institute of Geophysics and Planetology, SOEST, University of Hawaii Honolulu, HI 96822, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2013 Received in revised form 18 January 2014 Accepted 24 January 2014 Available online 5 February 2014
Mysterious hematite spherules, also known as “blueberries”, observed at Meridiani Planum on Mars have been widely accepted as concretions which are formed by precipitation of aqueous fluids. One of the biggest mysteries is that all observed Martian blueberries are limited in size with maximum diameter of 6.2 mm. In contrast, terrestrial concretions are not size limited. In this article, we discuss significant differences between Martian blueberries and Earth concretion analogs. Puzzling observations from Mars Exploration Rovers Opportunity and Spirit suggest that the spherules may not be concretions but are cosmic spherules formed by ablation of meteorites. The perfect spherical shape of spherules, their observed size limit, and all other physical properties are easily explained by a meteorite ablation model. Evidence that some of these spherules are only few years old strongly constrains concretion and other growth mechanisms related to aqueous processes that require the existence of water on Mars in its recent history. The large number of hematite spherules in Meridiani Planum may be due to a big rare iron meteorite impact event in this region sometime in the past. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Mars spherules Mars blueberries Hematite spherules Meteorite Concretions Cosmic spherules
1. Introduction On January 24, 2004, the Mars Exploration Rover Opportunity landed in Eagle crater on Meridiani Planum. This site was chosen because earlier MARS Global Surveyor TES data showed it to contain large amounts of the mineral hematite (Christensen et al., 2000). Because on Earth, hematite is formed through aqueous processes, the discovery of hematite in Martian soil suggests the past presence of water. One of the surprising discoveries of the Rover was the presence of large numbers of hematite spherules (Klingelhöfer et al., 2004; Squyres et al., 2004a; Squyres et al., 2004b), now commonly known as “blueberries”. Initial scientific investigation (McLennan et al., 2005; Squyres et al., 2004a; Squyres et al., 2004b) suggested that the blueberries and their fragments are the primary carriers of hematite at the site. The hematite in the Martian soil is mostly observed on the surface, and the subsurface soil in the excavated trenches is dominated by basaltic sand. The blueberries are typically 4 mm in diameter, mostly perfect hard spheres with very fine grain and no observed internal structure. The entire inventory of the observed spherules is suggested to be within the upper 1 cm thickness of the surface soil (Calvin et al., 2008; Christensen et al., 2004; McLennan et al., 2005; Squyres et al., 2004a; Squyres et al., 2006a; Squyres et al., 2004b).
n
Corresponding author. Tel.: +1 808 9565095. E-mail address: [email protected] (A.K. Misra).
http://dx.doi.org/10.1016/j.pss.2014.01.020 0032-0633 & 2014 Elsevier Ltd. All rights reserved.
For the past 9 years, most of the scientific community has widely accepted that the Mars blueberries are concretions (Calvin et al., 2008; Christensen et al., 2004; Glotch and Bandfield, 2006; Glotch et al., 2006; Kula and Baldwin, 2012; McLennan et al., 2005; SeftonNash and Catling, 2008; Squyres et al., 2004a; Squyres et al., 2006a; Squyres et al., 2004b; Squyres et al., 2009; Squyres et al., 2006b). Possible concretion terrestrial analogs have been found in southern Utah, in the Jurassic Navajo Sandstone (Chan et al., 2004; Chan et al., 2005), and in Lake Brown, Australia (Bowen et al., 2008). However, a comparison of the iron oxide concretions from the Navajo Sandstone and images and scientific data made available by NASA reveals remarkable differences between the terrestrial concretions and Martian blueberries. Here we present evidence that indicates that the blueberries are not concretions and suggest that instead they are of meteoritic origin.
2. Physical properties of Earth concretions and Martian spherules By definition, a concretion is a compact mass of mineral matter, usually spherical or disk-shaped, embedded in a host rock of a different composition. Concretions are formed by precipitation of mineral matter from the liquid phase, usually about a nucleus. The physical mechanism of nucleation and growth is required for the formation of concretions. This growth mechanism puts no limit on
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the size of the concretion. Concretions on Earth are large (centimeters to meters) (Abdel-Wahab and McBride, 2001; Chan et al., 2004) and seem to have no size limit. In contrast, all blueberries found on Mars are limited in size (diameter less than 6.2 mm) (Calvin et al., 2008; McLennan et al., 2005; Sefton-Nash and Catling, 2008). Apart from the size difference, concretions come in variety of shapes and are rarely perfect spheres (Bowen et al., 2008; Chan et al., 2004). The interior of a Mars spherule is hard, uniform, and extremely fine grained (Arvidson et al., 2004; Herkenhoff et al., 2004). Terrestrial concretions are not spherical but metallic looking shells, which enclose the grains of the host matrix as part of the concretion mechanism (Bowen et al., 2008). Fig. 1 shows the picture of few terrestrial concretions from Jurassic Navajo Sandstone, southern Utah. The concretions have been cut in half to reveal the interior structure. The figure illustrates that these concretions are not perfect spheres and do not show a size limit. The sizes of these Earth spherules are also much larger than the Martian spherules which have the upper size limit of 6.2 mm . The interior structure of the concretions is different than the outer layer. Micro-Raman spectroscopy was used to identify the chemical compositions of terrestrial concretions. Raman spectra were measured at multiple locations on all the samples. Fig. 2 shows representative Raman spectra of terrestrial concretions showing
