The geology and mineralogy of Ritchey crater, Mars: Evidence for

Transcription

PUBLICATIONS
Journal of Geophysical Research: Planets
RESEARCH ARTICLE
10.1002/2013JE004602
Key Points:
• Hesperian age Ritchey crater hosts clay,
hydrated silica, and mafic minerals
• The central peak contains clays within
an impact melt layer
• Uplifted bedrock lacks clays, suggesting
they are absent in the deeper crust
Correspondence to:
V. Z. Sun,
[email protected]
Citation:
Sun, V. Z., and R. E. Milliken (2014), The
geology and mineralogy of Ritchey
crater, Mars: Evidence for post-Noachian
clay formation, J. Geophys. Res. Planets,
119, 810–836, doi:10.1002/
2013JE004602.
Received 22 DEC 2013
Accepted 13 MAR 2014
Accepted article online 18 MAR 2014
Published online 17 APR 2014
The geology and mineralogy of Ritchey crater, Mars:
Evidence for post-Noachian clay formation
Vivian Z. Sun1 and Ralph E. Milliken1
1
Department of Geological Sciences, Brown University, Providence, Rhode Island, USA
Abstract Widespread detection of phyllosilicates (clay minerals) in Noachian (>3.5 Ga) terrains on Mars and
their paucity in younger terrains have led to the hypothesis that Noachian conditions were more clement than
the colder, drier conditions that have since followed. However, recent clay detections in several Hesperian
impact craters suggest that fluvial transport and alteration were possible after the posited early era of
phyllosilicate formation. Here we present evidence that rocks within Hesperian age Ritchey crater (28.5°S, 51°W)
record a period of post-Noachian fluvial transport and in situ alteration. This resulted in the transport of clays
from the crater wall to the crater floor and the formation of hydrated silica and Fe/Mg smectite in Ritchey’s
central uplift. Clay minerals associated with central uplifts are commonly interpreted to represent preexisting
clays excavated from depth, potentially providing insight into older crustal clay-forming processes. Here we
present detailed geomorphic and mineralogic maps and show that the clays in Ritchey’s central peak formed
after or as a direct result of the impact and are thus Hesperian or younger. Clays on the crater wall were either
preexisting clays exposed by the impact or formed in situ through postimpact water-rock interaction. In either
scenario, some of these clays were likely subsequently transported to the crater floor by fluvial-alluvial
processes in a source-to-sink system. In this context, the hydrated phases in Ritchey indicate several different
formation and transport mechanisms and provide further evidence that near-surface clay mineral formation,
and thus habitable conditions, existed on Mars after the Noachian.
1. Introduction
Hydrated minerals provide direct evidence for water-related processes and have been shown to be
widespread on Mars based on visible near-infrared reflectance spectra acquired by the Observatoire pour la
Mineralogie L’Eau, les Glaces et l’Activite [Bibring et al., 2005, 2006; Poulet et al., 2005] and CRISM (Compact
Reconnaissance Imaging Spectrometer for Mars) [Murchie et al., 2007; Mustard et al., 2008] orbital imaging
spectrometers. The presence of phyllosilicates in particular, hereafter referred to as “clay minerals,” could
result from a variety of formation scenarios on Mars. These include surface weathering under warmer and
wetter conditions [Poulet et al., 2005; Bibring et al., 2006], subsurface weathering under cooler surface
conditions more similar to those of present-day Mars [Ehlmann et al., 2011], alteration in a hydrothermal
environment induced by volcanic [Farmer, 1996; Mangold et al., 2007] or impact activity [Newsom, 1980; Allen
et al., 1982; Rathbun and Squyres, 2002; Abramov and Kring, 2005; Schwenzer and Kring, 2009; Marzo et al.,
2010], or neoformation (direct precipitation) in aqueous depositional environments [Milliken and Bish, 2010;
Bristow and Milliken, 2011].
The vast majority of clay mineral occurrences identified on Mars are in ancient Noachian age terrains (≥3.5 Ga
absolute age based on results of Hartmann and Neukum [2001]), whereas younger Hesperian and Amazonian
terrains are marked by a paucity of clay minerals [Poulet et al., 2005; Bibring et al., 2006; Murchie et al., 2009;
Arvidson et al., 2014]. This association between mineralogy and geologic time suggests clay minerals formed
early in Martian history when surface conditions may have been warmer and wetter, whereas younger
terrains lack clay minerals because environmental conditions changed to preclude their widespread
formation [Bibring et al., 2006; Murchie et al., 2009]. Clay formation during the early history of Mars also
coincides with morphologic evidence of widespread valley network formation, which had largely ceased by
the Early Hesperian [Carr, 1996; Fassett and Head, 2008]. These independent morphologic and mineralogic
observations suggest significant water-rock interaction prior to ~3.5 Ga on Mars, followed by a decrease in
near-surface aqueous activity as the Martian surface became colder and drier. However, the nature of this
purported climate change on Mars, the variability in local climatic conditions, and whether or not it was
geologically rapid or gradual is poorly understood.
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Though predominantly located in Noachian age terrains, clay minerals have also been identified in several
younger Hesperian impact craters [e.g., Carter et al., 2010; Mangold et al., 2012]. However, the mineralogic and
geologic contexts of these hydrated phases must be studied in detail to determine whether they represent
postimpact alteration products [Marzo et al., 2010], excavated ancient crustal materials [Carter et al., 2010;
Quantin et al., 2012], or sediments remobilized by fluvial [Mustard et al., 2008] or eolian transport [Loizeau et al.,
2007]. For instance, clay mineral-bearing rocks associated with the central uplifts of impact craters have been
interpreted as altered ancient crust excavated from depth [e.g., Mustard et al., 2008; Carter et al., 2010; Quantin
et al., 2012], but they may also represent postimpact in situ alteration [Marzo et al., 2010; Osinski et al., 2013].
Testing these contrasting hypotheses, especially for post-Noachian craters, is critical for understanding the
timing and duration of clay formation on Mars.
Examples of craters believed to have excavated Noachian clay minerals are found in Nili Fossae [Mangold
et al., 2007], the southern highlands [Mustard et al., 2008], the northern plains [Carter et al., 2010], and
Leighton crater [Michalski and Niles, 2010]. Potentially recycled (transported) Noachian clay minerals include
deposits in Holden crater [Grant et al., 2008; Milliken and Bish, 2010], Jezero crater [Ehlmann et al., 2008],
Ismenius Cavus [Dehouck et al., 2010], Terby crater [Ansan et al., 2011], and Columbus crater [Wray et al., 2011].
Clay minerals possibly generated by impact-induced hydrothermal or low-temperature alteration during the
Noachian include occurrences in craters in the Isidis region [Ehlmann et al., 2009], the southern highlands
[Ehlmann et al., 2010], and Holden crater [Osinski et al., 2013]. In contrast, post-Noachian alteration has been
inferred for clay minerals in the central uplift of Toro crater [Marzo et al., 2010; Fairén et al., 2010], in an alluvial
fan in Majuro crater [Mangold et al., 2012], and possibly in fan deposits at Ismenius Cavus [Dehouck et al.,
2010]. These disparate scenarios and age estimates highlight the complexity associated with the geological
characterization of clay minerals, which can form in a wide variety of settings and are often not indicative of
unique formation conditions. In order to evaluate various hypotheses for clay minerals found in impact craters,
it is first necessary to understand the detailed spatial relationships between local stratigraphy (e.g., bedding,
facies changes, and superposition) and variations in mineralogy.
In this context, we discuss here the diverse mineralogy and geology of Ritchey crater (28.5°S, 51°W), a postNoachian impact crater that hosts a variety of hydrated minerals and morphologic features indicative of fluvial
activity and in situ alteration. We focus on previously detected hydrated silica [Quantin et al., 2012] and present
evidence for the detection of clay minerals, where the latter occur in a variety of geologic units that predate and
postdate the Ritchey impact event. Through detailed geomorphic and mineralogic mapping, we evaluate
whether the observed clay minerals are consistent with impact excavation, transport (sediment recycling),
and/or postimpact formation and alteration. We conclude by discussing the implications of these observations
for understanding the timing and duration of clay-forming conditions on Mars as well as the limitations of
inferring crustal composition from clay minerals associated with central uplifts of impact craters.
2. Data and Methods
2.1. CTX, HiRISE, and CRISM
Geomorphic mapping was conducted using Context Camera (CTX) and High-Resolution Imaging Science
Experiment (HiRISE) data. CTX acquires images at a spatial resolution of 5–6.5 m/pixel in 30 km wide swaths
and provides context for HiRISE and targeted CRISM data [Malin et al., 2007]. The processed and mapprojected CTX data used in this study were obtained from Arizona State University’s Mars Image Viewer
website. CTX digital elevation models (DEMs) were generated using the Ames Stereo Pipeline [Moratto et al.,
2010] and were used for qualitative purposes, including assessment of the relative stratigraphic positions of
different geologic units.
