Note Orbital identification of clays and carbonates in Gusev crater John Carter a,b,⇑ , Francois Poulet a a Institut d’Astrophysique Spatiale, Bat 121, Universite Paris-Sud, Orsay, France b European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile article info Article history: Received 3 December 2011 Revised 15 February 2012 Accepted 21 February 2012 Available online 10 March 2012 Keywords: Mars, Surface Infrared observations Mineralogy abstract Gusev crater was selected as the landing site of one of the two NASA Mars Exploration Rovers because water once could have ponded within the crater and partly filled it with sediments as suggested by the presence of a feeder chan- nel and the fluvial-lacustrine morphology of the in-filling. However, the paucity of mineralogical evidence for fluvial- lacustrine activity revealed by the Spirit rover has remained a puzzle for years. Using orbital, near-infrared imaging spectroscopy, we report the detection of phyllosilicates and carbonates within and around the landing site of Spirit on the floor of Gusev crater. Placed in their geomorphological context, these minerals shed new light on the aqueous history of this crater, and offer a framework for the Spirit in situ measurements. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Gusev is a 160 km large impact crater located at the outlet of the 900 km-long Maadim Vallis on the martian divide between the ancient Noachian highlands and the younger northern plains (Fig. 1a). It exhibits an overall flat morphology indica- tive of infilling, with occasional hilly units. The crater infilling comprises mostly of volcanic and putative fluvial-lacustrine sediments, which deposited over thick- nesses of hundreds-of-meters during the Noachian to Hesperian eons (e.g. Irwin et al., 2002; Cabrol et al., 2003; Parker et al., 2010). The predicted aqueous origin for some of the infilling was the main rationale for landing the Spirit Mars Explora- tion Rover (MER) there. Finding only basaltic lava on the crater floor (Squyres et al., 2004), Spirit drove almost three kilometers to the Columbia Hills, a set of low hills to the east of the landing site (Fig. 1b). Older rocks in the hills have shown evidence for more substantial aqueous alteration (Morris et al., 2010; Ruff et al., 2011; Squy- res et al., 2008), however, this alteration is interpreted to be of localized hydrother- mal origin and no clear evidence for lacustrine sedimentation had been found to date. Previous orbital investigations of the mineralogy of Gusev did not yield positive detections of hydrated minerals (Lichtenberg et al., 2007; Arvidson et al., 2008). Using new data processing tools, we report the orbital detection of hydrated clays within sedimentary units of Gusev as well as carbonates in the Columbia Hills using the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) visible and near- infrared imaging spectrometer on-board Mars Reconnaissance Orbiter. 2. Data analysis Mineral composition is determined by analyzing data from the CRISM instru- ment (Murchie et al., 2007). CRISM measures the reflectance of the surface in the visible to near-infrared (0.3–3.9 lm) spectrally sampled at 6.55 nm and spatially binned down to 18 m/pix. For the purpose of this work, we focus on targeted obser- vations with spatial binning of 18 and 36 m/pix in the near-infrared (1.1– 2.6 lm). We process the ‘I/F’ data from the ‘TRR2’ calibration version available on the Planetary Data System (PDS) node which is calibrated radiance divided by a so- lar radiance spectrum for which the intensity is scaled to the Mars-Sol distance. I/F data is then converted to surface reflectance through a two-step process accounting for photometric effects and atmospheric gas absorptions as described in Erard and Calvin (1997). We use the updated atmospheric correction parameters provided by McGuire et al. (2009) which reduce the atmospheric CO 2 absorption residuals in the 1.9–2.0 lm region. Hydrated minerals have diagnostic absorption features in the 1.3–2.6 lm range (e.g. Clark et al., 1990) and while most exhibit a band at 1.4 and 1.9 lm, bands in the 2.2–2.6 lm region are more specific and allow the unambiguous identification of Fe/Mg-bearing phyllosilicates, Al-bearing phyllosilicates and carbonate salts. These minerals are detected using simple and time-efficient spectral criteria: SC ¼ 1 hri k band hri k continuum , where k band and k continuum are each a small interval of spectral channels within which we compute the median value hri of the reflectance spec- trum. We use 3 criteria with band centers at 1.93, 2.19 and 2.32 lm to detect and map the phyllosilicates and carbonates. The CRISM instrument suffers from high stochastic and non stochastic noise and systematic instrument artifacts as reported in Murchie et al. (2007) and Seelos et al. (2009). Collectively, they affect investigations of hydrated minerals by inducing spurious pixels in the maps (including false positives) and by reducing the signal- to-noise and thus complicating spectral identifications. These effects are particu- larly troublesome when dealing with