Christopher S. Edwards

The stratigraphy and evolution of lower Mount Sharp from spectral, morphological, and thermophysical orbital data sets

Abstract We have developed a refined geologic map and stratigraphy for lower Mount Sharp using coordinated analyses of new spectral, thermophysical, and morphologic orbital data products. The Mount Sharp group consists of seven relatively planar units delineated by differences in texture, mineralogy, and thermophysical properties. These units are (1–3) three spatially adjacent units in the Murray formation which contain a variety of secondary phases and are distinguishable by thermal inertia and albedo differences, (4) a phyllosilicate‐bearing unit, (5) a hematite‐capped ridge unit, (6) a unit associated with material having a strongly sloped spectral signature at visible near‐infrared wavelengths, and (7) a layered sulfate unit. The Siccar Point group consists of the Stimson formation and two additional units that unconformably overlie the Mount Sharp group. All Siccar Point group units are distinguished by higher thermal inertia values and record a period of substantial deposition and exhumation that followed the deposition and exhumation of the Mount Sharp group. Several spatially extensive silica deposits associated with veins and fractures show that late‐stage silica enrichment within lower Mount Sharp was pervasive. At least two laterally extensive hematitic deposits are present at different stratigraphic intervals, and both are geometrically conformable with lower Mount Sharp strata. The occurrence of hematite at multiple stratigraphic horizons suggests redox interfaces were widespread in space and/or in time, and future measurements by the Mars Science Laboratory Curiosity rover will provide further insights into the depositional settings of these and other mineral phases.

The sustainability of habitability on terrestrial planets: Insights, questions, and needed measurements from Mars for understanding the evolution of Earth‐like worlds

What allows a planet to be both within a potentially habitable zone and sustain habitability over long geologic time? With the advent of exoplanetary astronomy and the ongoing discovery of terrestrial-type planets around other stars, our own solar system becomes a key testing ground for ideas about what factors control planetary evolution. Mars provides the solar systems longest record of the interplay of the physical and chemical processes relevant to habitability on an accessible rocky planet with an atmosphere and hydrosphere. Here we review current understanding and update the timeline of key processes in early Mars history. We then draw on knowledge of exoplanets and the other solar system terrestrial planets to identify six broad questions of high importance to the development and sustaining of habitability (unprioritized): (1) Is small planetary size fatal? (2) How do magnetic fields influence atmospheric evolution? (3) To what extent does starting composition dictate subsequent evolution, including redox processes and the availability of water and organics? (4) Does early impact bombardment have a net deleterious or beneficial influence? (5) How do planetary climates respond to stellar evolution, e.g., sustaining early liquid water in spite of a faint young Sun? (6) How important are the timescales of climate forcing and their dynamical drivers? Finally, we suggest crucial types of Mars measurements (unprioritized) to address these questions: (1) in situ petrology at multiple units/sites; (2) continued quantification of volatile reservoirs and new isotopic measurements of H, C, N, O, S, Cl, and noble gases in rocks that sample multiple stratigraphic sections; (3) radiometric age dating of units in stratigraphic sections and from key volcanic and impact units; (4) higher-resolution measurements of heat flux, subsurface structure, and magnetic field anomalies coupled with absolute age dating. Understanding the evolution of early Mars will feed forward to understanding the factors driving the divergent evolutionary paths of the Earth, Venus, and thousands of small rocky extrasolar planets yet to be discovered.