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the spectral differences between the interior brown grains and outer black grains. It is shown that the brown grains located inside the concretions are quartz. For comparison, Raman spectra of rose quartz, hematite and goethite minerals are also shown. The Raman spectrum of rose quartz is identical to the Raman spectra of brown grains from inside of the concretions which confirm that the interior structure is mostly quartz, with grain size of the order of 150 μm. The black grains located at the outer layer of concretion also show dominating quartz peaks along with additional Raman peaks associated with goethite. The Raman analysis indicates that the exterior shell layer is mainly quartz with a very thin coating of goethite. Our Raman analysis of iron oxide concretions is consistent with that of Knauth et al. (2005). Concretion cements around the grains of the host matrix, and hence grains of host matrix are always included in the terrestrial concretions (Bowen et al., 2008). The concretion mechanism does not explain or account for the perfectly size-limited, homogeneous hematite spherules excluding the grains of host soil found on Mars. Another important fact is that on Earth erosion plays a critical role in the formation of concretions by removing the surrounding matrix and releasing the concretions. Therefore, the Earth concretion analogs are dull metallic objects that show signs of erosion and evidence of pitting. Often the outer surface also shows flow patterns, indicating the presence of aqueous media (Bowen et al., 2008; Chan et al., 2004). The erosion process also dictates that the Earth concretions are several thousand years old (Bowen et al., 2008). Another puzzle is that Martian blueberries are mostly concentrated within 1 cm of the surface (Arvidson et al., 2004; Bowen et al., 2008; Squyres et al., 2004a; Squyres et al., 2004b). This observation is not consistent with the formation of mineral precipitation from ground-water. As the level of ground-water goes down, the concentrations of dissolved minerals are expected to increase in the subsurface soil. For the terrestrial concretions, relatively larger numbers are observed in the deeper soil (Bowen et al., 2008) below the 1 cm top layer. However, no blueberries were observed in the deeper soil when trenches were excavated on Mars (Christensen et al., 2004; Squyres et al., 2004a; Squyres et al., 2006a). This observation suggests that a physical mechanism capable of depositing spherules from the top is more favorable than mechanisms involving ground-water. Previous research suggests (Squyres et al., 2004a; Squyres et al., 2004b) that hematite-rich spherules are embedded in the rock and erod from them. One of the images (Fig. 3) taken on sol 251
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Fig. 1. Earth concretions analog. Picture of Earth concretion analog samples from Jurassic Navajo Sandstone, southern Utah (courtesy William Mahaney). Samples have been cut in half to show the interior structure.
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Raman Shift ((cm-1) Fig. 2. Raman analysis of Earth concretions. Micro-Raman spectra of inner and outer grains of Earth concretions measured with 785 nm laser excitation. The Raman spectra of interior brown grains are identical to that of reference Raman spectrum of rose quartz. The black grains in the outer layer are goethite coated quartz.
Fig. 3. Blueberries on Wopmay Rock suggesting they landed from the top. Wopmay rock observed on sol 251 showing hundreds of fully exposed blueberries on it. Concentration of blueberries in region A is higher than that away from the rock (region B), and no blueberries are observed in the shadow of the rock as shown in region C; suggesting that these blueberries fell from the top. Image courtesy of NASA/JPL.
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from the Opportunity Rover is that of the unusual 1 m lumpy rock “Wopmay” found on the lower slopes of Endurance Crater (MER mission image a, Wopmay, 2004). The image displays hundreds of blueberries on its surface. A closer look confirms that all the spherules on the rock are fully formed spheres and are fully exposed, i.e., none of the spheres are seen as partially emerged. If the spherules are concretions, we would expect to see some partially exposed blueberries protruding from the rock. An erosion process as a part of concretion mechanism would produce both completely and partially exposed spherules on all the surfaces of the rock. The distribution of the blueberries around the rock strongly suggests that they fell from above. This is indicated by comparing the high concentration of the blueberries on the left side of the rock (region A) with the lower concentration of blueberries in region B away from the rock. Another supporting observation is that no blueberries can be seen in region C, which is in the shadow of the rock. The absence of blueberries in region C is not consistent with a concretion mechanism. We would also like to point out that the likelihood of dust devils transporting the blueberries is also ruled out here. A dust devil event would fill region C with blueberries and would also remove blueberries from the slopes of the Wopmay rock.
3. Proposed meteorite ablation mechanism and evidences supporting the cosmic spherule model We propose that the observed spherules originate from the ablation of meteorites. Meteoroids, of various sizes, enter the Mars atmosphere at high speed and low temperature. Upon entering the atmosphere, they are heated by ram pressure (defined as a pressure exerted on a body moving through a fluid medium) and friction. The smaller meteoroids are completely melted like micrometeorites. Bigger meteoroids, because of their large mass and thermal conductivity, need more time to melt completely. Melting begins at the meteoroid's surface and as soon as the surface liquid reaches a critical depth, the liquid falls away and immediately breaks down into smaller spherical drops because of atmospheric drag and surface tension. This liquid drop formation also limits the thickness of the fusion crust of the meteorite, which is only a few mm thick. The maximum size of droplets is controlled by surface tension of liquid and the atmospheric drag force. Because the terminal velocity of the small drops is lower than that of the bigger meteoroid, the liquid drops decrease in speed, which results in lower ram pressure, in turn causing the temperature of the drops to decrease and become solid spherules. The melting mechanism also produces imprints on the meteorite surface from the falling liquid drops. Strong evidence in support of this theory is the observed iron meteorite named “Heat Shield Rock” on sol 349 (Fig. 4), which was found on Mars (MER mission image b, Heat Shield Rock. R. D. G. Frederick, 2005). The MER science team confirmed that the composition of this meteorite is mainly iron, with kamacite as the primary Fe-bearing mineral and around 7 wt% nickel (Fairén et al., 2011; Schröder et al., 2008). A careful examination of the surface of the fusion crust shows regmaglypts and several circular imprints. This suggests an estimation of the approximate size of the molten drops removed from the meteorite, which is consistent with the size of the blueberries lying on the ground. The impact of the meteorite on the ground is expected to form several micro-berries and mini-berries from the liquid which was still attached to the meteorite before the impact. Fig. 4 clearly shows a larger concentration of smaller blueberries in the immediate surroundings of the meteorite forming a halo. There are also several blueberries shown on top of the meteorite. The presence of these blueberries on the meteorite is not
Fig. 4. Blueberries and imprints on the fusion crust of an iron meteorite found on Mars. Iron meteorite named “heat shield rock” observed on sol 339 by the Opportunity Rover showing a few blueberries on it and several imprints formed by spherules removed from the fusion crust. Image 1P159435226EFF40DPP taken from Sol 352. Image courtesy of NASA/JPL/Cornell.
