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The Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) spacecraft landed successfully on Mars and imaged the surface to characterize the surficial geology. Here we report on the geology and subsurface structure of the landing site to aid in situ geophysical investigations. InSight landed in a degraded impact crater in Elysium Planitia on a smooth sandy, granule- and pebble-rich surface with few rocks. Superposed impact craters are common and eolian bedforms are sparse. During landing, pulsed retrorockets modified the surface to reveal a near surface stratigraphy of surficial dust, over thin unconsolidated sand, underlain by a variable thickness duricrust, with poorly sorted, unconsolidated sand with rocks beneath. Impact, eolian, and mass wasting processes have dominantly modified the surface. Surface observations are consistent with expectations made from remote sensing data prior to landing indicating a surface composed of an impact-fragmented regolith overlying basaltic lava flows.

A numerical model of heat conduction through particulate media made of spherical grains cemented by various bonding agents is presented. The pore‐filling gas conductivity, volume fraction, and thermal conductivity of the cementing phase are tunable parameters. Cement fractions <0.001–0.01% in volume have small effects on the soil bulk thermal conductivity. A significant conductivity increase (factor 3–8) is observed for bond fractions of 0.01 to 1% in volume. In the 1 to 15% bond fraction domain, the conductivity increases continuously but less intensely (25–100% conductivity increase compared to a 1% bond system). Beyond 15% of cements, the conductivity increases vigorously and the bulk conductivity rapidly approaches that of bedrock. The composition of the cements (i.e. conductivity) has little influence on the bulk thermal inertia of the soil, especially if the volume of bond <10%. These results indicate that temperature measurements are sufficient to detect cemented soils and quantify the amount of cementing phase, but the mineralogical nature of the bonds and the typical grain size are unlikely to be determined from orbit. On Mars, a widespread surface unit characterized by a medium albedo (0.19–0.26) and medium/high thermal inertia (200–600 J s−0.5 m−2 K−1) has long been hypothesized to be associated with a duricrust. The fraction of cement required to fit the thermal data is less than ∼1–5% by volume. This small amount of material is consistent with orbital observations, confirming that soil cementation is an important factor controlling the thermal inertia of the Martian surface.

We present a model of heat conduction for mono‐sized spherical particulate media under stagnant gases based on the kinetic theory of gases, numerical modeling of Fourier's law of heat conduction, theoretical constraints on the gas thermal conductivity at various Knudsen regimes, and laboratory measurements. Incorporating the effect of the temperature allows for the derivation of the pore‐filling gas conductivity and bulk thermal conductivity of samples using additional parameters (pressure, gas composition, grain size, and porosity). The radiative and solid‐to‐solid conductivities are also accounted for. Our thermal model reproduces the well‐established bulk thermal conductivity dependency of a sample with the grain size and pressure and also confirms laboratory measurements finding that higher porosities generally lead to lower conductivities. It predicts the existence of the plateau conductivity at high pressure, where the bulk conductivity does not depend on the grain size. The good agreement between the model predictions and published laboratory measurements under a variety of pressures, temperatures, gas compositions, and grain sizes provides additional confidence in our results. On Venus, Earth, and Titan, the pressure and temperature combinations are too high to observe a soil thermal conductivity dependency on the grain size, but each planet has a unique thermal inertia due to their different surface temperatures. On Mars, the temperature and pressure combination is ideal to observe the soil thermal conductivity dependency on the average grain size. Thermal conductivity models that do not take the temperature and the pore‐filling gas composition into account may yield significant errors.