HiRISE is a multispectral camera that acquires images at a high spatial scale of up to 25 cm/pixel. The camera
acquires data using three channels: red (0.57–0.83 μm) in 6 km wide swaths, and blue green (0.4–0.58 μm), and
near-infrared (0.79–1.1 μm) in 1.2 km wide swaths [McEwen et al., 2007]. The processed and map-projected
HiRISE images and DEMs used in this study were obtained from the University of Arizona’s HiRISE website.
In addition to morphology-based mapping, mineralogy was also mapped using CRISM spectral data. CRISM is
a hyperspectral imaging spectrometer that acquires reflectance data from 0.36 to 3.92 μm over 544 channels
at a sampling of 6.55 nm/channel. A visible to near-infrared (VNIR) “S” detector obtains data from 0.36 to
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Table 1. List of CRISM Observation IDs Used in This Study
CRISM Observation ID
FRT00007C34
FRT00013678
FRT00009620
FRT0000AB54
FRT0000B5C3
FRT0001253F
FRT00012DFA
HRL00010D87
FRT000052EE
FRT00017D66
FRT0000A0B7
FRT0000AC1F
HRL0000D370
HRL00011D5C
FRT00012ACC
FRT0001376C
FRT0000654F
Location
Central Peak
Dune Field/Central Peak
Eastern Wall/Rim
Interior Deposits
Southern Wall
Western Wall/Floor
10.1002/2013JE004602
1.05 μm and an infrared (IR) “L” detector from 1.00
to 3.92 μm. In its highest spatial and spectral
resolution “targeted” mode, CRISM acquires fullresolution targeted (FRT) observations and halfresolution long (HRL) observations, which have
spatial resolutions of 15–19 m/pixel and
38 m/pixel respectively [Murchie et al., 2007]. For
this study we examined 14 FRT and three HRL
images (Table 1) to assess mineralogy and
compositional variations within Ritchey.
CRISM data (Derived Data Record and calibrated
Targeted Reduced Data Record version 3 files)
were obtained from the Planetary Data System
Geosciences Node and processed with the CRISM
Northern Wall
analysis tool v6.6 using the Environment for
Visualizing Images (ENVI) software package.
Northern Floor
Standard photometric corrections were performed
by dividing each I/F spectrum by the cosine of the
incidence angle, and atmospheric removal was based on the volcano scan method (scaling of atmospheric path
as derived from observations over Olympus Mons) [McGuire et al., 2009]. The IR data were filtered to remove
spectral spikes and data striping [Parente, 2008], and both VNIR and IR data were map projected and layer
stacked to produce a single image cube spanning the full CRISM wavelength range for each observation.
Standard CRISM VNIR and IR spectral summary parameter maps [Pelkey et al., 2007] were also generated to
guide mineral identification.
2.2. Mineral Identification
In this study we examine the 0.36–2.6 μm wavelength range of the CRISM data. Although the ~1.2 to 2.6 μm
wavelength range is sufficient for detecting hydrated silicates [Ehlmann et al., 2009], the shorter wavelength data
were also analyzed to confirm the detection and shape of absorptions near 1 μm associated with Fe-bearing
phases such as olivine, pyroxene, and Fe-bearing clay minerals.
The CRISM spectral summary parameters were first used to identify areas with spectral signatures indicative of
mafic or hydrated minerals. Of interest to this study are the OLINDEX, LCPINDEX, HCPINDEX, SINDEX, BD1900,
BD2210 and D2300 parameters, which are designed to indicate the presence of olivine, low-Ca pyroxene,
high-Ca pyroxene, hydrous sulfates, H2O-bearing minerals, Al-OH or Si-OH vibrations, and Fe/Mg-OH
vibrations, respectively. The OLINDEX, LCPINDEX, and HCPINDEX rely on the detection of Fe-related
absorptions near ~1 and ~2 μm, but Fe-bearing clay minerals (e.g., nontronite and chlorite) are also known
to give rise to positive detections in the OLINDEX. Though examined for all CRISM observations, the SINDEX
did not exhibit evidence for the presence of hydrous sulfates in Ritchey and is not considered further here.
The BD1900 parameter highlights the presence of H2O-bearing minerals and is often found to yield
positive detections when clay minerals (or sulfates) are present. However, this absorption is the result of a
combination H2O bend and stretch vibration and is nonunique; thus, it cannot typically be used to identify
specific hydrous minerals. In contrast, the BD2210 parameter is indicative of Al-OH-bearing phases such as
aluminous clays (e.g., montmorillonite, illite, and kaolinite) or Si-OH-bearing phases (e.g., opaline silica and
hydrated volcanic/impact glass). Similarly, the D2300 parameter measures a drop in reflectance (I/F) near
2.3 μm that is often indicative of Fe/Mg-OH vibrations as found in certain clay minerals (e.g., nontronite,
saponite, serpentine, and chlorite).
After initial detection in CRISM spectral parameter summary images, we utilized the spectral ratio method to
further enhance spectral features in regions of interest to aid in mineral identification. In this method, the
mean spectrum of a region of interest, which may consist of hundreds to thousands of pixels, is divided by
the mean spectrum of a spectrally bland region of interest within the same image [e.g., Mustard et al., 2008;
Milliken et al., 2008]. This ratio method enhances absorption features and suppresses residual atmospheric
effects and instrumental noise, highlighting the differences in mineralogy between the region of interest and
whatever phases are associated with the “background” region. We note that spectral ratios calculated using
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Figure 1. North is up in all figures unless otherwise noted. (a) Global MOLA topography map with context for Figures 1b
and 1c. (b) Ritchey’s location indicated by the white arrow on MOLA topography. (c) The same region as Figure 1b with
a geologic map [Tanaka, 1986; Skinner et al., 2006] overlain. Hesperian units are purple toned while Noachian units are brown
and tan toned. Ritchey is located at the boundary between Hesperian ridged plains to its west and heavily cratered Noachian
terrains to its east.
this method have the potential to be highly dependent on the choice of the background region (denominator
spectrum), and it is common in CRISM images for the “bland” background region to be associated with a dark
sand dune or dust-covered region. This can have the effect of suppressing pyroxene features near ~1 and
~2 μm in ratio spectra [Mustard et al., 2005] because similar weak features are often present in the spectra of
these background materials.
The derived ratio spectra were then compared to data from the Reflectance Experiment Laboratory (RELAB)
and CRISM [Murchie et al., 2007] spectral libraries to determine mineralogy. When necessary, the default
continuum removal method in the ENVI software package was used over the 0.36–2.65 μm wavelength range
to determine specific absorption band centers and widths for comparison to library spectra. Finally, offsets in
I/F near 1.02 μm between VNIR and IR ratio spectra [Murchie et al., 2009] were corrected when necessary by
scaling the VNIR data to match IR values. This scalar correction does not affect the presence, position, or
shape of absorption features discussed below.
2.3. Mapping
CRISM, CTX, and HiRISE data were integrated in the geographic information system ArcGIS software using an
equidistant cylindrical projection of the Mars IAU 2000 spheroid [Seidelmann et al., 2002]. A “morphologic”
map of the entirety of Ritchey crater was created based on geomorphic and stratigraphic features observed
in CTX images and guided by HiRISE where available. Distinct units were determined on the basis of texture
and tone in gray scale images, accounting for effects due to varying illumination angles. Mapping was carried
out by creating GIS shapefiles for each unit as identified in CTX images, and boundaries between units
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Figure 2. (a) White outlines represent crater counting areas on Ritchey’s ejecta and the ridged plains to the west. Counts
were conducted using CTX, and the counting areas reflect where data was available. We restrict the ejecta area to within
0.5 crater radii of Ritchey’s rim as most ejected material lies within several crater radii [Melosh, 1989]. (b) Closeup of Ritchey’s
defined crater count area, with white circles indicating craters >1 km diameter that were counted. (c) Cumulative size/frequency
distribution of craters on Ritchey’s ejecta (filled circles) and the Hesperian ridged plains (triangles) that it superposes to the west.
2
A total of 3746 craters as small as 10 m [Hartmann et al., 2010] are counted for Ritchey over an area of 10,761 km , and 128
2
craters 1 km and larger [McEwen et al., 2005] are counted over 47,003 km of ridged plains as the oldest limit to Ritchey’s age.
Although secondaries and preimpact craters are discarded from the count to the best of our ability, any uncertainties are still
included to estimate the oldest ages possible. Note that isochron shifts at smaller crater diameters are likely due to eolian
resurfacing events (e.g., at 2.26 Ga).
identified at CTX scale (5–6 m/pixel) were examined in HiRISE to establish relative stratigraphic relationships.