consistent with the concretion phenomenon. First, a meteorite has to contain enough liquid water to initiate concretion. Second, the host matrix must erode significantly to expose the spherules. After such a concretion–erosion process, one would not expect to see any imprint left on the fusion crust. In addition to the cosmic spherules formed by the meteorite ablation mechanism, the impact of a significantly large meteorite on the ground can also produce impact spherules by melting the ground. The cosmic spherules are not expected to make any impact spherules due to their low mass. The impact spherules are possible on Mars but are expected to be formed in the localized impact regions associated with large meteorite landing sites and not distributed over a large area on Meridiani Planum. One strong support for the origin of spherules from meteorites is that it puts an upper limit on the size of the spherules. By assuming that the spherules will break up if the atmospheric drag force is greater than the surface tension force, and at terminal velocity the drag force is equal to the gravitational force, one can estimate (Clift et al., 1978) the diameter of the drop (D) to be proportional to rffiffiffiffiffiffi s Dp gρ Where s is surface tension of the liquid, g is acceleration due to gravity and ρ is the density of the liquid. Simplifying the above equation rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 s2 g 1 ρ 1 ¼ D1 s1 g 2 ρ 2 where subscripts 1 and 2 represent two different types of liquids. Raindrops are usually 1 mm to 2 mm in size. Drops as large as 3 mm that are not spherical (Beard and Chuang, 1987, Perlman 2005) are sometimes observed. When the raindrop reaches 4.5 mm, it immediately breaks up into small drops and microdrops by opening up like a parachute (Perlman 2005, Fox 2009). Using s1 ¼0.073 N/m surface tension of water, g1 ¼9.8 m/s2 as Earth gravity, ρ1 ¼ 1000 kg/m3 as density of water, s2 ¼ 1.46 N/m surface tension of molten iron (Kim et al. 2006), g2 ¼ 3.675 m/s2 as Mars gravity, and ρ2 ¼7860 kg/m3 as the density of iron one can estimate the diameter (D2) of blueberries, assuming that they are formed from the melting of iron meteorites. Substituting these values gives D2 ¼2.6 D1, which estimates the size of blueberry D2 ¼3.9 mm if D1 ¼1.0035 mm. The blueberry of size 7.8 mm corresponding to a 3 mm raindrop size is not expected to be spherical at all. Any
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blueberry reaching 12 mm in size (corresponding to D1 ¼4.5 mm) would immediately break up into smaller drops and microdrops by opening up as a parachute. This theory can be easily disproved if even one blueberry out of thousands observed so far on Mars violates the size limit. However, so far no blueberry is shown to be larger than 6.2 mm (Calvin et al., 2008; McLennan et al., 2005; Sefton-Nash and Catling, 2008; Squyres et al., 2004a; Squyres et al., 2004b) and the proposed theory holds. The theory also explains the presence of large numbers of micro-berries observed only on the top surface layer of Mars. The observation of a large number of spherules and microspherules is not an indication of a large number of meteoritic events on Mars. This is because a small meteorite of size 4 cm in diameter entering the Martian atmosphere at an angle is expected to make 1000 spherules of 4 mm in size which will be distributed over a large area along the meteorite trajectory. The vast majority of spherules are isolated spheres and very rarely doublets and triplets have been observed (McLennan et al., 2005; Squyres et al., 2004b; Squyres et al., 2006b, Calvin et al., 2008). Rare occurrences of doublets and triplets have been suggested to be a strong evidence of the concretion mechanism by McLennan et al. (2005) because they are statistically unlikely in volcanic and impact spherules. With the meteorite ablation mechanism it is possible to form doublets and triplets. The meteorite ablation mechanism also explains why the occurrence of doublets and triplets will be very rare in comparison to isolated spherules. As two liquid drops come in contact with each other, doublets can be formed. The mechanism, however, suggests that the probability of the formation of doublets and triplets would be very small as this would require a recombination of drops near the liquid–solid phase transition. A recombination well above the melting point in the liquid phase would form a big liquid drop which will break up again into smaller drops. An interesting prediction is that it is more likely to observe doublets and triplets consisting of individual drops of different diameters rather than the same diameter because the recombination is more likely to happen if one drop is moving faster than the other drop. Another prediction is that one should also observe berries other than blueberries representing meteorites of different compositions. All these predictions are in agreement with observations on Mars (Calvin et al., 2008; McLennan et al., 2005; Squyres et al., 2004b; Squyres et al., 2006b). Another issue that is not consistent with the concretion mechanism is the age of the spherules. Because erosion is an important step in formation of the concretion, the spherules are expected to be thousands of years old, like those found on Earth. Fig. 5 shows mostly eroded spherules from sol 369 (MER mission image c, sol
Fig. 5. Evidence supporting spherules of different ages. Image 1M160940487 EFF42D9P2956M2M1-BR (sol 369) showing coexistence of old and new berries indicating large age difference. Courtesy: NASA/JPL & M Lyle.
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Fig. 6. Evidence supporting spherules of different compositions. Image from sol 319 showing yellow berries along with blue berries indicating chemical difference. Image courtesy of NASA/JPL.