The water content of recurring slope lineae on Mars

Observations of recurring slope lineae (RSL) from the High-Resolution Imaging Science Experiment have been interpreted as present-day, seasonally variable liquid water flows; however, orbital spectroscopy has not confirmed the presence of liquid H2O, only hydrated salts. Thermal Emission Imaging System (THEMIS) temperature data and a numerical heat transfer model definitively constrain the amount of water associated with RSL. Surface temperature differences between RSL-bearing and dry RSL-free terrains are consistent with no water associated with RSL and, based on measurement uncertainties, limit the water content of RSL to at most 0.5–3 wt %. In addition, distinct high thermal inertia regolith signatures expected with crust-forming evaporitic salt deposits from cyclical briny water flows are not observed, indicating low water salinity (if any) and/or low enough volumes to prevent their formation. Alternatively, observed salts may be preexisting in soils at low abundances (i.e., near or below detection limits) and largely immobile. These RSL-rich surfaces experience ~100 K diurnal temperature oscillations, possible freeze/thaw cycles and/or complete evaporation on time scales that challenge their habitability potential. The unique surface temperature measurements provided by THEMIS are consistent with a dry RSL hypothesis or at least significantly limit the water content of Martian RSL.

The thermophysical properties of the Bagnold Dunes, Mars: Ground truthing orbital data

In this work, we compare the thermophysical properties and particle sizes derived from the Mars Science Laboratory (MSL) rovers Ground Temperature Sensor (GTS) of the Bagnold dunes, specifically Namib dune, to those derived orbitally from Thermal Emission Imaging System (THEMIS), ultimately linking these measurements to ground-truth particle sizes determined from Mars Hand Lens Imager (MAHLI) images. In general, we find that all three datasets report consistent particle sizes for the Bagnold dunes (~110-350 microns, and are within measurement and model uncertainties), indicating that particle sizes of homogeneous materials determined from orbit are reliable. Furthermore, we examine the effects of two physical characteristics that could influence the modeled thermal inertia and particle sizes, including: 1) fine-scale (cm-m scale) ripples, and 2) thin layering of indurated/armored materials. To first order, we find small scale ripples and thin (approximately centimeter scale) layers do not significantly affect the determination of bulk thermal inertia from orbital thermal data determined from a single nighttime temperature. Modeling of a layer of coarse or indurated material reveals that a thin layer (< ~5 mm; similar to what was observed by the Curiosity rover) would not significantly change the observed thermal properties of the surface and would be dominated by the properties of the underlying material. Thermal inertia and grain sizes of relatively homogeneous materials derived from nighttime orbital data should be considered as reliable, as long as there are not significant sub-pixel anisothermality effects (e.g. lateral mixing of multiple thermophysically distinct materials).

Widespread distribution of OH/H 2 O on the lunar surface inferred from spectral data

Remote-sensing data from lunar orbiters have revealed spectral features consistent with the presence of OH or H2O on the lunar surface. Analyses of data from the Moon Mineralogy Mapper spectrometer onboard the Chandrayaan-1 spacecraft have suggested that OH/H2O is recycled on diurnal timescales and persists only at high latitudes. However, the spatial distribution and temporal variability of the OH/H2O, as well as its source, remain uncertain. Here we incorporate a physics-based thermal correction into analysis of reflectance spectra from the Moon Mineralogy Mapper and find that prominent absorption features consistent with OH/H2O can be present at all latitudes, local times and surface types examined. This suggests the widespread presence of OH/H2O on the lunar surface without significant diurnal migration. We suggest that the spectra are consistent with the production of OH in space-weathered materials by the solar wind implantation of H+ and formation of OH at crystal defect sites, as opposed to H2O sourced from the lunar interior. Regardless of the specific composition or formation mechanism, we conclude that OH/H2O can be present on the Moon under thermal conditions more wide-ranging than previously recognized.Hydroxyl produced by space weathering processes may be widespread and persistent on the lunar surface according to analysis of spectral data from the Chandrayaan-1 spacecraft.

Structural relationship between the Karakoram and Longmu Co fault systems, southwestern Tibetan Plateau, revealed by ASTER remote sensing

The western margin of the Tibetan Plateau is defined by the NE-striking, sinistral Longmu Co fault system and the NW-striking, dextral Karakoram fault system. The region of convergence of these two systems is remote and politically sensitive, precluding systematic geologic mapping in the field. As a consequence, there is considerable controversy regarding the relationship between these regionally important structural features. Analysis of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) multispectral data and validation with field mapping provide new insights via the production of a lithologic map. Detailed analysis of the ASTER data shows that the two fault systems do not intersect but instead become parallel. Additionally, the geometry and sense of motion of the two fault systems imply that they are acting as a conjugate fault pair, in conjunction with the normal Angmong fault system, allowing for the extrusion of Tibetan lithosphere toward the east.

