Quantifying Sub-Meter Surface Heterogeneity on Mars Using Off-Axis Thermal Emission Imaging System (THEMIS) Data

Surface heterogeneities below the spatial resolution of thermal infrared (TIR) instruments result in anisothermality and can produce emissivity spectra with negative slopes toward longer wavelengths. Sloped spectra arise from an incorrect assumption of either a uniform surface temperature or a maximum emissivity during the temperature-emissivity separation of radiance data. Surface roughness and lateral mixing of different sub-pixel surface units result in distinct spectral slopes with magnitudes proportional to the degree of temperature mixing. Routine Off-nadir Targeted Observations (ROTO) of the Thermal Emission Imaging Spectrometer (THEMIS) are used here for the first time to investigate anisothermality below the spatial resolution of THEMIS. The southern flank of Apollinaris Mons and regions within the Medusae Fossae Formation are studied using THEMIS ROTO data acquired just after local sunset. We observe a range of sloped TIR emission spectra dependent on the magnitude of temperature differences within a THEMIS pixel. Spectral slopes and wavelength-dependent brightness temperature differences are forward-modeled for a series of two-component surfaces of varying thermal inertia values. Our results imply that differing relative proportions of rocky and unconsolidated surface units are observed at each ROTO viewing geometry and suggest a local rock abundance six times greater than published results that rely on nadir data. High-resolution visible images of these regions indicate a mixture of surface units from boulders to dunes, providing credence to the model.

The Analysis of Cones within the Tianwen-1 Landing Area

On 15 May 2021, the Zhurong rover of China’s first Mars mission, Tianwen-1 (TW-1), successfully landed in southern Utopia Planitia on Mars. Various landforms were present in the landing area, and this area recorded a complex geological history. Cones are one of the typical landforms in the landing area and Utopia Planitia, and they have a great significance to the local geological processes due to the diversity of their origins. Using High-Resolution Imaging Camera (HiRIC) images collected by the TW-1 orbiter, we identified a total of 272 well-preserved circular cones in the landing area. Detailed surveys of their spatial distribution, morphological characteristics, and morphometric parameters were conducted. A preliminary analysis of the surface characteristics of these cones also provides additional information to strengthen our understanding of them. The results of the high-resolution topographic analysis show that the cone heights are in the range of 10.5–90.8 m and their basal diameters range from 178.9–1206.6 m. We compared the morphometric parameters of the cones in the landing area with terrestrial and Martian analogous features and found that our measured cones are consistent with the ranges of mud volcanoes and also a small subset of igneous origin cones. However, the result of spatial analysis is more favorable to mud volcanoes, and the lower thermal inertia of the cones in the landing area compared to their surrounding materials is also a typical characteristic of mud volcanoes. Based on current evidence and analysis, we favor interpreting the cones in the TW-1 landing area as mud volcanoes.

Characteristics, Origins, and Biosignature Preservation Potential of Carbonate-Bearing Rocks Within and Outside of Jezero Crater

Carbonate minerals have been detected in Jezero crater, an ancient lake basin that is the landing site of the Mars 2020 Perseverance rover, and within the regional olivine-bearing (ROB) unit in the Nili Fossae region surrounding this crater. It has been suggested that some carbonates in the margin fractured unit, a rock unit within Jezero crater, formed in a fluviolacustrine environment, which would be conducive to preservation of biosignatures from paleolake-inhabiting lifeforms. Here, we show that carbonate-bearing rocks within and outside of Jezero crater have the same range of visible-to-near-infrared carbonate absorption strengths, carbonate absorption band positions, thermal inertias, and morphologies. Thicknesses of exposed carbonate-bearing rock cross-sections in Jezero crater are ∼75-90 m thicker than typical ROB unit cross-sections in the Nili Fossae region, but have similar thicknesses to ROB unit exposures in Libya Montes. These similarities in carbonate properties within and outside of Jezero crater is consistent with a shared origin for all of the carbonates in the Nili Fossae region. Carbonate absorption minima positions indicate that both Mg- and more Fe-rich carbonates are present in the Nili Fossae region, consistent with the expected products of olivine carbonation. These estimated carbonate chemistries are similar to those in martian meteorites and the Comanche carbonates investigated by the Spirit rover in Columbia Hills. Our results indicate that hydrothermal alteration is the most likely formation mechanism for non-deltaic carbonates within and outside of Jezero crater.