A map of fluvial channels and possible fan deposits was also produced using CTX images and DEMs to
determine potential source-to-sink transport pathways for any fluvially derived materials.
A mineralogic map was produced by first georeferencing CRISM false-color images (red/green/blue = IR
channels 233/78/13) to CTX images. Color composite spectral parameter maps (RGB = OLINDEX, LCPINDEX,
D2300) were then aligned to the georeferenced CRISM false-color images to ensure spatial alignment
between the morphologic and mineralogic maps. Mineral detections/assemblages were mapped as GIS
shapefiles using the CRISM parameter composites as a guide, and all positive parameter detections were
confirmed by examining the actual spectra and/or spectral ratios for each region. False positives or spectrally
unremarkable regions were excluded from the final map products.
3. Morphologic Mapping of Ritchey Crater
3.1. Geologic Setting
Ritchey crater (28.5°S, 51°W) is a 78 km diameter impact crater situated at the boundary between Noachian
age terrain to the east and Hesperian [Skinner et al., 2006] or Late Noachian [Tanaka et al., 2013] ridged plains
to the west (Figure 1). Crater count dating on Ritchey’s ejecta places the impact event at ~3.46 Ga during the
Early Hesperian, an age that is consistent in a relative sense to the older (~3.74 Ga) ridged plains that it
superposes (Figure 2). In this work we adopt absolute ages for time-stratigraphic boundaries on Mars as
presented by Hartmann and Neukum [2001], with the Noachian-Hesperian boundary occurring at 3.5–3.7 Ga
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Figure 3. Geomorphologic map of Ritchey crater overlaid at 70% transparency on a CTX mosaic. Map units are in approximate or inferred stratigraphic order. Not all map units are observed at each location in Ritchey, either due to erosion or
nondeposition, making absolute determination of stratigraphic order for all units difficult. In addition, gradational contacts
and local superposition suggest some units may be stratigraphically and/or laterally equivalent.
and the Hesperian-Amazonian boundary occurring at 2.9–3.3 Ga. In comparison to other craters on the
Noachian highlands, Ritchey is a relatively fresh complex crater with terraced walls, a well-preserved rim,
and visible ejecta. The impact superposes an older crater of similar diameter to the southeast such that the
southeastern edge of Ritchey’s rim lies where the older crater’s central uplift once existed. From these
mapping and stratigraphic relationships, it is likely that Ritchey sampled both Noachian and Hesperian
terrains, and the central uplift may contain crustal materials exhumed from a depth as great as 9.1 km
[Caudill et al., 2012]. The remainder of this section describes the various morphologic map units in detail.
3.2. Crater Floor
Deposits that characterize the crater floor (CF) of Ritchey have been divided into four map units. These
include undivided (CFu), fractured (CFf ), light-toned and smooth (CFls), and dark-toned and fractured
(CFdf ) (Figure 3). Much of the crater floor material in Ritchey is mapped as undivided (CFu). This is because
although the floor deposits vary in texture throughout the crater, much of it appears as a continuous
cohesive unit and lacks definitive, traceable contacts at CTX scale. The southeastern portion of CFu
exhibits small topographic terraces that follow a northeast-southwest orientation and are roughly parallel
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Figure 4. (a) CTX mosaic showing the mapped extent of CFf in white and locations of spectra in Figure 4b and CFf unit
examples in Figure 4c. (b) CRISM spectra from FRT observations: (1) 654 F, (2) 7C34, (3 and 4) 52EE, and (5) 9620. (c) Examples
of CFf texture from HiRISE observations: (A) PSP_005372_1515, (B) PSP_009090_1515, (C) PSP_003526_1510, (D) ESP_011635_1510,
(E) ESP_017371_1505, and (F) PSP_006862_1510.
to the exposed face of the nearby and overlying light-toned massive (Ilm) unit (see description below).
These small topographic steps may be indicative of thin bedding and erosional retreat along bedding
planes. In contrast, the southwestern region of the crater floor exhibits a hummocky morphology. The
morphology of the northern portion of map unit CFu is less hummocky and grades into a small-scale
(visible in HiRISE images) yardang-like texture of parallel to subparallel ridges oriented NE-SW that suggest
eolian erosion.
The CFf floor unit is characterized by a light to medium tone in gray scale CTX images and exhibits fractures,
some of which are polygonal, at HiRISE resolution. The fractures are variable in appearance and in some
locations exhibit a “flaky” texture, possibly due to erosion (see Figure 4c for examples). Because the fracturing
is only clearly visible at HiRISE resolution, the mapped extent of this unit is likely biased by the limitation of
HiRISE coverage, which is greatest in the central and northeastern regions of the crater (Figure 4a). In other
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Figure 5. HiRISE images showing examples of the stratigraphic relations of morphologic units. Scale bars indicate 200 m.
(a) ESP_025797_1515, (b) PSP_003526_1510, and (c–e) PSP_006005_1515.
parts of the crater floor where HiRISE coverage is lacking, the CFf unit was mapped by its light-toned
mottled texture as observed in CTX images. The actual extent of map unit CFf is potentially much greater
than that shown in Figure 4, particularly in the northern, southeastern, and possibly western portions of the
crater floor. In several locations the CFf fracture pattern superposes the CFu textures (Figure 5c).
Map unit CFls is light toned, smooth textured, and occurs in localized/isolated regions of the crater floor. It
superposes the underlying CFu textures, and in one northern floor location it grades into the fracture
pattern that characterizes map unit CFf, suggesting this unit may represent and/or be stratigraphically
equivalent to unfractured CFf (Figure 5c).
The final crater floor map unit, CFdf, is smooth and dark toned in CTX images, exhibits linear fractures, and
occurs only on the eastern crater floor between interior deposits (see section 3.4) and the crater wall. Its
northern contact with map unit CFf suggests it superposes both CFf and the interior deposits (Figure 5b).
3.3. Crater Center
The crater center (CC), which includes the central uplift of Ritchey, was subdivided into three map units.
These include light-toned massive deposits (CClm), dark-toned rough deposits (CCdr), and smooth,
fractured deposits (CCsf ). The CClm unit is light toned in CTX images, appears massive (lacks clear bedding
at CTX and HiRISE scale), and is heavily fractured. It is stratigraphically beneath the other central uplift units
as well as portions of map unit CFu, consistent with it representing bedrock uplifted from depth (Figure 5d).
CClm may actually consist of two different units that appear teal or white in standard HiRISE near-infrared,
red, and blue-green false-color images (Figure 6b) and pink or white in CRISM false-color images (e.g.,
Figure 6c). However, these potential subunits are indistinguishable in gray scale CTX images, and due to
lack of CRISM and HiRISE color coverage throughout the entire central uplift, they were mapped together
as a single unit (CClm).
The CCdr unit is darker and rougher textured than the surrounding CC units in CTX images, and it typically
appears dark green in CRISM false-color images (Figure 6c). This unit is most continuous in the northeastern
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Figure 6. (a) CTX mosaic showing context at the central uplift for Figures 6 and 7. (b) HiRISE PSP_006005_1515 with morphologic units annotated and their associated mineralogies. Note the presence of both white and teal CClm, which are
indistinguishable in gray scale imagery. (c) False-color CRISM FRT 7C34 (RGB: 233/78/13) showing pink and white CClm
exposed by a 2 km impact crater and variations in CClm hydrated mineralogy. Scale bars indicate 1 km.
region of the central uplift and has been partially eroded in other areas, resulting in a somewhat lighter tone
that may result from the underlying light-toned CClm unit. The CCdr unit superposes the uplifted CClm
bedrock (Figure 7a), and in some locations it occurs as a breccia with clasts that range up to several meters in
diameter (Figures 7c–7e). A HiRISE DEM also shows flow-like features in portions of CCdr that occur along
slopes, suggesting the material was partially fluidized during emplacement (Figure 7b). Notably, the CCdr unit
appears to drape preexisting topography and does not exhibit any substantial variation in thickness even
when present on steeply sloping portions of CClm.
The CCsf unit is located primarily in the north central region of the crater center and is characterized by a
medium tone and smooth texture in CTX images that is highly fractured in HiRISE images. This unit is higher
in relief than CFu, and its fractured texture sometimes appears to superpose the CFu texture (Figure 6b). The
boundary between CFu and CCsf appears gradational, and it is unclear if CCsf postdates (is stratigraphically
higher than) CFu or if CCsf is stratigraphically equivalent to CFu and simply more resistant to erosion, resulting
in the higher relief. CCsf also appears to be stratigraphically equivalent to and sometimes grades into CCdr
(Figure 7a) and exhibits localized brecciation (Figures 7c and 7d), but it is distinctly smoother and more
heavily fractured than CCdr.
