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The square indicates color-coded results superimposed on the location of the reported subglacial lake at 193°E, 81°S.^[1] Blue represents the area with the brightest radar reflection. (Image credit: USGS Astrogeology Science Center, Arizona State University, INAF.)

Salty subglacial lakes are controversially inferred from radar measurements to exist below the South Polar Layered Deposits (SPLD) of Mars' southern ice cap.^[1]^[2]^[3] The idea of subglacial lakes due to basal melting at the polar ice caps on Mars was first hypothesized in 1987.^[4]^[5]^[6] For liquid water to persist below the SPLD, researchers propose that perchlorate is dissolved in the water, which lowers the freezing temperature,^[1]^[2] but other explanations such as saline ice or hydrous minerals have been offered. 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.^[7] As a result, it is disputed whether these radar detections were instead caused by other materials such as saline ice^[8] or deposits of minerals such as clays^[8]^[9]. While lakes with salt concentrations 20 times that of the ocean pose challenges for life,^[10] 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.^[11]^[12]^[13]^[14]^[15]

Features

A study from 2018 first reported radar observations of a potential 20-km wide subglacial lake centered at 193°E, 81°S at the base of the SPLD ^[1] 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 noticed radar echoes stronger than what ice or rock would reflect coming from 1.5 km below the surface at the base of the SPLD.^[16] They interpreted the bright radar reflections to indicate high permittivity (the ability of a material to become polarized and store energy in response to an electric field), consistent with liquid water.^[3] Three additional subglacial lakes on the km-wide scale next to the original lake were also proposed from a more detailed study^[10]^[17], though the study also indicates the possibility that the three locations could contain wet sediment instead of lakes^[2]^[18].

Though the SHAllow RADar (SHARAD) on the Mars Reconnaissance Orbiter operates at higher frequencies, a subglacial lake should be detectable but bright radar reflectors are absent.^[19] However, with the discovery of many widespread occurrences of the radar features in the SPLD area,^[20] corroboration between the two instruments might become possible.^[21]

Physical constraints

Geothermal heating and perchlorate

The radar evidence can be difficult to understand due to scattering effects of the layers in the SPLD on radar reflections (according to an eLetter by Hecht et al. replying to the original publication^[1] along with other sources^[22] ). As a result, recent efforts have been focused on explaining how the freezing temperature at the base of the SPLD might be lowered due to a combination of perchlorate salt and enhanced regional geothermal flux. Following the detection of perchlorate in the northern plains of Mars by the Phoenix lander,^[23] it was predicted that perchlorate could allow a brine of 1-3 meters deep to exist at the base of the northern ice cap of Mars.^[24] Perchlorate is a salt now considered to be widespread on Mars^[25] and is known to lower the freezing point of water. The recent 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.^[1]^[2] 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. This is due to a small amount of pressure melting (Mars' gravity is about a third of Earth's) that would only lower the melting point by 0.3-0.5 K and an estimated low geothermal heat flux of 14-30 mW/m².^[7] A geothermal heat flux greater than 72 mW/m² would support the subglacial lake, 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.^[7] 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.^[26]

Since magnesium and calcium perchlorate solutions can be supercooled to as low as 150 K^[2]^[27] and the surface temperature at the south pole is approximately 160 K^[1], 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.^[2] However, a study found high basal signatures to be widespread across the SPLD. Consequently, it is unlikely that the source of the bright reflectors was due to liquid water because the reflectors can reach close to the surface where the mean annual surface temperature would not be low enough for super-cooled perchlorate brines to exist.^[20]

Surface features

Additional approaches to determining the plausibility of the subglacial lakes included a study looking for related surface features.^[28] On Earth, examples of surface feature expressing a subglacial lake below can include fractures or ridge features like at Pine Island Glacier in Antarctica.^[29] While the study only found surface features that match CO₂ and aeolian processes and none corresponding to the putative subglacial lakes, the lack of surface features also do not rule out the possibility of the subglacial lake.^[30] This is because while the surface of the SPLD is expected to be at least thousands of years old and possibily millions of years old, it is hard to constrain when the putative subglacial lake would have modified the surface features.^[28]

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,^[8] a conductive mineral deposit such as clays^[8]^[9], and igneous materials.^[31] Future work is necessary to better constrain how these alternative hypotheses hold under Mars-like conditions using instruments like MARSIS.