Although not the prime focus of the InSight mission, the near-surface geology and physical properties investigations provide critical information for both placing the instruments (seismometer and heat flow probe with mole) on the surface and for understanding the nature of the shallow subsurface and its effect on recorded seismic waves. Two color cameras on the lander will obtain multiple stereo images of the surface and its interaction with the spacecraft. Images will be used to identify the geologic materials and features present, quantify their areal coverage, help determine the basic geologic evolution of the area, and provide ground truth for orbital remote sensing data. A radiometer will measure the hourly temperature of the surface in two spots, which will determine the thermal inertia of the surface materials present and their particle size and/or cohesion. Continuous measurements of wind speed and direction offer a unique opportunity to correlate dust devils and high winds with eolian changes imaged at the surface and to determine the threshold friction wind stress for grain motion on Mars. During the first two weeks after landing, these investigations will support the selection of instrument placement locations that are relatively smooth, flat, free of small rocks and load bearing. Soil mechanics parameters and elastic properties of near surface materials will be determined from mole penetration and thermal conductivity measurements from the surface to 3–5 m depth, the measurement of seismic waves during mole hammering, passive monitoring of seismic waves, and experiments with the arm and scoop of the lander (indentations, scraping and trenching). These investigations will determine and test the presence and mechanical properties of the expected 3–17 m thick fragmented regolith (and underlying fractured material) built up by impact and eolian processes on top of Hesperian lava flows and determine its seismic properties for the seismic investigation of Mars’ interior.

The heat flow and physical properties package measured soil thermal conductivity at the landing site in the 0.03–0.37 m depth range. Six measurements spanning solar longitudes from 8.0° to 210.0° were made and atmospheric pressure at the site was simultaneously measured using InSight's Pressure Sensor. We find that soil thermal conductivity strongly correlates with atmospheric pressure. This trend is compatible with predictions of the pressure dependence of thermal conductivity for unconsolidated soils under martian atmospheric conditions, indicating that heat transport through the pore filling gas is a major contributor to the total heat transport. Therefore, any cementation or induration of the soil sampled by the experiments must be minimal and soil surrounding the mole at depths below the duricrust is likely unconsolidated. Thermal conductivity data presented here are the first direct evidence that the atmosphere interacts with the top most meter of material on Mars. Plain Language Summary A soil's ability to transport heat is a fundamental parameter that holds information on quantities like soil bulk porosity, composition, grain size, and the state of cementation or induration. In the soil, heat is transported through grain‐to‐grain contacts as well as through the pore filling CO2 gas. The heat flow and physical properties package (HP3) of the InSight Mars mission measured soil thermal conductivity at the landing site repeatedly over the course of a martian year. As atmospheric pressure changes between seasons due to the redistribution of CO2 across the planet, we found that soil thermal conductivity also changes. Thermal conductivity increased for increased atmospheric pressure, a behavior typical for unconsolidated material. This implies that the amount of cement or induration of the sampled soil must be minimal. Key Points We measured thermal conductivity of the martian soil and found that its conductivity strongly correlates with atmospheric pressure We conclude that heat conduction through the pore‐filling gas is significant and that cementation of the soil must be minimal Our data show that the atmosphere directly interacts with the top most meter of material on Mars

The selection of the Discovery Program InSight landing site took over four years from initial identification of possible areas that met engineering constraints, to downselection via targeted data from orbiters (especially Mars Reconnaissance Orbiter (MRO) Context Camera (CTX) and High-Resolution Imaging Science Experiment (HiRISE) images), to selection and certification via sophisticated entry, descent and landing (EDL) simulations. Constraints on elevation ( ≤ − 2.5 km for sufficient atmosphere to slow the lander), latitude (initially 15°S–5°N and later 3°N–5°N for solar power and thermal management of the spacecraft), ellipse size (130 km by 27 km from ballistic entry and descent), and a load bearing surface without thick deposits of dust, severely limited acceptable areas to western Elysium Planitia. Within this area, 16 prospective ellipses were identified, which lie ∼600 km north of the Mars Science Laboratory (MSL) rover. Mapping of terrains in rapidly acquired CTX images identified especially benign smooth terrain and led to the downselection to four northern ellipses. Acquisition of nearly continuous HiRISE, additional Thermal Emission Imaging System (THEMIS), and High Resolution Stereo Camera (HRSC) images, along with radar data confirmed that ellipse E9 met all landing site constraints: with slopes <15° at 84 m and 2 m length scales for radar tracking and touchdown stability, low rock abundance (<10 %) to avoid impact and spacecraft tip over, instrument deployment constraints, which included identical slope and rock abundance constraints, a radar reflective and load bearing surface, and a fragmented regolith ∼5 m thick for full penetration of the heat flow probe. Unlike other Mars landers, science objectives did not directly influence landing site selection.