369 (2005)), however, a careful look at the photograph reveals a very shiny (and hence younger) spherule in the figure (indicated by arrow). The shine of the spherule indicates that this spherule is much younger than the other dull looking spherules because most polished metallic surfaces starts looking dull after few years due to weathering. An Earth analog would be comparing metallic parts of a new car to an older car. The self-portrait pictures of Opportunity Rover from 2004, 2007 and 2011 show weathering effect on the Rover due to Martian atmosphere. The coexistence of old and new spherules on Meridiani Planum is compatible with the meteorite ablation model, but not with the concretion model. The concretion model would require the surface water to come back after thousands of years and form new concretions on the surface of Meridiani Planum. Later in this article we show that some of the spherules are only a few years old, which would require an extremely fast concretion phenomenon. In Fig. 6 (MER mission image d, sol 319, 2005) we observe the co-existence of both blue and yellow berries, indicating that other types of meteorites do fall on Mars and form size-limited spherules. Although this is a false color image, the false color scheme does indicate a change in optical reflectance and hence a different chemical compositions for blue and yellow berries. And finally, we look at the evidence that strongly supports the cosmic spherules mechanism but is impossible to explain the concretion mechanism. If spherules do originate from meteorites then there is a small possibility that they will fall on top of the Rovers and other introduced man-made objects. For the meteorite model to hold, any spherules observed on the Rovers must hold the size limit and also be shiny with no sign of erosion. The ability to collect only the shiny spherules over the eroded spherules would rule out possibility of collecting spherules from the ground due to dust devil events. The evidence of spherules on man-made objects will indicate that they fell after January 24, 2004. One of the places to look for evidence is on the heat shield. However, the heat shield landed with a high speed, which may have disturbed the spherules lying on the ground and hence it is possible that it picked up spherules from the ground during the impact. Therefore, the presence of spherules on the heat shield may not be a convincing case. Other man-made objects are the Rovers themselves and unlike the heat shield they landed smoothly and traveled at a low speed. Fig. 7 (sol 322) shows few spherules collected by the Opportunity Rover (MER mission image, Opportunity Self-Portrait (2005)). The size of these spherules matches with the size of the blueberries shown on the ground in Fig. 7 at right bottom corners of the photograph. Fig. 8 shows what seem to be micro-spherules at the two locations labeled “A” on the solar panel of the Opportunity Rover. The image (1M156679326EFF3981P2979M2M1) was taken by the microscopic imager on Opportunity Rover on sol 321. The image also shows a burned
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Fig. 7. Evidence of blueberries landed on the Opportunity Rover. Image taken from sol 322 showing blueberries on the Opportunity Rover.
Fig. 8. Evidence of micro-blueberries landed on the Opportunity Rover. Image (sol 321) shows microberries on the solar panel of the Opportunity Rover at locations labeled “A” and burned mark at location “B”.
spherical object has appeared on the bottom of the solar panel marked “E” and the effects of dust devils can be seen in the area marked “F”, which shows a collection of dust only. The ability of these round objects to leave marks and roll off suggests that they fell on the Rover from above with enough speed to imprint solar panels. We propose that these objects are indeed cosmic spherules and that their presence on the Rovers is evidence that these spherules are only few years old. This strongly constrains concretion and other growth mechanisms (Fan et al., 2010) related to aqueous processes because it would require the existence of water on Mars in its recent history and a much faster growth process for spherules formation. According to Mössbauer data (Klingelhöfer et al., 2004), blueberries at Meridiani Planum are made of hematite. A recent publication by Minitti et al. (2005) showed that it is possible to form hematite in a CO2 atmosphere at temperatures above 900 1C from high-FeO glass-rich basalts. Cooper et al. (1996) determined that with FeO concentration 4 1.9 wt%, and temperature of 700 1C, the Fe migration is favored over Ca and Mg migration. Hence it is possible for hematite (and a hematite coating) to form from meteorites that have some iron in them under the present or past conditions existing on Mars. Aqueous alteration after being deposited in the ground could also account for the formation of hematite. The formation of hematite from iron is possible in the atmosphere and has been suggested by Bohren and Olivero (1984) in an article on the evidence for formation of hematite particles at 60 km altitude on Earth. The Mössbauer instrument on the Opportunity Rover is mostly a surface technique due to the limited penetration depth of gamma rays used. Hence both pure hematite spherules and hematite-coated spherules will appear the same. A combined analysis of Mössbauer with Mini-TES indicates that in some cases spherules are composed of a mixture of hematite and basaltic material (McLennan et al., 2005). Both Mössbauer and Mini-TES instruments provide measurements on a large area and represent average composition of several spherules along with background soil. Mini-TES, similar to Mössbauer is also a surface technique. The instruments on the Opportunity Rover are not capable of analyzing chemical composition of individual spherules and would not be able to separate out different types of spherules in a mixed population. Such analytical capability is available to the ChemCam LIBS instrument on the Curiosity Rover.