Saline ice

While the initial study assumed negligible conductivity in their calculation of the permittivity values,^[1] by accounting for permittivity, conductive materials that are not liquid water may also be considered.^[21] 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^[1], a more recent study suggested that the bright reflection is instead due to a large contrast in electric conductivity in the materials.^[8] Saline ice, observed on Earth beneath the Taylor Glacier in Antarctica^[32], is one potential source for the bright basal reflections, though the electric conductivity of saline ice at martian temperatures is unknown.^[8]

Hydrous minerals

The mineralogical explanation is the most favored in recent studies, especially with specific hydrous minerals such as jarosite^[8] and smectite.^[33] 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.^[9] 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.^[8]^[9]^[20]

Igneous materials

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.^[31] They found that 0.3%-2% of the surface of Mars could produce similar signals, most of which belong to volcanic regions.^[31] 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.^[31]

Terrestrial analogue sites and habitability

The putative subglacial lakes are of interest for the possibility of supporting life.^[34] If physical conditions allowed one location of subglacial liquid water on Mars to exist, then this might extend to other subsurface biospheres on the planet.^[35] On Earth, subglacial lakes exist below hundreds of meters of ice in both the Arctic^[36] and Antarctic^[15]^[37] and act as a planetary analog for both the potential subglacial lakes on Mars and liquid oceans below icy shells of moons like Europa.^[36] To study the life in subglacial lakes on Earth, ice core drilling is used to reach the water, but contamination is commonly considered to have compromised attempts to sample from the water of both Lake Vostok and Lake Ellsworth.^[15] However, microbes have been sampled from the accretion ice (frozen lake water) of Lake Vostok.^[38] Also, Lake Whillans was a successful sampling endeavor from under 800 m of ice, where over 4000 species of chemoautotrophs have been identified.^[11]^[13]^[14] ^[15] Whether similar microbes could survive in the putative salty subglacial lakes on Mars is still unknown, but if liquid water is present, it could preserve inactive microbial life.^[22]^[39]^[40]

