Geomorphic knobs of Candor Chasma, Mars: New Mars Reconnaissance Orbiter data and comparisons to terrestrial analogs

Icarus 205 (2010) 138–153

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Geomorphic knobs of Candor Chasma, Mars: New Mars Reconnaissance Orbiter data and comparisons to terrestrial analogs Marjorie A. Chan a,*, Jens Ormö b, Scott Murchie c, Chris H. Okubo d, Goro Komatsu e, James J. Wray f, Patrick McGuire g, James A. McGovern c, the HiRISE Team h a

Department of Geology and Geophysics, University of Utah, 115 S. 1460 E., Salt Lake City, UT 84112, USA Centro de Astrobiologı´a (CSIC-INTA), Instituto Nacional de Técnica Aeroespacial, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain c John Hopkins University/Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, USA d US Geological Survey, Flagstaff, AZ 86001, USA e International Research School of Planetary Sciences, Università d’Annunzio, Viale Pindaro 42, 65127 Pescara, Italy f Department of Astronomy, Cornell University, Ithaca, NY 14853, USA g McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO 63130, USA h Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA b

a r t i c l e

i n f o

Article history: Received 31 October 2008 Revised 3 April 2009 Accepted 4 April 2009 Available online 7 May 2009 Keyword: Mars Geological processes Mineralogy Image processing Terrestrial planets

a b s t r a c t High Resolution Imaging Science Experiment (HiRISE) imagery and digital elevation models of the Candor Chasma region of Valles Marineris, Mars, reveal prominent and distinctive positive-relief knobs amidst light-toned layers. Three classifications of knobs, Types 1, 2, and 3, are distinguished from a combination of HiRISE and Thermal Emission Imaging System (THEMIS) images based on physical expressions (geometries, spatial relationships), and spectral data from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Type 1 knobs are abundant, concentrated, topographically resistant features with their highest frequency in West Candor, which have consistent stratigraphic correlations of the peak altitude (height). These Type 1 knobs could be erosional remnants of a simple dissected terrain, possibly derived from a more continuous, resistant, capping layer of pre-existing material diagenetically altered through recrystallization or cementation. Types 2 and 3 knobs are not linked to a single stratigraphic layer and are generally solitary to isolated, with variable heights. Type 3 are the largest knobs at nearly an order of magnitude larger than Type 1 knobs. The variable sizes and occasional pits on the tops of Type 2 and 3 knobs suggest a different origin, possibly related to more developed erosion, preferential cementation, or textural differences from sediment/water injection or intrusion, or from a buried impact crater. Enhanced color HiRISE images show a brown coloration of the knob peak crests that is attributable to processing and photometric effects; CRISM data do not show any detectable spectral differences between the knobs and the host rock layers, other than albedo. These intriguing knobs hold important clues to deducing relative rock properties, timing of events, and weathering conditions of Mars history. Ó 2009 Elsevier Inc. All rights reserved.

1. Introduction Mars is well known for its numerous craters and mountains, but small and subtle surface features also yield valuable clues to the possible hydrodynamic and weathering regimes on Mars, especially as the wealth of new high-resolution images increases. There is a variety of geomorphic landforms and surface features on Mars (e.g., Mutch et al., 1976; Thomas, 1982; Greeley and Batson, 2001; Bue and Stepinski, 2006; Chapman, 2007; Head, 2007) including depositional features such as: eolian ridges and bedforms (Greeley

* Corresponding author. Fax: +1 801 581 7162. E-mail address: [email protected] (M.A. Chan). 0019-1035/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2009.04.006

and Iversen, 1985; Greeley et al., 1996; Bourke et al., 2004, 2008); erosional features such as small gullies (Malin and Edgett, 2000); exhumed terrains (Arvidson et al., 2002); and weathering products from cracks (Thomas et al., 2005; Chan et al., 2008) and exfoliation (McSween et al., 1999; Thomas et al., 2005). Mariner 9 and the Viking orbiters photographed hills and knobs that were interpreted to be eroded remnants of fractured rock (e.g., Manent and El-Baz, 1986). The Mars Orbiter Camera (MOC) onboard Mars Global Surveyor (MGS) returned images of subtle surface details at the 1999 Mars Polar Lander site (Edgett and Malin, 2000; Malin and Edgett, 2001) that showed features described as knobs and pinnacles. Here we analyze new High Resolution Imaging Science Experiment (HiRISE) images that have unprecedented detail and resolution. The images show details that reflect different geologic

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characteristics of conical, steep-walled, positive-relief, geomorphic landforms (herein referred to as knobs) in southwest Candor Chasma. We describe the knobs’ characteristics and identify possible origins for these knobs based on Earth analogs with similar characteristics.

2. Methods 2.1. Mapping methods A combination of several different data sets was utilized for characterizing knobs in southwest Candor Chasma. The Thermal Emission Imaging System (THEMIS, on the Mars Odyssey spacecraft) combines infrared (100 m/pixel) and visible (20 m/pixel) wavelength imagery (e.g., Christensen et al., 2003, 2004; Christensen et al., 2008 via the Planetary Data System THEMIS Data Node). The Context Camera (CTX, on the Mars Reconnaissance Orbiter, or MRO) supplies panchromatic visible images of Mars at 6 m/pixel scale over swaths 30 km wide (Malin et al., 2007), providing regional context for the MRO’s HiRISE and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) (Murchie et al., 2007, 2009a) instruments. HiRISE imagery has greater than 100:1 signal-to-noise ratios and 30 cm/pixel scale, an increase in resolution by one to two orders of magnitude over the data available in previous studies (McEwen et al., 2007). All of the aforementioned data sets are the supporting context for digital elevation models (DEMs; 1 m postings), built from HiRISE imagery to analyze specific study areas. The specific areas were targeted from the THEMIS and CTX images, where suitable HiRISE imagery could discern individual landforms. HiRISE DEMs were generated from PSP_003540_1735 and PSP_003474_1735 via the stereo procedure of Kirk et al. (2008). HiRISE red, blue-green, and near-infrared channels provided data for RGB ‘enhanced’ color images (cf., Delamere et al., 2009). Color coverage was co-registered to the DEM using a combination of manually and software selected tie points between PSP_003540_1735 and a shaded relief image of the DEM. We additionally checked all the Mars Orbiter Camera (MOC, Malin and Edgett, 2001) images, yet found the THEMIS, CTX, and HiRISE images to be more useful for our comparisons. 2.2. Mineralogy methods CRISM data were superimposed on the HiRISE data to search for correlations between the knobs and mineral composition. CRISM covers the wavelength range 0.36–3.92 lm at 6.55 nm/channel. There are two major operational modes, nadir-pointed global multispectral mapping at 200 m/pixel, and high-resolution hyperspectral ‘‘targeted observations” which were used to measure the knobs at 20 m/pixel. These data were converted to apparent I/F using procedures described by Murchie et al. (2007, 2009a). To correct for variations in illumination and atmospheric attenuation, a Lambertian surface was assumed. I/F was divided by the cosine of the incidence angle, and by a scaled atmospheric transmission spectrum obtained during an observation crossing Olympus Mons (Bibring et al., 2005, 2006; Mustard et al., 2008). Propagated detector noise was reduced using a filtering algorithm (Parente, 2008), and the data were map-projected on the MOLA shape model using telemetered instrument and spacecraft pointing and position (Murchie et al., 2007, 2009a). The current status of data accuracy, precision, and artifacts is summarized by Murchie et al. (2009a). We mapped the occurrences and strengths of key absorptions using ‘‘summary products” (Pelkey et al., 2007), standardized representations of absorption band strengths using wavelengths represented in both targeted and multispectral survey data.