small exposures (a few CRISM pixels) as it is not possible to perform large spatial averages to increase the signal-to-noise. In addition, observational biases including localized photometric effects (e.g. shad- ows), surface dust and aerosols induce spectral slopes that affect the criteria and also cause false positives and negatives. Finally, spatial mixtures with non-hydrated minerals both reduce the absorption depths and affect the spectral continuum which in turn bias the criteria. We tackle these sources of error by developing new processing steps which correct for instrument artifacts and observational biases: (i) Along-track, spatial striping is removed. (ii) Spikes and bursts are cor- rected in the spectral dimension. (iii) Residual spurious pixels are removed. (iv) Prior to running the spectral criteria, we subtract the neutral component in each spectrum by assuming it is homogeneous throughout each CRISM observation (whereas hydrated mineral exposures are localized). The neutral component is ta- ken as a weighted average in the along-track direction. (v) We remove the spectral continuum and run the criteria. (vi) Residual systematic biases and stochastic noise are filtered out in each spectral criterion map. A detailed description of these pro- cessing steps is available in Carter et al. (2012, submitted to PSS). 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.10.1016/j.icarus.2012.02.024 ⇑ Corresponding author at: Institut d’Astrophysique Spatiale, Bat 121, Universite Paris-Sud, Orsay, France. E-mail address: [email protected](J. Carter). Icarus 219 (2012) 250–253 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus
Transcript
Icarus 219 (2012) 250–253
Contents lists available at SciVerse ScienceDirect
Orbital identification of clays and carbonates in Gusev crater
John Carter a,b,⇑, Francois Poulet a
a Institut d’Astrophysique Spatiale, Bat 121, Universite Paris-Sud, Orsay, Franceb European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
a r t i c l e i n f o
Article history:Received 3 December 2011Revised 15 February 2012Accepted 21 February 2012Available online 10 March 2012
Gusev crater was selected as the landing site of one of the two NASA Mars Exploration Rovers because water oncecould have ponded within the crater and partly filled it with sediments as suggested by the presence of a feeder chan-nel and the fluvial-lacustrine morphology of the in-filling. However, the paucity of mineralogical evidence for fluvial-lacustrine activity revealed by the Spirit rover has remained a puzzle for years. Using orbital, near-infrared imagingspectroscopy, we report the detection of phyllosilicates and carbonates within and around the landing site of Spiriton the floor of Gusev crater. Placed in their geomorphological context, these minerals shed new light on the aqueoushistory of this crater, and offer a framework for the Spirit in situ measurements.
� 2012 Elsevier Inc. All rights reserved.
1. Introduction
Gusev is a �160 km large impact crater located at the outlet of the 900 km-longMaadim Vallis on the martian divide between the ancient Noachian highlands andthe younger northern plains (Fig. 1a). It exhibits an overall flat morphology indica-tive of infilling, with occasional hilly units. The crater infilling comprises mostly ofvolcanic and putative fluvial-lacustrine sediments, which deposited over thick-nesses of hundreds-of-meters during the Noachian to Hesperian eons (e.g. Irwinet al., 2002; Cabrol et al., 2003; Parker et al., 2010). The predicted aqueous originfor some of the infilling was the main rationale for landing the Spirit Mars Explora-tion Rover (MER) there. Finding only basaltic lava on the crater floor (Squyres et al.,2004), Spirit drove almost three kilometers to the Columbia Hills, a set of low hillsto the east of the landing site (Fig. 1b). Older rocks in the hills have shown evidencefor more substantial aqueous alteration (Morris et al., 2010; Ruff et al., 2011; Squy-res et al., 2008), however, this alteration is interpreted to be of localized hydrother-mal origin and no clear evidence for lacustrine sedimentation had been found todate.
Previous orbital investigations of the mineralogy of Gusev did not yield positivedetections of hydrated minerals (Lichtenberg et al., 2007; Arvidson et al., 2008).Using new data processing tools, we report the orbital detection of hydrated clayswithin sedimentary units of Gusev as well as carbonates in the Columbia Hills usingthe CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) visible and near-infrared imaging spectrometer on-board Mars Reconnaissance Orbiter.
2. Data analysis
Mineral composition is determined by analyzing data from the CRISM instru-ment (Murchie et al., 2007). CRISM measures the reflectance of the surface in thevisible to near-infrared (0.3–3.9 lm) spectrally sampled at �6.55 nm and spatiallybinned down to 18 m/pix. For the purpose of this work, we focus on targeted obser-vations with spatial binning of �18 and �36 m/pix in the near-infrared (1.1–2.6 lm). We process the ‘I/F’ data from the ‘TRR2’ calibration version available on
ll rights reserved.