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Orosei, R.; Lauro, S. E.; Pettinelli, E.; Cicchetti, A.; Coradini, M.; Cosciotti, B.; Di Paolo, F.; Flamini, E.; Mattei, E.; Pajola, M.; Soldovieri, F. (2018-08-03). "Radar evidence of subglacial liquid water on Mars". Science. 361 (6401): 490–493. doi:10.1126/science.aar7268. ISSN 0036-8075.
^ ^a ^b ^c ^d ^e ^f Lauro, Sebastian Emanuel; Pettinelli, Elena; Caprarelli, Graziella; Guallini, Luca; Rossi, Angelo Pio; Mattei, Elisabetta; Cosciotti, Barbara; Cicchetti, Andrea; Soldovieri, Francesco; Cartacci, Marco; Di Paolo, Federico (January 2021). "Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data". Nature Astronomy. 5 (1): 63–70. doi:10.1038/s41550-020-1200-6. ISSN 2397-3366.
^ ^a ^b "Liquid water spied deep below polar ice cap on Mars". www.science.org. Retrieved 2022-05-30.
^ fischer (2018-07-27). "Evidence of Subsurface Martian Liquid Water Bolstered". Planetary Science Institute. Retrieved 2022-06-03.
^ Witze, Alexandra (2018-07-25). "There's water on Mars! Signs of buried lake tantalize scientists". Nature. 560 (7716): 13–14. doi:10.1038/d41586-018-05795-6.
^ Clifford, Stephen M. (1987). "Polar basal melting on Mars". Journal of Geophysical Research. 92 (B9): 9135. doi:10.1029/JB092iB09p09135. ISSN 0148-0227.
^ ^a ^b ^c Sori, Michael M.; Bramson, Ali M. (2019-02-16). "Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today". Geophysical Research Letters. 46 (3): 1222–1231. doi:10.1029/2018GL080985. ISSN 0094-8276.
^ ^a ^b ^c ^d ^e ^f ^g ^h Bierson, C. J.; Tulaczyk, S.; Courville, S. W.; Putzig, N. E. (2021-07-16). "Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals". Geophysical Research Letters. 48 (13). doi:10.1029/2021GL093880. ISSN 0094-8276.
^ ^a ^b ^c ^d Smith, I. B.; Lalich, D. E.; Rezza, C.; Horgan, B. H. N.; Whitten, J. L.; Nerozzi, S.; Holt, J. W. (August 2021). "A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice". Geophysical Research Letters. 48 (15). doi:10.1029/2021GL093618. ISSN 0094-8276.
^ ^a ^b O'Callaghan, Jonathan (2020-09-28). "Water on Mars: discovery of three buried lakes intrigues scientists". Nature. doi:10.1038/d41586-020-02751-1.
^ ^a ^b "Microbes Discovered in Subglacial Antarctic Lake May Hint at Life in Space". Animals. 2014-08-20. Retrieved 2022-05-31.
^ Magazine, Smithsonian; Thompson, Helen. "Thousands of Microbe Species Live in This Buried Antarctic Lake". Smithsonian Magazine. Retrieved 2022-05-30.
^ ^a ^b Christner, Brent C.; Priscu, John C.; Achberger, Amanda M.; Barbante, Carlo; Carter, Sasha P.; Christianson, Knut; Michaud, Alexander B.; Mikucki, Jill A.; Mitchell, Andrew C.; Skidmore, Mark L.; Vick-Majors, Trista J. (August 2014). "A microbial ecosystem beneath the West Antarctic ice sheet". Nature. 512 (7514): 310–313. doi:10.1038/nature13667. ISSN 1476-4687.
^ ^a ^b Mikucki, J. A.; Lee, P. A.; Ghosh, D.; Purcell, A. M.; Mitchell, A. C.; Mankoff, K. D.; Fisher, A. T.; Tulaczyk, S.; Carter, S.; Siegfried, M. R.; Fricker, H. A. (2016-01-28). "Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 374 (2059): 20140290. doi:10.1098/rsta.2014.0290.
^ ^a ^b ^c ^d Siegert, Martin J.; Priscu, John C.; Alekhina, Irina A.; Wadham, Jemma L.; Lyons, W. Berry (2016-01-28). "Antarctic subglacial lake exploration: first results and future plans". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 374 (2059): 20140466. doi:10.1098/rsta.2014.0466. PMC 4685969. PMID 26667917.
^ ^a ^b Kaufman, Marc (2018-07-25). "Large Reservoir of Liquid Water Found Deep Below the Surface of Mars". Astrobiology at NASA Life in the Universe.
^ Reid, Tim (2020-10-19). "New evidence for subglacial lakes on Mars". Nature Italy. doi:10.1038/d43978-020-00008-z.
^ "Mars may be hiding tantalizing lakes beneath its glaciers". Science. 2020-09-28. Retrieved 2022-05-31.
^ "Underground Lake Found on Mars? Get the Facts". Science. 2018-07-25. Retrieved 2022-05-31.
^ ^a ^b ^c Khuller, Aditya R.; Plaut, Jeffrey J. (2021-07-16). "Characteristics of the Basal Interface of the Martian South Polar Layered Deposits". Geophysical Research Letters. 48 (13). doi:10.1029/2021GL093631. ISSN 0094-8276.
^ ^a ^b ^c Schroeder, Dustin M.; Steinbrügge, Gregor (2021-09-27). "Alternatives to Liquid Water Beneath the South Polar Ice Cap of Mars". Geophysical Research Letters. 48 (19). doi:10.1029/2021gl095912. ISSN 0094-8276.
^ ^a ^b Chang, Kenneth; Overbye, Dennis (2018-07-25). "A Large Body of Water on Mars Is Detected, Raising the Potential for Alien Life". The New York Times. ISSN 0362-4331. Retrieved 2022-06-03.
^ Hecht, M. H.; Kounaves, S. P.; Quinn, R. C.; West, S. J.; Young, S. M. M.; Ming, D. W.; Catling, D. C.; Clark, B. C.; Boynton, W. V.; Hoffman, J.; DeFlores, L. P. (2009-07-03). "Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site". Science. 325 (5936): 64–67. doi:10.1126/science.1172466. ISSN 0036-8075.
^ Fisher, David A.; Hecht, Michael H.; Kounaves, Samuel P.; Catling, David C. (2010-02-16). "A perchlorate brine lubricated deformable bed facilitating flow of the north polar cap of Mars: Possible mechanism for water table recharging". Journal of Geophysical Research. 115: E00E12. doi:10.1029/2009JE003405. ISSN 0148-0227.
^ Clark, Benton C.; Kounaves, Samuel P. (October 2016). "Evidence for the distribution of perchlorates on Mars". International Journal of Astrobiology. 15 (4): 311–318. doi:10.1017/S1473550415000385. ISSN 1473-5504.
^ Arnold, N. S.; Conway, S. J.; Butcher, F. E. G.; Balme, M. R. (August 2019). "Modeled Subglacial Water Flow Routing Supports Localized Intrusive Heating as a Possible Cause of Basal Melting of Mars' South Polar Ice Cap". Journal of Geophysical Research: Planets. 124 (8): 2101–2116. doi:10.1029/2019JE006061. ISSN 2169-9097.
^ Toner, J. D.; Catling, D. C.; Light, B. (2014-05-01). "The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars". Icarus. 233: 36–47. doi:10.1016/j.icarus.2014.01.018. ISSN 0019-1035.
^ ^a ^b Landis, M. E.; Whitten, J. L. (2022-05-28). "Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars". Geophysical Research Letters. 49 (10). doi:10.1029/2022GL098724. ISSN 0094-8276.
^ Vaughan, David G.; Corr, Hugh F. J.; Bindschadler, Robert A.; Dutrieux, Pierre; Gudmundsson, G. Hilmar; Jenkins, Adrian; Newman, Thomas; Vornberger, Patricia; Wingham, Duncan J. (2012-08-03). "Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica". Journal of Geophysical Research: Earth Surface. 117 (F3): n/a–n/a. doi:10.1029/2012jf002360. ISSN 0148-0227.
^ Landis, Margaret; Whitten, Jennifer (2021). "Geologic Context of the Bright Marsis Reflector in Ultimi Scopuli, South Polar Layered Deposits, Mars". Geological Society of America Abstracts with Programs. Geological Society of America. doi:10.1130/abs/2021am-366811.
^ ^a ^b ^c ^d ^e Grima, C.; Mouginot, J.; Kofman, W.; Hérique, A.; Beck, P. (2022-01-24). "The Basal Detectability of an Ice‐Covered Mars by MARSIS". Geophysical Research Letters. 49 (2). doi:10.1029/2021gl096518. ISSN 0094-8276.
^ Montross, Scott; Skidmore, Mark; Christner, Brent; Samyn, Denis; Tison, Jean-Louis; Lorrain, Reginald; Doyle, Shawn; Fitzsimons, Sean (2014-01-02). "Debris-Rich Basal Ice as a Microbial Habitat, Taylor Glacier, Antarctica". Geomicrobiology Journal. 31 (1): 76–81. doi:10.1080/01490451.2013.811316. ISSN 0149-0451.
^ Choi, Charles (2021-07-29). "Mars' buried polar 'lakes' may just be frozen clay". Space.com. Retrieved 2022-05-31.
^ Rothery, David. "Mars: mounting evidence for subglacial lakes, but could they really host life?". The Conversation. Retrieved 2022-05-31.
^ Billings, Lee (July 25, 2018). "Deep within Mars, Liquid Water Offers Hope for Life". Scientific American. Retrieved 2022-05-31.
^ ^a ^b Rutishauser, Anja; Blankenship, Donald D.; Sharp, Martin; Skidmore, Mark L.; Greenbaum, Jamin S.; Grima, Cyril; Schroeder, Dustin M.; Dowdeswell, Julian A.; Young, Duncan A. (2018-04-06). "Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic". Science Advances. 4 (4): eaar4353. doi:10.1126/sciadv.aar4353. ISSN 2375-2548. PMC 5895444. PMID 29651462.
^ Wright, Andrew; Siegert, Martin (December 2012). "A fourth inventory of Antarctic subglacial lakes". Antarctic Science. 24 (6): 659–664. doi:10.1017/S095410201200048X. ISSN 0954-1020.
^ Bulat, Sergey A. (2016-01-28). "Microbiology of the subglacial Lake Vostok: first results of borehole-frozen lake water analysis and prospects for searching for lake inhabitants". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 374 (2059): 20140292. doi:10.1098/rsta.2014.0292.
^ Rothery, David. "Mars: mounting evidence for subglacial lakes, but could they really host life?". The Conversation. Retrieved 2022-05-31.
^ Hallsworth, John E. "What on Earth could live in a salt water lake on Mars? An expert explains". The Conversation. Retrieved 2022-05-31.