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3. Results and interpretations The results of this study are presented in sections that first discuss the physical characteristics of host rock stratification and knobs. The knobs are subdivided into three different types based on their geomorphic characteristics. Following a description of each knob type, we present models and terrestrial analogs for that knob type. The spectral characteristics from CRISM data are presented next and are related to the physical characteristics of the three knob types, to use mineralogy interpreted from the spectral data to test the models of knob formation. 3.1. Host rock stratification HiRISE images of southwest Candor Chasma (Figs. 1–3) show host rock of light-toned deposits with relatively flat layers. Layers in this area generally slope northward, toward the center of the chasma, typically at dips of less than 20° in areas of minimal post-depositional deformation (Okubo et al., 2008). The layers show different degrees of resistance to erosion, where some form prominent topographic steps and other less resistant layers are slope-forming. The beds appear to be moderately continuous at an outcrop scale (meters to kilometers), although tracing of individual beds is difficult due to varying degrees of exposure. Some layers also show folding and deformation (Okubo et al., 2008). The host rock stratification shows repetitive layering that suggests variable environmental conditions for sedimentary deposition, possibly as lacustrine sediments (e.g., Mangold et al., 2007) or as eolian sediments that were infiltrated by groundwater (Murchie et al., 2009b). Intervening swales between knobs commonly contain eolian dunes and rippled sand, that likely represent eroded and redeposited material from the host rock. 3.2. Knob characteristics Different types of knobs and their characteristics are distinguished by a combination of THEMIS and HiRISE images (Figs. 1– 3). The knobs occur in large parts of Candor Chasma, but they seem to be especially abundant in southwestern Candor Chasma (Fig. 1). Here there can be concentrations of hundreds per 100 km2 of conical, positive-relief knobs (Fig. 1a). The knobs exhibit peaked summits to rounded or flat tops with a possible cap layer. Many knob tops show a brown coloration in enhanced color HiRISE data, giving rise to initial suggestions of the presence of a chemical composition (internally or on the cap surface) distinct from the layered host rock (Fig. 3). Small knobs range from several meters up to several tens of meters in diameter, with ‘‘nearest-neighbor” spacings similar to the diameter scales. Large knobs are tens of meters tall, with broad bases up to tens of meters to more than 100 m in diameter (Fig. 1b). Some knobs are elongated and exhibit shoulders, forming ‘‘mesas” (Fig. 1a). Others occur in elongated clusters or in isolation far from topographic features such as hills and escarpments (Fig. 1b). The distinguishing features and characteristics of the knobs are outlined below, with comparisons of the different Types 1, 2 and 3 in Table 1. The descriptive categories adopted in this work may reflect different origins of the knobs, although different characteristics (e.g., size) do not necessarily require separate origins. 3.2.1. Type 1 knob description THEMIS images indicate areas of a dissected landscape where abundant Type 1 knobs are intimately related to and/or show connections to elongate ridges or fins (Figs. 1 and 2a and b). Elongated ridge and fin-like landforms (related to the knobs) parallel apparent joint or fracture systems, indicating the structural influence

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Fig. 1. THEMIS visible images of dissected terrain of western Candor Chasma, Valles Marineris, showing strong lineaments and parallel joints or fractures. (a1) Type 1 knobs in Image V04410002 with upper inset context image, center latitude 6.232048N, center longitude 282.979 E. (b1) Type 1 knobs in the upper half of Image V02163002, center latitude 4.824N, center longitude 283.299E. In the lower half of this same image, more solitary Type 2 knobs are seen in the lighter area, and in the lowermost portion of the image. Enlargement (a2) shows cropped image sections from the middle portion of image (a1) to show the numerous and abundant Type 1 knobs and fins/ridges, eroded out of a once more continuous layer with a likely resistant capping layer. Enlargement (b2) shows a cropped image section from the bottom of (b1), where Type 2 knobs are more common with a Type 3 knob shown in the lower right. Images from THEMIS Public Data Releases (Christensen et al., 2008).

on dissection of layered rocks. Many of the knobs in Fig. 1a as well as some in Fig. 2 in areas with subhorizontal beds appear to have similar altitudes and heights, suggesting the knobs are eroded out of the same (or a correlative) layer. However, there are also knobs that seem to be controlled by other factors such as faults (Fig. 2c). They are discussed under the category Type 2 knobs below. Many of the knobs themselves appear to show layering on the sides, consistent with the layering in the landscape in which they occur. The most diagnostic structures of Type 1 knobs (summarized in Table 1) are the related clustering or linkage of knobs and their consistent (same) peak height altitude. 3.2.2. Type 1 knob interpretations The linkage and geomorphic expression of Type 1 knobs indicates an erosional origin from layered deposits that were dissected. This is illustrated by Fig. 1a where one unit or bed to the left in the image inset seems broken up and the fractures etched out so that a

labyrinthine system of trenches appears between positive morphological features (early stage in Type 1 knob development). Further to the right of the inset box, it is assumed the erosion may have acted longer and the trenches have widened to be wider than the size of the remnant positive morphological features (late stage in Type 1 knob development). Terrestrial analogs of erosional remnants of once more extensive, pre-existing overlying strata produce a variety of mesas, buttes, pinnacles, and knobs (Fig. 4). Some of the dissected terrains of North America’s desert southwest (Fig. 4a–c) strongly resemble the scale and forms of images from western Candor Chasma (Figs. 1 and 2). Relatively hard strata overlying weaker, softer strata produce a resistant cap from inverted weathering over time (e.g., also known as hoodoos, and pillars). Hoodoos commonly occur in groups where deep gullies cut through easily eroded sediments (Fairbridge, 1968). Buttes and hoodoos are common across the southwestern US (Fig. 4e and f) and in badland topography (Hall,

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to this interpretation of erosional remnants is a consistent cap rock layer and correlation to an adjacent escarpment, indicating that the remnants and escarpment were formerly connected. Capped pinnacles and hoodoos rarely occur solitary far from larger mesas and escarpments of the strata from which they originated. The erosion-resistance of Type 1 knobs might originate from a slightly different composition (that may not be detected by CRISM, as discussed in a later section), diagenetic reactions, recrystallization, and/or lithification (possibly from moisture) that helped strengthen the rock, a lack of weak horizons (e.g., well-defined bedding planes), or from a difference between the grain-size distributions in the knob-forming material (a possible coarser grained pre-existing/capping layer is likely to be more resistant to weathering). A possible analog for moisture-affected lithification is cemented aggregates of wind-blown dust termed ‘‘parna” (Butler, 1956; Hesse, 2004), which was proposed to form a fraction of the sandsized material at the Mars Pathfinder landing site (Greeley and Williams, 1994). The persistence of sulfate in the knobs (discussed later) as monohydrate suggests that if recrystallization occurred it may have taken place in a hypersaline environment to prevent alteration to a polyhydrated form (cf., Vaniman et al., 2004). Candidate sources of cap rocks in southwest Candor Chasma are the stratigraphic units identified by Okubo et al. (2008). These areally extensive units are massive, roughly 100 m thick and interbedded throughout the exposed stratigraphic section. The units appear conformable with the thinner (5 m thick) beds that form the majority of the prominent layering seen in this region. Both the units and thinner layers are interpreted to be part of a continuous stratigraphic sequence (Okubo et al., 2008).

