Clay-bearing Units Research Articles

Combination of MRO SHARAD and deep-learning-based DTM to search for subsurface features in Oxia Planum, Mars

Context. Oxia Planum is a mid-latitude region on Mars that attracts a great amount of interest worldwide. An orbiting radar provides an effective way to probe the Martian subsurface and detect buried layers or geomorphological features. The Shallow radar orbital radar system on board the NASA Mars reconnaissance orbiter transmits pulsed signals towards the nadir and receives returned echoes from dielectric boundaries. However, radar clutter can be induced by a higher topography of the off-nadir region than that at the nadir, which is then manifested as subsurface reflectors in the radar image. Aims. This study combines radar observations, terrain models, and surface images to investigate the subsurface features of the ExoMars landing site in Oxia Planum. Methods. Possible subsurface features are observed in radargrams. Radar clutter is simulated using the terrain models, and these are then compared to radar observations to exclude clutter and identify possible subsurface return echoes. Finally, the dielectric constant is estimated with measurements in both radargrams and surface imagery. Results. The resolution and quality of the terrain models greatly influence the clutter simulations. Higher resolution can produce finer cluttergrams, which assists in identifying possible subsurface features. One possible subsurface layering sequence is identified in one radargram. Conclusions. A combination of radar observations, terrain models, and surface images reveals the dielectric constant of the surface deposit in Oxia Planum to be 4.9–8.8, indicating that the surface-covering material is made up of clay-bearing units in this region.

A CaSSIS and HiRISE map of the Clay-bearing Unit at the ExoMars 2022 landing site in Oxia Planum

In preparation for the operations of the ExoMars Rosalind Franklin rover, characterising its landing site in Oxia Planum is essential. Of particular interest is the extensive Clay-bearing Unit present at the site, a key target in the search for biosignatures. In this paper we provide a map based on variations in colour and spectral information within this unit, covering the 1σ landing envelope of the rover along with a 1 ​km buffer to account for minor shifts of the landing envelope ahead of launch (referred to going forward as the 1σ+ landing envelope). We used imagery from the Colour and Stereo Surface Imaging System (CaSSIS) and High Resolution Imaging Science Experiment (HiRISE) instruments, along with CaSSIS Band Ratio Composites with enhanced colour sensitivity to the presence of ferric (Fe3+) and ferrous (Fe2+) iron bearing materials. Our map is of a far higher resolution (map-scale 1:2000) than those previously available and, in contrast to previously available maps of this unit, differentiates between an Orange Subunit and a Blue Subunit which make up the Clay-bearing Unit. This mapping covered the ∼91% of the 1σ+ landing envelope where there was CaSSIS coverage and split the Clay-bearing Unit into three categories: one for each of the clay subunits, and another for exposures of the Clay-bearing Unit where either both subunits were too intermixed to reliably separate, or where it was difficult to determine which of the two were present.The results from our mapping shows that at least ∼35% of the 1σ+ envelope is covered by exposures of the Clay-bearing Unit: ∼18% by the Orange Subunit, ∼9% the Blue Subunit, and ∼12% were classified as Indeterminate. The spread of these two subunits varied substantially over the 1σ+ landing envelope, with the south-east half of the landing envelope dominated by the Orange Subunit (∼70% exposures in this area belonging to the Orange Subunit, ∼10% to the Blue Subunit and ∼20% to the Indeterminate category), while the north-west has more sporadic exposures of the Clay-bearing Unit (∼22% Orange, ∼37% Blue and ∼41% Indeterminate). The colour distinction between the two subunits is thought to be due to constituent mineralogical differences rather than differences in dust coverage of the two subunits. The scale of the fracturing present in the two subunits has also been assessed in this study, via qualitative observations of the fracture length and quantitative mapping out of fracture networks. While there were differences in the scale of fracturing between the two subunits, these were not as great as had previously been identified.

Alternating wet and dry depositional environments recorded in the stratigraphy of Mount Sharp at Gale crater, Mars

AbstractThe Curiosity rover is exploring Hesperian-aged stratigraphy in Gale crater, Mars, where a transition from clay-bearing units to a layered sulfate-bearing unit has been interpreted to represent a major environmental transition of unknown character. We present the first description of key facies in the sulfate-bearing unit, recently observed in the distance by the rover, and propose a model for changes in depositional environments. Our results indicate a transition from lacustrine mudstones into thick aeolian deposits, topped by a major deflation surface, above which strata show architectures likely diagnostic of a subaqueous environment. This model offers a reference example of a depositional sequence for layered sulfate-bearing strata, which have been identified from orbit in other locations globally. It differs from the idea of a monotonic Hesperian climate change into long-term aridity on Mars and instead implies a period characterized by multiple transitions between sustained drier and wetter climates.