4. Discussion and summary impact mark at location “B”. The circular burn marks on the solar panels of the Rover are in agreement with the meteorite ablation model as the cosmic spherules and micro-spherules would be hot. Other more convincing evidence is the observation of spherical shiny objects on the solar panels of the Spirit Rover, which landed near Gusev Crater. While investigating the Gusev Crater area the Spirit Rover rarely observed spherules on the ground and, therefore, it is highly unlikely that it selectively picked spherules from the ground during dust devil events. One can further argue that these spherules are not airborne from the ground. We would expect to see more dust and older and smaller spherules because they would be aerodynamically responsive to air lifting due to larger surface area-to-mass ratio. Any spherule collected by the Spirit Rover is strong a evidence of cosmic spherule. The images from the Spirit Rover taken on sol 330 show few spherical objects on the solar panels marked as “C” and “D” (Fig. 9a) (MER mission image, Spirit Self-Portrait, 2005). An image (Fig. 9b) of the same solar panels taken almost a year later on sol 586 (MER mission image, Spirit Self-Portrait, 2006) shows that two of the objects have rolled off and only the impact prints can be seen. A new
The evidence presented in previous sections strongly suggests that Martian spherules are cosmic spherules formed from the ablation of meteorites. If the Martian spherules originate from meteorites, they would be expected to be hard, extremely fine grained, perfect spheres, polished, size-limited, and show no sign of core nucleation. In addition they will be observed only on the top soil and will be missing in deeper soil. All these properties are consistent with observed data on the Martian blueberries. Table 1 lists the observed data on Martian spherules and shows that all of the observations are easily explained with the meteorite ablation model. Same tests are also shown for a concretion model for comparison. The thin Mars atmosphere creates a unique environment for spherules forming from meteorites. On the Earth's Moon, the lack of atmosphere deprives the meteorites of a heating mechanism; hence, no cosmic spherules are expected to be on the Moon. Meteorites falling on the Moon are expected to make the impact glass spherules. On Earth, cosmic spherules are of mm-size and are found in abundance (Brownlee, 1985; Brownlee et al., 1997; Taylor and Brownlee, 1991; Taylor et al., 2000) and roughly 10,000 t of
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Sol 330
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Sol 586 F C D
E Fig. 9. Evidence of blueberries landed on the Spirit Rover. Images (a) and (b) comparing same solar panels of Spirit Rover from sol 330 and sol 586 showing blueberries that have rolled off a year later from solar panels “C” and “D”. A new blueberry is seen in solar panel “E”. Area “F” shows a collection of dust only due to dust devil events.
Table 1 Observations and physical properties of Martian spherules and their comparison with concretion and meteorite ablation models. Observations
Agreement with concretion model
Agreement with meteorite model
1. All blueberries are less than 6.2 mm in diameter (size limitation) 2. Blueberries show no grain structure 3. Blueberries show no nucleation 4. Coexistence of old and fresh spherules (age difference) 5. Coexistence of blue and yellow berries (chemical difference) 6. Existence of blueberries limited to top soil and missing in deeper soil 7. Spherules found on manmade objects (Opportunity and Spirit Rovers) 8. Interiors of spherules missing grains of host soil 9. Hematite (composition) for blueberries 10. Predominantly perfect spheres (shape) 11. Large amount of micro-berries observed only on top surface 12. Blueberries appear embedded in soil 13. Hard (mechanical strength) 14. Circular burn spot on solar panel
No No No No No (may be) No No No Noa No No Yes Yes No
Yes Yes Yes Yes Yes Yes Yes Yes Yesb Yes Yes Yes Yes Yes
a b
Hematite-coated host soil grains (quartz, gypsum, etc.). Other chemical compositions (e.g. yellow berries) are also possible.
interplanetary dust impact the atmosphere each year (Brownlee, 1985). Mars has unique, ideal conditions (low gravity and thin CO2 atmosphere) for forming larger spherules. Mars is also located just next to the asteroid belt and therefore would be a suitable place to find large amounts of cosmic spherules (Fairén et al., 2011). The large number of blueberries in Meridiani Planum might be due to a big rare iron meteorite breaking up and partially melting over this region. A big iron meteorite entering the Martian atmosphere at low angle and exploding in the atmosphere can form the large amount of blueberries and microberries distributed over a several kilometer area on Meridiani Planum. This possibility is consistent with the fact that most of the blueberries on Meridiani Planum are likely to be very old, in excess of 3.5 Ga as suggested by McLennan et al. (2005). The global mapping of hematite coverage in Sinus Meridiani shows an elliptical distribution (Plate 2, Christensen et al. 2001). The hematite-rich unit is sharply defined and has not been mobilized. A relatively fresh, unnamed 22 km crater located at 31S, 6.61W and its continuous ejecta blanket showing no detectable evidence of hematite (Christensen et al. 2001) suggests that
hematite deposit is older than the crater and has not been redistributed. The elliptical distribution and sharply defined hematite boundary is consistent with a meteoroid origin. An iron meteorite of 40 m in diameter is equivalent to 1012 blueberries of 4 mm in diameter and can cover a large area. Several large iron meteorites have been discovered in Meridiani Planum (Ashley et al. (2011); Fairén et al. (2011)). Chappelow and Golombek (2010) argue that six iron meteorites are the result of a single large rare event. We suggest that the hematite spherules and the iron meteorites are the result of the same large rare impact. We infer that the Martian impact event resembled the Canyon Diablo impact in that most of the projectile was converted into melt droplets and multi-kg fragments (Artemieva and Pierazzo, 2011). However, in the Martian impact, a very low impact angle to the horizontal may have ensured that most of the impact energy was deposited in the atmosphere, as in the recent Chelyabinsk event (Borovička et al., 2013). The new shiny spherule (shown in Fig. 5), yellow berries (shown in Fig. 6) and cosmic spherules on the rovers are not part of the same event and may be due to stony or stony iron meteorites.
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5. Conclusion The evidence presented in this paper strongly suggests that the Martian berries (yellow and blue) originate from meteorites and are not concretions. In the cosmic spherule model, a meteoroid entering the Martian atmosphere melts and forms berries, microberries and nano phase materials. The maximum size of the spherules is limited by the surface tension of molten material and the atmospheric drag force. The estimated maximum size of blueberries, assuming they are formed from ablation of iron meteorites, is consistent with the observed data on Mars. In addition, the cosmic spherule model explains all observed properties of Martian hematite spherules including the mostly perfect spherical shape, largely isolated spheres, fine grain structure and no internal structure and nucleation. The observation of spherules and micro-spherules on Rovers is also in agreement with the cosmic spherule model. The circular burn spot on solar panel of rover is a strong evidence of hot microberry impact and this could be of concern to future Mars Rovers and other exploration missions and should be taken into account when designing equipments. The instruments should be able to withstand the impact of hot berries falling from sky. Finally, the cosmic spherule model also explains why the blueberries, yellow berries and micro-berries are only observed on the top surface layer and missing in the deeper soil.
