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LRO-LAMP detection of geologically young craters within lunar permanently shaded regions
Icarus 273 (2016) 114–120
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LRO-LAMP detection of geologically young craters within lunar permanently shaded regions Kathleen E. Mandta,∗, Thomas K. Greathousea, Kurt D. Retherforda, G. Randall Gladstonea, Andrew P. Jordanb, Myriam Lemelinc, Steven D. Koeberd, Ernest Bowman-Cisnerosd, G. Wesley Pattersone, Mark Robinsond, Paul G. Luceyc, Amanda R. Hendrixf, Dana Hurleye, Angela M. Sticklee, Wayne Pryorg a
Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78228, USA Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mãnoa, 1680 East-West Road, Honolulu, HI 96822, USA d Arizona State University, Tempe, AZ 85281, USA e Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA f Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA g Central Arizona College, 8470 N. Overfield Rd., Coolidge, AZ 85128, USA b c
a r t i c l e
i n f o
Article history: Received 30 April 2015 Revised 20 July 2015 Accepted 25 July 2015 Available online 1 August 2015 Keywords: Moon, surface Impact processes Ultraviolet observations Cratering Regoliths
a b s t r a c t The upper 25–100 nm of the lunar regolith within the permanently shaded regions (PSRs) of the Moon has been demonstrated to have significantly higher surface porosity than the average lunar regolith by observations that the Lyman-α albedo measured by the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) is lower in the PSRs than the surrounding region. We find that two areas within the lunar south polar PSRs have significantly brighter Lyman-α albedos and correlate with the ejecta blankets of two small craters (<2 km diameter). This higher albedo is likely due to the ejecta blankets having significantly lower surface porosity than the surrounding PSRs. Furthermore, the ejecta blankets have much higher Circular Polarization Ratios (CPR), as measured by LRO Mini-RF, indicating increased surface roughness compared to the surrounding terrain. These combined observations suggest the detection of two craters that are very young on geologic timescales. From these observations we derive age limits for the two craters of 7–420 million years (Myr) based on dust transport processes and the radar brightness of the disconnected halos of the ejecta blankets. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Geologically young, or “fresh”, craters provide an important opportunity to calibrate space weathering processes (Denevi et al., 2014; Robinson et al., 2015). Observations of fresh craters within the permanently shaded regions (PSRs) of the Moon, however, are challenging due to the absence of direct sunlight. PSRs are of great scientific and exploration interest because of their expected ability to trap and retain volatiles, potentially for billions of years (Watson et al., 1961; Paige et al., 2010a) but little is known about their surface properties or space weathering rates compared to other regions of the Moon. Fortunately, various spacecraft instruments have managed to collect data from PSRs at a range of wavelengths and through particle detections. These observations pro-
∗
Corresponding author.
http://dx.doi.org/10.1016/j.icarus.2015.07.031 0019-1035/© 2015 Elsevier Inc. All rights reserved.
vide information about the volatile content, surface properties and geological history of the PSR interiors. Reduced epithermal neutron fluxes (Feldman et al., 2001; Mitrofanov et al., 2010), elevated Circular Polarization Ratios (CPR; Nozette et al., 1996; Spudis et al., 2013) and surface reflectance at visible (Haruyama et al., 2008; Lucey et al., 2014) and ultraviolet wavelengths (Gladstone et al., 2012) provide limits on the amount of surface frost and subsurface ice present in the PSRs. At the same time, images of the interiors of south polar PSRs produced using sunlight scattered off of crater walls (Haruyama et al., 2008), radar (Spudis et al., 2013) and laser altimetry (Zuber et al., 2012; Lucey et al., 2014) provide details about the topography, surface features and material properties. We describe here the detection of two geologically young craters within south polar PSRs using maps of the Lyman-α (121.57 nm) albedo of their interiors. These maps were produced using data taken by the Lyman Alpha Mapping Project (LAMP), a
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far-ultraviolet (FUV) imaging spectrograph (Gladstone et al., 2010) on the NASA Lunar Reconnaissance Orbiter (LRO; Chin et al., 2007). This work demonstrates a new method for detecting fresh craters on the Moon and provides observations that are useful for studying space weathering processes within the lunar PSRs. Space weathering occurs through several processes, including impact gardening (Arnold, 1975), solar wind sputtering (Hapke, 1973) and possibly dielectric breakdown (Jordan et al., 2014, 2015). Impact gardening is the turnover of regolith by micrometeoroid impacts, which physically breaks down grains, buries more material than it exposes, and heats and vaporizes regolith material that is then deposited on the surrounding material (Hapke et al., 1975; Pieters et al., 20 0 0; Noble et al., 2007). Exposure to the solar wind sputters atoms from the surface that are redeposited as a coating on the surrounding regolith (Hapke, 1973). Finally, dielectric breakdown, a proposed process that may be unique to the PSRs, occurs when large solar energetic particle events cause dielectric breakdown, or sparking, in locations where cold temperatures greatly lengthen the lunar regolith’s electrical discharging timescale (Jordan et al., 2014, 2015). If effective in the PSRs, the impact of dielectric breakdown would be to (1) increase the percentage of fine-grained particles in the uppermost 1 mm of regolith within the PSRs, (2) increase evidence of weathering due to vaporization of material, and (3) increase surface porosity (because smaller particles can form “fairy castle” structures more easily; Hapke and van Horn, 1963). Surface porosity is an important parameter in albedo evaluations (Hapke, 2008). It is the fraction of free space between and inside individual grains in the upper few centimeters of the lunar regolith. It is unclear which process dominates in the PSRs