On Earth, solar radiation can transmit down to multiple metres within ice, depending on its optical properties. Organisms within ice can harness energy from photosynthetically active radiation while being protected from damaging ultraviolet radiation. On Mars, the lack of an effective ozone shield allows ~30% more damaging ultraviolet radiation to reach the surface in comparison with Earth. However, our radiative transfer modelling shows that despite the intense surface ultraviolet radiation, there are radiatively habitable zones within exposed mid-latitude ice on Mars, at depths ranging from a few centimetres for ice with 0.01–0.1% dust, and up to a few metres within cleaner ice. Numerical models predict that dense dusty snow in the martian mid-latitudes can melt below the surface at present. Thus, if small amounts of liquid water are available at these depths, mid-latitude ice exposures could represent the most easily accessible locations to search for extant life on Mars.

Mars Reconnaissance Orbiter (MRO), with its arrival in 2006 and nearly continuous operation since, has provided data for the study of martian polar processes spanning nine Mars years. Mars' polar deposits have long been thought to preserve records of past climates, potentially readable like terrestrial ice cores. However, unraveling millions of years of history in the ice depends on understanding Mars' current and recent-past climate, including the interactions of atmospheric and surface processes. MRO has allowed for revolutionary discoveries, long-term monitoring of ongoing processes, and multiple complementary datasets to address the question of how the polar ice deposits reflect climatological changes. In part, MRO has been able to do this from its variety of instrumentation simultaneously observing interannual changes in geomorphology of the surface in up to ~25 cm/pixel detail, repeatable processes and changes in the atmosphere, and surface composition, as well as investigating signs of past changes recorded in the icy polar layered deposits. In this paper, we summarize the contribution of MRO to our current understanding of Mars polar science, and in particular how MRO's long-duration mission has improved our understanding of the fundamental volatile cycles on Mars.

Turbulent fluxes of heat, momentum, and humidity in the atmospheric boundary layer are pivotal to the evolution of geology, weather and climate, and the possibility of life. Here we extend recent advances in calculating these near-surface turbulent fluxes in the Earth's atmospheric boundary layer to any terrestrial planetary body with an atmosphere. These improvements include: (a) incorporating Monin-Obukhov similarity functions that encompass the entire range of atmospheric stability expected on terrestrial planetary bodies, (b) accounting for the additional shear associated with buoyant plumes under unstable conditions, (c) using surface renewal theory to calculate transfer rates within the interfacial layer adjacent to the surface, and (d) explicitly accounting for key humidity effects that become especially important when a volatile is more buoyant than the ambient gas (e.g., on Mars where H₂O is lighter than CO₂). We tested and validated our model using in situ data collected on Earth, Mars, and Titan under a wide range of atmospheric stability, pressure, and surface roughness conditions. The model shows up to 71% better agreement with measurements compared to methods commonly used on Mars for evaporation/sublimation. Compared to previous estimates for H₂O ice on Mars, our model predicts up to 1.5–190x lower latent heat fluxes under stable atmospheric conditions (depending on the wind speed) and 1.78x higher latent heat fluxes under unstable conditions. Our results provide improved constraints on the stability of ice on Mars and will help determine whether ice can melt under present-day conditions.

HIGHLIGHTS

Our mapping area is 100x larger than previous studies of these regions.

91% of these ridge networks are confined to ancient Noachian terrain.

Most ridge networks are present on relatively flat areas.

Groundwater processes might be widespread across Noachian terrain.