Fig. 2. Distinctive knobs of southwest Candor Chasma, Valles Marineris. (a) Study area locale. THEMIS daytime infrared basemap (Christensen et al., 2008) data (100 m per pixel) with illumination from the upper left. Small box with arrow shows location (b) of CTX image. (b) Context Camera image showing the numerous small knobs and locations of HiRISE images in southwest Candor Chasma. Subscene from CTX image P06_003474_1735_XI_05S076W_070424. In large areas to the north and north-east, the knobs occur with regular spacing and with their peaks of similar altitude. Such examples would qualify as Type 1 knobs in our classification (see Table 1). However, many knobs in the central part of the image (partially covered by the frame boxes) fall outside this regularity. (c) Some knobs show an apparent alignment with faults. High concentrations of knobs occur along some bedrock layers as well. Other knobs show no apparent spatial correlation with faults or layering, indeed appearing at different stratigraphic levels in the layered deposits. Such knobs are included in the category labeled Type 2 (see Table 1). HiRISE image PSP_003540_1735 with illumination from the left.

1993). Sandstone layers typically comprise terrestrial resistant capping lithology, where grain size is an important factor. A key

3.2.3. Type 2 knob description Type 2 knobs generally possess the same characteristics of steep-sided, conical shape with some layering (possibly the original host rock) on the side like Type 1 knobs, but show more variable characteristics and are generally more dispersed (not as strongly clustered) and have greater variation in peak height altitudes. The Type 2 knobs (Figs. 1–3, 5 and Table 1) are common and are similar in size, morphology and expression to Type 1 knobs but have more spotty occurrences than Type 1, and in some cases appear to coexist on the fringes of clusters of Type 1 knobs (Fig. 2). In Fig. 1b there is a thin cluster of knobs some tens of meters wide (Type 2) as well as one solitary kilometer-wide large knob (Type 3, to be discussed). None of these Type 2 knobs seem to be linked to any adjacent resistant layer. In Fig. 2b, the top part of the image (above the center inset box) shows abundant elongate Type 1 knobs. Towards the central and lower parts of the image they appear more conical and sparsely spaced like Type 2 knobs. Although Type 2 knobs are numerous as well, they are not as distinctly linked to a single dissected layer as the Type 1 knobs. A constructed topographic profile across several Type 2 knobs (Fig. 3d) shows brown-toned cap rock of variable peak heights that appears to be consistent with the projected structural attitudes of several stratigraphic members, as mapped by Okubo et al. (2008) (Fig. 3d). The Type 2 knobs can occur amidst (co-mingled) or spatially distant from Type 1 knobs. Some rows of Type 2 knobs cut across the general trend of the layers suggesting control by geologic discontinuities such as faults and major fracture zones (Fig. 2c). Several series of ten or more knobs of approximately similar height are aligned in rows, following a lineation or trend, possibly related to erosion of a bed or raised ridge (Fig. 2c) or to preferential preservation of cemented, resistant materials along a fault or fracture (Fig. 2c) (cf., Okubo et al., 2009). Although some knobs are aligned along geologic discontinuities (possible faults or fractures), where knobs are more isolated and more widely spaced, there appears to be less of a relationship to discontinuities.

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Fig. 3. Detailed views of Type 2 Candor knobs in HiRISE enhanced color images. A cap or mantling of brown-toned material that is not conformable to the light-toned layering is present on some knobs. Eolian dune patterns (sinuous ridges) show wind deflection and edge effects around the knobs in topographic lows. The blue tone of these dunes is consistent with basaltic sand. Light-toned layering of the surrounding bedrock occurs along the sides of the knobs. Type 2 knobs labeled A, B and C are shown from different perspectives in each panel. Perspective views have no vertical exaggeration. (a) Map view of brown-topped knobs with illumination from the left. (b) Although the browntones in some places appear to be the result of scattered light illuminating topographically shaded east-facing slopes, other brown coloration is also clearly present on Sun-lit slopes. Perspective view looks toward the southeast. (c) The brown-toned capping material exhibits variable elevation and aerial extent among the knobs. Perspective view looks toward the east. HiRISE image PSP_003540_1735. (d) Topographic profile across several Type 2 knobs of (a). Occurrences of the brown-toned cap rock are shown in color where they are observed along the profile. Cap rock locations are consistent with the projected structural attitudes of several stratigraphic units mapped by Okubo et al. (2008). Profile topography (HiRISE digital elevation model) and measurements of bedding dip are discussed in Okubo et al. (2008).

Some concentric topographic ring-ridge-like features (Fig. 5a) are visible around the base of some knobs, and their ring-like (‘‘bulls eye”) pattern differs in orientation from exposed layers in the surrounding host rock. The ring-like features associated with some knobs show slight angular unconformities with the surrounding host rock layers e.g., knob above north arrow of Fig. 3a showing basal exposure of bedding that cross cuts the eolian infill and the general host rock layering, and enlargements of a knob cluster in Fig. 5a. Although appearing at the limit of the spatial resolution of the images, the observed discordances may be important clues to the origin of Type 2 knobs (Fig. 5), which do not show a clear relationship to erosion from a once-continuous resistant layer.

A feature exclusive to Type 2 knobs (as well as Type 3 to be discussed) is the occasional peaked top that exhibits a central pit, essentially a resistant outer rim with a central depression (Fig. 5d). Type 2 knobs with pits comprise less than 10% of the total number of knobs, but where present, the pits are distinctive even at the resolution of the THEMIS images. The pits are discussed further with Type 3 knobs. 3.2.4. Type 2 knob interpretations The tops of Type 2 knobs typically occur at different heights (peak height altitudes) (Fig. 3b and c), which makes them appear unrelated to any single bed in the underlying host rock. However, a constructed topographic profile through several of the Type 2

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Table 1 Summary of knob characteristics and possible origins based on terrestrial analogs. Bold font indicates relevant characteristics of Type 1 knobs. Italics indicates relevant characteristics of Type 2 knobs. Bold italics indicates relevant characteristics of Type 3 knobs (the largest of the knobs). On the possible analog interpretations for the knobs, symbols indicate the likelihood of the characteristics: ‘‘+” for data supporting this feature, ‘‘ ” for data that does not seem to support this feature, and ‘‘?” for uncertainty or ambiguity of data to support this feature. Knob characteristics

Knob type Small (several m to 100 m diameter)

Possible origins (+ supports,

problematic)

Type 1

Type 2

Large (100 m diameter) Type 3

Conical, steep-sided landforms Linked to ridge/fins, mesas Numerous with spacing similar to diameter scale Solitary or clustered knobs, with spacing several times the diameter scale Can have central pit, depression on top Shoulders and flat tops Host rock layering visible on sides Penetrative, continuous layering

Yes Yes Yes

Yes No No

Yes No No

+ + +

+ +/ +

+

No

Yes, all solitary

+/ (possible with longer erosion, or variable dipping layers, Fig. 3d)

+/

+/

+

+

No

Yes, solitary or clustered Yes

+

+

+/

+

Yes Yes

Yes Yes

Yes Yes

+ + (Fig. 4)

? + (Fig. 6a–c)

+ (Fig. 6e)

Yes

+

No

Yes/No (see Fig. 5) ?

+

Concentric rings cutting adjacent strata Adjacent resistant layer Peaks on same altitude as adjacent resistant layer Occasionally aligned along faults Aligned along bedding

Yes/No (see Fig. 5) Yes

Yes Yes

Rare No

Rare No

+ +

Yes Yes

Yes No

? No

+ +

Yes

Yes

?

+

None in CRISM

None in CRISM

None in CRISM

No (not likely)

Capping false coloration (from HiRISE enhancement) Center/cap mineralogy different from host layers

Erosional remnants: fins, mesas and ‘‘hoodoos” Type 1, 2?

Diagenetic preferential cement Type 2, 3?