or if the space weathering rates from impact gardening and solar wind sputtering would be the same in the lunar PSRs as in other regions of the Moon. Latitudinal variation in the impact gardening rate is not expected (Arnold, 1975), but it is possible that the PSRs experience less exposure to solar wind than other areas of the Moon (Lucey et al., 2014). In any case, fresh craters provide a snapshot in time of space weathering conditions and are, thus, valuable tools for evaluating the influence of space weathering on lunar regolith. Fresh craters are brighter at visible wavelengths due to the exposure of material not previously subjected to space weathering (Denevi et al., 2014; Robinson et al., 2015) and have high CPR due to increased surface roughness at centimeter to decimeter scale (Bell et al., 2012; Spudis et al., 2013). Observations of fresh craters in the near UV (30 0–40 0 nm) show that space weathering causes the spectrum of the regolith to mature faster at UV wavelengths than at visible or infrared wavelengths (VNIR – 40 0–140 0 nm; Denevi et al., 2014), which means that fresh craters detected in the far UV (10 0–20 0 nm) could be significantly younger than those identified as fresh in the VNIR. Therefore, the use of starlight and sky-glow illumination by LRO-LAMP to map the PSRs (Gladstone et al., 2010, 2012) provides a unique tool for detecting fresh craters in these difficult-to-study regions. Within its wavelength coverage of 57–196 nm, LRO-LAMP is sensitive to the surface reflectance of the uppermost 25–100 nm of the lunar regolith. Previous studies of the LAMP FUV albedo measurements indicate that the PSRs in the lunar south polar region contain between 0.3% and 2% surface frost and that the porosity of the regolith in the PSRs is >70% (Gladstone et al., 2012) while the lunar average is ∼52%. This is in good agreement with results from the Lunar Crater Observation and Sensing Satellite (LCROSS) which found that the regolith in the Cabeus crater PSR has a porosity of ∼70% (Schultz et al., 2010). Higher surface porosity will reduce the albedo across all wavelengths due to a greater absorption of photons (Hapke, 2008). Therefore, porosity is the best explanation for low Lyman-α albedo observed by LAMP (Gladstone et al., 2012; Lucey et al., 2014) within PSRs.
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However, maps of the normal albedo at 1064 nm (near-infrared) made using data from the LRO Lunar Orbiter Laser Altimeter (LOLA; Smith et al., 2010) show that the normal albedo at this wavelength within the PSRs is greater than the surrounding areas (Lucey et al., 2014), despite the higher porosity observed by LAMP (Gladstone et al., 2012). This higher normal albedo is interpreted to suggest either the presence of significant amounts of water ice or that the material within the PSR is not as weathered as material outside of the PSR (Lucey et al., 2014). 2. Observations For this study we focus on two small, anomalously bright regions (A and B) within Faustini and Slater craters that otherwise exhibit low LAMP Lyman-α albedo, which has been shown to coincide with PSRs (Gladstone et al., 2012). These bright regions are shown in Fig. 1, are located in the south polar region highlighted in the inset. These PSRs are located within the south pole Aitkin Basin, the largest impact crater on the Moon. Faustini has a diameter of 41.6 km and is more than 3 km deep. The material on its floor Faustini is estimated to be 3.50 billion years (Gyr) old (Tye et al., in press) and elevated CPR inside Faustini has been interpreted to indicate that the crater is filled with plains material related to the Orientale basin (Campbell and Campbell, 2006), although the elevated CPR is found to be patchy (Spudis et al., 2013). 2.1. Albedo and surface porosity The bright regions (designated A and B in Fig. 1) correlate with what appears to be the ejecta blankets of small (<2 km) craters. The Lyman-α albedos of the polar region south of 80° latitude, south pole PSRs and the two bright regions are illustrated in Fig. 2 along with their modeled porosity (Hapke, 2008; Gladstone et al., 2012). It is clear from this figure that the Lyman-α albedos of the ejecta blankets of craters A and B are much higher than their surrounding PSRs, suggesting a 40–50% decrease in their porosity compared to the PSR. Moreover, the ejecta blanket Lyman-α albedos are higher than the average albedo for the south polar region, implying a porosity that is up to 5% lower than the south pole region. 2.2. Topography Figs. 3 and 4 illustrate LOLA (Smith et al., 2010) shaded relief maps of the two PSRs. Sample topographic profiles of craters A and B are given in Figs. 3b and 4b. These are small craters: crater A has a maximum diameter of 1.4 km and a minimum of ∼1.0 km, while crater B is roughly circular with a diameter of 0.8 km. Their depth-to-diameter (d/D) ratios are 0.12 (A) and 0.21 (B). Based on the Lyman-α albedo map, crater A’s ejecta blanket appears in Fig. 1 to extend to the right and above the crater while the ejecta blanket of crater B surrounds the crater symmetrically. 2.3. Observations at other wavelengths The LOLA albedo, measured for the wavelength of 1064 nm (Lucey et al., 2014), is shown in Fig. 5. Unlike Lyman-α wavelengths, the ejecta blankets of craters A and B show no evidence of a substantially different normal albedo at 1064 nm than their surrounding PSRs. Fig. 6a and b are composite images of the interior of Faustini and Slater craters taken by the Lunar Reconnaissance Orbiter Camera (LROC; Robinson et al., 2010) in visible wavelengths (390–700 nm) using sunlight scattered off crater walls as an
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Fig. 1. LAMP Lyman-α albedo overlaid on a LOLA shaded relief map of the two PSRs of interest. The Lyman-α albedo in the PSRs is lower than areas that are exposed to sunlight, the exception being the ejecta blankets of two small craters, designated A and B. The circles surrounding the craters have a radius of 3 km and are centered on the crater (inset). LOLA shaded relief map of the lunar south polar region (area south of 80°S latitude) produced using the JMoon beta tool (http://jmars.asu.edu/node/2055). Lines of latitude and longitude are shown for every 10° (right hand panel). The box indicates the region of focus for this study.