Acknowledgments The authors would like to thank Nancy Hulbirt and May Izumi for their valuable help with figures and editing. We would also like to thank William Mahaney for providing the Earth concretion analog samples from Jurassic Navajo Sandstone, southern Utah. We are also thankful to Dr. Christopher McKay, Gary R. Huss and another anonymous reviewer for their critical review and constructive comments, which greatly helped to improve the manuscript. All the data and images from Mars are courtesy of NASA and JPL. JPL image policy allows use of images without prior permission as cited on their website 〈http://www.jpl.nasa.gov/images/ policy/〉 References Abdel-Wahab, A., McBride, E.F., 2001. Origin of giant calcite-cemented concretions, temple member, Qasr El sagha formation (Eocene), faiyum depression, Egypt. J. Sediment. Res. 71, 70–81. Artemieva, N., Pierazzo, E., 2011. The canyon diablo impact event: 2. Projectile fate and target melting upon impact. Meteorit. Planet. Sci. 46 (6), 805–829. Arvidson, R.E., Anderson, R.C., Bartlett, P., Bell, J.F., Christensen, P.R., Chu, P., Davis, K., Ehlmann, B.L., Golombek, M.P., Gorevan, S., Guinness, E.A., Haldemann, A.F.C., Herkenhoff, K.E., Landis, G., Li, R., Lindemann, R., Ming, D.W., Myrick, T., Parker, T., Richter, L., Seelos, F.P., Soderblom, L.A., Squyres, S.W., Sullivan, R.J., Wilson, J., 2004. Localization and physical property experiments conducted by Opportunity at Meridiani Planum. Science 306, 1730–1733. Ashley, J.W., Golombek, M.P., Christensen, P.R., Squyres, S.W., McCoy, T.J., Schröder, C., Fleischer, I., Johnson, J.R., Herkenhoff, K.E., Parker, T.J., 2011. Evidence for mechanical and chemical alteration of iron‐nickel meteorites on Mars: process insights for Meridiani Planum. J. Geophys. Res. 116, http://dx.doi.org/10.1029/ 2010JE003672 (E00F20). Beard, K.V., Chuang, C., 1987. A new model for the equilibrium shape of raindrops. J. Atmos. Sci. 44, 1509–1524. Bohren, C.F., Olivero, J.J., 1984. Evidence for haematite particles at 60 km altitude. Nature 310, 216–218. Borovička, J., Spurný, P., Brown, P., Wiegert, P., Kalenda, P., Clark, D., Shrbený, L., 2013. The trajectory, structure and origin of the Chelyabinsk asteroidal impactor. Nature 503, 235–237. Bowen, B.B., Benison, K.C., Oboh-Ikuenobe, F.E., Story, S., Mormile, M.R., 2008. Active hematite concretion formation in modern acid saline lake sediments, Lake Brown, Western Australia. Earth Planet. Sci. Lett. 268, 52–63. Brownlee, D.E., 1985. Cosmic dust: collection and research. Annu. Rev. Earth Planet. Sci. 13, 147–173.
Brownlee, D.E., Bates, B., Schramm, L., 1997. The Leonard Award Address Presented 1996 July 25, Berlin, Germany: the elemental composition of stony cosmic spherules. Meteorit. Planet. Sci. 32, 157–175. Calvin, W.M., Shoffner, J.D., Johnson, J.R., Knoll, A.H., Pocock, J.M., Squyres, S.W., Weitz, C.M., Arvidson, R.E., Bell, J.F., Christensen, P.R., de Souza, P.A., Farrand, W.H., Glotch, T.D., Herkenhoff, K.E., Jolliff, B.L., Knudson, A.T., McLennan, S.M., Rogers, A. D., Thompson, S.D., 2008. Hematite spherules at Meridiani: results from MI, MiniTES, and Pancam. J. Geophys. Res.: Planets, 113; (E12S37). Chan, M.A., Beitler, B., Parry, W.T., Ormo, J., Komatsu, G., 2004. A possible terrestrial analog for haematite concretions on Mars. Nature 429, 731–734. Chan, M.A., Bowen, B.B., Parry, W.T., 2005. Red rock and red planet diagenesis: comparisons of Earth and Mars concretions. GSA Today 15, 4–10. Chappelow, J.E., Golombek, M.P., 2010. Event and conditions that produced the iron meteorite Block Island on Mars. J. Geophys. Res. 115, http://dx.doi.org/10.1029/ 2010JE003666 (E00F07). Christensen, P.R., Bandfield, J.L., Clark, R.N., Edgett, K.S., Hamilton, V.E., Hoefen, T., Kieffer, H.H., Kuzmin, R.O., Lane, M.D., Malin, M.C., Morris, R.V., Pearl, J.C., Pearson, R., Roush, T.L., Ruff, S.W., Smith, M.D., 2000. Detection of crystalline hematite mineralization on Mars by the thermal emission spectrometer: evidence for near-surface water. J. Geophys. Res.: Planets 105, 9623–9642. Christensen, P.R., Morris, R.V., Lane, M.D., Bandfield, J.L., Malin, M.C., 2001. Global mapping of Martian hematite mineral deposits: remnants of water-driven processes on early Mars. J. Geophys. Res.: Planets 106, 