Injected pipes cemented by groundwater, springs? Type 2, 3?

Volcanic or Fumaroles Type 2? 3?

Exhumed impact crater Type 3?

+

+

Yes

+/

?

+

+

?

?

NA

NA

NA

NA NA

+

+

?

(not required) +

(not required) +

Possible, but not required

Possible, but not required

(not required) + Yes, likely

NA NA +/ No

NA, not applicable.

knobs (Fig. 3d) indicates that differential erosion and weathering of several stratigraphic layers of variable dip angles could explain some different peak height altitudes. Hypothetical possibilities of internal structures of the knobs that could create ‘‘bulls eye” or discordant ‘‘rings” are shown in Fig. 5c and d. A vertical injection cylinder of clastic material into layered deposits (Fig. 5e) as well as some parts of the host sediment could be cemented by fluids migrating through the injection. Uncemented central parts of the injection could erode more easily, forming a peak pit (see discussion on pitted knobs below). The top ‘‘layers” could appear inclined depending on the angle at which the top has been eroded. Remnant parts of cemented host rock layers can appear ‘‘attached” to the flanks of the knob giving it a layered appearance although this layering is not penetrative. Where some rows of Type 2 knobs seem to cross-cut the general strike of the exposed layers (Fig. 2c), faults could have been conduits for fluid flow or waters that cemented knob-forming material even without significant or recognizable changes in mineral composition detectable by CRISM. Some knobs show shoulders and are closely spaced and of uniform height. However, in both central and southern parts of southwest Candor Chasma, distinct sporadic knobs or knob clusters occur kilometers from escarpments of layered deposits (Fig. 1b), further indicating an internally coherent and resistant nature, and unlikely survival as solitary erosional forms (especially without a resistant capping lithology). The origin for Type 2 knobs (Figs. 2c, 3, 5 and Table 1) is more perplexing than Type 1 due to the variability and complexity of forms, yet in some instances they still look similar to Type 1 knobs (Fig. 2). Some Type 2 knobs could represent advanced erosional stages, or differential weathering and erosion of several stratigraphic layers with variable dips to develop the landscape with

knobs of different heights/altitudes. Desert landscapes of classic terrestrial buttes show some more isolated knobs (e.g., Monument Valley buttes nicknamed ‘‘The Mittens”, Fig. 4d–e) on the order of several kilometers from nearby escarpments. However, although ‘‘The Mitten” buttes are about two kilometers apart, they still have nearly identical heights or peak elevations (measured from US Geological Survey 7.5 minute topographic maps at 1897.7 m and 1882.4 m) due to the resistant lithologic boundary of the top/capping layer (Chenoweth, 2000). The other possible mechanisms (besides advanced or differential weathering/erosional states) to generate the plan view expressions shown in Fig. 5 are presented as A–C below for Type 2 knobs, with examples of terrestrial analogs: (A) Diagenetic (post-depositional) cemented knobs form from fluid movement through porous, granular material (Fig. 6a–c). Acidic or reducing fluids mobilize minerals such as iron oxides with later re-precipitation of iron as concentrated, concretionary cement (Chan et al., 2004, 2005) that preserves original bedding or lamination. Resistant concretionary cones and cylinders tens of centimeters in diameter to several meters high occur in the Jurassic Navajo Sandstone in Utah (Chan et al., 2000; Ormö et al., 2004). Although these Navajo examples are small, large forms on Mars can result from widespread fluid movement with more easily mobilized elements and a more reactive host rock (McLennan et al., 2005). The concretionary forms typically have an organized nearest-neighbor spacing (Ortoleva, 1994) and could also occur as more solitary isolated spots of mineralization. Concretionary cylinders show well-cemented rinds and some internal concentric layering, with central pit interiors that are less cemented (Fig. 5d, vertical fluid flow and cementation option, with analogies in Fig. 6a–c). Although the Navajo examples show iron oxide cementation,

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Fig. 4. Terrestrial examples of Type 1 knob landscapes. Aerial views of dissected southeast Utah landscapes of jointed sedimentary layers, eroding into fins, knobs, and pinnacles from Hamblin (2004, used by permission): (a) Arches National Park, (b) Moab anticline area, and (c) Needles district of Canyonlands National Park. In (a) and (b) the elongate fins are typically on the order of several tens of meters across or more. These images (a–c) of the dissected terrain look analogous to certain parts of Figs. 1 and 2 with Type 1 knobs. Classic buttes of Monument Valley at the Utah–Arizona border (d–e) are shown in two perspectives. Google 2008 satellite map (d) shows the topographic buttes due to the capping, resistant Permian DeChelly Sandstone of the Permian Cutler Group. The erosional west (W) and east (E) ‘‘Mittens” (e) are over 500 m diameter at the base and rise up over 200 m in height, with their highest altitude points being within less than 16 m of each other due to the correlation of the capping strata. Typical hoodoos (f) of Bryce Canyon National Park in southwestern Utah range up to 45 m high. Densely populated pinnacles erode and weather out from formerly continuous preexisting strata.

which has been proposed for concretionary spherules on Mars (‘‘blueberries” of Meridiani Planum – Squyres et al., 2004; Chan et al., 2004; McLennan et al., 2005; Ormö et al., 2004; Squyres and Knoll, 2005), concretionary forms could involve other mineralogies as long as there are fluid conduits and mineral-rich waters (perhaps where the cement is not significantly different from the host rock). (B) Synsedimentary megapipes from fluidized, liquefied and injected sediment cause repacking of grains and disruption of origi-

nal bedding (Schlee, 1963; Lowe, 1975, and see summaries of literature in Netoff, 2002; Chan et al., 2007) to produce knobs with massive internal structure (Figs. 5e and 6d and e). Fluid-rich sediment is ‘‘released” upwards due to overpressurized sediment (e.g., sediment loading) or strong ground motion (e.g., seismic waves from marsquakes or bolide impacts) that forces fluids upwards as fluidization or dewatering. Injectites occur at large scales visible on seismic sections, to outcrop scales (e.g., Netoff and Shroba, 2001; Netoff, 2002; Hurst et al., 2003a,b; Chan et al., 2007), and

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Fig. 5. Diagrammatic interpretations of possible causes of apparent layering and ‘‘ring” features within and adjacent to the knob structures, and mechanisms for producing a pit type of depression on the top of Type 2 and 3 knobs. (a) HiRISE view of Type 2 knobs (Fig. 3a) with red arrows indicating concentric ‘‘ring” features of the knobs that appears to be independent of the host rock layering. (b) Areal view of a seemingly layered knob. This appearance is, however, possible to generate either with ‘‘through-going”, continuous, penetrative layers (c) or with differentially cemented cylinders cutting the host strata (d). (e) Sedimentary injectional type of knob in cross-section and perspective view provides another alternative for generating concentric ring layering. Uncemented central parts of the injection could erode more easily, forming a peak pit. Possible terrestrial examples are shown in Fig. 6e. Differences in concentric ring cementation could cause the lowermost ring to cross-cut surrounding host rock layering as in (a).

may follow faults or fractures, or paleoenvironment structures (Netoff, 2002; Chan et al., 2007). Typically, the sides of the sandstone pipes retain the layering of the original host rock that was intruded (cf., Figs. 5 and 6e). Liquefied sand pipes are significant conduits for fluid flow that can preferentially cement grains with iron oxides (Fig. 6d) or carbonates. If injectites carrying upwardly mobile fluids with sediment break the surface, this would result

