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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.

Multiple nations and private entities are pushing to make landing humans on Mars a reality. The majority of proposed mission architectures envision “living off the land” by leveraging Martian water-ice deposits for fuel production and other purposes. Fortunately for mission designers, water ice exists on Mars in plentiful volumes. The challenge is isolating accessible ice deposits within regions that optimize other preferred landing-site conditions. Here, we present the first results of the Mars Subsurface Water Ice Mapping (SWIM) project, which has the aim of searching for buried ice resources across the mid-latitudes. Through the integration of orbital datasets in concert with new data-processing techniques, the SWIM project assesses the likelihood of ice by quantifying the consistency of multiple, independent data sources with the presence of ice. Concentrating our efforts across a significant portion of the northern Hemisphere, our composite ice consistency maps indicate that the broad plains of Arcadia and the extensive glacial networks across Deuteronilus Mensae match the greatest number of remote sensing criteria for accessible ice-rich, subsurface material situated equatorward of the contemporary ice-stability zone.

Martian landmarks on the radar The northern polar cap of Mars, containing enough water to cover the entire planet to a depth of several metres, features two major landforms that stand above all others. These are the enormous canyon, Chasma Boreale, and a series of spiral troughs. The processes leading to their formation have remained unclear. Now two papers in this issue present detailed histories of both systems. John Holt and colleagues use penetrating radar imagery from Mars Reconnaissance Orbiter's radar sounder to show that depositional processes — rather than a catastrophic event — formed the Chasma Boreale. Isaac Smith and John Holt use Mars Reconnaissance Orbiter data to rule out erosional cutting of polar ice as the cause of the central troughs, and instead conclude that they too are largely depositional, having migrated polewards and upwards in elevation in the past two million years. The Chasma Boreale is a large canyon — 500 km long, up to 100 km wide, and nearly 2 km deep — that cuts into the north polar layered deposits on Mars. Quite how it formed has been unclear. However, new penetrating radar imagery has now been used to show that depositional processes, rather than catastrophic events, were responsible. The polar layered deposits of Mars contain the planet’s largest known reservoir of water ice 1 , 2 and the prospect of revealing a detailed Martian palaeoclimate record 3 , 4 , but the mechanisms responsible for the formation of the dominant features of the north polar layered deposits (NPLD) are unclear, despite decades of debate. Stratigraphic analyses of the exposed portions of Chasma Boreale—a large canyon 500 km long, up to 100 km wide, and nearly 2 km deep—have led most researchers to favour an erosional process for its formation following initial NPLD accumulation. Candidate mechanisms include the catastrophic outburst of water 5 , protracted basal melting 6 , erosional undercutting 7 , aeolian downcutting 7 , 8 , 9 and a combination of these processes 10 . Here we use new data from the Mars Reconnaissance Orbiter to show that Chasma Boreale is instead a long-lived, complex feature resulting primarily from non-uniform accumulation of the NPLD. The initial valley that later became Chasma Boreale was matched by a second, equally large valley that was completely filled in by subsequent deposition, leaving no evidence on the surface to indicate its former presence. We further demonstrate that topography existing before the NPLD began accumulating influenced successive episodes of deposition and erosion, resulting in most of the present-day topography. Long-term and large-scale patterns of mass balance achieved through sedimentary processes, rather than catastrophic events, ice flow or highly focused erosion, have produced the largest geomorphic anomaly in the north polar ice of Mars.

The surface of Mars has been well mapped and characterized, yet the subsurface — the most likely place to find signs of extant or extinct life and a repository of useful resources for human exploration — remains unexplored. In the near future this is set to change.

ABSTRACTThe thermal inertia of Mars is a physical property that controls the diurnal and seasonal cycles in surface temperature. It is defined as a function of the thermal conductivity, heat capacity, and density, all of which depend primarily on the physical structure of the surface layer. As such, thermal inertia provides information about the nature of the surface of Mars and the types of materials from which it is composed. Interpreting thermal inertia can be complicated by the variety of structures and material properties that result in the same thermal inertia value. In general, variations in the thermal conductivity have the greatest influence on the thermal inertia. Factors such as soil grain size, cementing or induration, rock abundance, the presence of bedrock, and surface heterogeneity all play an important role. The physical processes that effect the thermal conductivity are discussed to provide a framework from which thermal inertia of the Martian surface may be better understood.Over the years, thermal inertia has been derived from numerous Earth-based and spacecraft temperature observations of Mars. In particular, thermal inertia from Viking, Mars Global Surveyor (MGS), and Mars Odyssey data has been derived and mapped with increasing spatial resolution, in each case providing an improved understanding of the surface layer. In addition, local-scale observations from the Mars Exploration Rovers (MERs) have provided in situ thermal inertia ground truth of characteristic soils and rocks. Overall, the surface of Mars is dominated by soils to a depth of a few centimeters or more.

ABSTRACTSurface characteristics at the five sites where spacecraft have successfully landed on Mars can be related favorably to their signatures in remotely sensed data from orbit and from the Earth. Comparisons of the rock abundance, types and coverage of soils (and their physical properties), thermal inertia, albedo, and topographic slope all agree with orbital remote-sensing estimates and show that the materials at the landing sites can be used as “ground truth” for the materials that make up most of the equatorial and mid-latitude regions of Mars. The five landing sites sample two of the three dominant global thermal inertia and albedo units that cover ∼ 80% of the surface of Mars. The Viking Landers 1 and 2, Spirit, and Mars Pathfinder landing sites are representative of the moderate-to-high thermal inertia and intermediate-to-high albedo unit that is dominated by crusty, cloddy, and blocky soils (duricrust) with various abundances of rocks and bright dust. The Opportunity landing site is representative of the moderate-to-high thermal inertia and low-albedo surface unit that is relatively dust-free and composed of dark eolian sand and/or increased abundance of rocks. Interpretation of radar data confirms the presence of load bearing, relatively dense surfaces controlled by the soil type at the landing sites, regional rock populations from diffuse scattering similar to those observed directly at the sites, and root-mean-squared (RMS) slopes that compare favorably with 100 m scale topographic slopes extrapolated from altimetry profiles and meter scale slopes from high-resolution stereo images.

Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO₂) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO₂ volatile release. If released into the atmosphere at times of high obliquity, the CO₂ reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

Radar measurements reveal a substantial buried deposit of carbon dioxide in the south pole of Mars. Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO 2 ) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO 2 volatile release. If released into the atmosphere at times of high obliquity, the CO 2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.