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Figure 7. See Figure 6a for the locations of these features. (a and b) From a HiRISE DEM of PSP_005372_1515 and
PSP_006005_1515 with a vertical exaggeration of 3. (c–e) Examples of breccia clasts embedded in CCdr and CCsf units,
from HiRISE PSP_006005_1515. Scale bars indicate 50 m.
3.4. Interior Deposits
The interior deposits consist of units that clearly postdate the crater formation event (superpose the original
crater floor) and lie along the outer margin of the crater floor, locally superposing the crater wall. The two
mapped interior deposit units (I) consist of a light-toned massive unit (Ilm) overlain by a dark, rough-textured
capping unit (Idr) (Figure 5a). The dark capping unit appears to be more resistant to erosion than the
underlying light-toned unit and likely protects the Ilm unit from erosion, as evidenced by undercutting
(Figures 8b and 8c). In HiRISE images, map unit Idr exhibits a rough texture with small fractures and numerous
boulders, whereas map unit Ilm is smoother in appearance, forms steep scarps, and lacks clear stratification.
Although the best and most laterally continuous examples of these interior deposits are concentrated in the
eastern portion of the crater, several smaller occurrences are located along the northern crater floor and
potentially near the central uplift (Figures 8d–8f). These isolated examples are also consistent with unit Idr
superposing the lighter-toned Ilm unit, though the latter appear much thinner than the prototypical Ilm units
in the east. This spatial distribution (Figure 8a) suggests that the Idr and perhaps Ilm units were previously
much more laterally extensive, possibly spanning much of the crater, but they have experienced significant
erosion since their emplacement and are now quite limited in spatial extent.
3.5. Surface Deposits
Surface deposits in Ritchey are identified as materials that do not clearly represent bedrock or cohesive
geologic units. These materials consist of dark-toned cover (Sc) and dark-toned dunes (Sd), both of which are
likely eolian in origin. The cover (Sc) occurs in local topographic lows, and in some locations it may be weakly
cemented/indurated. However, this unit can exhibit linear dunes and may consist primarily of unconsolidated
material; active movement/saltation of this material has not been observed but cannot be ruled out.
Occurrences of the Sc unit are concentrated in the northwestern portion of the crater and form a distinct
northwest-southeast trending pattern across the crater floor. The dark dunes, map unit Sd, are eolian bed
forms that are found only in the southwestern portion of the crater. It is not clear if these dunes are cemented
and immobile or if they are instead actively migrating across the crater floor.
3.6. Fan Deposits
Smooth-textured deposits are present along portions of the crater wall and floor and are mapped as two
distinct units. The first unit, Ss, consists of smooth-textured deposits that superpose the crater wall or floor
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Figure 8. (a) CTX mosaic showing distribution of Ilm and Idr units in white. (b) HiRISE PSP_003526_1510 shows an example
of the prototypical Idr cap overlying Ilm. (c) A HiRISE anaglyph of PSP_003526_1510 and PSP_003249_1510. The undercut
and cliff-forming nature of the Ilm unit is further observable in Figure 8c. (d and e) HiRISE PSP_006005_1515 and (f )
HiRISE PSP_029542_1510 show likely Idr and Ilm units near the central uplift.
but lack a clear association with fluvial features or a sediment source region. The second unit, Sf, is similar in
texture and tone to Ss but consists of deposits with a fan-shaped morphology that occur at the terminus of
fluvial features that incise the crater wall. The fan deposits occur along most portions of the crater wall, but
there is a notable paucity along the southwestern edge of the crater, in accordance with limited evidence
of fluvial incision in this region of the crater wall. Some fan surfaces also exhibit inverted channels that are
several kilometers long and tens of meters wide (Figure 9b), indicating erosion (e.g., possible eolian
deflation) of at least several meters of material. Mars Orbiter Laser Altimeter (MOLA) topographic profiles
over these fan surfaces as measured from the apex to the visible toes of the deposits yield gradients
ranging from 0.03 to 0.09 and an average slope of 0.063 or 3.60° (Figure 9c). In most cases these fan
deposits are associated with fluvial channels that incise the crater wall, and some of these channels can
even be traced to sources beyond the rim of Ritchey crater (Figure 10).
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Figure 9. (a) CTX mosaic showing location of inverted channels in Figure 9b and sampled fan deposits in Figure 9c. (b) HiRISE
ESP_027142_1510 on CTX mosaic. (c) MOLA topographic profiles of fan deposits around the crater, color coded to Figure 9a.
3.7. Crater Wall
The crater wall appears to consist of exposed bedrock and is mapped as a single undivided unit (CWu) as it
lacks visible stratification, possibly due to slumping along the walls. As mentioned above, fluvial channels
incise the crater walls with the exception of the southwestern region. Portions of the crater wall that
are unobscured by dark cover generally appear to be light or moderate toned in gray scale CTX and
HiRISE images.
4. Mineralogy of Ritchey
The mineralogic map (Figure 11) was produced independently of the morphologic map and guided by
color composite spectral parameter maps (RGB = OLINDEX, LCPINDEX, D2300/BD2210). The parameter
maps and associated spectra were inspected in ENVI manually to assess their accuracy. Individual regions
of interest (ROIs) (each typically a few hundred or fewer pixels) were defined by the parameter maps and/
or distinctly colored regions in CRISM false-color images (RGB = IR channels 233/78/13). The ratio
spectrum for each ROI was visually inspected to identify diagnostic absorption features, and ROI spectra
with similar absorption features and similar absorption strengths were then grouped together. In this
manner, we were able to produce higher-quality spectra (decreased noise) by averaging multiple ROIs
(often totaling up to thousands and sometimes tens of thousands of pixels) without including false
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Figure 10. Fluvial channel and fan deposit map of Ritchey crater on a CTX mosaic. Dotted lines represent inferred fan
deposits or possible fluvial features that do not exhibit distinct boundaries.
positives or unrepresentative mineralogies. By picking small individual ROIs, we could also determine
detailed mineralogic boundaries and detect subtle variations in mineralogy (e.g., Fe versus Mg smectites,
which are indistinguishable in the simple D2300 parameter maps). This approach was especially effective
for regions of the crater with particularly diverse and complex mineral assemblages, including the
northern crater floor and central peak (Figure 12). After spectral analysis in ENVI, confirmed positive
detections of the various minerals and mineral assemblages were then mapped in ArcGIS. The remainder
of this section discusses the detected minerals and their spatial distribution in detail. Approximate ROIs
representative of the spectra presented in Figures 14–17 are shown in Figure 13.
4.1. Spectrally Dominant Minerals and Mineral Assemblages
4.1.1. Olivine
Olivine detections in Ritchey are rarely spectrally pure, and spectral ratios for these areas often exhibit clay or
hydrated silica signatures (Figure 14). Olivine is present in some portions of the central uplift (unit CCsf) as
well as the crater walls and east crater rim. The short-wavelength band center of the 1 μm absorption and the
presence of a distinct absorption near 0.85 μm suggest that these olivine occurrences are consistent with
forsteritic (Mg-rich) olivine [Burns, 1970; Sunshine and Pieters, 1998] or fine-grained fayalite [Mustard et al.,
2008]. Olivine with Mg smectite signatures is associated with the central uplift (CCdr), crater wall bedrock, and
the northern crater floor. Olivine is also mixed with hydrated silica on the northern crater floor near the
central uplift, a small portion of central uplift (CCdr), and at the terminal regions of a fan deposit on the
western crater floor.
4.1.2. Pyroxene
Pyroxene is the most spatially prevalent mineral detected in the crater and is often spectrally “pure” in
spectral ratios (Figure 15), particularly on the crater floor, crater wall, and in the CClm unit of the central uplift.
The majority of detected pyroxenes is of the low-calcium variety [Adams, 1974], with iron crystal field splitting
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Figure 11. Mineralogic map of mafic and hydrated minerals and their mixtures in Ritchey crater, overlaid at 70% transparency
on a CTX mosaic. Refer to Table 1 and Figure 13 for the CRISM observations used in the construction of this map.
absorptions centered at 0.925 and 1.91 μm in the central uplift (CClm) and 0.95 and 1.94–2.0 μm in the crater
walls and floors (CWu, CFu, CFf, and CFdf).
Pyroxene-bearing materials whose spectra also exhibit relatively weak Fe/Mg smectite absorptions are
located in portions of CFu and CFf on the crater floor, as well as parts of the crater wall. Pyroxene with
Figure 12. Closeup of the central peak region. (left) HiRISE PSP_006005_1515 on a CTX mosaic. (right) The same image with
the mineralogic map (from Figure 11) overlaid at 70% transparency.
