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Martian ice holds an important key to interpreting Mars' past climate, but much is still unknown regarding the distribution and properties of Mars' ice deposits. Previous surveys identified craters that contain "outlying" deposits of ice separate from, but nearby, the north and south polar layered deposits (NPLD and SPLD). There are many differences between the characteristics of the NPLD and SPLD, which may or may not be shared by these outlying deposits, and may provide clues to the climate conditions under which ice in the polar regions formed. Ground penetrating radar is one of the crucial datasets for understanding Martian ice, as it can probe the subsurface and place constraints on the properties of buried materials. We have analyzed 517 SHARAD radar tracks across 24 ice deposits housed within craters, including quantifying surface reflectivity and identifying the presence of subsurface reflectors. After examining the subsurface radar observations, we determined that the northern outlier deposits share many common characteristics with the NPLD, and thus may have been emplaced concurrently or at least under similar environmental conditions. The southern outlying crater deposits exhibit a variety of subsurface characteristics, and likely represent two or more populations of ice‐rich deposits that may have differing emplacement histories. Plain Language Summary: Mars has two large ice caps at its poles, which combined contain a similar volume of ice as Greenland on Earth. Near these large ice caps are craters that are also filled with ice, referred to as outlying crater ice deposits. These outlying crater ice deposits may or may not have formed at the same time as the polar caps. Like scientists on Earth use ice cores to analyze how our climate has changed over time, radar observations of ice on Mars can penetrate deep into the ice and act as digital ice cores, recording changes to the ice deposited over time. We use these radar observations to analyze the history of the outlying crater deposits and find that the northern deposits may have had a similar history to the north polar cap, while the southern deposits may have a more varied history. Key Points: We analyzed the radar surface and subsurface properties of outlying ice deposits in craters near the Martian polesSouthern outlying deposits likely represent two or more different depositional historiesNorthern outlying deposits likely record the same depositional history as the north polar layered deposit [ABSTRACT FROM AUTHOR]

It has recently been suggested that clay minerals, which are widespread on the Martian surface, could be the possible source of the basal bright reflections detected by MARSIS at Ultimi Scopuli, instead of briny water. This hypothesis is based on dielectric measurements on a wet Ca‐Montorillonite (STx‐1b) sample conducted at 230 K, which reported permittivity values (apparent permittivity of 39 at 4 MHz) compatible with the median value of 33 retrieved by MARSIS 4 MHz data inversion in the high reflectivity area. These experimental results are, however, incompatible with well‐established dielectric theory and with laboratory measurements on clays, at MARSIS frequency and Martian temperatures, reported in the literature. Here, we replicate the experiment using a setup to precisely control the rate of cooling/warming and the temperature inside and outside the clay sample. We found that the rate of cooling, the position of the temperature sensor and, consequently, the thermal equilibrium between the sample and the sensor play a fundamental role in the reliability of the measurements. Our results indicate that even for a large water content in the clay sample, at 230 K and 4 MHz, the apparent permittivity is only 8.4, dropping to 4.1 at 200 K, ruling out clays as a possible source of the bright reflections detected by MARSIS at the base of the SPLD. Plain Language Summary: The recent discovery of bright basal reflections below the South polar ice cap at Ultimi Scopuli (Mars) by MARSIS has sparked an intense debate on their origin. Because bright radar reflections detected below the ice sheets in Antarctica and Greenland usually indicate the presence of basal water, and based on additional data processing methods, these anomalies were interpreted to be caused by liquid briny water. Recent laboratory measurements questioned this hypothesis and instead suggested that the presence of clay could explain the observed results. According to these measurements, the dielectric properties of wet clay at 230 K (−43°C) mimic the high reflectivity of briny water. Here, we replicate the experiment, making sure to reach thermal equilibrium during each measurement run to guarantee that the temperature is correctly evaluated when the dielectric properties of the sample are measured. This was accomplished using a dedicated setup to precisely control the thermal cycle and to carefully check the internal and external temperatures of the wet clay sample. Our experimental results clearly show that the dielectric properties of clay at 230 K are incompatible with the observed MARSIS anomalies and thus briny water remains the most plausible explanation for the bright basal reflections detected by MARSIS. Key Points: Anomalous bright basal reflections have been detected by MARSIS at Ultimi Scopuli, MarsDielectric measurements of wet clays at low temperature show that they cannot generate strong basal reflectionsReliable measurements of dielectric behavior of planetary analogs require a careful control of the temperature inside the sample [ABSTRACT FROM AUTHOR]

