Water ice cliffs on Mars: Some locations on Mars are known to have water ice just below the surface, but how much has remained unclear. Dundaset al.used data from two orbiting spacecraft to examine eight locations where erosion has occurred. This revealed cliffs composed mostly of water ice, which is slowly sublimating as it is exposed to the atmosphere. The ice sheets extend from just below the surface to a depth of 100 meters or more and appear to contain distinct layers, which could preserve a record of Mars' past climate. They might even be a useful source of water for future human exploration of the red planet.Science, this issue p.199

Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low‐density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three‐layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m⁻³. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

Many discoveries of active surface processes on Mars have been made due to the availability of repeat high-resolution images from the High Resolution Imaging Science Experiment (HiRISE) onboard the Mars Reconnaissance Orbiter. HiRISE stereo images are used to make digital terrain models (DTMs) and orthorectified images (orthoimages). HiRISE DTMs and orthoimage time series have been crucial for advancing the study of active processes such as recurring slope lineae, dune migration, gully activity, and polar processes. We describe the process of making HiRISE DTMs, orthoimage time series, DTM mosaics, and the difference of DTMs, specifically using the ISIS/SOCET Set workflow. HiRISE DTMs are produced at a 1 and 2 m ground sample distance, with a corresponding estimated vertical precision of tens of cm and ∼1 m, respectively. To date, more than 6000 stereo pairs have been acquired by HiRISE and, of these, more than 800 DTMs and 2700 orthoimages have been produced and made available to the public via the Planetary Data System. The intended audiences of this paper are producers, as well as users, of HiRISE DTMs and orthoimages. We discuss the factors that determine the effective resolution, as well as the quality, precision, and accuracy of HiRISE DTMs, and provide examples of their use in time series analyses of active surface processes on Mars.

Transport of fine-grained dust is one of the most widespread sedimentary processes occurring on Mars today. In the present climate, eolian abrasion and deflation of rocks are likely the most pervasive and active dust-forming mechanism. Martian dust is globally enriched in S and Cl and has a distinct mean S:Cl ratio. Here we identify a potential source region for Martian dust based on analysis of elemental abundance data. We show that a large sedimentary unit called the Medusae Fossae Formation (MFF) has the highest abundance of S and Cl, and provides the best chemical match to surface measurements of Martian dust. Based on volume estimates of the eroded materials from the MFF, along with the enrichment of elemental S and Cl, and overall geochemical similarity, we propose that long-term deflation of the MFF has significantly contributed to the global Martian dust reservoir.

Long-lived hydrothermal systems are prime targets for astrobiological exploration on Mars. Unlike magmatic or impact settings, radiogenic hydrothermal systems can survive for >100 million years because of the Ga half-lives of key radioactive elements (e.g., U, Th, and K), but remain unknown on Mars. Here, we use geochemistry, gravity, topography data, and numerical models to find potential radiogenic hydrothermal systems on Mars. We show that the Eridania region, which once contained a vast inland sea, possibly exceeding the combined volume of all other Martian surface water, could have readily hosted a radiogenic hydrothermal system. Thus, radiogenic hydrothermalism in Eridania could have sustained clement conditions for life far longer than most other habitable sites on Mars. Water radiolysis by radiogenic heat could have produced H₂, a key electron donor for microbial life. Furthermore, hydrothermal circulation may help explain the region’s high crustal magnetic field and gravity anomaly.