Sylvain Piqueux

Context . Thermal measurements provide key constraints on the physical properties of icy satellite surfaces. On Europa, previous analyses of the Galileo Photopolarimeter-Radiometer (PPR) dataset revealed heterogeneities in thermal inertia, but the limited spatial resolution prevented a detailed thermophysical characterization. Aims . We derive high-resolution maps of the Europa surface albedo and thermal inertia and infer the microphysical properties of its icy regolith from a reanalysis of the Galileo PPR dataset by discussing the processes controlling its thermophysical evolution. Methods . We used the KRC thermal model (K refers to the conductivity κ , R to the density ρ , and C to the specific heat C ) to analyze the PPR brightness temperatures and retrieve the albedo and thermal inertia. These values were then interpreted using conductivity models of porous ice to constrain the grain size and porosity. Results . We derived a mean Bond albedo of 0.64 ± 0.06 and a mean thermal inertia of 56 ± 17 J m ⁻² K ⁻¹ s ^−1/2 (1 σ ). The thermal inertia shows significant spatial variations, with a low-inertia equatorial band (39 ± 7) and higher values at mid-latitudes on the leading hemisphere (56 ± 11). The trailing-hemisphere equator also exhibits higher thermal inertia (63 ± 17), likely related to compositional differences. Conductivity models indicate a porous icy regolith with grain sizes ranging from a few micrometers to a few centimeters and an average porosity of 0.61 ± 0.1 in the upper centimeters of the Europa surface. Conclusions . The thermal inertia distribution shows little correlation with geological units. Its agreement with modeled magnetospheric ion fluxes instead suggests that sputtering-driven sintering plays a fundamental role in shaping the thermophysical properties of Europa. The absence of a high-inertia equatorial band analogous to the PacMan anomaly on the icy moons of Saturn indicates inefficient electron-driven sintering, while temperature-gradient metamorphism might enhance grain growth at depth. The modeled surface temperatures range between ∼67 and 148 K at mid to low latitudes, with peak daytime values counteracting radiolytic amorphization while limiting volatile stability.

Europa's double ridges often display lower albedo and redder color than their surroundings. Their unique topography may cause sublimation‐driven darkening due to illumination and self‐heating—the process of thermal segregation. We apply an advanced 3D thermophysical model, including shadowing and self‐heating through mutual exchange of radiation, to digital elevation models of double ridges at a range of latitudes and orientations. Results show that self‐heating in ridge troughs can markedly increase temperatures and sublimation rates, with a difference in maximum trough temperatures of up to 20 K, which may have implications for detection of endogenic heat. Incorporating a simple exosphere model and assuming an initial 10% concentration of 1 µm non‐ice particles, we find thermal segregation can produce reddening in the form of dark lag layers from the equator to the middle latitudes, but is generally negligible at 60° or higher. Lag formation timescales in ridge troughs are 10–100 years to produce an optically thick layer. Modeling suggests that low‐albedo lag layer formation provides positive feedback, further increasing surface heating. These effects may also darken Europa's surface in areas surrounding the ridges. However, the net mass balance controlling sublimation and lag formation is highly sensitive to the global water exosphere density: values molec/ produce reddening in the trough and ablation of of material, while values molec/ result in net deposition of . Model predictions of resulting low albedo material in double ridge troughs are provided, which can be tested with eventual data from Europa Clipper.

We investigate supersaturation in the Martian southern polar night using rederived vertical profiles from Mars Global Surveyor (MGS) radio occultation (RO) measurements. Supersaturation, which affects not only local condensation but also the polar and global atmospheric dynamics, occurs when temperatures fall below the saturation point. Conventional MGS‐RO retrievals suffer from uncertainties in atmospheric composition, as they do not account for depletion due to its condensation onto the seasonal polar ice caps, and from the use of fixed or empirically chosen uppermost temperatures. To address these limitations, we apply a revised retrieval scheme that incorporates a latitude‐ and season‐dependent estimate of the mixing ratio based on Mars Odyssey Gamma Ray Spectrometer argon data, along with a temperature climatology derived from Mars Climate Sounder (MCS) observations on board the Mars Reconnaissance Orbiter. We find that the original RO data underestimated supersaturation occurrence by 10%–20 % inside, and overestimated it by a similar fraction outside, the polar night. The updated profiles reveal that supersaturation commonly occurs up to 15 km altitude and S latitude, with seasonal and vertical variations peaking before the southern winter solstice. A comparison with MCS‐derived surface ice temperatures shows general consistency, although notable vertical gradients and near‐surface cold layers indicate complex boundary‐layer thermodynamics. These results demonstrate that reducing key assumptions in RO retrievals yields improved constraints on Martian polar processes, providing new insights into the spatial and temporal behavior of supersaturation during southern winter.

