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 × 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.
Introduction 1.1. Gullies on Mars Gullies are predominantly found within latitudes of ∼30°-50° in either martian hemisphere (Malin & Edgett, 2001), with most gullies in this latitude range occurring on pole-facing slopes (e.g., Harrison et al., 2015). These gullies typically consist of a broad upslope alcove that feeds into a channel, leading to a depositional apron of debris downslope. Numerous mechanisms for gully formation have been suggested: release of liquid water/brine from shallow (
Background 1.1. H 2 O Ice on Mars The presence of H 2 O ice and liquid water is key to the evolution of Martian geology, with implications for the potential for past or extant life, and the future of robotic and human exploration on Mars. H 2 O ice reservoirs hold key records of Mars' climate history, and understanding the state and evolution of subsurface ice can provide insight on past and present-day processes on Mars. The obliquity of Mars is thought to have oscillated over recent timescales (10 5-10 6 years), leading to the cycling of H 2 O ice from the poles to the midlatitudes (Head et al., 2003; Jakosky et al., 1995). The poles contain the largest known reservoirs (∼2.6 × 10 6 km 3) of ice (Byrne, 2009; Smith et al., 2001). The seasonal polar caps are composed mostly of CO 2 ice, and are present at the surface of each pole during the winter (Kieffer, 1979; Leighton & Murray, 1966). During the summer, the retreating seasonal caps reveal underlying perennial ice caps. The northern perennial ice cap is composed of H 2 O ice (Farmer et al., 1976; Kieffer et al., 1976), and is the principal source and sink of water vapor on Mars (Smith, 2004). The southern perennial cap consists
The presence, stability, and physical nature of H 2 O ice on Mars has major implications for understanding martian history, evolution, and future robotic and manned exploration. However, the pervasive presence of dust on Mars causes the ice to contain typically <∼1% dust, especially when exposed at the mid-latitudes (Dundas et al., 2018;. The ice is thought to have been deposited as snow during periods of high obliquity that occurred numerous times over the last few million years (Christensen, 2003;Jakosky & Carr, 1985;Madeleine et al., 2014). At visible wavelengths, dust is far more absorbing than H 2 O ice (Figure 1; Wolff et al., 2009), so small amounts of dust can lower the albedo at these wavelengths (e.g., Dozier et al., 2009;Painter et al., 2013). Lower albedos lead to enhanced radiative heating, which affects the ice's energy balance and its stability and evolution over time.Radiative heating is also enhanced by snow metamorphism. On Earth, fresh snow (with grain radius 50-100 μm) quickly metamorphoses due to vapor diffusion and and grain-boundary diffusion (Kaempfer & Schneebeli, 2007); surface snow grains can grow to radii of several hundred μm. The density also increases, in three stages by three different mechanisms (grain-boundary sliding, mechanical deformation, and bubble shrinkage): (a) In snow, densification proceeds by grain-boundary sliding, up to 550 kg/m 3 . This is the maximum density obtainable for snow at the surface. (b) In subsurface firn, density can increase by mechanical deformation up to 830 kg/m 3 ; at this density, the air becomes closed off into bubbles, becoming "glacier ice." (c) Overburden pressure in glacier ice causes the air bubbles to shrink, further increasing the density to approach that of pure ice, 917 kg/m 3 (Cuffey & Paterson, 2010). Density per se has no effect on the albedo of opaque media, but in snow and ice the density does often correlate with grain size, which is the dominant Abstract Recent evidence of exposed H 2 O 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 H 2 O ice are poorly constrained. Here, we present a model of the spectral albedo of H 2 O 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 H 2 O 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 ...
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