Morphological and Spectral Diversity of the Clay-Bearing Unit at the ExoMars Landing Site Oxia Planum.

The European Space Agency and Roscosmos' ExoMars rover mission, which is planned to land in the Oxia Planum region, will be dedicated to exobiology studies at the surface and subsurface of Mars. Oxia Planum is a clay-bearing site that has preserved evidence of long-term interaction with water during the Noachian era. Fe/Mg-rich phyllosilicates have previously been shown to occur extensively throughout the landing area. Here, we analyze data from the High Resolution Imaging Science Experiment (HiRISE) and from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instruments onboard NASA's Mars Reconnaissance Orbiter and the Colour and Stereo Surface Imaging System (CaSSIS) onboard ESA's Trace Gas Orbiter to characterize, at a high spatial resolution, the morphological and spectral variability of Oxia Planum's surface deposits. Two main types of bedrocks are identified within the clay-bearing, fractured unit observed throughout the landing site: (1) an orange type in HiRISE correlated with the strongest detections of secondary minerals (dominated by Fe/Mg-rich clay minerals) with, in some locations, an additional spectral absorption near 2.5 μm, suggesting the mixture with an additional mineral, plausibly carbonate or another type of clay mineral; (2) a more bluish bedrock associated with weaker detections of secondary minerals, which exhibits at certain locations a ∼1 μm broad absorption feature consistent with olivine. Coanalysis of the same terrains with the recently acquired CaSSIS images confirms the variability in the color and spectral properties of the fractured unit. Of interest for the ExoMars mission, both types of bedrocks are extensively outcropping in the Oxia Planum region, and the one corresponding to the most intense spectral signals of clay minerals (the primary scientific target) is well exposed within the landing area, including near its center.

Oxia Planum: The Landing Site for the ExoMars "Rosalind Franklin" Rover Mission: Geological Context and Prelanding Interpretation.

The European Space Agency (ESA) and Roscosmos ExoMars mission will launch the “Rosalind Franklin” rover in 2022 for a landing on Mars in 2023.The goals of the mission are to search for signs of past and present life on Mars, investigate the water/geochemical environment as a function of depth in the shallow subsurface, and characterize the surface environment. To meet these scientific objectives while minimizing the risk for landing, a 5-year-long landing site selection process was conducted by ESA, during which eight candidate sites were down selected to one: Oxia Planum. Oxia Planum is a 200 km-wide low-relief terrain characterized by hydrous clay-bearing bedrock units located at the southwest margin of Arabia Terra. This region exhibits Noachian-aged terrains. We show in this study that the selected landing site has recorded at least two distinct aqueous environments, both of which occurred during the Noachian: (1) a first phase that led to the deposition and alteration of ∼100 m of layered clay-rich deposits and (2) a second phase of a fluviodeltaic system that postdates the widespread clay-rich layered unit. Rounded isolated buttes that overlie the clay-bearing unit may also be related to aqueous processes. Our study also details the formation of an unaltered mafic-rich dark resistant unit likely of Amazonian age that caps the other units and possibly originated from volcanism. Oxia Planum shows evidence for intense erosion from morphology (inverted features) and crater statistics. Due to these erosional processes, two types of Noachian sedimentary rocks are currently exposed. We also expect rocks at the surface to have been exposed to cosmic bombardment only recently, minimizing organic matter damage.

Evidence for a changing Martian climate from the mineralogy at Mawrth Vallis

Layered outcrops in the Mawrth Vallis region of Mars contain the greatest diversity of aqueous alteration products on the planet, and these materials are used to infer past aqueous environments. Orbital investigations indicate Al/Si-rich clay-bearing units overly an Fe/Mg-smectite-rich unit. Many different secondary minerals have been identified in the upper Al/Si-rich clay units, but the presence of poorly crystalline phases has not been previously investigated. Identification of ∼10–30% allophane and imogolite in the clay-bearing units resolves previous mineralogical discrepancies between TES and CRISM of clay-bearing units on Mars. We demonstrate here that the poorly crystalline aluminosilicates allophane and imogolite comprise a significant portion of the uppermost stratum of the Al/Si-clay-rich units. These phases are unique to immature soils derived from volcanic ash in well-drained, mildly acidic environments on Earth, and we hypothesize that the deposits discovered here originate from supervolcanic activity in nearby Arabia Terra. The transition through time from smectite-bearing units to the uppermost allophane/imogolite unit in Mawrth Vallis signifies a change in climate from a warm and wet environment to one where water was sporadic and likely depleted rapidly.