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Figure 13. Example CRISM FRT observations with colored areas indicating locations with spectra representative of those
presented in Figures 14–17. White regions indicate the denominator background used for each CRISM image. Olivine is
indicated by red regions, LCP by green, hydrated silica by cyan (with LCP) or blue (with olivine), and clays with magenta.
hydrated silica signatures is primarily observed in parts of the central uplift (units CClm, CCsf, and CFu) and in
several fan deposits on the western crater floor. Pyroxene absorptions are also found to be associated with
eolian deposits and sand-covered regions in Ritchey, as is the case for other eolian deposits on Mars [e.g.,
Mustard et al., 2005].
4.1.3. Hydrated Silica
Hydrated silica is found at the central uplift in portions of uplifted CClm, overlying CCsf and CCdr units,
and in the nearby portions of the undivided crater floor (CFu) and several fan deposits on the western
crater floor. It is always spectrally mixed with olivine, LCP, Fe/Mg smectites, or a combination of these
phases (Figure 16). Under low relative humidity and/or at low water contents, opaline silica can be
distinguished from hydrated glass (impact or volcanic) by a shift in the minimum of the OH stretching
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vibration overtone from ~1.41 μm
to ~1.38 μm [Milliken et al., 2008].
Both opaline silica and hydrated
glass can exhibit H2O absorptions at
~1.9 μm and Si-OH absorptions
centered near ~2.2 μm, though the
latter tend to be more symmetrical
in volcanic glass when compared to
opaline silica. For the detections in
Ritchey, the 1.4 μm absorption is
typically weak or absent, but the
band center commonly occurs at
longer wavelengths (~1.4 μm),
suggesting that the materials may
be volcanic or impact glass with Si
values that range between basaltic
and rhyolitic compositions [Milliken
et al., 2008]. Alternatively, if these
materials represent highly siliceous
compositions, such as opaline silica
Figure 14. CRISM spectra of olivine and laboratory spectra, identified by
their spectral file ID, from the CRISM spectral library. The spectrum from
or very Si-rich glass, then the
17D66 is on the southern wall, AB54 on the eastern rim, and 7C34 at the
observed variations in the relative
central uplift. Approximate locations of the CRISM spectra are indicated by
strengths of absorptions at 1.9 μm
red regions in Figure 13. Note slight absorptions at 1.9 and 2.3 μm due to the
and
2.2 μm can indicate the level of
presence of Fe/Mg smectites.
crystallinity. A deeper 2.2 μm
absorption relative to the 1.9 μm absorption is consistent with an opal-A structure, whereas the inverse
suggests opal-CT [Rice et al., 2013]. Within portions of unit CClm at the central uplift that exhibit hydrated
silica, the 2.2 μm/1.9 μm band depth
ratios appear to have a spatial
dependence. Low 2.2 μm/1.9 μm
ratios occur in CClm exposed by a
2 km impact crater, and higher
ratios are present elsewhere in the
central uplift. The shape of the 1.9
and 2.2 μm absorption features in
the western fan deposit are
noticeably sharper, which may
indicate a mixture of an Al smectite
(e.g., montmorillonite) with opaline
silica [Bishop et al., 2008; Mangold
et al., 2012].
4.1.4. Fe/Mg Phyllosilicates
Fe/Mg phyllosilicates (e.g., smectite)
have been identified in numerous
locations on Mars by the presence of
absorptions at 1.4 μm (OH/H2O),
1.9 μm (H2O), and 2.28–2.31 μm
(Mg/Fe-OH) [Poulet et al., 2005;
Figure 15. CRISM spectra of pyroxene, with laboratory spectra from the
Mustard et al., 2008]. Similar features
CRISM spectral library (LCP) and NASA RELAB (HCP). All laboratory spectra
are observed in spectral ratios from
are identified by their spectral file ID. The LCP found in the uplifted CClm
Ritchey (Figure 17) and are
units at the central uplift (7C34) have 1 μm and 2 μm absorptions at shorter
associated with the central uplift
wavelengths than the LCP found at the crater floor (52EE; eastern floor) and
(CCdr, CCsf, and CClm), crater wall,
walls (1253 F; eastern wall). Approximate locations of the CRISM spectra are
crater floor (CFu and CFf ), and
indicated by green regions in Figure 13.
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interior deposits (Ilm). These spectra
also exhibit variations in the shape
(symmetry) and center of the
~2.3 μm absorption, for which the
latter ranges from 2.29 to 2.32 μm.
Most clay mineral detections in
Ritchey are consistent with Fe/Mg
smectite or mixed-layer chlorite/
smectite [Milliken et al., 2010], but
we have also identified one
localized occurrence of chlorite
along the northern crater wall
(indicated by the magenta dotted
circle in Figure 13a).
4.2. Dominant Minerals and
Mineral Assemblages by Map Unit
Figure 16. CRISM spectra of hydrated silica and a laboratory spectrum of an opal
coating on a basalt, taken under Mars pressure-temperature conditions courtesy
of Gregg Swayze. The spectra from 7C34 are at the central uplift, and the spectrum from AC1F is found in a fan deposit on the western floor. Approximate
locations of the CRISM spectra are indicated by blue or cyan regions in Figure 13.
Figure 17. CRISM spectra of Fe/Mg phyllosilicates and laboratory spectra from
NASA RELAB (nontronite) and the CRISM spectral library (chlorite and saponite).
All laboratory spectra are identified by their spectral file ID. Approximate locations of the CRISM spectra are indicated by magenta regions in Figure 13.
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This section discusses the results of
the integration of the independently
produced morphologic and
mineralogic maps. Spectrally
dominant minerals and mineral
assemblages are variable both within
and between various morphologic
map units, and these distributions
are discussed in detail below.
4.2.1. Crater Center
Fe/Mg smectite absorptions are
strongest in the dark-toned CCdr unit
and are spectrally mixed with olivine
(Figure 18a). The majority of spectra
from the light-toned, uplifted CClm
unit is dominated by low-Ca
pyroxene and shows no clay mineral
absorptions, except for some
portions of CClm near a 2 km wide
impact crater. However, these areas
exhibit a very weak 2.3 μm
absorption (Figure 18b) and are
adjacent to the smectite-bearing
CCdr and CFu (described below)
units. This suggests the weak clay
mineral features in CClm are likely
due to mixing with detritus derived
from the adjacent CCdr unit
(Figure 18c). Spectral ratios from
some CCsf regions also exhibit weak
absorptions at 2.3 μm. All smectites
found in the central uplift region
have a rather symmetrical absorption
centered at ~2.31 μm, consistent
with Mg-rich clay minerals.
Hydrated silica is found in portions of
all three of the central peak units
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Figure 18. The coverage here is similar to those in Figures 6 and 7. (a and b) Spectra for the corresponding units in
Figure 18c. The vertical scale of Figures 18a and 18b are equivalent. (c) Mineralogic map overlaid on a HiRISE DEM of
PSP_005372_1515 and PSP_006005_1515 with a vertical exaggeration of 3. Morphologic units and their associated
mineralogies are annotated. The colors correspond to mineralogy and their boundaries cut across morphologic map unit
boundaries. Note that Sm-bearing CClm occurs in a topographic low, while pure LCP-bearing CClm occurs on a slope.
(Figure 18). In the uplifted CClm unit, hydrated silica absorptions occur with LCP and vary in the relative band
strengths of their 1.9 and 2.2 μm absorptions (previously discussed in section 4.1.3). Hydrated silica in the
overlying CCsf and CCdr units are almost always spectrally mixed with olivine.
4.2.2. Crater Wall
Some of the strongest Fe/Mg clay mineral signatures in Ritchey are found in exposed bedrock along the
crater wall where they are often spectrally mixed with olivine (i.e., strong 1 μm absorptions). Slightly weaker
clay signatures, commonly mixed with LCP, occur in dustier areas or portions of the crater wall that appear to
be covered by sediments (Figures 19b and 19c). This trend is noticeable on the north and east walls, which
tend to have better exposures of bedrock (Figure 3) and exhibit stronger olivine and clay mineral absorptions
(Figure 11). Clay mineral detections in the north and west wall are consistently Mg rich with metal-OH
vibration band centers at 2.30–2.31 μm. Spectral ratios from the upper east crater wall and rim display similar
features, although some spectra in the lower portions of the east wall exhibit band centers closer to 2.29 μm,
suggesting a more Fe-rich clay may be present. Similarly, clay minerals in the south wall are predominantly Fe
rich with absorptions centered at 2.29 μm (see the 17D66 spectrum in Figure 17), although more Mg-rich clay
minerals are also present.