illumination source. Craters A and B can be seen in these images, but it is difficult to tell if the visible reflectance of their ejecta blankets is different from other craters within the PSRs, possibly due to the poor illumination conditions. At high contrast, the rims of the craters appear sharp, as is common for fresh craters. The annual average temperature is measured by the LRO Diviner infrared radiometer in infrared wavelengths (0.3–400 μm; Paige et al., 2010b). Fig. 6c shows that the average temperature for crater A is not significantly different than the surrounding PSR. The average temperature for crater B may be slightly lower than the surrounding PSR although there appears to be two other areas within the PSR with lower average temperatures that do not have higher Lyman-α albedo, suggesting that this is not unique to crater B. Mini-RF measures surface properties using 12.6 cm wavelength radar backscatter (Nozette et al., 2010). A very distinctive property of these craters is the CPR for the ejecta blankets, which appears clearly to be elevated, as illustrated in Fig. 6d. Crater A appears to have a more extensive ejecta blanket in the CPR map than in the LAMP Lyman-α albedo map, while the ejecta blanket of crater B looks similar in both the CPR and Lyman-α albedo maps. 2.4. Summary of observations The collective observations from several LRO instruments present a complex picture of these two exemplary craters. Both are simple craters formed by the impact of meteoroids that were 10– 10 0 0 m in diameter, depending on the impactor’s density and velocity. The dimensions and ejecta blanket position of crater A suggest that it was formed by an oblique impact with the impactor coming in at an incidence angle <45° from the horizon (Gault and Wedekind, 1978). The height of Faustini’s crater wall is ∼2.7 km in the likely direction from which the impactor came and the crater is ∼16 km from the wall peak giving a minimum incidence angle θ = tan−1 (2.7/16) = 9.6°. The d/D for craters A and B are within the expected range for observations of small fresh craters on the Moon (Craddock and Howard, 20 0 0). Despite no observable difference in visible albedo or in the annual average temperature for crater A, the Lyman-α albedo of the ejecta blankets of both craters is higher than not only the PSR but also the average south polar albedo. Additionally, the CPR of both ejecta blankets is elevated,
Fig. 2. LAMP Lyman-α albedo and modeled porosity (Hapke, 2008; Gladstone et al., 2012). Error bars for the albedo are based on counting statistics, which are then propagated to the modeled porosity value.
consistent with an increased surface roughness expected from fresh craters. The mild difference in the average temperature of crater B compared to the surrounding terrain may indicate that it is younger than crater A. In general, we conclude that the main difference between these ejecta blankets and the surrounding region is best explained by a significant reduction in surface porosity of the uppermost nm of regolith and an increase in surface roughness. These observations suggest that craters A and B are geologically younger than other similarly sized craters within this region. 3. Determining the age of the fresh craters The primary method for determining the age of surface features on the Moon is counting craters imposed on the surface feature. This method is difficult to use for small craters like the ones we have observed. Other approaches include evaluating parameters of surface morphology, such as the depth-to-diameter ratio, or evaluating the degradation state of optically bright ejecta. In the case of the two craters within PSRs we are evaluating, neither of these methods is very informative. We attempt to set an upper limit for the age of these craters based on processes that could increase the porosity of the upper few microns of regolith. We assume that the formation of the ejecta blanket collapsed all fairy castle structures and that over time they are rebuilt through dust
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Fig. 3. LOLA observations of Faustini. As in Fig. 1, crater A is identified with a circle drawn to have a radius of 3 km and centered on the crater: (a) LOLA shaded relief map of the Faustini crater and (b) sample cross-section of crater A.
elevation and deposition within the PSRs until the observed porosity of the ejecta blanket matches the surrounding region. The porosity determinations are based on LAMP albedo observations. The albedo in the LAMP pixels covering the ejecta blanket differs from the pixels outside of the ejecta blanket but within the PSR because they are lacking a deep enough layer of fairy castle structures to increase the surface porosity to the value of the surrounding PSR. We set an upper limit for the age of the ejecta blankets based on an estimated time period required to increase the surface porosity from 0% for freshly placed crater ejecta to ∼80%, which is the average PSR porosity derived from LAMP Lyman-α albedo measurements. To increase the surface porosity, several layers of micron-sized dust must be deposited gently onto the surface. Laboratory studies have determined that only particles with a diameter less than 15 μm could participate in the formation of these structures (Hapke and van Horn, 1963). We estimate the time scale for deposition of this layer of dust based on the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment diurnally averaged dust intensity measurements of 2– 6 × 10−4 particles/m2 /s/2π sr (Berg et al., 1974). This averaged intensity excludes terminator measurements that show up to two orders of magnitude greater flux from the sunward direction. It is unclear what average particle sizes were detected by LEAM, but particles with diameters as great as 1 mm were observed
(Grün and Horányi, 2013). The Lunar regolith particle radius number distribution of Goguen et al. (2010) indicates that the majority of particles have a radius of 1–2 μm. Assuming that the regolith particles are approximately spherical with an average diameter of 1.2 μm, dust that is levitated and transported across the surface of the PSR must fill the LAMP pixel with an average particle density, ρ , of ∼2.2 × 1017 particles/m3 to produce 80% surface porosity based on the volume of space occupied by the grains
ρ=
(1 − P ) V
(1)
where P is the surface porosity and V is the average volume of a regolith grain based on the average diameter given above. The time required to deposit ∼10 layers of fine-grained highly-porous material can be estimated by
t1 =
ρd I
(2)
where t1 is the time required for deposition, d is the depth of the layer required to be deposited, and I is the dust intensity. In order to convert the average dust intensity measured by the LEAM experiment to flux at the surface we must divide the intensity given above by 2. We assume that the depth of a layer required
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Fig. 4. LOLA observations of Slater. As in Fig. 1, crater B is identified with a circle drawn to have a radius of 3 km and centered on the crater: (a) LOLA shaded relief map of the Slater crater and (b) sample cross-section of crater B.