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in associated sand blows or sand volcanoes. A model including liquefied sand pipes is proposed to explain geomorphic features possibly linked with the hematite deposits at Meridiani Planum (Ormö et al., 2004). Sandstone megapipes weather out as large resistant towers tens of meters high and up to a few hundred meters in diameter and/or spread (Netoff, 2002) (Fig. 6e). These sandstone examples typically have only a different textural packing (internally massive vs. the bedded host rock) and thus show no significant compositional differences, and they can preserve clasts of the bedded host rock. If the synsedimentary fluidization is happening in place (in situ), there is little disturbance of the host rock with relatively smooth tapered sides to the knob. Some forcefully injected breccia pipes (Hannum, 1980) may be related to springs. Where there is more forceful injection through previously cemented and lithified rocks, the injectite can include breccia blocks, and the erosional form tends to separate more from the host rock due to textural changes in the injected core (vs. the layered host rock). (C) A variety of other internal subsurface processes could produce feeder necks and raised topographic knobs including volcanoes and steam vents or fumaroles (Symonds et al., 1992; Smith and Siegel, 2000). However, both volcanoes and hot springs typically precipitate distinctive minerals that should be visible in CRISM data. Fumaroles emit steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide that alter host rock and/or precipitate new minerals. Fumaroles are suggested as one origin of knobs on Mars (Chapman and Tanaka, 2002). Mounds either from springs or buildups (e.g., Cavalazzi et al., 2007) have morphological and dimensional similarities to the Candor knobs, and springs could be transitional to the injectites in the sense that they are still fed and cemented by upward moving fluids (and/or sediment as noted in B, above). To date, one conclusive hot spring documented on Mars is ‘‘Home Plate” in Gusev Crater at the MER Spirit landing site (Squyres et al., 2008), where opaline silica deposits have been distinguished. This Gusev Crater study of the Columbia Hills pointed out the difficulty in initially recognizing the opaline silica. So although CRISM should show distinctive mineralogies to identify springs, dust or other complicating factors could obscure subtle differences. Raised features of ‘‘pseudocraters” or ‘‘rootless cones” (e.g., Greeley and Fagents, 2001) are distributed in small clusters and superimposed on lava flows, but seem unlikely as explanations for knobs given the lack of any obvious lava flows. Similarly, pingos (e.g., de Pablo and Komatsu, 2009) typically show permafrost features such as fracturing on their tops that are lacking in Type 2 knobs. Most of these other miscellaneous mechanisms seem to require distinctive mineralogies (different from the layered host rock) or other distinctive associations we have not been able to detect. Thus, the pit geometry of some Type 2 knobs could likely result from any of the proposed mechanisms A–C above. 3.2.5. Type 3 knob description Rare Type 3 knobs (Fig. 1b) are typically dispersed and, with a size greater than 100 m in diameter, are an order of magnitude larger than Type 1 or 2 knobs. The large knobs typically exhibit spacings on the order of tens to hundreds of meters apart and are herein distinguished as Type 3 because of their unusually large size and solitary occurrence (Figs. 1 and 7b). These Type 3 knobs occur sporadically in all of Candor. A common feature to the Type 3 knobs is a pit-like summit feature with a raised outer rim and a central depression (Fig. 7). Although there is relatively scant description for these large knobs and therefore little known about their origins, they are mentioned because of their comparisons and potential relationships to the more numerous small knobs. This points out the need for more information about large knobs, and can help show areas that HiRISE images could focus on in the future.

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Fig. 6. Alternative terrestrial analogs for Type 2 Candor Chasma knobs: (a–c) Jurassic Navajo Sandstone iron oxide concretionary cylinder forms (tens of cm scale that is generally 1–2 orders of magnitude smaller than the Candor Chasma knobs), southeastern Utah; (d) Jurassic Carmel Formation injection pipes at arrows (10 m high) cemented with iron oxides, southeastern Utah; and (e) oblique aerial view of several Jurassic Entrada Sandstone injection megapipes (20+ m high, and similar to Candor Chasma knobs in size), Lake Powell – southern Utah (Photo: D. Netoff). Note: These types of analogs could occur without significant mineralogy changes on Mars if the cement were similar to the rock mineralogy, or where there is just a textural difference (e.g., grain size) in the rock that affects fluid flow and hence the resultant weathering form.

3.2.6. Type 3 knob interpretations The larger dimensions and solitary nature of Type 3 knobs suggest that the origin could be different than the numerous Type 1 or 2 small knobs that occur grouped together (e.g., Fig. 1b). The large, solitary size of >100 m in diameter could be produced by diagenetic waters that precipitate cement or by forcefully moved water via injectites, as even some injectites up to tens of meters diameter are recognized on seismic sections (Hurst et al., 2003b). Springs, volcanic or fumarole origins could also individually produce this solitary size and geometry of Type 3 knobs. However, such origins would be likely to show distinctive mineralogies separating the knob from the host rock layers. Although mud volcanoes have been proposed for ‘‘pitted cones” in other areas of Mars (e.g., Allen et al., 2009; Oehler and Allen, 2009; McGowan and McGill, 2009), the features interpreted as mud volcanoes typically occur in groups or clusters, and have high albedo characteristics that are lacking in the Type 3 knobs of this study. A possible origin of the large Type 3 knobs – particularly where there is a pit – is an impact-related origin considered below. Differential erosion of a cemented layer could include some small impact craters. Ormö et al. (2004, Fig. 9 therein) discuss a model for how impact craters may act as conduits for ground water fluids causing mineral precipitation at their brecciated rims. When applied to the formation of Type 3 pitted knobs, the impact craters could have accumulated on the layer before the cementation. Then flow of solutions through the permeable layer(s) could cement a brecciated zone around and below craters (with less cement in the crater interior, where part of the infill could be a less permeable material). Erosion could remove much of the layer so that the cemented rims and breccia lenses of the craters appear most resistant, protecting a column of material below the crater as the surrounding layered deposit and uncemented infill were stripped

away. One problem with this idea is that the pits do not vary in size as expected for impact craters on the surface of Mars. However, if the layer was exposed just for a geologically relatively short period, only a few small impact craters (i.e., equal or smaller than the largest of the observed pitted knobs) would have accumulated. Possibly the smallest impacts would not have made much difference for the groundwater flow, whereas some of the larger could have had greater influence (i.e., more extensive brecciation), explaining the relatively uniform sizes. Due to the relatively small size of the pitted knobs, we exclude their resistance to erosion to be a consequence of impact melt formation or an impact-induced hydrothermal process. At this time the only available MRO coverage of Type 3 knobs is HiRISE imagery over one pitted knob (Fig. 7a). The interpretation of this pit as an erosional remnant of an impact crater has one complication: it is not round. Instead, it is almost kidney-shaped where the southern part of the rim is higher than the northern rim. There are six general reasons that impact craters of this size might not be ‘‘perfectly” round, and we relate this specifically to the Fig. 7a kidney-shaped depression. (1) Pre-existing structures in the target can affect the crater shape. However, in this case no such structures are obvious. There is some layering, but it does not seem to have a dip that would cause an irregular crater shape. (2) Differential erosion might create different shapes. However, the ‘‘rim” seems to be of equal height and it would be difficult to explain the inward bulge of one section of the rim. (3) Post-impact tectonism can modify the shape. This is common on the Earth (with plate tectonics), but there is no indication for later modification at this martian structure.