[:0-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Orosei, R.; Lauro, S. E.; Pettinelli, E.; Cicchetti, A.; Coradini, M.; Cosciotti, B.; Di Paolo, F.; Flamini, E.; Mattei, E.; Pajola, M.; Soldovieri, F. (2018-08-03). "Radar evidence of subglacial liquid water on Mars". Science. 361 (6401): 490–493. doi:10.1126/science.aar7268. ISSN 0036-8075.

[:2-2] ^ ^a ^b ^c ^d ^e ^f Lauro, Sebastian Emanuel; Pettinelli, Elena; Caprarelli, Graziella; Guallini, Luca; Rossi, Angelo Pio; Mattei, Elisabetta; Cosciotti, Barbara; Cicchetti, Andrea; Soldovieri, Francesco; Cartacci, Marco; Di Paolo, Federico (January 2021). "Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data". Nature Astronomy. 5 (1): 63–70. doi:10.1038/s41550-020-1200-6. ISSN 2397-3366.

[:3-3] "Liquid water spied deep below polar ice cap on Mars". www.science.org. Retrieved 2022-05-30.

[4] scher (2018-07-27). "Evidence of Subsurface Martian Liquid Water Bolstered". Planetary Science Institute. Retrieved 2022-06-03.

[5] Witze, Alexandra (2018-07-25). "There's water on Mars! Signs of buried lake tantalize scientists". Nature. 560 (7716): 13–14. doi:10.1038/d41586-018-05795-6.