Fig. 5. LOLA 1064 nm normal albedo of the two PSRs (shaded) covered in this study. As in Fig. 1, craters A and B are identified with circles drawn to have a radius of 3 km and centered on the crater.
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Fig. 6. LROC, Diviner and Mini-RF observations of the PSRs of interest. As in Fig. 1, craters A and B are identified with circles drawn to have a radius of 3 km and centered on the craters: (a) LROC composite image of the interior of Faustini taken with an 24.23 ms exposure time with an image resolution is 20 m/pixel. (b) LROC composite image of the interior of Slater crater taken with an 24.23 ms exposure time with an image resolution of 20 m/pixel. (c) Annual average temperature measured by Diviner. (d) Mini-RF CPR of the two PSRs and the surrounding area.
to change the Lyman-α albedo is 10 layers of 1.2 μm-sized grains, or a depth of 12 μm. These values are summarized in Table 1. Using Eq. (2), we find that the maximum timescale for the ejecta blanket in a PSR to be visible to LAMP is 420 million years based on the average dust flux measured by LEAM. However, it is important to note that the conditions within PSRs are probably more comparable to conditions at sunset when LEAM measured the dust flux to be more than two orders of magnitude
greater than other times of the day. If we assume that the dust flux within the PSR is two orders of magnitude greater due to these unusual conditions, then the time scale could be limited to less than 2.8 million years. Another new method for dating fresh craters with diameters less than 3 km was developed using Mini-RF data (Bell et al., 2012). In this case, radar reflectance measurements were used to derive a lifetime of the discontinuous halo from fresh craters with
Table 1 Parameters used to estimate the age of the fresh craters observed by LAMP and Mini-RF. Variable
Parameter
Value
Notes
avg max
Dc
Crater diameter
Dh
Discontinuous halo diameter
t2
Age based on discontinuous halo
4 × 10−4 particles/m2 /s/2π sr 6 × 10−2 particles/m2 /s/2π sr 9.0 × 10−19 m3 2.2 × 1017 particles/m3 1.2 × 10−5 m 420 Myr (average flux) 2.8 Myr (maximum flux) 1.4 km (A) 0.8 km (B) ∼3.8 km (A) ∼3.6 km (B) >75 Myr (A) ∼16 Myr (B)
LEAM measurements (Berg et al., 1974) LEAM sunset measurements (Colwell et al., 2007)
d t1
Average dust flux Maximum dust flux Volume of a dust grain Particle density Depth Time required to reduce surface porosity
V
ρ
Assuming average diameter of 1.2 μm and 80% porosity 10 layers of 1.2 μm diameter grains Eq. (2) and LAMP measurements LOLA topography Mini-RF maps
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known ages. The discontinuous halo is defined as the part of the ejecta blanket beginning at four crater radii from the center of the crater and extending outward. Based on the model results provided in Figs. 3 and 4 of Bell et al. (2012), the discontinuous halo lifetimes for craters A and B are ∼75 (A) and ∼25 (B) Myr. From the Mini-RF data illustrated in Fig. 6d and presented in Table 1, we determined that the discontinuous halo extends to ∼3 (A) and ∼4.5 (B) crater diameters indicating that the craters have undergone 100% (A) and 65% (B) of their discontinuous halo lifetimes (Bell et al., 2012). This suggests that crater A is between 75 and 420 Myr old, and that crater B is no more than 16 Myr old. It is important to note that crater A overlaps another crater, as seen in Fig. 6a. This could result in an enhanced number of blocky surfaces that could extend the timescale for degradation of the discontinuous halo and increases the lower limit for the age of crater A. 4. Conclusion We have determined that craters A and B are geologically younger than other similarly sized craters within this region based on information provided by the LAMP and Mini-RF instruments on LRO showing that their ejecta blankets have significantly higher surface roughness and lower porosity compared to the surrounding terrain (Spudis et al., 2013; Denevi et al., 2014; Robinson et al., 2015). Over time, processes that transport very small regolith grains (Berg et al., 1974) will deposit μm-scale layers of highly porous regolith in “fairy castle” structures (Hapke and van Horn, 1963), thus eliminating the evidence observed by LAMP for a fresh crater. The evidence for increased surface roughness would remain until impact gardening reduces the surface roughness observed by Mini-RF (Spudis et al., 2013). An upper limit of 420 Myr can be placed on the age of both craters based on the timescales for grain transport on the lunar surface (Berg et al., 1974). Furthermore, the observed extent of the discontinuous halo (Bell et al., 2012) for craters A and B in radar brightness gives a narrower age limit of 75–420 Myr for crater A and an approximate age of 16 Myr for crater B. The younger age of crater B compared to crater A agrees with the annual average temperature difference and the fact that part of the ejecta blanket for crater A that is visible in the CPR map is not visible in the LAMP map. Through these observations we have established a new method for detecting fresh craters on the Moon. It is important to note that this method is most useful in the PSRs as the difference between the FUV albedo of the fresh craters and the average for sunlit areas is easily detectable. Outside of the PSRs the relative contrast is possibly too small for fresh craters to be readily apparent in current LRO datasets, but otherwise provides a new technique for constraining ages within the youngest population of craters. Acknowledgment This work was funded by the Lunar Reconnaissance Orbiter project. References Arnold, J.R., 1975. A Monte Carlo model for the gardening of the lunar regolith. Moon 13, 159–172.