Corinto crater is a fresh, 13.9 km diameter impact crater in Elysium Planitia that produced one of the most extensive systems of thermally distinct rays and secondary craters on Mars, extending up to ∼2,000 km and covering a nearly 180° arc. It is the youngest crater of its size on Mars and impacted between 0.1 and 2.5 Ma into Early Amazonian basalt flows (∼1.7 Ga) that may have been water or ice rich at depth. The location of most rays and secondaries and the shape of the crater and the layered, continuous ejecta indicate it was produced by an oblique impact (20°–30°). Four facies of radial rays consisting of dense secondary craters, vary predominantly with distance from the crater. Thermal rays have low thermal inertia indicating surface materials dominated by fine sand. The total number of secondary craters >10 m diameter produced by Corinto is estimated to be ∼2 billion (∼10 ⁹ to 7 × 10 ¹⁰ ) from counts of the size‐frequency distribution of secondaries. Four martian meteorites (basaltic Shergottites) have ejection and crystallization ages that most closely match the impact and protolith ages of Corinto, but their crystallization ages are 4–5 times younger than the Elysium basalts that Corinto impacted. Differences in thermal rays and secondary crater morphology and size with distance from Corinto suggest that material properties and/or composition of the impacting material, properties of the surface material being impacted, and the impact velocity and size/mass of the ejected material, are all important in determining the nature of the rays.

The current climate of Mars is characterized by frequent dust storms, during which dust is raised from the surface and transported throughout the atmosphere via global circulation and wind patterns. The addition or removal of dust can cause the surface to brighten or darken significantly. This process not only modifies the appearance of the surface, but alters the thermal insulation properties due to the low thermal conductivity of dust. Of particular interest is the inter‐annual/seasonal pattern of dust redistribution and the spatial scales at which such process take place. While global climate models predict changes in surface dust coverage before and after global storms, their output concerning the amount and locations of transported dust differs significantly. Changes in both dust coverage and thickness are constrained by employing a numerical thermal model and analyzing surface temperature differences from overlapping Thermal Emission Imaging System‐Infrared images. A multi‐dimensional lookup table relating thermal inertia, surface temperature difference, and dust layer thickness is constructed to derive changes in surface dust redistribution at high spatial resolution for both inter‐annual/seasonal timescales. Modeled surface albedos are also derived to provide an additional constraint on surface dust redistribution. Upper and lower limits for derived changes in surface dust thickness are established from variations in surface temperature and modeled surface albedo respectively. Upon analyzing two distinct image pairs, we find that dust deposition/removal can occur at small spatial scales (i.e., 100s of m) and that changes in surface dust thickness can range from a few to hundreds of microns.

Characterizing the exchange of water between the Martian atmosphere and the (sub)surface is a major challenge for understanding the mechanisms that regulate the water cycle. Here we present a new data set of water ice detected on the Martian surface with the Thermal Emission Imaging System (THEMIS). The detection is based on the correlation between bright blue‐white patterns in visible images and a temperature measured in the infrared that is too warm to be associated with ice and interpreted instead as water ice. Using this method, we detect ice down to 21.4°S, 48.4°N, on pole‐facing slopes at mid‐latitudes, and on any surface orientation poleward of 45° latitude. Water ice observed with THEMIS is most likely seasonal rather than diurnal. Our data set is consistent with near‐infrared frost detections and predictions by the Mars Planetary Climate Model. Water frost average temperature is 170 K, and the maximum temperature measured is 243 K, lower than the water ice melting point. Melting of pure water ice on the surface is unlikely due to cooling by latent heat during its sublimation. However, 243 THEMIS images show frosts that are hot enough to form brines if salts are present on the surface. The water vapor pressure at the surface, calculated from the ice temperature, indicates a dry atmosphere in early spring, during the recession of the ice cap. The large amount of water vapor released by the sublimation of warm frost cannot stabilize subsurface ice at mid‐latitudes.