Perennial water storage in firn aquifers has been observed within the lower percolation zone of the southeast Greenland ice sheet. Spatially distributed seismic and radar observations, made ~50 km upstream of the Helheim Glacier terminus, reveal spatial variations of seismic velocity within a firn aquifer. From 1.65 to 1.8 km elevation, shear‐wave velocity (Vs) is 1,290 ± 180 m/s in the unsaturated firn, decreasing below the water table (~15 m depth) to 1,130 ± 250 m/s. Below 1.65 km elevation, Vs in the saturated firn is 1,270 ± 220 m/s. The compressional‐to‐shear velocity ratio decreases in the downstream saturated zone, from 2.30 ± 0.54 to 2.01 ± 0.46, closer to its value for pure ice (2.00). Consistent with colocated firn cores, these results imply an increasing concentration of ice in the downstream sites, reducing the porosity and storage potential of the firn likely caused by episodic melt and freeze during the evolution of the aquifer. Plain Language Summary: An integrated geophysical analysis of seismic, radar, and borehole measurements has been completed over a firn aquifer in southeast Greenland. We show the stiffness of the aquifer increases at lower elevations, closer to sea level, which leads to a decrease in pore space for the meltwater to be stored. This corresponds to an increase in ice content within the firn at lower elevations, as observed in borehole measurements, and likely caused by the meltwater refreezing within and below the aquifer. Key Points: Characterizing the storage potential of firn aquifers is important for predicting water budgets in a warming worldMultiple geophysical methods are combined to evaluate the elastic properties and porosity of the firn aquiferSeismic velocities detect an increase in ice content in the aquifer downslope, reducing the porosity and storage potential of the firn [ABSTRACT FROM AUTHOR]

Greenland Ice Sheet surface melting has increased since the 1990s, affecting the rheology and scattering properties of the near‐surface firn. We combine firn cores and modeled firn densities with 7 years of CryoVEx airborne Ku‐band (13.5 GHz) radar profiles to quantify the impact of melting on microwave radar penetration in West Central Greenland. Although annual layers are present in the Ku‐band radar profiles to depths up to 15 m below the ice sheet surface, fluctuations in summer melting strongly affect the degree of radar penetration. The extreme melting in 2012, for example, caused an abrupt 6.2 ± 2.4 m decrease in Ku‐band radar penetration. Nevertheless, retracking the radar echoes mitigates this effect, producing surface heights that agree to within 13.9 cm of coincident airborne laser measurements. We also examine 2 years of Ka‐band (34.5 GHz) airborne radar data and show that the degree of penetration is half that of coincident Ku‐band. Plain Language Summary: Radar waves emitted by satellites can be used to measure changes in surface elevation of the Greenland Ice Sheet. However, they do not reflect off the ice sheet surface itself but penetrate into the snow to a depth of about 15 m for radar wavelengths of 2.3 cm. When the snow melts, meltwater can percolate into the snow or refreeze at the surface. Layers of refrozen ice sharply reduce the degree of radar penetration and may be mistaken for an elevation increase in radar measurements. Here, we combine firn cores and modeled firn densities with 7 years of airborne radar data collected during field campaigns in West Central Greenland to quantify this effect. We identify internal layers corresponding to annual stratigraphy within the snowpack, and we show that more melt means less radar penetration into the firn. The unprecedented surface melting which occurred across Greenland in 2012 caused a sharp reduction in the degree of radar penetration, from 11.5 to 5.3 m. However, if the effects of penetration are corrected for, radar altimeters can accurately measure the surface elevation of the ice sheet. Key Points: We use airborne radar, firn cores, and firn models to confirm that summer melting is the principal source of radar penetration fluctuationsThe largest fluctuations are recorded after the extreme melt event of 2012, which caused a 6.2 ± 2.4 m reduction in radar penetrationEcho retracking compensates for radar penetration fluctuations, leading to surface heights that agree with laser data to within 13.9 cm [ABSTRACT FROM AUTHOR]