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(4) Obliquity of the impact angle can cause elliptical crater shapes, but are rather unlikely to cause a kidney-shaped crater unless two impactors hit side by side. (5) Clustered impactors could hit after atmospheric breakup, and modify a crater shape. On Earth with its thicker atmosphere, craters up to a few hundred meters in diameter may show signs of being clustered from individual smaller craters (e.g., Henbury craters, Milton, 1968). Larger craters tend to be circular due to less separation of the projectile fragments (e.g., Melosh, 1989). Thus, the thinner atmosphere on Mars makes this even more unlikely for a half kilometerwide depression such as that in Fig. 7b. (6) Clustered impactors from a ‘‘rubble-pile” or peanut-shaped impactor is a last option that cannot be excluded. However, the shape of the depression does not seem to be created from two or more adjacent smaller craters. Overall, the impact crater origins may better explain Type 3 knobs than the more commonly distributed Type 2 knobs with their smaller and more uniform/related sizes, yet there are still some geomorphic features that are difficult to fully explain. 3.3. CRISM data analysis In remotely sensed images of the Mars landscape, possible explanations of the knobs and their analogs hinge on whether the knobs are mineralogically distinct from the host rock layers. The spectral data from CRISM (Fig. 8) is thus crucial to distinguish what origins might be more likely than others, to the extent the absorption features indicative of minerals of interest are detectable over the instrument’s 0.4–3.9 lm wavelength range. Mineralogic detections important to characterizing the layered deposits include pyroxenes, ferric oxides and oxyhydroxides, and hydrated sulfates (Fig. 9b). The igneous mineral pyroxene is distinguished by the broad electronic transition absorptions near 1 and 2 lm (Adams, 1974; Clark et al., 1990; Cloutis and Gaffey, 1991; Cloutis et al., 2006; Sunshine et al., 1990). Ferric minerals are distinguished mainly by the positions and relative strengths of electronic transition absorptions near 0.53, 0.66, and 0.9 lm (Sherman et al., 1982; Morris, 1985; Morris et al., 2000). Sulfates containing different amounts of bound water are distinguished by their vibrational absorptions (Hunt et al., 1971). Monohydrated sulfates have absorptions near 2.1 and 2.4 lm. In sulfates with multiple bound water molecules (polyhydrated sulfates), the shorter absorption occurs near 1.9 lm, and the 2.4-lm absorption becomes so strong that it transforms into a falloff in brightness past 2.3 lm.

Fig. 7. Type 3 knobs (up to a km in diameter) with pitted peaks in eastern Candor Chasma that are generally solitary and more rare. Examples from HiRISE (a) and Context Camera images (b and c). Note: (a) is an enlargement from the upper right outline box in (b).

3.3.1. General spectral context A map of CRISM coverage of western Candor Chasma shows regional spectral variations and the locations of the measured knobs (Fig. 8). The properties of the materials elsewhere in that chasma (Murchie et al., 2009b) provide context for spectral variations associated with the knobs. The interior of the chasma exposes deeply eroded plateau plains on the chasma walls, and the center of the chasma is occupied by a 5-km relief plateau of interior layered deposits called Ceti Mensa. Both the chasma walls and the layered materials have a relatively high thermal inertia and spectral variations that correlate with geologic unit boundaries, suggesting that coverage by wind-blown dust is minimal and the spectral signature of the underlying substrate is being measured. Representative spectra are shown in Fig. 9 using thin lines. To a first order, most of the spectral variance in the chasma walls and layered materials can

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Fig. 8. (a) CRISM multispectral (long strips) and targeted (hourglass-shaped) observations covering western Candor Chasma, overlain at 40% transparency on a THEMIS dayIR mosaic. The white box in (a) shows the location of Fig. 10. Elevation contours are shown in white at a 1-km interval. The red image plane represents the depth of the 0.53lm absorption in nanophase or finely crystalline ferric oxide; the green plane represents depth of the 0.9-lm absorption in crystalline ferric minerals; and the blue plane represents the integrated area in 1-lm absorption resulting from the mafic mineral pyroxene, from 0.75 to 1.02 lm. The dynamic ranges of the red, green, and blue planes are absorption band depths of 0.22, 0.03, and 0.02, respectively. (b) The red and blue image planes are the same as in ‘‘a”, and the green plane is replaced with strength of the inflection in the spectrum due to the decrease of reflectance into the 2.4-lm band in hydrated sulfates. The dynamic range is an absorption band depth of 0.04.

be explained if they are composed of materials resembling the dust and pyroxene-containing rock that dominate the surrounding plateau plains (Mustard et al., 2008). The chasma walls and floor (gray spectrum in Fig. 9) both exhibit relatively low reflectances and 1-

and 2-lm absorptions indicative of pyroxenes (in the blue image planes in Fig. 8a and b), In the layered deposits, similar dark materials are restricted to patches ranging from kilometers to tens of meters in scale (dark green spectrum in Fig. 9). The HiRISE and

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Fig. 9. (a) Type spectra of materials in western Candor Chasma (thin lines), and spectra of the knobs and surrounding materials in southwestern Candor Chasma (thick lines), covering wavelengths 0.44–3.1 lm. Each spectrum covers a nearly square area between 90 and 200 m on a side. The data have been calibrated to apparent I/F, divided by the cosine of the incidence angle, and divided by an atmospheric transmission spectrum scaled to the strength of the 2.0-lm CO2 absorption. Breaks in the spectra are wavelengths where data calibration is problematic. (b) Laboratory spectra of pure mineral analogs for material observed in western Candor Chasma. The broad pyroxene absorption centered at 1.0 lm is observed in the bottom three spectra in (a). The narrow 2.1-lm absorption indicative of monohydrated sulfate is observed in the spectra in (a) labeled as being monohydrate-bearing. The region covered by the brown spectrum in (a) was identified in TES data as containing gray hematite, and the CRISM spectrum exhibits a band center near 0.9 lm and a strong slope at 1.0–1.7 lm indicative of an additional more finely crystalline component.

CTX images show that these dark patches are rippled sand, which in some cases seems to originate from discrete strata in the layered deposits (Murchie et al., 2009b). The layered deposits are dominated by medium- to high-reflectance materials that lack 1- and 2-lm pyroxene absorptions but do exhibit a pervasive 0.53-lm absorption indicative of nanophase ferric oxide (red image planes in Fig. 8a and b, light orange spectrum in Fig. 9). The layered deposits resemble dust on the surrounding plateau in their moderate to high albedo and presence of a pervasive 0.53-

lm absorption, but they are distinguished by absorptions indicative of hydrated sulfates and crystalline ferric minerals absent from the chasma walls and surrounding plateau plains. Absorption features showing occurrences of hydrated sulfates and crystalline ferric minerals are shown in the green image planes in Fig. 8b and a, respectively. Fig. 8b shows hydrated sulfates (green image plane) as indicated by strength of the inflection in the spectrum due to the 2.4-lm absorption. The signature of hydrated sulfates occurs in medium- and high-reflectance outcrops (light green spectrum