23873–23885. Christensen, P.R., Wyatt, M.B., Glotch, T.D., Rogers, A.D., Anwar, S., Arvidson, R.E., Bandfield, J.L., Blaney, D.L., Budney, C., Calvin, W.M., Fallacaro, A., Fergason, R.L., Gorelick, N., Graff, T.G., Hamilton, V.E., Hayes, A.G., Johnson, J.R., Knudson, A.T., McSween, H.Y., Mehall, G.L., Mehall, L.K., Moersch, J.E., Morris, R.V., Smith, M.D., Squyres, S.W., Ruff, S.W., Wolff, M.J., 2004. Mineralogy at Meridiani Planum from the Mini-TES experiment on the Opportunity Rover. Science 306, 1733–1739. Clift, R., Grace, J.R., Weber, M.E., 1978. Bubbles, Drops, and Particles. Academic Press, New York Cooper, R.F., Fanselow, J.B., Poker, D.B., 1996. The mechanism of oxidation of a basaltic glass: chemical diffusion of network-modifying cations. Geochim. et Cosmochim. Acta 60, 3253–3265. Fairén, A.G., Dohm, J.M., Baker, V.R., Thompson, S.D., Mahaney, W.C., Herkenhoff, K.E., RodrÍGuez, J.A.P., Davila, A.F., Schulze-Makuch, D., El Maarry, M.R., Uceda, E.R., Amils, R., Miyamoto, H., Kim, K.J., Anderson, R.C., McKay, C.P., 2011. Meteorites at Meridiani Planum provide evidence for significant amounts of surface and nearsurface water on early Mars. Meteorit. Planet. Sci. 46, 1832–1841. Fan, C., Xie, H., Schulze-Makuch, D., Ackley, S., 2010. A formation mechanism for hematite-rich spherules on Mars. Planet. Space Sci. 58, 401–410. K. Fox, How a raindrop is like an exploding parachute, ScienceNOW, 20 July 2009. 〈http://news.sciencemag.org/sciencenow/2009/07/20-02.html〉. Glotch, T.D., Bandfield, J.L., 2006. Determination and interpretation of surface and atmospheric miniature thermal emission spectrometer spectral end-members at the Meridiani Planum landing site. J. Geophys. Res.: Planets 111 (E12S06). Glotch, T.D., Christensen, P.R., Sharp, T.G., 2006. Fresnel modeling of hematite crystal surfaces and application to martian hematite spherules. Icarus 181, 408–418. Herkenhoff, K.E., Squyres, S.W., Arvidson, R., Bass, D.S., Bell, J.F., Bertelsen, P., Ehlmann, B.L., Farrand, W., Gaddis, L., Greeley, R., Grotzinger, J., Hayes, A.G., Hviid, S.F., Johnson, J.R., Jolliff, B., Kinch, K.M., Knoll, A.H., Madsen, M.B., Maki, J.N., McLennan, S.M., McSween, H.Y., Ming, D.W., Rice, J.W., Richter, L., Sims, M., Smith, P.H., Soderblom, L.A., Spanovich, N., Sullivan, R., Thompson, S., Wdowiak, T., Weitz, C., Whelley, P., 2004. Evidence from Opportunity's microscopic imager for water on Meridiani Planum. Science 306, 1727–1730. Kim, H.S., Kobayashi, Y., Nagai, K., 2006. Modeling of the surface tension of liquid Fe–P alloy by calculation of liquidus line in Fe–P binary system. J. Mater. Res. 21, 1399–1408. Klingelhöfer, G., Morris, R.V., Bernhardt, B., Schröder, C., Rodionov, D.S., de Souza, P.A., Yen, A., Gellert, R., Evlanov, E.N., Zubkov, B., Foh, J., Bonnes, U., Kankeleit, E., Gütlich, P., Ming, D.W., Renz, F., Wdowiak, T., Squyres, S.W., Arvidson, R.E., 2004. Jarosite and Hematite at Meridiani Planum from Opportunity's Mössbauer spectrometer. Science 306, 1740–1745. Knauth, L.P., Burt, D.M., Wohletz, K.H., 2005. Impact origin of sediments at the opportunity landing site on Mars. Nature 438, 1123–1128. Kula, J., Baldwin, S.L., 2012. On hematite as a target for dating aqueous conditions on Mars. Planet. Space Sci. 67, 101–108. McLennan, S.M., Bell Iii, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., de Souza, P.A., Farmer, J., Farrand, W.H., Fike, D.A., Gellert, R., Ghosh, A., Glotch, T.D., Grotzinger, J.P., Hahn, B., Herkenhoff, K.E., Hurowitz, J.A., Johnson, J.R., Johnson, S.S., Jolliff, B., Klingelhöfer, G., Knoll, A.H., Learner, Z., Malin, M.C., McSween, H.Y., Pocock, J., Ruff, S.W., Soderblom, L.A., Squyres, S.W., Tosca, N.J., Watters, W.A., Wyatt, M.B., Yen, A., 2005. Provenance and diagenesis of the evaporite-bearing burns formation, Meridiani Planum, Mars. Earth Planet. Sci. Lett. 240, 95–121. MER mission image a, Wopmay, 2004. Mars Exploration Rover Mission, JPL/NASA. Press Release Images: Opportunity “Wopmay in False Color”, available at: 〈http://marsrover.nasa.gov/gallery/press/opportunity/20041104a/08-SS-01 -Wopmay-B278R1.jpg〉. MER mission image b, Heat Shield Rock. R. D. G. Frederick, 2005. The lying colors of Mars Opportunity's eye-candy, online image courtesy NASA/JPL/Cornell available at: 〈http://www.norwebster.com/mars/mer/images/meteorite.jpg〉. MER mission image c, sol 369, 2005. M. Lyle, Mars Exploration rover imagery, available at: 〈http://mars.lyleresearch.com/imagery/1M160940487EFF42D9P2956M2M1.JPG.html〉.