Recent evidence of exposed H₂O ice on Mars suggests that this ice was deposited as dusty (<~1% dust) snow. This dusty snow is thought to have been deposited and subsequently buried over the last few million years. On Earth, freshly fallen snow metamorphoses with time into firn and, if deep enough, into glacier ice. While spectral measurements of martian ice have been made, no model of the spectral albedo of dusty martian firn or glacier ice exists at present. Accounting for dust and snow metamorphism is important because both factors reduce the albedo of snow and ice by large amounts. However, the dust content and physical properties of martian H₂O ice are poorly constrained. Here, we present a model of the spectral albedo of H₂O snow and ice on Mars, which is based on validated terrestrial models. We find that small amounts (<1%) of martian dust can lower the albedo of H₂O ice at visible wavelengths from ~1.0 to ~0.1. Additionally, our model indicates that dusty (>0.01% dust) firn and glacier ice have a lower albedo than pure dust, making them difficult to distinguish in visible or near-infrared images commonly used to detect H₂O ice on Mars. Observations of excess ice at the Phoenix landing site are matched by 350-μm snow grains with 0.015% dust, indicating that the snow has not yet metamorphosed into glacier ice. Our model results can be used to characterize orbital observations of martian H₂O ice and refine climate-model predictions of ice stability.

We expand on previous studies of the South Polar Layered Deposits' (SPLD) basal interface using data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) to obtain detailed maps of elevation, topography, and reflected radar power. Using these maps, we derive the thickness (ranging from 0 to 3.7 km) and volume of the SPLD (~1.60 x 10⁶ km³). While most basal interfaces reflect less power than the average SPLD surface, areas with basal echo power exceeding that of the surface are widespread throughout the SPLD, including at the location of potential subglacial water bodies in Ultimi Scopuli. The occurrence of these high basal echo power signatures appears to be largely frequency independent in MARSIS data. While the cause of the relatively high basal echo power values is uncertain, our observations suggest that this behavior is widespread, and not unique to Ultimi Scopuli.

KEY POINTS

CO₂ frost is detected at all latitudes, and near the equator it is largely restricted to low thermal inertia regions

Small amounts of H₂O frost can form at most latitudes during the night, but are difficult to detect with current instrumentation at Mars

47% of all gullies overlap with CO₂ frost detections but present-day activity appears to occur in loose, unconsolidated materials

Midlatitude slopes on Mars are mantled by deposits proposed to contain H₂O ice and dust, overlaid by a desiccated lag. However, direct evidence of their volatile content is lacking. Here we present novel evidence of light-toned materials within midlatitude gully alcoves eroded into these mantles. The appearance and Lambert albedo of these materials suggests that they are either dust or H₂O ice. We interpret them to be H₂O ice because it is unlikely for a short-term, localized dust deposit to form only within the mantle walls. The temperatures are generally too warm (>~240 K) for the ice to be a frost in equilibrium. Therefore, this ice is likely similar to the dusty ice documented within midlatitude scarps, but with more dust, and exposed in smaller patches by slumping. It has been proposed that CO₂ frosts remove the overlying lag, causing the exposed H₂O ice to sublimate, liberate dust within the ice for transport, and erode gullies in the mantle. But we observe gullies eroded in wall rock that continue into the mantle, implying that the same process erodes both substrates. H₂O ice melt can explain gullies eroded in the wall rock and the mantle. Numerical models show that relatively dense H₂O snow on Mars melts only when it contains small amounts of dust. The observed exposure of dusty ice provides a mechanism for it to melt under some conditions and form some gullies. Access to liquid water within this ice could provide potential abodes for any extant life.

Pulsed plasma thrusters (PPTs) typically utilize a solid propellant such as
Polytetrafluoroethylene (PTFE) to generate moderate specific impulse at the low-power scale. This work discusses the design and development of a multi-axis PPT module with a compact form factor of 0.9U and a maximum mass of ~1 kg, thereby minimizing system volume and mass to enable larger science payloads. The PPT has eight thrusters integrated into a single thruster module to allow for multi-axis control, a key feature not currently available in any COTS (commercial, off-the shelf) PPT. Copper-tungsten alloys and molybdenum, both of which have been shown to have relatively low electrode erosion rates, will be used for the PPT design’s electrodes. Analytical results have shown that the multi-axis PPT configuration enables fine pointing, which can allow for precise scientific measurements of targets in deep space, as well as the potential to enable formation flying missions. Additionally, the PPT can also be used for station keeping, deorbiting and reaction-wheel momentum dumping with a predicted minimum specific impulse of 1000s and a minimum impulse bit of 100 μN.