Incorporation of portable infrared spectral imaging into planetary geological field work: Analog studies at Kīlauea Volcano, Hawaii and Potrillo Volcanic Field, New Mexico

During geological work for future planetary missions, portable/hand-held infrared spectral imaging instruments have the potential to significantly benefit science objectives. We assess how ground-based infrared spectral imaging can be incorporated into geological field work in a planetary setting through a series of field campaigns at two analog sites: Kīlauea Volcano, Hawaii, and Potrillo Volcanic Field, New Mexico. For this study, we utilize thermal infrared emission spectroscopy (8–13 μm) because this wavelength range is sensitive to major silicate spectral features and covers the terrestrial atmospheric window; however, our conclusions are applicable to other forms of infrared imaging (e.g., near-infrared reflectance spectroscopy). We demonstrate the ways in which spectral imaging could potentially enhance the science return and/or efficiency of traditional geological field work. Benefits include the following: documentation of major compositional variations within scenes, the ability to detect visually subtle and/or concealed variability in (sub) units, and the ability to characterize remote and/or inaccessible outcrops. These advantages could help field workers rapidly document sample context and develop strategic work plans. Furthermore, ground-based imaging provides a critical link between orbital/aerial imaging scales and sampling scales. Last, infrared spectral imaging data may be combined with in situ measurement techniques, such as X-ray fluorescence, as well as other ground-based remote sensing techniques, such as LIDAR (Light Detection And Ranging), to maximize geological understanding of the work area. Plain Language Summary Future missions to planetary objects are expected to have increasingly more human and rover components in surface exploration. To aid the explorers in conducting scientific tasks, portable instruments will likely be invaluable. Currently, knowledge of instrument suitability and most effective incorporation strategies are not sufficiently developed. As one of the first steps in this development process, we assess the fundamental capabilities of portable imaging technique in providing critical information for geological field work on planetary surfaces. Portable imaging, operating in the thermal infrared (8–13 μm), captured crucial data regarding rock/mineral types at field sites analogous to planetary settings. Value brought forth by portable infrared imaging is substantial, and this technique has the potential to benefit effective geological field work, which may lead to maximizing scientific return from missions. This finding and accompanying analyses presented here serve as a foundation for further development of instruments and mission strategies.

A Complex Fluviolacustrine Environment on Early Mars and Its Astrobiological Potentials

Chloride-bearing deposits and phyllosilicates-bearing units are widely distributed in the southern highlands of Mars, but these phases are rarely found together in fluviolacustrine environments. The study of the coexistence of these minerals can provide important insights into geochemistry, water activity, and ultimately the climate and habitability of early Mars. Here we use high-resolution compositional and morphological orbiter data to identify and characterize the context of diverse minerals in a Noachian fluviolacustrine environment west of Knobel crater (6.7°S, 226.8°W). The chlorides in this region are likely formed through the evaporation of brines in a closed topographic basin. The formation age of chlorides is older than 3.7 Ga, based on stratigraphic relationships identified and previously obtained crater retention ages. The timing of the alteration of basaltic materials to iron-magnesium smectites in relation to the chloride formation in this location is enigmatic and is unable to be resolved with currently available remote sensing data. Importantly, we find that this close relationship between these key minerals revealed by the currently available data details a complex and intimate history of aqueous activity in the region. Of critical importance are the evaporitic deposits as analogous terrestrial deposits have been shown to preserve ancient biosignatures and possibly even sustain microbial communities for hundreds of millions of years. These salts could have protected organic matter from ultraviolet radiation, or even allow modern habitable microenvironments in the shallow subsurface through periodic deliquescence. The high astrobiology potential of this site makes it a good candidate for future landed and sample return missions (e.g., the Chinese 2020 Mars mission).