Eridania Basin: An ancient paleolake floor as the next landing site for the Mars 2020 rover

The search for traces of past Martian life is directly connected to ancient paleolakes, where ponding water or low-energy water fluxes were present for long time intervals. The Eridania paleolakes system, located along the 180° meridian, is one of the largest lacustrine environments that were once present on Mars. Morphological features suggest that it was constituted by connected depressions filled by water to maximum depths of ∼2400m and a volume of at least 562,000km3. We focused our attention on the northern side of the Eridania Basin, where high-albedo, uneven patches of material characterized by the absence of dust are present. Based on OMEGA and CRISM orbital imaging spectroscopy data, a large clay-bearing unit has been identified there. In particular, a set of aqueous minerals in present in the stratigraphy, being visible through erosional windows in the first several tens of meters of the sedimentary sequence. Below this capping unit, a thin Al-rich clay stratum attributable to Al-smectite and/or kaolins is present. This overlies a Fe-rich clay stratum, attributable to the nontronite smectite. At the base of the mineralogic sequence a stratum that could be either a zeolite or more likely a hydrated sulfate is present. In addition, small deposits of alunite (a rare phase on Mars), and jarosite are here found at several locations. Such stratigraphy is interpreted as originating from a surface weathering process similar to terrestrial abiotic pedogenesis; nonetheless, possible exobiologic processes can be also invoked to explain it. NASA's Spirit rover landed on Gusev crater in 2004, near the mouth of the Ma'adim Vallis, which connects this crater with the considered paleolakes system. The Eridania site provides the unique opportunity to complete the measurements obtained in Gusev crater, while investigating the exposed mineralogical sequence in its depositionary setting. In addition, the extremely favorable landing parameters, such as elevation, slope, roughness, rock distribution, thermal inertia and dust coverage, support this location as a possible landing site for the NASA Mars 2020 rover.

Embedded clays and sulfates in Meridiani Planum, Mars

The area of Meridiani Planum on Mars became of particular interest after the detection of coarse-grained, gray hematite, which led to the choice of this region as final landing site for the Mars Exploration Rover Opportunity. Multiple additional minerals have since been detected in the region, both from orbit and in situ. The present paper reassesses in detail the mineralogy and geomorphology of the area (between 3.5°S to 6.5°N latitude and 8.0°W to 8.0°E longitude) using visible and near-infrared hyperspectral data, merged with high spatial resolution images into a Geographic Information System. Fe/Mg-rich phyllosilicates, as well as several types of sulfates are identified within the kilometer of sedimentary deposits constituting the etched terrains and the overlying hematite-rich plains. The mineralogical stratigraphy of the etched terrains is characterized by a sulfate-rich unit, enriched in Fe/Mg-rich clays in its uppermost part. This clay-rich horizon is capped by another sulfate-rich unit, of different composition, associated with hematite detections and constituting the material of the hematite plains unit. The clay-bearing unit, which directly underlies the hematite plains, may be exposed in the rim of Endeavour Crater and could correspond to the material that is currently being analyzed by Opportunity. Diverse landforms including karsts and pan features are observed at the surface of the hydrated etched terrains and indicate past surface water and potential groundwater aquifers. Our analysis reveals that both surficial water and groundwater processes are required to explain the diversity of morphologies and mineralogies observed in the area. Although orbital detections are consistent with the hematite and sulfate detections made in situ by Opportunity in the hematite plains, they also show the presence of a clay-rich horizon at depth, overlying another, thicker, possibly chemically distinct, sulfate-rich unit within the etched terrains. Therefore we conclude that the very acidic conditions that prevailed during the formation of the topmost sulfate-rich unit, as observed in Meridiani Planum and in the hematite plains by the rover Opportunity, are not representative of the entire history of the etched terrains. In contrast, most hydrated minerals in Meridiani Planum likely formed at more neutral pH, at the beginning of the Hesperian. The overall sulfate/clay/sulfate sequence is similar to the one observed in Gale Crater, the landing site of the Mars Science Laboratory rover.