[6] Clifford, Stephen M. (1987). "Polar basal melting on Mars". Journal of Geophysical Research. 92 (B9): 9135. doi:10.1029/JB092iB09p09135. ISSN 0148-0227.

[:4-7] Sori, Michael M.; Bramson, Ali M. (2019-02-16). "Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today". Geophysical Research Letters. 46 (3): 1222–1231. doi:10.1029/2018GL080985. ISSN 0094-8276.

[:1-8] ^ ^a ^b ^c ^d ^e ^f ^g ^h Bierson, C. J.; Tulaczyk, S.; Courville, S. W.; Putzig, N. E. (2021-07-16). "Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals". Geophysical Research Letters. 48 (13). doi:10.1029/2021GL093880. ISSN 0094-8276.

[:6-9] Smith, I. B.; Lalich, D. E.; Rezza, C.; Horgan, B. H. N.; Whitten, J. L.; Nerozzi, S.; Holt, J. W. (August 2021). "A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice". Geophysical Research Letters. 48 (15). doi:10.1029/2021GL093618. ISSN 0094-8276.

[:5-10] O'Callaghan, Jonathan (2020-09-28). "Water on Mars: discovery of three buried lakes intrigues scientists". Nature. doi:10.1038/d41586-020-02751-1.

[:10-11] "Microbes Discovered in Subglacial Antarctic Lake May Hint at Life in Space". Animals. 2014-08-20. Retrieved 2022-05-31.

[12] Magazine, Smithsonian; Thompson, Helen. "Thousands of Microbe Species Live in This Buried Antarctic Lake". Smithsonian Magazine. Retrieved 2022-05-30.

[:11-13] Christner, Brent C.; Priscu, John C.; Achberger, Amanda M.; Barbante, Carlo; Carter, Sasha P.; Christianson, Knut; Michaud, Alexander B.; Mikucki, Jill A.; Mitchell, Andrew C.; Skidmore, Mark L.; Vick-Majors, Trista J. (August 2014). "A microbial ecosystem beneath the West Antarctic ice sheet". Nature. 512 (7514): 310–313. doi:10.1038/nature13667. ISSN 1476-4687.

[:12-14] Mikucki, J. A.; Lee, P. A.; Ghosh, D.; Purcell, A. M.; Mitchell, A. C.; Mankoff, K. D.; Fisher, A. T.; Tulaczyk, S.; Carter, S.; Siegfried, M. R.; Fricker, H. A. (2016-01-28). "Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 374 (2059): 20140290. doi:10.1098/rsta.2014.0290.

[:13-15] Siegert, Martin J.; Priscu, John C.; Alekhina, Irina A.; Wadham, Jemma L.; Lyons, W. Berry (2016-01-28). "Antarctic subglacial lake exploration: first results and future plans". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 374 (2059): 20140466. doi:10.1098/rsta.2014.0466. PMC 4685969. PMID 26667917.

[:16-16] Kaufman, Marc (2018-07-25). "Large Reservoir of Liquid Water Found Deep Below the Surface of Mars". Astrobiology at NASA Life in the Universe.

[17] Reid, Tim (2020-10-19). "New evidence for subglacial lakes on Mars". Nature Italy. doi:10.1038/d43978-020-00008-z.

[18] "Mars may be hiding tantalizing lakes beneath its glaciers". Science. 2020-09-28. Retrieved 2022-05-31.

[19] "Underground Lake Found on Mars? Get the Facts". Science. 2018-07-25. Retrieved 2022-05-31.

[:7-20] Khuller, Aditya R.; Plaut, Jeffrey J. (2021-07-16). "Characteristics of the Basal Interface of the Martian South Polar Layered Deposits". Geophysical Research Letters. 48 (13). doi:10.1029/2021GL093631. ISSN 0094-8276.

[:14-21] Schroeder, Dustin M.; Steinbrügge, Gregor (2021-09-27). "Alternatives to Liquid Water Beneath the South Polar Ice Cap of Mars". Geophysical Research Letters. 48 (19). doi:10.1029/2021gl095912. ISSN 0094-8276.

[:17-22] Chang, Kenneth; Overbye, Dennis (2018-07-25). "A Large Body of Water on Mars Is Detected, Raising the Potential for Alien Life". The New York Times. ISSN 0362-4331. Retrieved 2022-06-03.

[23] Hecht, M. H.; Kounaves, S. P.; Quinn, R. C.; West, S. J.; Young, S. M. M.; Ming, D. W.; Catling, D. C.; Clark, B. C.; Boynton, W. V.; Hoffman, J.; DeFlores, L. P. (2009-07-03). "Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site". Science. 325 (5936): 64–67. doi:10.1126/science.1172466. ISSN 0036-8075.