4.2.3. Crater Floor and Fan Deposits
The crater floor units (CFu and CFf ) and fan deposits exhibit relatively weak clay mineral signatures
compared to those on the crater wall. When present, the clays are almost always mixed with LCP, except
for the north crater floor where it is also mixed with olivine (Figure 11). Clay minerals in these units appear
to be Mg rich with an absorption centered near 2.3–2.31 μm, except for those in a fan deposit on the east
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Figure 19. (a) CTX mosaic showing context for Figures 19b–19e on the eastern crater wall. (b) CRISM FRT 1253 F with
colored markers indicating regions with spectra representative of those shown in Figure 19c. (c) CRISM spectra showing
increasing band strength of Fe/Mg smectite absorptions with decreasing LCP signatures. (d–f) HiRISE ESP_012914_1515
showing bedrock exposures and their associated mafic and hydrated mineralogies. Note that the smectites occurring with
LCP appear in closer proximity to dark sand deposits, while those occurring with olivine are at a relatively dust-free topographic high. The smectites also appear to be embedded within the bedrock.
wall that exhibit a band center closer to 2.29 μm. More generally, fan deposits and crater floor units are
spectrally dominated by LCP and/or material with reflectance properties similar to typical Martian dust.
However, CRISM coverage of the many fan-like deposits in Ritchey and portions of the crater floor is quite
limited (Figure 11); thus, the mineralogical variability of these units may be greater than what is observed
in the current CRISM coverage.
Hydrated silica is found in a fan deposit by the west wall and on the northern crater floor close to the central
uplift. The fan deposit appears indurated and has hydrated silica signatures across its entire visible surface
(indicated by the cyan ROI in Figure 13e). On the northern crater floor, hydrated silica is spectrally mixed with
a variety of other minerals (olivine, LCP, and clay minerals). The hydrated silica is not associated with a
particular deposit on the crater floor, but it occurs on the undivided crater floor map unit (CFu) only in
proximity to the central uplift.
4.2.4. Interior Deposits
The light-toned Ilm unit exhibits Fe/Mg smectite signatures mixed with LCP, with absorption band centers
ranging from 2.29 to 2.31 μm. The 2.29 μm occurrences (possible Fe smectites) are located in Ilm units near a
fan deposit containing similar mineralogy. The 2.3 μm occurrences (possible Mg smectites) are found in a few
buttes, and 2.31 μm examples (possible Mg smectites) are found in the more laterally extensive Ilm units.
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In contrast, the overlying dark capping unit (Idr) lacks spectral signatures of hydrated minerals and is spectrally
similar to dusty regions in Ritchey and elsewhere on Mars.
5. Discussion
5.1. Evidence for Impact Melt and Associated Clays in the Central Uplift
Fe/Mg clay mineral signatures consistent with smectites or mixed-layer smectite/chlorite at the central uplift
of Ritchey are likely native to only the overlying, draping CCdr unit, despite slight 2.3 μm features in some
spectra of CCsf and uplifted CClm materials. These slight 2.3 μm absorptions are mostly in the uplifted CClm
that has been exposed by a 2 km diameter impact crater, which is too small to independently develop a
hydrothermal system that could promote clay formation [Osinski et al., 2013]. The vast majority of uplifted
CClm materials exhibit strong low-Ca pyroxene features and do not exhibit clay mineral signatures, and
spectra of the few exposures that do exhibit the latter only show a slight kink at 2.3 μm (Figure 18b). The
shallowness of this absorption is due either to dilution by the strong overlapping 2 μm pyroxene signature
and/or physical contamination of clay minerals in these locations from the adjacent clay-bearing CCdr unit
(Figure 18a). The latter is a likely cause because the CClm locations with weak 2.3 μm features are local
topographic lows and may act as a trap for clay-bearing sediment eroded from the nearby CCdr unit. In
comparison, other nearby CClm units that are LCP rich and lack clay mineral features are higher in relief and
appear sloped (Figure 18c).
Based on these observations, we conclude that the light-toned CClm unit, which represents bedrock
uplifted from depth, is dominated by low-Ca pyroxene and lacks evidence for the presence of clay minerals.
Instead, Fe/Mg clay minerals in the central uplift region are restricted to the CCdr unit that drapes and
unconformably overlies the uplifted CClm materials (Figures 6 and 18). This stratigraphic relationship
implies that the Fe/Mg clay minerals at the central uplift were not excavated from depth and were instead
emplaced and/or formed during or after the Ritchey impact event. This constrains their maximum
depositional age to be equivalent to the crater age of ~3.46 Ga, or Early Hesperian. It is unclear whether clay
minerals in the CCdr unit were formed in situ or transported by fluvial or eolian processes. However, the
CCdr unit is brecciated and exhibits some evidence for flow-like features, and it clearly drapes the
underlying topography (Figure 7). These characteristics, as well as its location in the crater and stratigraphic
position, indicate that the clay-bearing CCdr unit may be impact melt.
Impact melt products can be altered through hydrothermal or low-temperature alteration, and clay minerals
have been observed in terrestrial impacts as a result of these processes [e.g., Osinski et al., 2013]. Classic
hydrothermal minerals such as chlorite and prehnite [Ehlmann et al., 2009] and vent-like mounds [Marzo et al.,
2010; Mangold et al., 2012] are seemingly absent from Ritchey’s central uplift, making low-temperature
alteration a more likely scenario. Furthermore, both clay minerals and olivine in unit CCdr are consistent with
Mg-rich varieties, consistent with in situ alteration of olivine to smectite, as has been suggested for claybearing sediments observed in Gale crater by the Curiosity rover [Grotzinger et al., 2014; Vaniman et al., 2014;
McLennan et al., 2014]. Intriguingly, there is no clear evidence for aqueous alteration of any kind in the
uplifted CClm bedrock even though it is highly fractured and would be susceptible to fluid migration in the
presence of an impact-induced hydrothermal system. However, if such alteration in the uplifted bedrock was
limited in extent or confined primarily to small-scale fractures, then it is unlikely that it would actually be
detected at the moderate spatial resolution of CRISM.
Spectra for certain locations of the CCsf unit also exhibit weak clay mineral absorptions that are likely not
native to the unit. The majority of CCsf within the central uplift region is associated with spectrally pure
olivine, and the only portions whose spectra exhibit slight 2.3 μm absorptions occur in proximity to CCdr in
a relative topographic low (Figure 18c), consistent with physical contamination by clay minerals. As discussed
above, the CCsf unit appears to be stratigraphically equivalent to the CCdr unit in that it directly superposes
CClm and locally grades into the CCdr unit. Because it also appears to drape the underlying topography
and exhibits local breccia zones, the CCsf unit is also consistent with an impact melt origin. Though not
definitive, it is possible that the CCsf and CCdr units are both an impact melt layer, with the CCdr representing
a rougher, more weathered lithology that is revealed by the erosion and removal of the smoother, upper CCsf
lithology. In this scenario, the olivine-rich CCsf could be the predecessor of CCdr, in which the former is
chemically altered and weathered to form the latter.
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If the CCsf unit is in fact impact melt, its olivine signature may represent secondary olivine formed by the
impact [Schwenzer and Kring, 2013] or possibly a glassy impact melt product. The clay minerals in CCdr may
be low-temperature aqueous alteration products of such materials, possibly produced by episodic periods of
surface runoff, local precipitation, or near-surface groundwater after the primary materials were emplaced. If
this mechanism of alteration is correct, then there is effectively no minimum age constraint on clay formation
in these units, though the clays can be no older than the (Hesperian) age of Ritchey itself. Alternatively, if the
impact target lithologies already contained volatile-rich materials such as hydrated clays or sulfates in the
near-surface, then the resulting impact melt may also have hosted volatiles. This could lead to direct
formation of clay minerals during emplacement and cooling of the impact melt. Though speculative, it is
possible that the thin, upper CCsf material could represent the uppermost portion of an impact melt layer
that was quenched. This could retard degassing and lead to a preferential increase in volatile content and
thus clay formation in the interior portions of the melt layer, represented by the CCdr unit.
Additional support for the impact melt products in Ritchey includes the detection of hydrated silica at the
central uplift. The spectral signatures are consistent with either impact glass or highly hydrated opaline silica
[Milliken et al., 2008] and are found in the uplifted CClm, overlying CCsf, and nearby CFu units. If the signatures
in CClm are opaline silica, the detection of possible opal-CT near the 2 km impact crater and opal-A elsewhere
may (see description above) suggest additional alteration (opal maturation) may have been induced by the
smaller impact. Together, the observations described here indicate that clay minerals in the central uplift of
Ritchey are associated with likely impact melt products and formed as a direct result of the impact event or
from postimpact alteration. There is no clear evidence for clay minerals being uplifted from depth in this
location. Uplifted bedrock, possibly from as deep as 9 km [Caudill et al., 2012; Quantin et al., 2012], is
morphologically and mineralogically distinct and consistent with unaltered or very minimally altered low-Ca
pyroxene-bearing lithologies. Though heavily fractured, the uplifted bedrock does not exhibit evidence of
extensive aqueous alteration at the spatial resolution of CRISM.