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LRO-LAMP detection of geologically young craters within lunar permanently shaded regions Kathleen E. Mandta,∗, Thomas K. Greathousea, Kurt D. Retherforda, G. Randall Gladstonea, Andrew P. Jordanb, Myriam Lemelinc, Steven D. Koeberd, Ernest Bowman-Cisnerosd, G. Wesley Pattersone, Mark Robinsond, Paul G. Luceyc, Amanda R. Hendrixf, Dana Hurleye, Angela M. Sticklee, Wayne Pryorg a
Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78228, USA Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Mãnoa, 1680 East-West Road, Honolulu, HI 96822, USA d Arizona State University, Tempe, AZ 85281, USA e Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA f Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA g Central Arizona College, 8470 N. Overfield Rd., Coolidge, AZ 85128, USA b c
a r t i c l e
i n f o
Article history: Received 30 April 2015 Revised 20 July 2015 Accepted 25 July 2015 Available online 1 August 2015 Keywords: Moon, surface Impact processes Ultraviolet observations Cratering Regoliths
a b s t r a c t The upper 25–100 nm of the lunar regolith within the permanently shaded regions (PSRs) of the Moon has been demonstrated to have significantly higher surface porosity than the average lunar regolith by observations that the Lyman-α albedo measured by the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP) is lower in the PSRs than the surrounding region. We find that two areas within the lunar south polar PSRs have significantly brighter Lyman-α albedos and correlate with the ejecta blankets of two small craters (<2 km diameter). This higher albedo is likely due to the ejecta blankets having significantly lower surface porosity than the surrounding PSRs. Furthermore, the ejecta blankets have much higher Circular Polarization Ratios (CPR), as measured by LRO Mini-RF, indicating increased surface roughness compared to the surrounding terrain. These combined observations suggest the detection of two craters that are very young on geologic timescales. From these observations we derive age limits for the two craters of 7–420 million years (Myr) based on dust transport processes and the radar brightness of the disconnected halos of the ejecta blankets. © 2015 Elsevier Inc. All rights reserved.
1. Introduction Geologically young, or “fresh”, craters provide an important opportunity to calibrate space weathering processes (Denevi et al., 2014; Robinson et al., 2015). Observations of fresh craters within the permanently shaded regions (PSRs) of the Moon, however, are challenging due to the absence of direct sunlight. PSRs are of great scientific and exploration interest because of their expected ability to trap and retain volatiles, potentially for billions of years (Watson et al., 1961; Paige et al., 2010a) but little is known about their surface properties or space weathering rates compared to other regions of the Moon. Fortunately, various spacecraft instruments have managed to collect data from PSRs at a range of wavelengths and through particle detections. These observations pro-
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Corresponding author.
http://dx.doi.org/10.1016/j.icarus.2015.07.031 0019-1035/© 2015 Elsevier Inc. All rights reserved.
vide information about the volatile content, surface properties and geological history of the PSR interiors. Reduced epithermal neutron fluxes (Feldman et al., 2001; Mitrofanov et al., 2010), elevated Circular Polarization Ratios (CPR; Nozette et al., 1996; Spudis et al., 2013) and surface reflectance at visible (Haruyama et al., 2008; Lucey et al., 2014) and ultraviolet wavelengths (Gladstone et al., 2012) provide limits on the amount of surface frost and subsurface ice present in the PSRs. At the same time, images of the interiors of south polar PSRs produced using sunlight scattered off of crater walls (Haruyama et al., 2008), radar (Spudis et al., 2013) and laser altimetry (Zuber et al., 2012; Lucey et al., 2014) provide details about the topography, surface features and material properties. We describe here the detection of two geologically young craters within south polar PSRs using maps of the Lyman-α (121.57 nm) albedo of their interiors. These maps were produced using data taken by the Lyman Alpha Mapping Project (LAMP), a
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far-ultraviolet (FUV) imaging spectrograph (Gladstone et al., 2010) on the NASA Lunar Reconnaissance Orbiter (LRO; Chin et al., 2007). This work demonstrates a new method for detecting fresh craters on the Moon and provides observations that are useful for studying space weathering processes within the lunar PSRs. Space weathering occurs through several processes, including impact gardening (Arnold, 1975), solar wind sputtering (Hapke, 1973) and possibly dielectric breakdown (Jordan et al., 2014, 2015). Impact gardening is the turnover of regolith by micrometeoroid impacts, which physically breaks down grains, buries more material than it exposes, and heats and vaporizes regolith material that is then deposited on the surrounding material (Hapke et al., 1975; Pieters et al., 20 0 0; Noble et al., 2007). Exposure to the solar wind sputters atoms from the surface that are redeposited as a coating on the surrounding regolith (Hapke, 1973). Finally, dielectric breakdown, a proposed process that may be unique to the PSRs, occurs when large solar energetic particle events cause dielectric breakdown, or sparking, in locations where cold temperatures greatly lengthen the lunar regolith’s electrical discharging timescale (Jordan et al., 2014, 2015). If effective in the PSRs, the impact of dielectric breakdown would be to (1) increase the percentage of fine-grained particles in the uppermost 1 mm of regolith within the PSRs, (2) increase evidence of weathering due to vaporization of material, and (3) increase surface porosity (because smaller particles can form “fairy castle” structures more easily; Hapke and van Horn, 1963). Surface porosity is an important parameter in albedo evaluations (Hapke, 2008). It is the fraction of free space between and inside individual grains in the upper few centimeters of the lunar regolith. It is unclear which process dominates in the PSRs or if the space weathering rates from impact gardening