Spatiale, Bat 121, Universite
).
the Planetary Data System (PDS) node which is calibrated radiance divided by a so-lar radiance spectrum for which the intensity is scaled to the Mars-Sol distance. I/Fdata is then converted to surface reflectance through a two-step process accountingfor photometric effects and atmospheric gas absorptions as described in Erard andCalvin (1997). We use the updated atmospheric correction parameters provided byMcGuire et al. (2009) which reduce the atmospheric CO2 absorption residuals in the1.9–2.0 lm region.
Hydrated minerals have diagnostic absorption features in the 1.3–2.6 lm range(e.g. Clark et al., 1990) and while most exhibit a band at �1.4 and �1.9 lm, bands inthe 2.2–2.6 lm region are more specific and allow the unambiguous identificationof Fe/Mg-bearing phyllosilicates, Al-bearing phyllosilicates and carbonate salts.These minerals are detected using simple and time-efficient spectral criteria:
SC ¼ 1� hrikbandhrikcontinuum
, where kband and kcontinuum are each a small interval of spectral
channels within which we compute the median value hri of the reflectance spec-trum. We use 3 criteria with band centers at 1.93, 2.19 and 2.32 lm to detectand map the phyllosilicates and carbonates.
The CRISM instrument suffers from high stochastic and non stochastic noise andsystematic instrument artifacts as reported in Murchie et al. (2007) and Seelos et al.(2009). Collectively, they affect investigations of hydrated minerals by inducingspurious pixels in the maps (including false positives) and by reducing the signal-to-noise and thus complicating spectral identifications. These effects are particu-larly troublesome when dealing with small exposures (a few CRISM pixels) as itis not possible to perform large spatial averages to increase the signal-to-noise. Inaddition, observational biases including localized photometric effects (e.g. shad-ows), surface dust and aerosols induce spectral slopes that affect the criteria andalso cause false positives and negatives. Finally, spatial mixtures with non-hydratedminerals both reduce the absorption depths and affect the spectral continuumwhich in turn bias the criteria. We tackle these sources of error by developingnew processing steps which correct for instrument artifacts and observationalbiases: (i) Along-track, spatial striping is removed. (ii) Spikes and bursts are cor-rected in the spectral dimension. (iii) Residual spurious pixels are removed. (iv)Prior to running the spectral criteria, we subtract the neutral component in eachspectrum by assuming it is homogeneous throughout each CRISM observation(whereas hydrated mineral exposures are localized). The neutral component is ta-ken as a weighted average in the along-track direction. (v) We remove the spectralcontinuum and run the criteria. (vi) Residual systematic biases and stochastic noiseare filtered out in each spectral criterion map. A detailed description of these pro-cessing steps is available in Carter et al. (2012, submitted to PSS).
Fig. 1. (a) Regional context of Gusev crater, Mars. Background is a THEMIS mosaic colorized with MOLA elevation. (b) Context Camera mosaic of the southeastern floor ofGusev crater and Columbia Hills. Footprints of the four CRISM observations used in this study are shown in dotted lines (FRT3192, FRT595C, HRS11212, HRS13F67). Mineralmapping of the Al and Fe/Mg phyllosilicates is shown in purple and red, respectively. Geological mapping of the fluvial-lacustrine unit from Kuzmin et al. (2000) is overlain inblue. (c) Spectral evidence for Fe/Mg and d Al phyllosilicates. CRISM spectra are in black (averages over 1114 and 134 pixels, respectively). Color spectra are spectral matchesfrom the NASA/Keck RELAB spectral library.
Note / Icarus 219 (2012) 250–253 251
The mineral investigation is complemented by high-resolution (26 cm/pix to6 m/pix) imagery data from the HiRISE and CTX instruments, geological mappingfrom Kuzmin et al. (2000) as well as thermal inertia and morphological mappingfrom Martinez-Alonso et al. (2005) and Milam et al. (2003).
3. Mineral detections
We surveyed the mineralogy of Gusev crater by investigating a dozen targetedCRISM observations over the various geological, morphological and thermophysicalunits described in Kuzmin et al. (2000), Milam et al. (2003), and Martinez-Alonsoet al. (2005). Two distinct phyllosilicates (Al and Fe/Mg bearing) and carbonate saltare identified in four CRISM observations (FRT3192, FRT595C, HRS11212,HRS13F67) in the SE part of the crater floor, as shown in Fig. 1b.