[24] Fisher, David A.; Hecht, Michael H.; Kounaves, Samuel P.; Catling, David C. (2010-02-16). "A perchlorate brine lubricated deformable bed facilitating flow of the north polar cap of Mars: Possible mechanism for water table recharging". Journal of Geophysical Research. 115: E00E12. doi:10.1029/2009JE003405. ISSN 0148-0227.

[25] Clark, Benton C.; Kounaves, Samuel P. (October 2016). "Evidence for the distribution of perchlorates on Mars". International Journal of Astrobiology. 15 (4): 311–318. doi:10.1017/S1473550415000385. ISSN 1473-5504.

[26] Arnold, N. S.; Conway, S. J.; Butcher, F. E. G.; Balme, M. R. (August 2019). "Modeled Subglacial Water Flow Routing Supports Localized Intrusive Heating as a Possible Cause of Basal Melting of Mars' South Polar Ice Cap". Journal of Geophysical Research: Planets. 124 (8): 2101–2116. doi:10.1029/2019JE006061. ISSN 2169-9097.

[27] Toner, J. D.; Catling, D. C.; Light, B. (2014-05-01). "The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars". Icarus. 233: 36–47. doi:10.1016/j.icarus.2014.01.018. ISSN 0019-1035.

[:18-28] Landis, M. E.; Whitten, J. L. (2022-05-28). "Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars". Geophysical Research Letters. 49 (10). doi:10.1029/2022GL098724. ISSN 0094-8276.

[29] Vaughan, David G.; Corr, Hugh F. J.; Bindschadler, Robert A.; Dutrieux, Pierre; Gudmundsson, G. Hilmar; Jenkins, Adrian; Newman, Thomas; Vornberger, Patricia; Wingham, Duncan J. (2012-08-03). "Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica". Journal of Geophysical Research: Earth Surface. 117 (F3): n/a–n/a. doi:10.1029/2012jf002360. ISSN 0148-0227.

[:8-30] Landis, Margaret; Whitten, Jennifer (2021). "Geologic Context of the Bright Marsis Reflector in Ultimi Scopuli, South Polar Layered Deposits, Mars". Geological Society of America Abstracts with Programs. Geological Society of America. doi:10.1130/abs/2021am-366811.

[:9-31] Grima, C.; Mouginot, J.; Kofman, W.; Hérique, A.; Beck, P. (2022-01-24). "The Basal Detectability of an Ice‐Covered Mars by MARSIS". Geophysical Research Letters. 49 (2). doi:10.1029/2021gl096518. ISSN 0094-8276.