and solar wind sputtering would be the same in the lunar PSRs as in other regions of the Moon. Latitudinal variation in the impact gardening rate is not expected (Arnold, 1975), but it is possible that the PSRs experience less exposure to solar wind than other areas of the Moon (Lucey et al., 2014). In any case, fresh craters provide a snapshot in time of space weathering conditions and are, thus, valuable tools for evaluating the influence of space weathering on lunar regolith. Fresh craters are brighter at visible wavelengths due to the exposure of material not previously subjected to space weathering (Denevi et al., 2014; Robinson et al., 2015) and have high CPR due to increased surface roughness at centimeter to decimeter scale (Bell et al., 2012; Spudis et al., 2013). Observations of fresh craters in the near UV (30 0–40 0 nm) show that space weathering causes the spectrum of the regolith to mature faster at UV wavelengths than at visible or infrared wavelengths (VNIR – 40 0–140 0 nm; Denevi et al., 2014), which means that fresh craters detected in the far UV (10 0–20 0 nm) could be significantly younger than those identified as fresh in the VNIR. Therefore, the use of starlight and sky-glow illumination by LRO-LAMP to map the PSRs (Gladstone et al., 2010, 2012) provides a unique tool for detecting fresh craters in these difficult-to-study regions. Within its wavelength coverage of 57–196 nm, LRO-LAMP is sensitive to the surface reflectance of the uppermost 25–100 nm of the lunar regolith. Previous studies of the LAMP FUV albedo measurements indicate that the PSRs in the lunar south polar region contain between 0.3% and 2% surface frost and that the porosity of the regolith in the PSRs is >70% (Gladstone et al., 2012) while the lunar average is ∼52%. This is in good agreement with results from the Lunar Crater Observation and Sensing Satellite (LCROSS) which found that the regolith in the Cabeus crater PSR has a porosity of ∼70% (Schultz et al., 2010). Higher surface porosity will reduce the albedo across all wavelengths due to a greater absorption of photons (Hapke, 2008). Therefore, porosity is the best explanation for low Lyman-α albedo observed by LAMP (Gladstone et al., 2012; Lucey et al., 2014) within PSRs.
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However, maps of the normal albedo at 1064 nm (near-infrared) made using data from the LRO Lunar Orbiter Laser Altimeter (LOLA; Smith et al., 2010) show that the normal albedo at this wavelength within the PSRs is greater than the surrounding areas (Lucey et al., 2014), despite the higher porosity observed by LAMP (Gladstone et al., 2012). This higher normal albedo is interpreted to suggest either the presence of significant amounts of water ice or that the material within the PSR is not as weathered as material outside of the PSR (Lucey et al., 2014). 2. Observations For this study we focus on two small, anomalously bright regions (A and B) within Faustini and Slater craters that otherwise exhibit low LAMP Lyman-α albedo, which has been shown to coincide with PSRs (Gladstone et al., 2012). These bright regions are shown in Fig. 1, are located in the south polar region highlighted in the inset. These PSRs are located within the south pole Aitkin Basin, the largest impact crater on the Moon. Faustini has a diameter of 41.6 km and is more than 3 km deep. The material on its floor Faustini is estimated to be 3.50 billion years (Gyr) old (Tye et al., in press) and elevated CPR inside Faustini has been interpreted to indicate that the crater is filled with plains material related to the Orientale basin (Campbell and Campbell, 2006), although the elevated CPR is found to be patchy (Spudis et al., 2013). 2.1. Albedo and surface porosity The bright regions (designated A and B in Fig. 1) correlate with what appears to be the ejecta blankets of small (<2 km) craters. The Lyman-α albedos of the polar region south of 80° latitude, south pole PSRs and the two bright regions are illustrated in Fig. 2 along with their modeled porosity (Hapke, 2008; Gladstone et al., 2012). It is clear from this figure that the Lyman-α albedos of the ejecta blankets of craters A and B are much higher than their surrounding PSRs, suggesting a 40–50% decrease in their porosity compared to the PSR. Moreover, the ejecta blanket Lyman-α albedos are higher than the average albedo for the south polar region, implying a porosity that is up to 5% lower than the south pole region. 2.2. Topography Figs. 3 and 4 illustrate LOLA (Smith et al., 2010) shaded relief maps of the two PSRs. Sample topographic profiles of craters A and B are given in Figs. 3b and 4b. These are small craters: crater A has a maximum diameter of 1.4 km and a minimum of ∼1.0 km, while crater B is roughly circular with a diameter of 0.8 km. Their depth-to-diameter (d/D) ratios are 0.12 (A) and 0.21 (B). Based on the Lyman-α albedo map, crater A’s ejecta blanket appears in Fig. 1 to extend to the right and above the crater while the ejecta blanket of crater B surrounds the crater symmetrically. 2.3. Observations at other wavelengths The LOLA albedo, measured for the wavelength of 1064 nm (Lucey et al., 2014), is shown in Fig. 5. Unlike Lyman-α wavelengths, the ejecta blankets of craters A and B show no evidence of a substantially different normal albedo at 1064 nm than their surrounding PSRs. Fig. 6a and b are composite images of the interior of Faustini and Slater craters taken by the Lunar Reconnaissance Orbiter Camera (LROC; Robinson et al., 2010) in visible wavelengths (390–700 nm) using sunlight scattered off crater walls as an
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Fig. 1. LAMP Lyman-α albedo overlaid on a LOLA shaded relief map of the two PSRs of interest. The Lyman-α albedo in the PSRs is lower than areas that are exposed to sunlight, the exception being the ejecta blankets of two small craters, designated A and B. The circles surrounding the craters have a radius of 3 km and are centered on the crater (inset). LOLA shaded relief map of the lunar south polar region (area south of 80°S latitude) produced using the JMoon beta tool (http://jmars.asu.edu/node/2055). Lines of latitude and longitude are shown for every 10° (right hand panel). The box indicates the region of focus for this study.