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in Fig. 9) as well as in the pyroxene-containing dark patches superposed on the layered deposits. Typically the sulfate is monohydrated, as evidenced by an absorption near 2.13 lm, consistent with the Mg-rich monohydrate kieserite. Fig. 8a shows crystalline ferric minerals in the green image plane, as the strength of a 0.9lm absorption. There are two types of exposures of crystalline ferric minerals: first, in some parts of the dust-like materials that form the bulk of the layered deposits, where there is also an enhanced 0.53-lm absorption due to nanophase oxides; the presence of both absorptions creates a yellow color in Fig. 8a. Invariably there is also a strong signature of hydrated sulfate. The second type of exposure is in darker, lower-albedo materials at the base of the plateaus’ flanks or on the lowermost slopes, that lack a 0.53-lm absorption. These materials have a green color in Fig. 8a, and are shown by the brown spectrum in Fig. 9a. The largest occurrence, west of Candor Mensa, corresponds to a TES detection of gray hematite (Mangold et al., 2007; Knudson et al., 2007; Murchie et al., 2009b). 3.3.2. Spectral properties of the knobs Full-resolution CRISM observations of southwestern Candor Chasma cover a representative area containing typical knobs (e.g., Fig. 10 shown at arrows). Representative spectra of Type 2 knobs and lower- and higher-reflectance materials in the surrounding, highly deformed layered materials are shown by heavy lines in Fig. 9. The standard representation of HiRISE color was synthesized (Fig. 10a) by convolving the CRISM reflectances through the band passes of HiRISE color filters, with a minimum–maximum stretch performed independently in each synthesized band. The light-colored outcrops of knob sides resemble medium- to highreflectance parts of the layered deposits in their reflectance, the presence of a moderate 0.53-lm absorption, and the presence of a 2.13-lm absorption due to monohydrated sulfate. Over HiRISE’s 0.4–1.0 lm wavelength range, spectral properties of the knobs closely resemble those of the surrounding high-standing, light-color materials (heavy orange spectrum in Fig. 9). That is, the knobs have a moderate 0.53-lm absorption except that some of the knobs have a brightness up to about 10% lower (heavy orange spectrum in Fig. 9). Ratios of the spectra of knobs to surrounding materials failed to reveal systematic differences in absorption strengths; the only significant differences are reflectance and effects due to topographic shading. On anti-sunward-facing slopes, the slope of the spectral continuum is redder; that is, reflectance increases toward longer wavelengths. This may be attributed to a greater proportion of illumination by sky radiance as opposed to solar irradiance. The knobs’ dust-like spectral properties at HiRISE wavelengths, combined with their commonly lower reflectance and the stretch applied to the data, impart a slightly brown color to some of the knobs in the synthesized HiRISE color (Fig. 3). This effect is accentuated on shaded, east-facing slopes, where a high fraction of illumination comes from light scattered through the relatively red sky instead of directly from the Sun. Fig. 10b shows strengths of absorptions due to nanophase ferric oxides, pyroxene, and crystalline ferric oxides, using the same spectral features and the same scaling as Fig. 8a. In the layered deposits surrounding the knobs, nanophase iron oxide-containing material dominates the higher-standing, light-colored outcrops, whereas pyroxene occurs in the rippled sands in the intervening topographic swales. The knobs (including their capping tops) are nearly undistinguished from the higher-standing, light-color parts of the deformed layered deposits. There is no evidence at the tens of meters pixel scale for concentrations of crystalline ferric minerals in the knobs like those that occur in other regions of the layered deposits.

Fig. 10. The floor of southwestern Candor Chasma with deformed layered materials and unconformably superposed knobs. Arrows show example knobs. (a) HiRISE 3color image synthesized from CRISM data convolved through HiRISE bandpasses, with each band stretched independently. The data are mosaiced and shown at 20 m/pixel on a background of THEMIS VIS imaging for context. (b) 0.53-lm, 0.9lm, and 1-lm absorptions due to nanophase ferric oxide, crystalline ferric oxide, and pyroxene, respectively, in the red, green, and blue image planes, using the same formulations as in Fig. 8. All three image planes have been overlain with 40% transparency on THEMIS-VIS images. (c) Absorption strengths due to hydrated sulfates, where the red and blue image planes show depth of the 1.9-lm absorption due to polyhydrated sulfates, and the green image plane shows depth of the 2.1-lm absorption due to monohydrated sulfates. The 1.9-lm band is evaluated at 1.95 lm relative to a continuum fit linearly between 1.87 and 2.07 lm. The 2.1-lm band is evaluated at 2.13 lm relative to a continuum fit linearly between 1.93 and 2.25 lm. All three image planes have a dynamic range of 0.04 and have been overlain with 40% transparency on map-projected THEMIS-VIS images for context.

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Fig. 10c shows strengths of absorptions due to polyhydrated and monohydrated sulfates. The region surrounding the knobs consistently exhibits a 2.1-lm absorption due to monohydrated sulfate (Fig. 9). Similarly with Ceti Mensa, this signature is enhanced in the dark, pyroxene-containing sands in topographic swales, but it is also present in the light-colored outcrops. The knobs themselves have a comparable to slightly weaker absorption than in the surrounding light-colored outcrops, demonstrating that the knobs also contain monohydrated sulfate but provide no evidence for an enhancement of its abundance. No obvious exposures of material containing polyhydrated sulfates occur on or surrounding the knobs.

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The CRISM data (at the current resolution) show that there is no significant spectral differences between Type 1 and 2 knobs and their surrounding terrain except for albedo, and in fact the knobs are representative of average properties of the layered deposits. The similar strengths of absorptions due to hydrated sulfates and crystalline ferric minerals fails to support a concentration of either class of minerals in the knobs (for example as a cement), but does not rule out a phase lacking a visible–infrared spectral signature such as anhydrous silica or anhydrous chlorides. In fact, the apparent ‘‘brown” color of the knobs in HiRISE images (Fig. 3) appears to be just an artifact of the slightly lower reflectance than surrounding higher-reflectance material, combined with shading effects and the high-contrast stretch applied to the color images. This analysis supports models for the origin of the knobs that do not require a significant compositional difference from the host rock layers. In a similar type of analysis to determine if visible color differences viewed with the Opportunity rover’s visible and infrared cameras in Endurance crater equated with real differences in chemical composition (from RAT and APXS analysis), analysis suggested that color variations were due to variation in surface texture and the ability of the rocks to capture sand grains (e.g., Farrand et al., 2007). The idea that Type 1 knobs are part of a continuous stratigraphic sequence of units identified by Okubo et al. (2008) is consistent with CRISM observations that the cap rock of the knobs is chemically similar to the surrounding layered rocks. Similarly, origins of Type 2 knobs by cementation and injectites or megapipes are most consistent with the CRISM data, if the cementing phase is simply recrystallized monohydrated sulfate. At present, there is no spectral evidence for phyllosilicate minerals or silica expected in the fumarole model. CRISM data specific to the Type 3 knobs is currently not available.

certain permeable layers that were more susceptible to diagenetic reactions to cause preferential cementation. Certainly the presence of water would help facilitate chemical or diagenetic reactions in the rock. Any layer with preferential cementation (even without a noticeable change in mineral composition) could contribute to the variable layers even as a cap rock that facilitates the type of dissected terrain and the subsequent erosion and weathering for Type 1 knobs, and possibly Type 2 knobs at more advanced stages of weathering, or with differential erosion of various dipping strata. Formation of erosional landforms would imply greater times to form the features than would episodic fluid flow movements that could be relatively short lived. Vertical movements via injection or upward fluid flow (similar to springs, although no distinctive mineralogy is distinguished) could explain the Type 2 knobs with ring structures and/or with pits, with subsequent erosion and weathering (reflecting differences of the internal core structure) to produce the more solitary and sometimes larger knobs. These vertical movements would not require different mineralogies from the host rock, although in some terrestrial analogs the fluid flow cements can have a distinctive mineralogy. An advanced stage of erosion from Type 1 knobs to a progressively more denuded phase of Type 2 knobs suggests multiple or prolonged episodes of erosion. The possible Type 2 origins of diagenetic cementation or injection in pipe forms also implies vertical motion of water or water-laden sediment through layered deposits near Mars’ surface or in the shallow subsurface (prior to the current exposure). The Type 3 knobs are more difficult to categorize and explain, although certainly the erosion to their present forms can be more easily facilitated in the presence of water as part of the hydrological cycle, reshaping the surface of Mars. A role for fluvial erosion in forming knobs is supported by HiRISE and CRISM imaging of Candor Mensa which shows evidence for erosion of the layered deposits by braided channels, with concentration of gray hematite where the channels debouch onto the chasma floor forming alluvial fans (Murchie et al., 2009b). However such channels are not preserved surrounding the knobs, so fluvial erosion must have been followed by severe eolian erosion to have removed the channels themselves. An origin of Type 3 knobs as erosional remnants of partially cemented impact craters cannot be ruled out at this time. The combination of different data sets (HiRISE and CRISM) is necessary to more fully understand the physical characteristics and chemical characteristics of knobs. Overall, the variety of geomorphic expressions the Candor Chasma area indicate an intriguing history that must have included moving water on the surface at some time, and perhaps multiple or sustained periods.