5.2. Excavation or Alteration of Materials in the Crater Wall
Clay minerals detected in the wall of Ritchey crater may represent preexisting clays exposed by the impact
process (preimpact origin) or in situ alteration products (postimpact origin). The former would be consistent
with a Noachian or Hesperian age depending on the position of the clay minerals in the preimpact
stratigraphy, whereas the latter scenario would imply a maximum age of Early Hesperian (the age of Ritchey
crater) and a minimum age of Amazonian. The clay minerals detected in the crater wall are associated with
what appears to be exposed bedrock, whereas areas covered by what appears to be unconsolidated
materials (dust, soil, and/or sand) either lack or have very weak clay mineral signatures (Figure 19). In the
latter case, the weak clay absorption features are superposed on LCP absorptions, suggesting physical mixing
with LCP-bearing dust [Mustard et al., 2008]. While it is possible that these clay minerals could have formed by
alteration of LCP, clay features in the crater as a whole are observed to be very weak when occurring with LCP
features. However, in the presence of increasing olivine signatures at 1 μm, the clay mineral absorptions also
increase in strength (Figure 19c). This suggests that clay minerals are primarily associated with olivinebearing materials in exposed bedrock. In contrast, weak clay features in covered regions of the crater wall are
likely the result of mechanical mixing and spectral dilution by LCP.
Excavation of preexisting clay minerals in the walls of Ritchey is also possible as it impacted a preexisting
crater of similar size whose mineralogy is unknown. In addition, a widespread subsurface layer of clay
minerals has been exposed by impact craters and valley networks in the broader region to the east of Ritchey
[Buczkowski et al., 2013]. In this scenario, a layer enriched in Fe/Mg clay minerals would exist close to the
surface, consistent with the gross distribution of clay minerals we observe in the north, east, and south
wall exposures. However, unlike the regional exposures of the clay layer identified by Buczkowski et al.
[2013], the clay minerals in the wall of Ritchey are not confined to a clear stratigraphic position or horizon;
the clay occurrences are patchy, discontinuous, and the crater wall bedrock lacks evidence of clear
stratification. The patchy distribution on the eastern and southern walls of Ritchey may be the result of
previous disruption by the impact that formed the similar-sized crater that Ritchey superposes. However,
the distribution of clay minerals is similarly patchy on the western and northern walls, where postimpact
slumping of the crater wall is evident. In either case, the clays are not confined to clear stratigraphic
horizons, making interpretations of their origin ambiguous. Potentially excavated clay minerals also occur
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in ejecta material on the eastern crater rim (Figure 11), although in situ alteration and clay formation are
also possible in ejecta deposits [Osinski et al., 2013].
In situ alteration of primary minerals to form clay minerals is also possible given the preponderance of
fluvial features that incise the crater wall. As discussed above, clay minerals in the crater wall are often
associated with olivine-bearing deposits (Figure 11), and olivine can readily alter to form smectites under
appropriate conditions. Recent findings by the Curiosity rover in Gale crater suggest that saponite/Fe
saponite detected in Martian mudstones may be the product of in situ alteration of olivine [Vaniman et al.,
2014; McLennan et al., 2014]. The apparent Mg-rich composition of both the olivine and clay minerals in
most of the Ritchey crater wall is also consistent with in situ alteration of olivine, but the limitations of
orbital data do not allow us to unambiguously discriminate between excavation and postimpact in situ
alteration models.
5.3. Clay Minerals on the Crater Floor
The dilution effect of clay minerals mixing with LCP-rich components described above is also evident in clay
deposits identified on the crater floor (commonly in unit CFf; Figure 4) and in fan deposits, for which clay
mineral signatures are relatively weak and almost always mixed with LCP. It is likely that at least some, if not
most, of the clay minerals in the crater floor and fans have been fluvially transported from the crater wall,
physically mixed and spectrally diluted with LCP, and redeposited on the floor as alluvial-fluvial deposits. This
source-to-sink hypothesis is supported by consistent absorption band centers between clay minerals in
crater floor deposits and clay minerals in the fan source regions, which implies similar clay compositions.
Spectral ratios for clay deposits in the north and west crater wall and floor systems exhibit features near
2.31 μm consistent with Mg-bearing clay minerals. In contrast, spectral ratios of clay minerals in the eastern
fan deposit have features closer to 2.29 μm (more Fe-rich clay compositions) that are consistent with the
2.29 μm absorptions observed for the lower portions of the eastern crater wall. In situ alteration to form these
smectites is also possible, as was suggested for the case of an alluvial fan in Majuro crater [Mangold et al.,
2012] and in hydrothermal environments in crater floor deposits [Rathbun and Squyres, 2002]. However, the
clear depositional pathways (fluvial features) and similarities between clays in source and sink regions
suggest many of the clays found in the crater floor and fan deposits are likely the products of fluvial transport
from topographically higher regions along the crater wall.
One exception to the above source-to-sink scenario is a portion of the north floor of the crater that exhibits
a large olivine-bearing swath that has no traceable source (Figure 11). This region might represent a
distinct deposition event sourced from the northern crater wall that is elsewhere covered by LCP-bearing
units (such as CFf ) from later sedimentation. To first order there appears to be a correlation between LCP
and the overlying CFf polygonal fracture texture, as well as between olivine and floor deposits that do not
exhibit the characteristic CFf texture. However, this scenario is complicated by the difference in spatial
resolution between CRISM and HiRISE and the lack of HiRISE coverage over most of the northern floor that
is covered in CRISM data (FRT 654 F). The swath of olivine may also have been deposited as “secondary”
olivine from the Ritchey impact event itself, after which CFf may have been deposited over it. This
possibility may be consistent with the mafic and hydrated silica spectral signatures in the floor deposit that
lies immediately south of the olivine swath near the central uplift (Figure 11). If this hydrated silica deposit
was produced from the impact process, then it is also possible that the olivine is impact related [Schwenzer
and Kring, 2013] and not detritus derived from the crater rim.
5.4. Extent and Timing of Fluvial Activity
In addition to the multiple postimpact in situ alteration scenarios possible in Ritchey, the abundance of fluvial
channels and fan deposits throughout the crater indicate that alluvial and fluvial processes were an
important part of the crater’s history. Postimpact fluvial activity (Figure 10) and associated sediment transport
may have extended as far as the central uplift region (Figure 4a). In this context, and because clay minerals are
observed in multiple portions of the crater wall, it must be considered that clay minerals detected in the
crater floor units, fan deposits, and interior layered units are detrital components whose source(s) can be
traced to the crater wall and rim.
Aside from portions of the undivided crater floor CFu, clay minerals are distinctly associated with the highly
fractured crater floor unit CFf, confirmed by multiple CRISM observations (Figure 4b). Clay mineral signatures
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Figure 20. (a) Mineralogic and fluvial and fan deposit map overlaid on CTX mosaic of a complex fan deposit on the northern
wall. White arrows point out possible sources for different deposits of the fan. Colored markers indicate approximate locations
of spectra shown in Figure 20c. (b) CTX DEM produced from P04_002682_1513 and B05_011635_1513, vertically exaggerated,
with the mineralogic map overlaid. (c) CRISM spectra of fan layers from FRT observation 654 F. As in Figure 19c, note the
strengthening of the 2.3 μm absorption with decreasing LCP signatures.
in this unit as well as eastern fan deposits and their clear superposition of the CFu unit are consistent with
deposition by fluvial processes. As previously noted, the extent of the polygonally fractured texture that
characterizes unit CFf is mapped based on HiRISE and conservative extrapolations to CTX images. The
original extent of this unit was likely larger as it appears to have eroded away in some regions (Figure 4c), and
it is likely that the actual spatial extent of the CFf texture is much greater than presently mapped. The
presence of CFf near the central uplift (most notably south and east of the uplift) and nearby fluvial channels
suggests that fluvial activity reached as far in as the central uplift region, but there is no evidence that fluvial
activity affected the inner elevated portions of the central uplift where the CCdr and CClm units are located.