A.K. Misra et al. / Planetary and Space Science 92 (2014) 16–23
MER mission image d, sol 319, 2005. R. D. G. Frederick, The lying colors of Mars Opportunity's eye-candy, online image courtesy NASA/JPL/Cornell available at: 〈http://www.norwebster.com/mars/mer/images/bbxroad.jpg〉. MER mission image, Opportunity Self-Portrait, 2005. Mars Exploration Rover Mission, JPL/NASA. Press Release Images: Opportunity Self-Portrait, Sols 322-323, 〈http://marsrovers.jpl.nasa.gov/gallery/press/opportunity/20050218a. html〉. MER mission image, Spirit Self-Portrait, 2005. Mars Exploration Rover Mission, JPL/ NASA. Press Release Images: Spirit Self-Portrait, Sols 329-330〈http://marsro vers.jpl.nasa.gov/gallery/press/spirit/20050218a.html〉. MER mission image, Spirit Self-Portrait, 2006. Mars Exploration Rover Mission, JPL/ NASA. Press Release Images: Spirit, Still Shining After All This Time (Vertical), 〈http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20060104a.html〉. Minitti, M.E., Lane, M.D., Bishop, J.L., 2005. A new hematite formation mechanism for Mars. Meteorit. Planet. Sci. 40, 55–69. H. Perlman. Are raindrops shaped like teardrops? USGS website (2005), 〈http://ga. water.usgs.gov/edu/raindropshape.html〉. Schröder, C., Rodionov, D.S., McCoy, T.J., Jolliff, B.L., Gellert, R., Nittler, L.R., Farrand, W.H., Johnson, J.R., Ruff, S.W., Ashley, J.W., Mittlefehldt, D.W., Herkenhoff, K.E., Fleischer, I., Haldemann, A.F.C., Klingelhöfer, G., Ming, D.W., Morris, R.V., de Souza, P.A., Squyres, S.W., Weitz, C., Yen, A.S., Zipfel, J., Economou, T., 2008. Meteorites on Mars observed with the Mars Exploration Rovers. J. Geophys. Res.: Planets 113 (E06S22). Sefton-Nash, E., Catling, D.C., 2008. Hematitic concretions at Meridiani Planum, Mars: their growth timescale and possible relationship with iron sulfates. Earth Planet. Sci. Lett. 269, 366–376. Squyres, S.W., Arvidson, R.E., Bell, J.F., Brückner, J., Cabrol, N.A., Calvin, W., Carr, M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Marais, D.J.D., d’Uston, C., Economou, T., Farmer, J., Farrand, W., Folkner, W., Golombek, M., Gorevan, S., Grant, J.A., Greeley, R., Grotzinger, J., Haskin, L., Herkenhoff, K.E., Hviid, S., Johnson, J., Klingelhöfer, G., Knoll, A.H., Landis, G., Lemmon, M., Li, R., Madsen, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., Morris, R.V., Parker, T., Rice, J.W., Richter, L., Rieder, R., Sims, M., Smith, M., Smith, P., Soderblom, L.A., Sullivan, R., Wänke, H., Wdowiak, T., Wolff, M., Yen, A., 2004a. The Opportunity Rover’s Athena science investigation at Meridiani Planum, Mars. Science 306, 1698–1703.
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Squyres, S.W., Grotzinger, J.P., Arvidson, R.E., Bell, J.F., Calvin, W., Christensen, P.R., Clark, B.C., Crisp, J.A., Farrand, W.H., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., Knoll, A.H., McLennan, S.M., McSween, H.Y., Morris, R.V., Rice, J.W., Rieder, R., Soderblom, L.A., 2004b. In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars. Science 306, 1709–1714. Squyres, S.W., Arvidson, R.E., Bollen, D., Bell, J.F., Brückner, J., Cabrol, N.A., Calvin, W.M., Carr, M.H., Christensen, P.R., Clark, B.C., Crumpler, L., Des Marais, D.J., d’Uston, C., Economou, T., Farmer, J., Farrand, W.H., Folkner, W., Gellert, R., Glotch, T.D., Golombek, M., Gorevan, S., Grant, J.A., Greeley, R., Grotzinger, J., Herkenhoff, K.E., Hviid, S., Johnson, J.R., Klingelhöfer, G., Knoll, A.H., Landis, G., Lemmon, M., Li, R., Madsen, M.B., Malin, M.C., McLennan, S.M., McSween, H.Y., Ming, D.W., Moersch, J., Morris, R.V., Parker, T., Rice, J.W., Richter, L., Rieder, R., Schröder, C., Sims, M., Smith, M., Smith, P., Soderblom, L.A., Sullivan, R., Tosca, N.J., Wänke, H., Wdowiak, T., Wolff, M., Yen, A., 2006a. Overview of the Opportunity Mars Exploration Rover mission to Meridiani Planum: Eagle Crater to Purgatory Ripple. J. Geophys. Res.: Planets, 111; a (E12S12). Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, B.C., Grotzinger, J.P., Jolliff, B.L., McLennan, S.M., Tosca, N., Bell, J.F., Calvin, W.M., Farrand, W.H., Glotch, T.D., Golombek, M.P., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., McSween, H.Y., Yen, A.S., 2006b. Two years at Meridiani Planum: results from the Opportunity Rover. Science 313, 1403–1407. Squyres, S.W., Knoll, A.H., Arvidson, R.E., Ashley, J.W., Bell, J.F., Calvin, W.M., Christensen, P.R., Clark, B.C., Cohen, B.A., de Souza, P.A., Edgar, L., Farrand, W.H., Fleischer, I., Gellert, R., Golombek, M.P., Grant, J., Grotzinger, J., Hayes, A., Herkenhoff, K.E., Johnson, J.R., Jolliff, B., Klingelhöfer, G., Knudson, A., Li, R., McCoy, T.J., McLennan, S. M., Ming, D.W., Mittlefehldt, D.W., Morris, R.V., Rice, J.W., Schröder, C., Sullivan, R.J., Yen, A., Yingst, R.A., 2009. Exploration of Victoria crater by the Mars Rover Opportunity. Science 324, 1058–1061. Taylor, S., Brownlee, D.E., 1991. Cosmic spherules in the geologic record. Meteoritics 26, 203–211. Taylor, S., Lever, J.H., Harvey, R.P., 2000. Numbers, types, and compositions of an unbiased collection of cosmic spherules. Meteorit. Planet. Sci. 35, 651–666.