4. Discussion

5. Conclusions

It is possible that several origins in combination could contribute to the complex variety of geomorphic expressions of knobs in southwest Candor Chasma. Analysis of CRISM data reveals no detectable signature that supports a significant mineralogical difference between any part of the knobs and its surrounding layered host rock. The lack of evidence for a mineralogical difference is most consistent with genetic mechanisms that do not require it. Of the terrestrial analogs that are explored in this paper, the volcanic or fumarole origins (which might produce distinctive mineralogies) seem unlikely with current CRISM data resolution. However, the cap rock may be texturally different, harder or more coherently cemented (possibly through recrystallization or diagenetic alteration). In chemically reactive host rock, certain beds could be easily cemented with changes or shifts in the ground water table or within

New details from recent HiRISE images, in combination with THEMIS and CRISM data, reveal the geomorphic features of knobs in Candor. The knobs show positive relief on the order of tens of meters, with resistant tops relative to the more easily eroded surrounding layered deposits. The abundant Type 1 knobs are most consistent with formation by erosional processes that dissected the layered deposits, and created sculpted knob forms. Type 2 knobs are more ambiguous in their origin. They could be erosional remnants of various dipping layers, or indurated layers possibly cemented by recrystallized hydrated sulfates, or they could have formed as sediment megapipes. In either case, fluid movement via diagenetic pathways or injection/dewatering is implied. It seems unlikely that simple surface erosion that formed Type 1 knobs could form the pits on the Type 2 and 3 knobs. There are multiple working hypotheses for the origin of the knobs that could

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be refined by more detailed mineralogic data which may eventually become available through better and more thorough imaging techniques. In all the possible origins to explain the geomorphic knobs, the Earth analog processes require water, and thus suggest that while there may be important differences in the actual aqueous chemistry and genesis, the records of water movement are still significant. The recent HiRISE and CRISM images of knobs show remarkable geomorphic features of Candor Chasma that reflect lithologic properties and interactions with surface processes, and may hold records of past history and the surface evolution of Mars. Acknowledgments We thank Mary Chapman, Robin Fergason, Amy Knudson, and two anonymous reviewers for comments on this manuscript. We gratefully acknowledge partial funding support of this project from NASA Mars Fundamental Research NNG06GI10G to Chan, NASA through JPL subcontract 1277793 to Murchie for CRISM work, NASA Mars Data Analysis NNX06AE01G for Okubo, Spanish Ministry for Science and Innovation (CGL2004-03215/BTE) for Ormö, a grant from the Italian Space Agency for Komatsu, the Fannie & John Hertz Foundation and NSF Graduate Research Fellowship for Wray, and fellowships from the Alexander von Humboldt Foundation and the McDonnell Center for the Space Sciences for McGuire. References Adams, J.A., 1974. Visible and near-infrared diffuse reflectance spectra of pyroxenes as applied to remote sensing of solid objects in the Solar System. J. Geophys. Res. 79, 4829–4836. Allen, C.C., Oehler, D.Z., Baker, D.M., 2009. Mud volcanoes – A new class of sites for geological and astrobiological exploration of Mars. In: 40th Lunar and Planetary Science Conference. Abstract 1749. Arvidson, R.E., Seelos IV, F.P., Deal, K.S., Koeppen, W.C., Snider, N.O., Kieniewicz, J.M., Hynek, B.M., Mellon, M.T., Garvin, J.B., 2002. Mantled and exhumed terrains in Terra Meridiani, Mars. J. Geophys. Res. 108 (E12), 8073. doi:10.1029/ 2002JE001982. Bibring, J.-P., Langevin, Y., Gendrin, A., Gondet, B., Poulet, F., Berthé, M., Soufflot, A., Arvidson, R., Mangold, N., Mustard, J., Drossart, P., the OMEGA team, 2005. Mars surface diversity as revealed by the OMEGA/Mars Express observations. Science 307, 1576–1581. Bibring, J.-P., Langevin, Y., Mustard, J.F., Poulet, F., Arvidson, R., Gendrin, A., Gondet, B., Mangold, N., Pinet, P., Forget, F., the OMEGA team, 2006. Global mineralogical and aqueous Mars history derived from OMEGA/Mars Express data. Science 312, 400–404. doi:10.1126/science.1122659. Bourke, M.C., Bullard, J., Barnouin-Jha, O., 2004. Aeolian sediment transport pathways and aerodynamics at troughs on Mars. J. Geophys. Res., 109 (E7), E07005. doi: doi:10.1029/2003JE002155. Bourke, M.C., Edgett, K.S., Cantor, B.A., 2008. Recent aeolian dune change on Mars. Geomorphology 94, 247–255. Bue, B.D., Stepinski, T.F., 2006. Automated classification of landforms on Mars. Comput. Geosci. 32, 604–614. ISSN: 0098-3004. Butler, B.E., 1956. Parna: An eolian clay. Aust. J. Sci. 18, 145–151. Cavalazzi, B., Barbier, R., Ori, G.G., 2007. Chemosynthetic microbialites in the Devonian carbonate mounds of Hamar Laghdad (Anti-Atlas, Morocco). Sediment. Geol. 200, 73–88. Chan, M.A., Parry, W.T., Bowman, J.R., 2000. Diagenetic hematite and manganese oxides and fault-related fluid flow in Jurassic sandstones, southeastern Utah. Am. Assoc. Petrol. Geol. Bull. 84, 1281–1310. Chan, M.A., Beitler, B., Parry, W.T., Ormö, J., Komatsu, G., 2004. A possible terrestrial analogue for hematite concretions on Mars. Nature 429, 731–734. Chan, M.A., Bowen, B.B., Parry, W.T., Ormö, J., Komatsu, G., 2005. Red rock and red planet diagenesis: comparisons of Earth and Mars concretions. GSA Today 15 (8), 4–10. Chan, M.A., Netoff, D., Blakey, R., Kocurek, G., Alvarez, W., 2007. Clastic-injection pipes and syndepositional deformation structures in Jurassic eolian deposits: Examples from the Colorado Plateau. In: Hurst, A., Cartwright, J. (Eds.), Sand Injectites: Implications for Hydrocarbon Exploration and Production. Am. Assoc. Petrol. Geol. Mem. 87, 233–244. Chan, M.A., Yonkee, A., Netoff, D.I., Seiler, W.M., Ford, R.L., 2008. Polygonal cracks in bedrock on Earth and Mars: Implications for weathering. Icarus 94, 65–71. Chapman, M. (Ed.), 2007. The Geology of Mars: Evidence from Earth-based Analogs. Cambridge University Press. 474p. Chapman, M.G., Tanaka, K.L., 2002. Related magma–ice interactions: Possible origins of chasmata, chaos, and surface materials in Xanthe, Margaritifer, and Meridiani Terrae, Mars. Icarus 155, 324–339.

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