The distribution and crosscutting relationships of fluvial features in Ritchey suggest that alluvial-fluvial
processes were widespread in the crater and may have occurred in multiple episodes. An alluvial fan deposit
sourced from the north crater wall has distinct olivine and clay mineral spectral signatures in its uppermost
layer, a mafic (olivine and LCP) and clay mineral intermediate layer, and a lowermost layer that is spectrally
dominated by LCP and which runs into the CFu crater floor unit (Figure 20). This fan is also adjacent to the LCP
and clay mineral-bearing CFf deposit, which could have been emplaced either before or during fan
deposition. Postimpact, bottom-up hydrothermal alteration has been suggested for clay minerals observed
in an alluvial fan in Majuro crater [Mangold et al., 2012]. In the case of Ritchey, we observe the strongest clay
mineral signatures in the upper layers of the fan and weaker or absent features in the lower layers, more
consistent with depositional processes in an alluvial-fluvial environment. Though the potential for in situ
alteration cannot be entirely ruled out, we note that the clay signatures appear to be confined to the
geomorphic boundaries of this fan in existing CRISM coverage and they are not present in other fans. Such a
localized occurrence would not necessarily be expected if the clays were the result of postdepositional
surface weathering. Changes in mineralogy with stratigraphic order in the fan suggest multiple episodes of
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deposition, changes in sediment source region (provenance mineralogy), variations in hydrodynamic sorting,
particle size, cementation, or a combination of these factors.
First, the lowermost LCP-rich layer was deposited (or this may simply be the CFu floor unit), followed by
deposition of the olivine-, LCP-, and clay-bearing layer, and finally the olivine- and clay-bearing layer. The
sediment source region for this fan is confined to the crater wall and is relatively well defined; thus, detrital
mineral variations due to changes in source region mineralogy is likely not a dominant effect. However, it is
possible that progressive chemical weathering in the source region over time led to increased clay formation,
leading to the younger, stratigraphically higher layers becoming more enriched in clay minerals as the source
region became more enriched in these components. It is also possible that the trend of decreasing LCP with
increasing stratigraphic order is due to dilution effects. LCP-rich components might be flushed down the
crater wall by early fluvial activity and become progressively depleted in the crater wall, reflected in the upper
LCP-poor layers of the alluvial fan. Alternatively, the stratigraphic variation in mineralogy may be the result of
grain-size variations within the fan deposit, with higher clay mineral concentrations in the upper layers
representing finer-grained sediments and lower, clay-poor layers representing coarser-grained sediments.
Additional evidence that may suggest multiple depositional episodes lies within the diversity of Fe/Mg clay
minerals in the Ilm unit of the eastern interior deposits. The clay minerals in the Ilm unit are spectrally mixed
with LCP and lack spectral signatures of olivine, implying either low abundances of olivine, significant in situ
alteration of olivine to clay (likely smectite), or transport of only LCP and clays during deposition of the Ilm
unit. The latter hypothesis still allows for olivine alteration in the source region, and LCP dilution as sediments
are transported from the crater wall and deposited in the Ilm unit, but the presence of LCP throughout the
stratigraphy of these deposits requires the continued presence of LCP in the sediment source region. It is
worth noting that the eastern crater rim, which is the source region for these deposits, overlaps with where
the central uplift of the older, similar-sized crater that Ritchey superposes would have been located. If the
central uplift of this older crater was rich in LCP, as is the case for the central uplifted rocks of Ritchey, then this
may explain why the sediments sourced from this region in Ritchey are also LCP rich. The absorption band
centers of clays in the Ilm unit range from 2.29 to 2.31 μm, whereas spectra of most clays in Ritchey are
centered closer to 2.31 μm. This apparent variation in clay mineral composition is also consistent with
variability in protolith compositions or, in the case of in situ alteration on the crater wall, changes in fluid
chemistry and degree of water-rock interaction.
The stratigraphy observed for the Ilm and Idr units, in which a light-toned unit is capped by a darker, rougher,
blockier unit, has been observed elsewhere on Mars, particularly in nearby Holden crater [Grant et al., 2008].
In Holden, the light-toned strata were interpreted as possible lacustrine or distal alluvial sediments, and the
Ilm unit in Ritchey is also associated with alluvial systems. The light-toned units at Holden are also similar to
those at Ritchey in that both units are clay bearing, although the former exhibit clear bedding [Grant et al.,
2008] and those in Ritchey appear massive. The darker capping unit (Idr unit at Ritchey) appears blocky and
spectrally bland at both craters and has been interpreted at Holden to represent deposition in a high-energy
flooding event [Grant et al., 2008]. The Ilm unit in Ritchey is proximal to alluvial fans and sediment source
regions such as the crater wall, and it may once have been much more laterally extensive (Figure 8). This may
indicate a similar distal alluvial/lacustrine origin as interpreted for Holden, but the current spatial extent of the
exposures does not allow this hypothesis to be fully tested using only orbital data (e.g., strata are not exposed
over long distances).
At 28.5°S, Ritchey falls into the 18 to 29°S [Moore and Howard, 2005] or 10 to 35°S [Kraal et al., 2008]
range of craters containing alluvial fans. Ritchey’s fan deposits show the typical concave slope of alluvial fans
and have steeper average gradients than those found by Moore and Howard [2005]. However, they do fall
within the range of slopes observed in terrestrial and Martian alluvial fans. The topographic profiles of these
fans (Figure 9c) and their morphology do not indicate deposition into a body of water (e.g., a crater lake) [Ori
et al., 2000; Mangold and Ansan, 2006], though the presence of such a lake is not precluded by any of the
observations presented here. The steeper surface slopes of fans measured in this study may result from
coarser sediments, denser flow, or a smaller contributing area. Given Ritchey’s relatively young age, it is also
possible that the fans are less degraded relative to many of the craters previously surveyed [Moore and
Howard, 2005]. With an average slope of 0.063, some of these fans could have formed over millions of years
[Armitage et al., 2011]. Whether the fans represent single sustained or multiple short-lived fluvial episodes,
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the earliest they could have formed is the Early Hesperian. This age is similar to maximum Hesperian age
alluvial fans found in southern Margaritifer Terra craters [Grant and Wilson, 2011], Majuro crater [Mangold
et al., 2012], and Saheki crater [Morgan et al., 2014], which potentially formed from snow precipitation and
melt in these latitudes and, in the case of Saheki crater, in multiple episodes over thousands of years.
6. Conclusions
Mineralogic and morphologic evidence for post-Noachian clay formation in Ritchey crater complements
previous studies that discussed Late Hesperian alteration in Toro [Marzo et al., 2010; Fairén et al., 2010] and
Majuro [Mangold et al., 2012] craters. In the Hesperian age Ritchey crater, Fe/Mg smectite or mixed-layer
smectite/chlorite signatures occur in and around the central uplift of the crater in a distinct geologic unit
whose morphology, distribution, and stratigraphic context is consistent with impact melt. Clay minerals in
this unit may have formed during emplacement and cooling of the melt itself or much later as a product of
low-temperature in situ alteration. In either scenario the clay minerals in the central peak region (map unit CCdr)
would be Hesperian or younger in age. In contrast, highly fractured bedrock (uplifted from ~9 km depth)
unconformably underlies the clay-bearing impact melt unit and is spectrally dominated by low-Ca pyroxene
and lacks evidence of clay minerals.
Clay minerals detected in the crater wall, crater floor, and fan deposits sourced from the crater wall could
represent either in situ formation (alteration of primary minerals or neoformation) or excavation of
preexisting clays and fluvial-alluvial transport. Clay mineral signatures in fans associated with fluvial incision
and similar spectral signatures in the sediment source region indicate that at least some of the clays in the
crater floor and fan deposits are likely detrital and sourced from the crater wall. However, the crater wall
materials lack clear stratification/bedding, and the distribution of clay minerals is patchy. Therefore,
determining whether clays in the crater wall are predominantly the result of in situ alteration or simply
exposures of older clay-bearing strata is precluded using current orbital data.
The lack of preimpact clay minerals in Ritchey’s central uplift emphasizes the need to correlate spectral data to
high-resolution imagery to determine the preimpact or postimpact origin of hydrated phases in central uplifts
[Marzo et al., 2010; Osinski et al., 2013]. As discussed here, a detailed understanding of the morphologic and
stratigraphic context of mineral detections is crucial in order to assess which components were uplifted from
depth and represent deeper crustal compositions and processes. Only at the 25–50 cm/pixel resolution of
HiRISE is one able to definitively resolve the stratigraphic position, brecciation, and draping morphology of the
clay-bearing unit to determine it is likely impact melt and distinct from the uplifted pyroxene-rich bedrock.
Although clay minerals are present in the central uplift of Ritchey in a general sense, there is no strong evidence
for the presence of clay minerals in the deeper crust at this location. Future work will focus on applying these
methods to the central uplifts of other impact craters that exhibit clay mineral signatures, allowing us to better
understand the true distribution of clay minerals at depth and assess the role of crustal fluids and burial
processes on Mars.
Acknowledgments
We are grateful to the CRISM and HiRISE
science and operations teams for
acquiring and making the data readily
available. We would also like to thank
two anonymous reviewers and the
Associate Editor for their thoughtful and
constructive comments. This research
was supported by NASA Mars Data
Analysis Program grant NNX12A063G.
SUN AND MILLIKEN
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The geology and mineralogy of Ritchey crater, Mars: Evidence for

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