Surface thermal inertia derived from satellite imagery offers a valuable tool for remotely mapping the physical structure and water content of planetary regolith. Efforts to quantify thermal inertia using surface temperatures on Earth, however, have consistently yielded large uncertainties and suffered from a lack of reproducibility. Unlike dry or airless bodies, Earth's abundant water and dense atmosphere lead to dynamic thermophysical conditions that are a greater challenge to model than on a world like Mars. In this work, an approach was developed using field experiments to inform and fine‐tune a thermophysical model of terrestrial sediment and calculate an inherent thermal inertia value with higher precision and less initial knowledge of the sediment than has previously been achieved remotely on Earth. A thermal inertia derived for a basaltic tephra site in Northern Arizona was replicated within 1% between different field seasons, demonstrating reproducibility. Model‐derived values were validated in situ by two different thermophysical field probes to within 8% of the measured mean values. Analog studies such as this hold the promise of improved interpretations of surface materials on Mars, and an accurate thermal model for Earth is the key step to enabling translation between the two worlds.

The Kieffer model is a widely accepted explanation for seasonal modification of the Martian surface by CO₂ ice sublimation and the formation of a “zoo” of intriguing surface features. However, the lack of in situ observations and empirical laboratory measurements of Martian winter conditions hampers model validation and refinement. We present the first experiments to investigate all three main stages of the Kieffer model within a single experiment: (i) CO₂ condensation on a thick layer of Mars regolith simulant; (ii) sublimation of CO₂ ice and plume, spot, and halo formation; and (iii) the resultant formation of surface features. We find that the full Kieffer model is supported on the laboratory scale as (i) CO₂ diffuses into the regolith pore spaces and forms a thin overlying conformal layer of translucent ice. When a buried heater is activated, (ii) a plume and dark spot develop as dust is ejected with pressurized gas, and the falling dust creates a bright halo. During plume activity, (iii) thermal stress cracks form in a network similar in morphology to certain types of spiders, dendritic troughs, furrows, and patterned ground in the Martian high south polar latitudes. These cracks appear to form owing to sublimation of CO₂ within the substrate, instead of surface scouring. We discuss the potential for this process to be an alternative formation mechanism for “cracked” spider-like morphologies on Mars. Leveraging our laboratory observations, we also provide guidance for future laboratory or in situ investigations of the three stages of the Kieffer model.

During the first part of the Martian year (L_s = 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 (L_s) 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 (L_s = 100°–140°) than previously published data sets using water ice opacity retrievals (L_s = 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.

The InSight mission is a geophysical mission aimed at better understanding the structure of Mars and of the other rocky planets of the solar system. To do so, a lander accommodating two cameras, a very sensitive seismometer, and a dynamic self-penetrating heat probe nicknamed the mole were placed on the Mars surface by the Instrument Deployment Arm (IDA). Besides geophysical data (which definitely enriched the existing knowledge on the structure of Mars), the InSight instruments significantly increased the knowledge of the geological and geotechnical characteristics of the surface material at the InSight site. Small strain (elastic) parameters were derived from wave velocity measurements during the hammering sessions between the self-penetrating probe and the seismometer. A detailed observation of the soil profile along a depth of 37 cm was made possible thanks to the photos taken by the cameras, and to a detailed analysis of the mole penetration process. Further information was provided by an intense campaign of scraping and piling conducted by the IDA on the surface sand/dust layer. It was shown that the soil profile was composed of a surface 1 cm thick sand/dust layer, overlaying an around 20 cm thick loose duricrust made up of a cohesive matrix containing some pebbles, located above a 12 cm layer of sand overlaying a gravel/sand deposit. It is believed that the geology and soil mechanics data provided by the InSight mission will help for further robotic exploration of Mars.