What the ancient phyllosilicates at Mawrth Vallis can tell us about possible habitability on early Mars

Phyllosilicate deposits on Mars provide an opportunity to evaluate aqueous activity and the possibility that habitable environments may have existed during the Noachian period there. Analysis of hyperspectral visible/near-infrared (VNIR) Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) images has shown thick, complex profiles of phyllosilicates at Mawrth Vallis, Mars that are consistent with long-term aqueous activity and active chemistry. The ancient phyllosilicates in places such as this could have served as reaction centers for organic molecules. Previous experiments even suggest that phyllosilicates could have played a role in the origin of life. Regardless of whether life formed on early Mars or not, evaluating the type and thickness of clay-bearing units on Mars provides insights into plausible aqueous processes and chemical conditions both during the time of formation of the phyllosilicates, but also the subsequent period following their formation. The phyllosilicate outcrops at Mawrth Vallis extend across a broad (∼1000km) region and exhibit a consistent general trend of Al-phyllosilicates and amorphous Al/Si species at the top of the clay profile and Fe/Mg-phyllosilicates on the bottom. This implies either a change in water chemistry, a change in material being altered, or an alteration profile where the upper clays were leached and altered more significantly than those below. A change in iron in the phyllosilicate units is also observed such that an Fe2+-bearing unit is frequently observed between the Fe3+- and Mg-rich phyllosilicates below and the Al/Si-rich materials above. Abrupt changes in chemistry like this are often indicative of biogeochemical activity on Earth. Possible microbe-clay interactions are considered in comparison with the CRISM observations. This study evaluates CRISM spectra from four images of different outcrops across the Mawrth Vallis region and evaluates the observed phyllosilicates and clay components in terms of plausible aqueous and microbial processes and the potential for retention of biosignatures, if present.

Geochemical Consequences of Widespread Clay Mineral Formation in Mars’ Ancient Crust

Clays form on Earth by near-surface weathering, precipitation in water bodies within basins, hydrothermal alteration (volcanic- or impact-induced), diagenesis, metamorphism, and magmatic precipitation. Diverse clay minerals have been detected from orbital investigation of terrains on Mars and are globally distributed, indicating geographically widespread aqueous alteration. Clay assemblages within deep stratigraphic units in the Martian crust include Fe/Mg smectites, chlorites and higher temperature hydrated silicates. Sedimentary clay mineral assemblages include Fe/Mg smectites, kaolinite, and sulfate, carbonate, and chloride salts. Stratigraphic sequences with multiple clay-bearing units have an upper unit with Al-clays and a lower unit with Fe/Mg-clays. The typical restriction of clay minerals to the oldest, Noachian terrains indicates a distinctive set of processes involving water-rock interaction that was prevalent early in Mars history and may have profoundly influenced the evolution of Martian geochemical systems. Current analyses of orbital data have led to the proposition of multiple clay-formation mechanisms, varying in space and time in their relative importance. These include near-surface weathering, formation in ice-dominated near-surface groundwaters, and formation by subsurface hydrothermal fluids. Near-surface, open system formation of clays would lead to fractionation of Mars’ crustal reservoir into an altered crustal reservoir and a sedimentary reservoir, potentially involving changes in the composition of Mars’ atmosphere. In contrast, formation of clays in the subsurface by either aqueous alteration or magmatic cooling would result in comparatively little geochemical fractionation or interaction of Mars’ atmospheric, crustal, and magmatic reservoirs, with the exception of long-term sequestration of water. Formation of clays within ice would have geochemical consequences intermediate between these endmembers. We outline the future analyses of orbital data, in situ measurements acquired within clay-bearing terrains, and analyses of Mars samples that are needed to more fully elucidate the mechanisms of martian clay formation and to determine the consequences for the geochemical evolution of the planet.

Surface Properties of the Mars Science Laboratory Candidate Landing Sites: Characterization from Orbit and Predictions

This work describes the interpretation of THEMIS-derived thermal inertia data at the Eberswalde, Gale, Holden, and Mawrth Vallis Mars Science Laboratory (MSL) candidate landing sites and determines how thermophysical variations correspond to morphology and, when apparent, mineralogical diversity. At Eberswalde, the proportion of likely unconsolidated material relative to exposed bedrock or highly indurated surfaces controls the thermal inertia of a given region. At Gale, the majority of the landing site region has a moderate thermal inertia (250 to 410 J m−2 K−1 s−1/2), which is likely an indurated surface mixed with unconsolidated materials. The primary difference between higher and moderate thermal inertia surfaces may be due to the amount of mantling material present. Within the mound of stratified material in Gale, layers are distinguished in the thermal inertia data; the MSL rover could be traversing through materials that are both thermophysically and compositionally diverse. The majority of the Holden ellipse has a thermal inertia of 340 to 475 J m−2 K−1 s−1/2 and consists of bed forms with some consolidated material intermixed. Mawrth Vallis has a mean thermal inertia of 310 J m−2 K−1 s−1/2 and a wide variety of materials is present contributing to the moderate thermal inertia surfaces, including a mixture of bedrock, indurated surfaces, bed forms, and unconsolidated fines. Phyllosilicates have been identified at all four candidate landing sites, and these clay-bearing units typically have a similar thermal inertia value (400 to 500 J m−2 K−1 s−1/2), suggesting physical properties that are also similar.

Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data

Gale Crater contains a 5.2 km-high central mound of layered material that is largely sedimentary in origin and has been considered as a potential landing site for both the MER (Mars Exploration Rover) and MSL (Mars Science Laboratory) missions. We have analyzed recent data from Mars Reconnaissance Orbiter to help unravel the complex geologic history evidenced by these layered deposits and other landforms in the crater. Results from imaging data from the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) confirm geomorphic evidence for fluvial activity and may indicate an early lacustrine phase. Analysis of spectral data from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument shows clay-bearing units interstratified with sulfate-bearing strata in the lower member of the layered mound, again indicative of aqueous activity. The formation age of the layered mound, derived from crater counts and superposition relationships, is ∼3.6–3.8 Ga and straddles the Noachian–Hesperian time-stratigraphic boundary. Thus Gale provides a unique opportunity to investigate global environmental change on Mars during a period of transition from an environment that favored phyllosilicate deposition to a later one that was dominated by sulfate formation.

Role of impact excavation in distributing clays over Noachian surfaces

[1] Spectrometers have detected clay-bearing units in and on much of the ancient Martian crust. Geothermally heated aquifers in basaltic rock provide conditions conducive to forming Fe/Mg phyllosilicates at depth. Throughout the Noachian period, a high flux of km-scale bolides excavated buried materials and distributed them over the surface. We use the Maxwell Z-model to quantify the volume and final location of excavated clay-bearing material. We focus on two potentially detectable properties: the volume of clay-bearing material ejected as a fraction of total ejected volume and the volume percentage of clay-bearing material in the ejecta as a function of distance from the crater's rim. Generally, the volume percentage of clays in the ejecta is greatest for craters less than 25 km in diameter. Larger crater sizes incorporate a higher fraction of clay-poor material because they excavate to greater depths at which clays are likely absent. Specific trends in bulk clay volume fraction and the distribution of clay fraction across the ejecta deposit as a function of crater size depend on the depth to the clay-rich layer and its thickness. Impact excavation likely explains clays associated with ejecta deposits and may reveal clues about the volatile content and stratigraphy of the upper Noachian crust. Applying our model to the Mawrth Vallis region suggests that a clay layer a few hundred meters thick is buried at the −3000 m elevation contour. Given that clay layers are likely thin and buried in the upper 3 km of the crust, we predict that small to mid-sized craters (<25 km) will best exhibit detectable amounts of clays and that these clays will be most abundant in the crater wall and rim and less so in distal ejecta.

Stratigraphy in the Mawrth Vallis region through OMEGA, HRSC color imagery and DTM

The Mawrth Vallis region contains an extensive (at least 300 km × 400 km) and thick (⩾300 m), finely layered (at meter scale), clay-rich unit detected by OMEGA. We use OMEGA, HRSC DTMs derived from stereoscopic imagery, HRSC color imagery and high resolution imagery such as MOC, CTX and HiRISE to characterize the geometry and the composition of the clay-rich unit at the regional scale. Our results show that the clay-bearing unit can be divided into sub-units on the basis of differences in color and composition. In false-color visible imagery, alternating white/bluish and orange/red colored units correspond to a compositional succession of, respectively, Al- and Fe- or Mg-phyllosilicate rich material. Geological cross-sections are presented along the principal outcrops of the region in order to define the stratigraphy of these sub-units. This method shows that the dips of the sub-units are frequently close to the slopes of the present topography, except for scarps visible at the dichotomy boundary, inside impact craters walls, and outcrops inside Mawrth Vallis. In addition to the Al- and Fe- or Mg-phyllosilicate rich sub-units, an altered surface is identified as the lower basement unit. We propose two possible end-member scenarios to explain the derived stratigraphy: (1) alteration of volcaniclastic, aeolian or aqueous layered deposits of various compositions by groundwater, resulting in distinct altered rocks; or (2) Alteration coeval with the deposition of sediments under varying chemical conditions, in wet pedodiagenetic environment.

Mineralogic Information from a New Airborne Thermal Infrared Multispectral Scanner

A new six-channel aircraft multispectral scanner has been developed to exploit mineral signature information at wavelengths between 8 and 12 micrometers. Preliminary results show that igneous rock units can be identified from their free silica content, and that carbonate as well as clay-bearing units are readily separable on the digitally processed images.