User:AmidstClouds/sandbox

= Subglacial lakes on Mars = Using radar measurements, salty subglacial lakes are claimed exist below the Mars’ southern ice cap, the South Polar Layered Deposits (SPLD). For the liquid water to persist, the researchers propose the presence of perchlorate in the water to lower the freezing temperature. Challenges for explaining sufficiently warm conditions for liquid water to exist below the southern ice cap include low amounts of geothermal heating from the subsurface and overlying pressure from the ice. As a result, it is disputed whether these radar detections were instead caused by other materials such as saline ice or mineral deposits such as hydrous minerals like clays. While lakes with salt concentrations 20 times that of the ocean pose challenges for life, potential subglacial lakes on Mars are of high interest for astrobiology because microbial ecosystems have been found in deep subglacial lakes on Earth, such as in Lake Whillans in Antarctica below 800 m of ice.

Features
A study from 2018 first reported radar observations of a potential 20-km wide subglacial lake at the base of the SPLD using data from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on the European Space Agency’s Mars Express spacecraft. The team interpreted the bright radar reflections to have high permittivity (the ability of a material to become polarized and store energy in response to an electric field), as consistent with liquid water. Three additional subglacial lakes on the km-wide scale next to the original lake were also proposed from a more detailed study, though the study also indicates the possibility that the three locations could contain wet sediment instead of lakes.

Though the SHAllow RADar (SHARAD) on the Mars Reconnaissance Orbiter operates at higher frequencies, they would expect to be able to see a subglacial lake and have not detected any of the bright radar reflectors. However, with the discovery of many widespread occurrences of the radar features in the SPLD area, corroboration between the two instruments might become possible.

Physical constraints: geothermal heating, perchlorate, and surface features
The studies in support of the subglacial lake hypothesis proposed that magnesium and calcium perchlorate at the base of the SPLD would lower the freezing point of water to temperatures as low as 204 and 198 K, thereby allowing the existence of briny liquid water. First detected by the Phoenix lander in the northern plains of Mars, perchlorate is a salt considered to be widespread on Mars and is known to lower the freezing point of water. However, even taking into account perchlorate, modeling predicts the temperature to still be too cold for liquid water to exist at the bottom of the southern ice cap due to the estimated low geothermal heat flux there of 14-30 mW/m2. For the subglacial lake to exist, their model requires a geothermal heat flux greater than 72 mW/m2, thus requiring a local enhancement in the heat flux, perhaps sourced by geologically recent (within hundreds of thousands of years ago) magmatism in the subsurface. Similarly, another study based on the surface topography and ice thickness found that the radar detection did not coincide with their predictions of locations for subglacial lakes based on hydrological potential, and as a result, they proposed the detection was due to a localized patch of basal melting rather than a lake.

Since magnesium and calcium perchlorate solutions can be supercooled to as low as 150 K and the surface temperature at the south pole is approximately 160 K, it is possible that the temperature at depth would be even higher than predicted by other studies based on the undetermined geothermal flux and thermal properties of the SPLD. However, a study that found high basal signatures to be widespread across the SPLD and determined it unlikely that the source of the bright reflectors was due to liquid water since the reflectors can reach close to the surface where the mean annual surface temperature would not be low enough to allow for super-cooled perchlorate brines to exist.

Additional approaches to determining the plausibility of the subglacial lakes included a study looking for related surface features. On Earth, examples of surface feature expressing a subglacial lake below can include fractures or ridge features like at Pine Island Glacier in Antarctica. While the features on the surface do not correspond to the putative subglacial lakes, the lack of surface features also do not rule out the possibility of the subglacial lake.

Alternative hypotheses
In contrast with the hypothesis of subglacial water at the base of the SPLD, recent work proposed other basal materials such as saline ice or a conductive mineral deposit such as clays. While the initial study assumed negligible conductivity in their calculation of the permittivity values, by accounting for permittivity, conductive materials that are not liquid water may also be considered. Instead of the assumption that the bright radar reflections at the base of the ice cap are due to a large contrast in dielectric permittivity, a more recent study suggested that the bright reflection is instead due to a large contrast in electric conductivity in the materials. Saline ice, observed on Earth beneath the Taylor Glacier in Antarctica, is one potential source for the bright basal reflections, though the electric conductivity of saline ice at martian temperatures is unknown.

The mineralogical explanation is the most favored in recent studies, especially with specific hydrous minerals such as jarosite and smectite. Smectites have high enough dielectric permittivity to account for the bright reflections (though at laboratory temperatures of 230 K higher than expected conditions on Mars), and they exist at the edges of the SPLD. Ultimately, although the studies propose these new hypotheses, they do not completely reject the possibility of liquid water as the source of the bright radar returns.

A recent study also applied forward modeling by looking for what other regions on Mars might cause similar bright basal reflectors if there was a 1.4-km thick ice shell covering the base material. They found that 0.3%-2% of the surface of Mars could produce similar signals, most of which belong to volcanic regions. While the permittivity of igneous materials requires more research, they pointed out how high density igneous content may also cause the observed bright radar reflectors.

Terrestrial analogue sites and habitability
The putative subglacial lakes are especially of interest for the possibility of supporting life. If physical conditions allowed one location of subglacial liquid water on Mars to exist, then this might extend to other subsurface biospheres also existing on the planet. On Earth, subglacial lakes exist below hundreds of meters of ice in both the Arctic and Antarctic and act as a planetary analog for both the potential subglacial lakes on Mars and liquid oceans below icy shells of moons like Europa. To study the life in subglacial lakes on Earth, ice core drilling is used to reach the water, though engineering issues threatened contamination, resulting in what are commonly considered to be failed attempts to sample from both Lake Vostok and Lake Ellsworth. However, Lake Whillans was a successful sampling endeavor from under 800 m of ice, where over 4000 species of chemoautotrophs have been identified. Whether similar microbes could survive in the putative salty subglacial lakes on Mars is still unknown, but if liquid water was present, it could preserve inactive microbial life.