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@@ Line 1: / Line 1: @@
 [[File:Mars-SubglacialWater-SouthPoleRegion-20180725.jpg|thumb|The square indicates color-coded results superimposed on the location of the reported subglacial lake at 193°E, 81°S.<ref name=":0" /> Blue represents the area with the brightest radar reflection. (Image credit: USGS Astrogeology Science Center, Arizona State University, INAF.) ]]
-Salty subglacial lakes are controversially inferred from radar measurements to exist below the South Polar Layered Deposits (SPLD) of Mars' southern ice cap.<ref name=":0">{{Cite journal |last=Orosei |first=R. |last2=Lauro |first2=S. E. |last3=Pettinelli |first3=E. |last4=Cicchetti |first4=A. |last5=Coradini |first5=M. |last6=Cosciotti |first6=B. |last7=Di Paolo |first7=F. |last8=Flamini |first8=E. |last9=Mattei |first9=E. |last10=Pajola |first10=M. |last11=Soldovieri |first11=F. |date=2018-08-03 |title=Radar evidence of subglacial liquid water on Mars |url=https://www.science.org/doi/10.1126/science.aar7268 |journal=Science |language=en |volume=361 |issue=6401 |pages=490–493 |doi=10.1126/science.aar7268 |issn=0036-8075}}</ref><ref name=":2">{{Cite journal |last=Lauro |first=Sebastian Emanuel |last2=Pettinelli |first2=Elena |last3=Caprarelli |first3=Graziella |last4=Guallini |first4=Luca |last5=Rossi |first5=Angelo Pio |last6=Mattei |first6=Elisabetta |last7=Cosciotti |first7=Barbara |last8=Cicchetti |first8=Andrea |last9=Soldovieri |first9=Francesco |last10=Cartacci |first10=Marco |last11=Di Paolo |first11=Federico |date=2021-01 |title=Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data |url=https://www.nature.com/articles/s41550-020-1200-6 |journal=Nature Astronomy |language=en |volume=5 |issue=1 |pages=63–70 |doi=10.1038/s41550-020-1200-6 |issn=2397-3366}}</ref><ref name=":3">{{Cite web |title=Liquid water spied deep below polar ice cap on Mars |url=https://www.science.org/content/article/liquid-water-spied-deep-below-polar-ice-cap-mars |access-date=2022-05-30 |website=www.science.org |language=en}}</ref> The idea of subglacial lakes due to basal melting at the polar ice caps on Mars was first hypothesized in 1987.<ref>{{Cite web |last=fischer |date=2018-07-27 |title=Evidence of Subsurface Martian Liquid Water Bolstered |url=https://www.psi.edu/news/cliffordmars |access-date=2022-06-03 |website=Planetary Science Institute |language=en}}</ref><ref>{{Cite journal |last=Witze |first=Alexandra |date=2018-07-25 |title=There’s water on Mars! Signs of buried lake tantalize scientists |url=https://www.nature.com/articles/d41586-018-05795-6 |journal=Nature |language=en |volume=560 |issue=7716 |pages=13–14 |doi=10.1038/d41586-018-05795-6}}</ref><ref>{{Cite journal |last=Clifford |first=Stephen M. |date=1987 |title=Polar basal melting on Mars |url=http://doi.wiley.com/10.1029/JB092iB09p09135 |journal=Journal of Geophysical Research |language=en |volume=92 |issue=B9 |pages=9135 |doi=10.1029/JB092iB09p09135 |issn=0148-0227}}</ref> For liquid water to persist below the SPLD, researchers propose that perchlorate is dissolved in the water, which [[Freezing-point depression|lowers the freezing temperature]],<ref name=":0" /><ref name=":2" /> but other explanations such as saline ice or hydrous minerals have been offered. 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.<ref name=":4">{{Cite journal |last=Sori |first=Michael M. |last2=Bramson |first2=Ali M. |date=2019-02-16 |title=Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2018GL080985 |journal=Geophysical Research Letters |language=en |volume=46 |issue=3 |pages=1222–1231 |doi=10.1029/2018GL080985 |issn=0094-8276}}</ref> As a result, it is disputed whether these radar detections were instead caused by other materials such as saline ice<ref name=":1">{{Cite journal |last=Bierson |first=C. J. |last2=Tulaczyk |first2=S. |last3=Courville |first3=S. W. |last4=Putzig |first4=N. E. |date=2021-07-16 |title=Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals |url=https://onlinelibrary.wiley.com/doi/10.1029/2021GL093880 |journal=Geophysical Research Letters |language=en |volume=48 |issue=13 |doi=10.1029/2021GL093880 |issn=0094-8276}}</ref> or deposits of minerals such as clays<ref name=":1" /><ref name=":6">{{Cite journal |last=Smith |first=I. B. |last2=Lalich |first2=D. E. |last3=Rezza |first3=C. |last4=Horgan |first4=B. H. N. |last5=Whitten |first5=J. L. |last6=Nerozzi |first6=S. |last7=Holt |first7=J. W. |date=2021-08 |title=A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice |url=https://onlinelibrary.wiley.com/doi/10.1029/2021GL093618 |journal=Geophysical Research Letters |language=en |volume=48 |issue=15 |doi=10.1029/2021GL093618 |issn=0094-8276}}</ref>. While lakes with salt concentrations 20 times that of the ocean pose challenges for life,<ref name=":5">{{Cite journal |last=O'Callaghan |first=Jonathan |date=2020-09-28 |title=Water on Mars: discovery of three buried lakes intrigues scientists |url=https://www.nature.com/articles/d41586-020-02751-1 |journal=Nature |language=en |doi=10.1038/d41586-020-02751-1}}</ref> 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.