illumination source. Craters A and B can be seen in these images, but it is difficult to tell if the visible reflectance of their ejecta blankets is different from other craters within the PSRs, possibly due to the poor illumination conditions. At high contrast, the rims of the craters appear sharp, as is common for fresh craters. The annual average temperature is measured by the LRO Diviner infrared radiometer in infrared wavelengths (0.3–400 μm; Paige et al., 2010b). Fig. 6c shows that the average temperature for crater A is not significantly different than the surrounding PSR. The average temperature for crater B may be slightly lower than the surrounding PSR although there appears to be two other areas within the PSR with lower average temperatures that do not have higher Lyman-α albedo, suggesting that this is not unique to crater B. Mini-RF measures surface properties using 12.6 cm wavelength radar backscatter (Nozette et al., 2010). A very distinctive property of these craters is the CPR for the ejecta blankets, which appears clearly to be elevated, as illustrated in Fig. 6d. Crater A appears to have a more extensive ejecta blanket in the CPR map than in the LAMP Lyman-α albedo map, while the ejecta blanket of crater B looks similar in both the CPR and Lyman-α albedo maps. 2.4. Summary of observations The collective observations from several LRO instruments present a complex picture of these two exemplary craters. Both are simple craters formed by the impact of meteoroids that were 10– 10 0 0 m in diameter, depending on the impactor’s density and velocity. The dimensions and ejecta blanket position of crater A suggest that it was formed by an oblique impact with the impactor coming in at an incidence angle <45° from the horizon (Gault and Wedekind, 1978). The height of Faustini’s crater wall is ∼2.7 km in the likely direction from which the impactor came and the crater is ∼16 km from the wall peak giving a minimum incidence angle θ = tan−1 (2.7/16) = 9.6°. The d/D for craters A and B are within the expected range for observations of small fresh craters on the Moon (Craddock and Howard, 20 0 0). Despite no observable difference in visible albedo or in the annual average temperature for crater A, the Lyman-α albedo of the ejecta blankets of both craters is higher than not only the PSR but also the average south polar albedo. Additionally, the CPR of both ejecta blankets is elevated,
Fig. 2. LAMP Lyman-α albedo and modeled porosity (Hapke, 2008; Gladstone et al., 2012). Error bars for the albedo are based on counting statistics, which are then propagated to the modeled porosity value.
consistent with an increased surface roughness expected from fresh craters. The mild difference in the average temperature of crater B compared to the surrounding terrain may indicate that it is younger than crater A. In general, we conclude that the main difference between these ejecta blankets and the surrounding region is best explained by a significant reduction in surface porosity of the uppermost nm of regolith and an increase in surface roughness. These observations suggest that craters A and B are geologically younger than other similarly sized craters within this region. 3. Determining the age of the fresh craters The primary method for determining the age of surface features on the Moon is counting craters imposed on the surface feature. This method is difficult to use for small craters like the ones we have observed. Other approaches include evaluating parameters of surface morphology, such as the depth-to-diameter ratio, or evaluating the degradation state of optically bright ejecta. In the case of the two craters within PSRs we are evaluating, neither of these methods is very informative. We attempt to set an upper limit for the age of these craters based on processes that could increase the porosity of the upper few microns of regolith. We assume that the formation of the ejecta blanket collapsed all fairy castle structures and that over time they are rebuilt through dust
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Fig. 3. LOLA observations of Faustini. As in Fig. 1, crater A is identified with a circle drawn to have a radius of 3 km and centered on the crater: (a) LOLA shaded relief map of the Faustini crater and (b) sample cross-section of crater A.
elevation and deposition within the PSRs until the observed porosity of the ejecta blanket matches the surrounding region. The porosity determinations are based on LAMP albedo observations. The albedo in the LAMP pixels covering the ejecta blanket differs from the pixels outside of the ejecta blanket but within the PSR because they are lacking a deep enough layer of fairy castle structures to increase the surface porosity to the value of the surrounding PSR. We set an upper limit for the age of the ejecta blankets based on an estimated time period required to increase the surface porosity from 0% for freshly placed crater ejecta to ∼80%, which is the average PSR porosity derived from LAMP Lyman-α albedo measurements. To increase the surface porosity, several layers of micron-sized dust must be deposited gently onto the surface. Laboratory studies have determined that only particles with a diameter less than 15 μm could participate in the formation of these structures (Hapke and van Horn, 1963). We estimate the time scale for deposition of this layer of dust based on the Apollo 17 Lunar Ejecta and Meteorites (LEAM) experiment diurnally averaged dust intensity measurements of 2– 6 × 10−4 particles/m2 /s/2π sr (Berg et al., 1974). This averaged intensity excludes terminator measurements that show up to two orders of magnitude greater flux from the sunward direction. It is unclear what average particle sizes were detected by LEAM, but particles with diameters as great as 1 mm were observed
(Grün and Horányi, 2013). The Lunar regolith particle radius number distribution of Goguen et al. (2010) indicates that the majority of particles have a radius of 1–2 μm. Assuming that the regolith particles are approximately spherical with an average diameter of 1.2 μm, dust that is levitated and transported across the surface of the PSR must fill the LAMP pixel with an average particle density, ρ , of ∼2.2 × 1017 particles/m3 to produce 80% surface porosity based on the volume of space occupied by the grains
ρ=
(1 − P ) V
(1)
where P is the surface porosity and V is the average volume of a regolith grain based on the average diameter given above. The time required to deposit ∼10 layers of fine-grained highly-porous material can be estimated by
t1 =
ρd I
(2)
where t1 is the time required for deposition, d is the depth of the layer required to be deposited, and I is the dust intensity. In order to convert the average dust intensity measured by the LEAM experiment to flux at the surface we must divide the intensity given above by 2. We assume that the depth of a layer required
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Fig. 4. LOLA observations of Slater. As in Fig. 1, crater B is identified with a circle drawn to have a radius of 3 km and centered on the crater: (a) LOLA shaded relief map of the Slater crater and (b) sample cross-section of crater B.