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.

Olivine-Carbonate Mineralogy of the Jezero Crater Region

A well-preserved, ancient delta deposit, in combination with ample exposures of carbonate outcrops, makes Jezero Crater in Nili Fossae a compelling astrobiological site. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations to characterize the surface mineralogy of the crater and surrounding watershed. Previous studies have documented the occurrence of olivine and carbonates in the Nili Fossae region. We focus on correlations between these two well-studied lithologies in the Jezero crater watershed. We map the position and shape of the olivine 1 μm absorption band and find that carbonates are found in association with olivine which displays a 1 μm band shifted to long wavelengths. We then use Thermal Emission Imaging Spectrometer (THEMIS) coverage of Nili Fossae and perform tests to investigate whether the long wavelength shifted (redshifted) olivine signature is correlated with high thermal inertia outcrops. We find that there is no consistent correlation between thermal inertia and the unique olivine signature. We discuss a range of formation scenarios for the olivine and carbonate associations, including the possibility that these lithologies are products of serpentinization reactions on early Mars. These lithologies provide an opportunity for deepening our understanding of early Mars and, given their antiquity, may provide a framework to study the timing of valley networks and the thermal history of the Martian crust and interior from the early Noachian to today.

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).

The complex relationship between olivine abundance and thermal inertia on Mars

We examine four olivine-bearing regions at a variety of spatial scales with thermal infrared, visible to near-infrared, and visible imagery data to investigate the hypothesis that the relationship between olivine abundance and thermal inertia (i.e., effective particle size) can be used to infer the occurrence of olivine chemical alteration during sediment production on Mars. As in previous work, Nili Fossae and Isidis Planitia show a positive correlation between thermal inertia and olivine abundance in Thermal Emission Spectrometer (TES) and Thermal Emission Imaging System (THEMIS) data, which could be interpreted as indicating olivine chemical weathering. However, geomorphological analysis reveals that relatively olivine-poor sediments are not derived from adjacent olivine-rich materials, and therefore, chemical weathering cannot be inferred based on the olivine-thermal inertia relationship alone. We identify two areas (Terra Cimmeria and Argyre Planitia) with significant olivine abundance and thermal inertias consistent with sand, but no adjacent rocky (parent) units having even greater olivine abundances. More broadly, global analysis with TES reveals that the most typical olivine abundance on Mars is ~5-7% and that olivine-bearing (5-25%) materials have a wide range of thermal inertias, commonly 25-600 J m^-2 K^-1 s^-1/2. TES also indicates that the majority of olivine-rich (>25%) materials have apparent thermal inertias less than 400 J m^-2 K^-1 s^-1/2. In summary, we find that the relationship between thermal inertia and olivine abundance alone cannot be used in infer olivine weathering in the examined areas, that olivine-bearing materials have a large range of thermal intertias, and therefore that a complex relationship between olivine abundance and thermal inertia exists on Mars.

Surface energy budget and thermal inertia at Gale Crater: Calculations from ground-based measurements

The analysis of the surface energy budget (SEB) yields insights into soil-atmosphere interactions and local climates, while the analysis of the thermal inertia (I) of shallow subsurfaces provides context for evaluating geological features. Mars orbital data have been used to determine thermal inertias at horizontal scales of ∼10⁴ m² to ∼10⁷ m². Here we use measurements of ground temperature and atmospheric variables by Curiosity to calculate thermal inertias at Gale Crater at horizontal scales of ∼10² m². We analyze three sols representing distinct environmental conditions and soil properties, sol 82 at Rocknest (RCK), sol 112 at Point Lake (PL), and sol 139 at Yellowknife Bay (YKB). Our results indicate that the largest thermal inertia I = 452 J m^-2 K^-1 s^-1/2 (SI units used throughout this article) is found at YKB followed by PL with I = 306 and RCK with I = 295. These values are consistent with the expected thermal inertias for the types of terrain imaged by Mastcam and with previous satellite estimations at Gale Crater. We also calculate the SEB using data from measurements by Curiosity's Rover Environmental Monitoring Station and dust opacity values derived from measurements by Mastcam. The knowledge of the SEB and thermal inertia has the potential to enhance our understanding of the climate, the geology, and the habitability of Mars.