Geological investigations planned for the Europa Clipper mission will examine the formation, evolution, and expression of geomorphic structures found on the surface. Understanding geologic features, their formation, and any recent activity are key inputs in constraining Europa’s potential for habitability. In addition to providing information about the moon’s habitability, the geologic study of Europa is compelling in and of itself. Here we provide a high-level, cross-instrument, and cross-discipline overview of the geologic investigations planned within the Europa Clipper mission. Europa’s fascinating collection of ice-focused geology provides an unparalleled opportunity to investigate the dynamics of icy shells, ice-ocean exchange processes, and global-scale tectonic and tidal stresses. We present an overview of what is currently known about the geology of Europa, from global to local scales, highlighting outstanding issues and open questions, and detailing how the Europa Clipper mission will address them. We describe the mission’s strategy for searching for and characterizing current activity in the form of possible active plumes, thermal anomalies, evidence for surface changes, and extremely fresh surface exposures. The complementary and synergistic nature of the data sets from the various instruments and their integration will be key to significantly advancing our understanding of Europa’s geology.

During the first part of the Martian year (Ls = 50°–160°) a phenomenon occurs on Mars in the tropical and equatorial regions (30°N–10°S) known as the Aphelion Cloud Belt (ACB). During this time, there is prominent formation and diurnal variability of water ice clouds. Limited empirical attempts have been made to characterize the magnitude of radiative flux contributions from clouds to nighttime surface temperatures. In this work, we estimated the infrared (IR) flux contribution at ground level from the clouds by comparing surface temperature data from the Thermal Emission Spectrometer (TES) onboard Mars Global Surveyor (MGS) to calculated temperatures using the KRC numerical thermal model. We then generated a database of IR fluxes at the ground contributed by clouds spanning the entirety of the tropical and equatorial regions as a function of Solar Longitude (Ls) on Mars in one degree bins. We compared results with work presented elsewhere in the literature and found good agreement. We also found that temporal trends followed the general established range for the ACB but our analysis demonstrated the peak ACB values occurred at later times (Ls = 100°–140°) than previously published data sets using water ice opacity retrievals (Ls = 90°–110°). This database may be used in comparison to calculated Global Climate Model fluxes as well as a lookup tool for more precise estimation of surface and subsurface thermal environments in these regions. Plain Language Summary Previous work has shown that during the spring and summer seasons of the Martian northern hemisphere bands of water ice clouds form in the tropical and equatorial regions of Mars. These clouds tend to persist into the night and they have the capability of warming the surfaces below. Quantifying this warming is important for isolating the thermal and physical characteristics of Martian surface and subsurface materials. In the past, climate models have been used to understand the contribution of surface warming from these clouds but no attempt has been made to derive these contributions using data. In this study we used data from the Mars Global Surveyor Thermal Emission Spectrometer instrument, coupled with a thermal model, to derive the warming contributions from these clouds. We generated a database of these derived contributions which can be used to better describe the surface and subsurface of Mars in the equatorial and tropical regions as well as to be used as a comparative tool for climate models. Key Points We developed a methodology to derive radiative contributions of nighttime cloud fluxes on Mars using orbitally retrieved data and a thermal model Our results show general seasonal and spatial agreement with cloud opacity data sets published in the literature We found that these data sets may be used to describe surface thermophysical properties with better accuracy