Spectral signatures of the Fe/Mg-rich phyllosilicates are best fit by the (Mg)-bearing saponite smectite or an (Fe,Mg)-vermiculite (Fig. 1c), while pure (Fe)-bear-ing smectites are a poor match. To the limit of the CRISM spectral resolution andsignal-to-noise, no variation in the spectra has been found between these sites. Thisvermiculite and/or saponite signature is typical of hydrated exposures foundthroughout Mars (Carter et al., 2011), but no definite mineral identification hasbeen made yet. Saponite smectite is a more likely alteration product of basalt thanvermiculites as the latter usually requires the prior presence of biotite, a mineralnot confirmed on Mars. The Fe/Mg phyllosilicate signatures are therefore likely tohave a strong (Mg)-smectite component.
We map vermiculite/saponite minerals using a combination of the 1.93 and2.32 lm criteria in order to minimize false positives. We find them both withinthe Columbia Hills and in rugged floor deposits to the southeast. All the detectionsoccur within a geological unit previously interpreted as being fluvial-lacustrine sed-iments deposited between the Late Noachian and Early Amazonian (Kuzmin et al.,2000) and mapped in blue in Fig. 1b. This unit also exhibits high thermal inertia rel-ative to the other intra-crater terrains and an etched morphology (Milam et al.,2003). Martinez-Alonso et al. (2005) also proposed a sedimentary origin but alter-natively a volcanosedimentary origin for this unit, pending mineralogy data fromCRISM.
The Fe/Mg clays of the SE rugged unit (morphological ‘etched’ unit ETm in Milamet al. (2003)) are in close proximity to, but separated from the younger volcanic
plains which do not exhibit hydrated signatures (Fig. 1b). Detections are concen-trated on the most eroded, rugged part of the unit while it is devoid of alterationsignature towards the rim of Gusev where it exhibits a smoother texture.
In addition, a dozen clay-bearing occurrences are mapped in the Columbia Hills(Fig. 2a). While small individual exposures (typically <3 CRISM pixels) do not al-ways have sufficient signal-to-noise to unambiguously identify phyllosilicates, byaveraging together all these pixels, the diagnostic 1.9 and 2.3 lm bands are indeeddetected. Some outcrops have definite Fe/Mg clay signatures less than 700 m fromthe location where Spirit unfortunately became stuck in soft soil (Arvidson et al.,2010, Fig. 2a). The outcrops are bright and exhibit meter-scale fractures (Fig. 2b).Sub-meter scale images by HiRISE reveal numerous additional similar outcrops inthe Columbia Hills that are not resolved at the coarser resolution of CRISM.
Aluminous clays are also detected in two outcrops of the same Fe/Mg clay-bear-ing fluvial-lacustrine unit (within thermophysical ‘mesa’ unit MSt in Milam et al.(2003)). Spectral signatures are best fit by kaolin minerals, particularly (Al)-Halloy-site (Fig. 1d).
Carbonates signatures are found mixed with olivine at the sub-pixel scale in theComanche outcrops of the Columbia Hills, i.e. at the same location where this min-eral assemblage was found with Spirit (Morris et al., 2010, Fig. 3a). The 2.3 and2.5 lm bands are weak but their shapes and central wavelengths are closest to amagnesium rich member (akin to magnesite, Fig. 3b), as detected in situ. Highdetector noise at longer wavelengths precludes any identification based on thediagnostic bands at 3.4 and 3.9 lm. The additional 1.9 lm feature may indicateeither chemically bound water or the presence of a hydrous silica phase mixed withcarbonate. No carbonate signature without this additional band was identified here.We used the same combination of spectral criteria to map the Fe/Mg clays and car-bonates. Every hydrated mineral exposure identified was investigated manually todiscriminate between Fe/Mg clays and carbonates.
4. Implications for past aqueous activity
Gusev is at the outlet of the 900 km-long Ma’adim Vallis which dissects thesouthern highlands. Irwin et al. (2002) suggested the source region for the valleyto be the Eridania drainage basin which would have accumulated a large volumeof water since the Noachian, followed by surface run-off forming Ma’adim and with
Fe/Mg phyllosilicatesSpirit traverse
Fe/Mg phyllosilicates
a b1 km 50 m
Carbonates
Fig. b
Fig. 3a
Home Plate
Fig. 2. (a) High Resolution Imaging Science Experiment (HiRISE) mosaic of the Columbia Hills. The CRISM carbonate and phyllosilicate spectral criterion is mapped in red andorange, Spirit‘s traverse is in green. (b) HiRISE close-up of the easternmost clay-bearing outcrop in the Columbia Hills.