Fig. 5. LOLA 1064 nm normal albedo of the two PSRs (shaded) covered in this study. As in Fig. 1, craters A and B are identified with circles drawn to have a radius of 3 km and centered on the crater.
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Fig. 6. LROC, Diviner and Mini-RF observations of the PSRs of interest. As in Fig. 1, craters A and B are identified with circles drawn to have a radius of 3 km and centered on the craters: (a) LROC composite image of the interior of Faustini taken with an 24.23 ms exposure time with an image resolution is 20 m/pixel. (b) LROC composite image of the interior of Slater crater taken with an 24.23 ms exposure time with an image resolution of 20 m/pixel. (c) Annual average temperature measured by Diviner. (d) Mini-RF CPR of the two PSRs and the surrounding area.
to change the Lyman-α albedo is 10 layers of 1.2 μm-sized grains, or a depth of 12 μm. These values are summarized in Table 1. Using Eq. (2), we find that the maximum timescale for the ejecta blanket in a PSR to be visible to LAMP is 420 million years based on the average dust flux measured by LEAM. However, it is important to note that the conditions within PSRs are probably more comparable to conditions at sunset when LEAM measured the dust flux to be more than two orders of magnitude
greater than other times of the day. If we assume that the dust flux within the PSR is two orders of magnitude greater due to these unusual conditions, then the time scale could be limited to less than 2.8 million years. Another new method for dating fresh craters with diameters less than 3 km was developed using Mini-RF data (Bell et al., 2012). In this case, radar reflectance measurements were used to derive a lifetime of the discontinuous halo from fresh craters with
Table 1 Parameters used to estimate the age of the fresh craters observed by LAMP and Mini-RF. Variable
Parameter
Value
Notes
avg max
Dc
Crater diameter
Dh
Discontinuous halo diameter
t2
Age based on discontinuous halo
4 × 10−4 particles/m2 /s/2π sr 6 × 10−2 particles/m2 /s/2π sr 9.0 × 10−19 m3 2.2 × 1017 particles/m3 1.2 × 10−5 m 420 Myr (average flux) 2.8 Myr (maximum flux) 1.4 km (A) 0.8 km (B) ∼3.8 km (A) ∼3.6 km (B) >75 Myr (A) ∼16 Myr (B)
LEAM measurements (Berg et al., 1974) LEAM sunset measurements (Colwell et al., 2007)
d t1
Average dust flux Maximum dust flux Volume of a dust grain Particle density Depth Time required to reduce surface porosity
V
ρ
Assuming average diameter of 1.2 μm and 80% porosity 10 layers of 1.2 μm diameter grains Eq. (2) and LAMP measurements LOLA topography Mini-RF maps
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known ages. The discontinuous halo is defined as the part of the ejecta blanket beginning at four crater radii from the center of the crater and extending outward. Based on the model results provided in Figs. 3 and 4 of Bell et al. (2012), the discontinuous halo lifetimes for craters A and B are ∼75 (A) and ∼25 (B) Myr. From the Mini-RF data illustrated in Fig. 6d and presented in Table 1, we determined that the discontinuous halo extends to ∼3 (A) and ∼4.5 (B) crater diameters indicating that the craters have undergone 100% (A) and 65% (B) of their discontinuous halo lifetimes (Bell et al., 2012). This suggests that crater A is between 75 and 420 Myr old, and that crater B is no more than 16 Myr old. It is important to note that crater A overlaps another crater, as seen in Fig. 6a. This could result in an enhanced number of blocky surfaces that could extend the timescale for degradation of the discontinuous halo and increases the lower limit for the age of crater A. 4. Conclusion We have determined that craters A and B are geologically younger than other similarly sized craters within this region based on information provided by the LAMP and Mini-RF instruments on LRO showing that their ejecta blankets have significantly higher surface roughness and lower porosity compared to the surrounding terrain (Spudis et al., 2013; Denevi et al., 2014; Robinson et al., 2015). Over time, processes that transport very small regolith grains (Berg et al., 1974) will deposit μm-scale layers of highly porous regolith in “fairy castle” structures (Hapke and van Horn, 1963), thus eliminating the evidence observed by LAMP for a fresh crater. The evidence for increased surface roughness would remain until impact gardening reduces the surface roughness observed by Mini-RF (Spudis et al., 2013). An upper limit of 420 Myr can be placed on the age of both craters based on the timescales for grain transport on the lunar surface (Berg et al., 1974). Furthermore, the observed extent of the discontinuous halo (Bell et al., 2012) for craters A and B in radar brightness gives a narrower age limit of 75–420 Myr for crater A and an approximate age of 16 Myr for crater B. The younger age of crater B compared to crater A agrees with the annual average temperature difference and the fact that part of the ejecta blanket for crater A that is visible in the CPR map is not visible in the LAMP map. Through these observations we have established a new method for detecting fresh craters on the Moon. It is important to note that this method is most useful in the PSRs as the difference between the FUV albedo of the fresh craters and the average for sunlit areas is easily detectable. Outside of the PSRs the relative contrast is possibly too small for fresh craters to be readily apparent in current LRO datasets, but otherwise provides a new technique for constraining ages within the youngest population of craters. Acknowledgment This work was funded by the Lunar Reconnaissance Orbiter project. References Arnold, J.R., 1975. A Monte Carlo model for the gardening of the lunar regolith. Moon 13, 159–172.
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