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP 3 to measure the surface heat flow of the planet. The package uses temperature sensors that would have been brought to the target depth of 3–5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, by precessing about a point midway along its hull, it carved a 7 cm deep and 5–6 cm wide pit and reached a depth of initially 31 cm. The root cause of the failure – as was determined through an extensive, almost two years long campaign – was a lack of friction in an unexpectedly thick cohesive duricrust. During the campaign – described in detail in this paper – the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and by direct support by the latter. The mole tip finally reached a depth of about 37 cm, bringing the mole back-end 1–2 cm below the surface. It reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the mole hull. The penetration record of the mole was used to infer mechanical soil parameters such as the penetration resistance of the duricrust of 0.3–0.7 MPa and a penetration resistance of a deeper layer ( > 30 cm depth) of 4.9 ± 0.4 MPa . Using the mole’s thermal sensors, thermal conductivity and diffusivity were measured. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 2–15 kPa depending on the internal friction angle of the duricrust. Pushing the scoop with its blade into the surface and chopping off a piece of duricrust provided another estimate of the cohesion of 5.8 kPa. The hammerings of the mole were recorded by the seismometer SEIS and the signals were used to derive P-wave and S-wave velocities representative of the topmost tens of cm of the regolith. Together with the density provided by a thermal conductivity and diffusivity measurement using the mole’s thermal sensors, the elastic moduli were calculated from the seismic velocities. Using empirical correlations from terrestrial soil studies between the shear modulus and cohesion, the previous cohesion estimates were found to be consistent with the elastic moduli. The combined data were used to derive a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick unconsolidated layer of sand mixed with dust and above another 10 cm of unconsolidated sand. Underneath the latter, a layer more resistant to penetration and possibly containing debris from a small impact crater is inferred. The thermal conductivity increases from 14 mW/m K to 34 mW/m K through the 1 cm sand/dust layer, keeps the latter value in the duricrust and the sand layer underneath and then increases to 64 mW/m K in the sand/gravel layer below.

A massive CO 2 ice deposit overlies 1 part of Mars’s primarily H 2 O ice 2 – 4 south polar cap 5 . This deposit rivals the mass of Mars’s current, 96% CO 2 , atmosphere 6 . Its release could substantially alter Mars’s pressure and climate 1 . The deposit consists of alternating CO 2 and H 2 O ice layers to a depth of up to approximately 1 km (refs. 1 , 7 , 8 ). The top layer is an enigmatic 9 – 11 1–10 m covering of perennial surface CO 2 ice 12 called the residual south polar cap. Typical explanations of the layering invoke orbital cycles 1 , 7 . Up to now, models assumed that the H 2 O ice layers insulate and seal in the CO 2 , allowing it to survive high-obliquity periods 7 , 13 . However, these models do not quantitatively predict the deposit’s stratigraphy or explain the residual south polar cap’s existence. Here we present a model in which the deposit’s near-surface CO 2 can instead exchange with the atmosphere through permeable H 2 O ice layers. Using currently observed albedo 14 , 15 and emissivity 16 properties of the Martian polar CO 2 ice deposits, our model predicts that the present massive CO 2 ice deposit is a remnant of larger CO 2 ice deposits laid down during periods of decreasing obliquity that are ablated, liberating a residual lag layer of H 2 O ice, when obliquity increases. Fractions of previous CO 2 deposits remain as layers because the amplitudes of the obliquity maxima have been mostly decreasing during the past ~510 kyr (ref. 17 ). Our model simultaneously explains the observed massive CO 2 ice deposit stratigraphy, the residual south polar cap’s existence and the presence of a massive CO 2 ice deposit only in the south. We use our model to calculate Mars’s pressure history and determine that the massive CO 2 ice deposit is 510 kyr old. The long-term evolution and stratigraphy of the CO 2 ice residual southern polar cap of Mars can be explained by a model that includes the active coupling of near-surface CO 2 with the atmosphere through the permeable H 2 O ice layers.