Ice streams are bounded by abrupt transitions in speed called shear margins. Some shear margins are fixed by subglacial topography, but others are thought to be self‐organizing, evolving by thermal feedback to ice viscosity and basal drag which govern the stress balance of ice sheets. Resistive stresses (and properties governing shear‐margin formation) manifest nonuniquely at the surface, motivating the use of subsurface observations to constrain ice sheet models. In this study, we use ice‐penetrating radar data to evaluate three 3‐D thermomechanical models of the Northeast Greenland Ice Stream, focusing on model reproductions of ice temperature (a primary control on viscosity) and subsurface velocity. Data/model agreement indicates elevated temperatures in the Northeast Greenland Ice Stream margins, with depth‐averaged temperatures between 2 °C and 6 °C warmer in the southeast margin compared to ice in streaming flow, driven by vertical heat transport rather than shear heating. This work highlights complexity in ice divergence across stagnant/streaming transitions. Plain Language Summary: Ice sheet models used to project future sea level rise are calibrated using modern observations of ice flow at the ice sheet surface. However, the subsurface ice and rock properties that ultimately control the patterns of ice flow cannot be uniquely determined using observations of the surface alone. In this study, we use the structural and electromagnetic characteristics of the Greenland Ice Sheet (determined from ice‐penetrating radar data) to evaluate the subsurface performance of three different ice‐flow models of the Northeast Greenland Ice Stream. We show that fast flow in Northeast Greenland is, in part, controlled by softer, warmer ice, and that correctly modeling heat transport at the boundaries of ice streams is critical for realistic projections of their future behavior. Ultimately, we provide insight into a sensitive region of Greenland together with a new approach to geophysical data use in model evaluation, with the goal of reducing the range of plausible models projecting the future of the Greenland and Antarctic Ice Sheets. Key Points: Thermal softening of ice is present in the Northeast Greenland Ice Stream (NEGIS) shear margins, despite low strain ratesVertical advection of heat dominates the shear‐margin temperature structure here, validated by radar reflectivity and isochron geometryRadar data can be used to constrain ice temperature and subsurface velocity to evaluate ice sheet model spin‐up and inversions [ABSTRACT FROM AUTHOR]

Temporal variations in ice sheet flow directly impact the internal structure within ice sheets through englacial deformation. Large‐scale changes in the vertical stratigraphy within ice sheets have been previously conducted on centennial to millennial timescales; however, intra‐annual changes in the morphology of internal layers have yet to be explored. Over a period of 2 years, we use autonomous phase‐sensitive radio‐echo sounding to track the daily displacement of internal layers on Store Glacier, West Greenland, to millimeter accuracy. At a site located ∼30 km from the calving terminus, where the ice is ∼600 m thick and flows at ∼700 m/a, we measure distinct seasonal variations in vertical velocities and vertical strain rates over a 2‐year period. Prior to the melt season (March–June), we observe increasingly nonlinear englacial deformation with negative vertical strain rates (i.e., strain thinning) in the upper half of the ice column of approximately −0.03 a−1, whereas the ice below thickens under vertical strain reaching up to +0.16 a−1. Early in the melt season (June–July), vertical thinning gradually ceases as the glacier increasingly thickens. During late summer to midwinter (August–February), vertical thickening occurs linearly throughout the entire ice column, with strain rates averaging 0.016 a−1. We show that these complex variations are unrelated to topographic setting and localized basal slip and hypothesize that this seasonality is driven by far‐field perturbations in the glacier's force balance, in this case generated by variations in basal hydrology near the glacier's terminus and propagated tens of kilometers upstream through transient basal lubrication longitudinal coupling. Plain Language Summary: Ice sheets deform when subject to changes in its flow regime. Such deformation impacts the shape of and distance between internal layers within ice sheets. Through the development of a high‐precision stationary radar, we can measure the vertical compression or expansion between internal layers through time. We used this methodology at an inland location on Store Glacier in western Greenland to obtain a 2‐year‐long record of ice deformation. The records show that this location on Store Glacier stretches and compresses throughout the year, sometimes with both occurring at the same time on top of each other. Using satellite imagery, we discover that the deformation regime at our study site is linked to seasonal changes in the flow farther down the glacier nearer to the ocean. If this section slows down, then the ice will thicken farther up the glacier; similarly, faster flow at the terminus will stretch and thin the ice behind it. Key Points: Autonomous phase‐sensitive radar can be used to track the fine‐scale motion of glacier internal layersVertical velocity profiles of internal layers evolve seasonally in a region of fast flowChanges in the local strain field are driven by downstream far‐field longitudinal stress coupling [ABSTRACT FROM AUTHOR]