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6
0.980
0.985
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1.000
Home Plate
El Dorado
Comanche
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CRISM
Magnesite
Rel
ativ
e re
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(offs
et fo
r cla
rity)
Wavelength (µm)a b
Siderite
Olivine
Fig. 3. (a) HiRISE close-up on the Comanche Outcrops showing the carbonate and Fe/Mg phyllosilicates detections and Spirit traverse. The circle indicates the location of thecarbonate measurement by Spirit (Morris et al., 2010). (b) Spectral evidence for magnesium carbonate at Comanche (magnesite, blue). The ferric carbonate siderite (orange) isa poor match to the absorption features. A forsterite olivine spectrum is also shown in green.
252 Note / Icarus 219 (2012) 250–253
Gusev acting as final detention pond accumulating over 15,000 km3 of sediments.Kuzmin et al. (2000) also interpreted some of the Gusev deposits (the SE ruggedunit and Columbia Hills) to be fluvial-lacustrine sediments. The earliest evidenceof fluvial deposition is dated to the Late Noachian while younger deposits of EarlyAmazonian age attest to at least episodic aqueous activity spanning over 800 Myrs.The sediments were fed by channels breaching the south and SE rim, and pondingmay have occurred.
The detection of clays restricted to these sedimentary units strengthens thehypothesis of a fluvial-lacustrine depositional process, and is not easily explainedin the case of a volcanosedimentary origin as proposed by Martinez-Alonso et al.(2005). The clays may have been detrital in which case they formed in the sourceregion as alteration products of the Noachian crust, or authigenic implying forma-tion/transformation into clays during the depositional event(s) and in pondingwater. The spectral similarity between these clays and those found in the Noachiancrust including the source region for Ma’adim (Carter et al., 2011) would argue infavor of a detrital/fluvial origin, although authigenic/lacustrine formation cannotbe ruled out entirely. The presence of both a kaolin mineral and smectites or verm-iculites hints at diverse geochemical environments. Kaolin minerals form underhigher leeching and/or lower pH environments than smectites and could be the re-sult of late-stage, in situ transformation of the detrital material, but does not nec-essarily imply the existence of a lake at that time.
5. Reconciling orbital and in situ measurements
Based on Spirit’s Pancam instrument, Rice et al. (2010) have proposed the exis-tence of numerous hydrated silica-bearing materials within the Columbia Hills,some of which have been confirmed with other instruments on board Spirit. The�10 of small clay-bearing occurrences detected with CRISM, some in close proxim-
ity to the Pancam hydrated silica measurements, may be consistent with some ofthe in situ detections.
The detrital origin of the hydrated clays contrast with the proposed origin of thecarbonates identified in situ at the Comanche outcrops: these carbonates are inter-preted to be of local origin, as a result of hydrothermal alteration (Morris et al.,2010). The independent detection of weak carbonate signatures by CRISM atComanche provides the first orbital confirmation of in situ detection of carbonateson Mars and reinforces the other detections of carbonates, especially those found inassociation with olivine in Nili Fossae (Ehlmann et al., 2009).
Another hydrated mineral, opaline silica, has also been reported in the Colum-bia Hills with Spirit, and a hydrothermal origin is also proposed for these deposits(Squyres et al., 2008; Ruff et al., 2011). Consistent with Arvidson et al. (2008), we donot detect opaline silica with CRISM, most likely because of the limited spatialresolution.
6. Conclusions
Using data from the CRISM imaging spectrometer, we have identified Fe/Mgclays on the floor of Gusev crater and in the Columbia Hills, within units interpretedas being fluvial-lacustrine in origin. We interpret these clays as detrital sedimentstransported from a source region in the Noachian highlands to Gusev crater, possi-bly but not necessarily in the presence of a lake. We also report the identification ofcarbonates in the Comanche outcrops of the Columbia Hills, confirming in situ mea-surements by the Spirit rover.
The Opportunity rover is currently traversing Sinus Meridiani towards the rimof Endeavor crater in the hopes of achieving the first in situ measurements of clay-rich terrains also identified with CRISM (Wray et al., 2009). Our detection of claysonly 100s of meters away from Spirit’s traverse and the likelihood that these clay
Note / Icarus 219 (2012) 250–253 253
deposits are more widespread than can be inferred from orbit imply that Spiritcould actually have been the first rover to perform such in situ measurements. Acareful re-analysis of data acquired during its traverse in the light of these newdetections could reveal the first direct in situ evidence for clays on Mars.
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