<ref name=":10">{{Cite web |date=2014-08-20 |title=Microbes Discovered in Subglacial Antarctic Lake May Hint at Life in Space |url=https://www.nationalgeographic.com/animals/article/140820-antarctic-microbe-lake-astrobiology-science |access-date=2022-05-31 |website=Animals |language=en}}</ref><ref>{{Cite web |last=Magazine |first=Smithsonian |last2=Thompson |first2=Helen |title=Thousands of Microbe Species Live in This Buried Antarctic Lake |url=https://www.smithsonianmag.com/science-nature/thousands-microbe-species-live-buried-antarctic-lake-180952415/ |access-date=2022-05-30 |website=Smithsonian Magazine |language=en}}</ref><ref name=":11">{{Cite journal |last=Christner |first=Brent C. |last2=Priscu |first2=John C. |last3=Achberger |first3=Amanda M. |last4=Barbante |first4=Carlo |last5=Carter |first5=Sasha P. |last6=Christianson |first6=Knut |last7=Michaud |first7=Alexander B. |last8=Mikucki |first8=Jill A. |last9=Mitchell |first9=Andrew C. |last10=Skidmore |first10=Mark L. |last11=Vick-Majors |first11=Trista J. |date=2014-08 |title=A microbial ecosystem beneath the West Antarctic ice sheet |url=https://www.nature.com/articles/nature13667 |journal=Nature |language=en |volume=512 |issue=7514 |pages=310–313 |doi=10.1038/nature13667 |issn=1476-4687}}</ref><ref name=":12">{{Cite journal |last=Mikucki |first=J. A. |last2=Lee |first2=P. A. |last3=Ghosh |first3=D. |last4=Purcell |first4=A. M. |last5=Mitchell |first5=A. C. |last6=Mankoff |first6=K. D. |last7=Fisher |first7=A. T. |last8=Tulaczyk |first8=S. |last9=Carter |first9=S. |last10=Siegfried |first10=M. R. |last11=Fricker |first11=H. A. |date=2016-01-28 |title=Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0290 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140290 |doi=10.1098/rsta.2014.0290}}</ref><ref name=":13">{{Cite journal |last=Siegert |first=Martin J. |last2=Priscu |first2=John C. |last3=Alekhina |first3=Irina A. |last4=Wadham |first4=Jemma L. |last5=Lyons |first5=W. Berry |date=2016-01-28 |title=Antarctic subglacial lake exploration: first results and future plans |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0466 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140466 |doi=10.1098/rsta.2014.0466 |pmc=PMC4685969 |pmid=26667917}}</ref>
+Salty subglacial lakes are controversially inferred from radar measurements to exist below the South Polar Layered Deposits (SPLD) of Mars' southern ice cap.<ref name=":0">{{Cite journal |last=Orosei |first=R. |last2=Lauro |first2=S. E. |last3=Pettinelli |first3=E. |last4=Cicchetti |first4=A. |last5=Coradini |first5=M. |last6=Cosciotti |first6=B. |last7=Di Paolo |first7=F. |last8=Flamini |first8=E. |last9=Mattei |first9=E. |last10=Pajola |first10=M. |last11=Soldovieri |first11=F. |date=2018-08-03 |title=Radar evidence of subglacial liquid water on Mars |url=https://www.science.org/doi/10.1126/science.aar7268 |journal=Science |language=en |volume=361 |issue=6401 |pages=490–493 |doi=10.1126/science.aar7268 |issn=0036-8075}}</ref><ref name=":2">{{Cite journal |last=Lauro |first=Sebastian Emanuel |last2=Pettinelli |first2=Elena |last3=Caprarelli |first3=Graziella |last4=Guallini |first4=Luca |last5=Rossi |first5=Angelo Pio |last6=Mattei |first6=Elisabetta |last7=Cosciotti |first7=Barbara |last8=Cicchetti |first8=Andrea |last9=Soldovieri |first9=Francesco |last10=Cartacci |first10=Marco |last11=Di Paolo |first11=Federico |date=January 2021 |title=Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data |url=https://www.nature.com/articles/s41550-020-1200-6 |journal=Nature Astronomy |language=en |volume=5 |issue=1 |pages=63–70 |doi=10.1038/s41550-020-1200-6 |issn=2397-3366}}</ref><ref name=":3">{{Cite web |title=Liquid water spied deep below polar ice cap on Mars |url=https://www.science.org/content/article/liquid-water-spied-deep-below-polar-ice-cap-mars |access-date=2022-05-30 |website=www.science.org |language=en}}</ref> The idea of subglacial lakes due to basal melting at the polar ice caps on Mars was first hypothesized in 1987.<ref>{{Cite web |last=fischer |date=2018-07-27 |title=Evidence of Subsurface Martian Liquid Water Bolstered |url=https://www.psi.edu/news/cliffordmars |access-date=2022-06-03 |website=Planetary Science Institute |language=en}}</ref><ref>{{Cite journal |last=Witze |first=Alexandra |date=2018-07-25 |title=There’s water on Mars! Signs of buried lake tantalize scientists |url=https://www.nature.com/articles/d41586-018-05795-6 |journal=Nature |language=en |volume=560 |issue=7716 |pages=13–14 |doi=10.1038/d41586-018-05795-6}}</ref><ref>{{Cite journal |last=Clifford |first=Stephen M. |date=1987 |title=Polar basal melting on Mars |url=http://doi.wiley.com/10.1029/JB092iB09p09135 |journal=Journal of Geophysical Research |language=en |volume=92 |issue=B9 |pages=9135 |doi=10.1029/JB092iB09p09135 |issn=0148-0227}}</ref> For liquid water to persist below the SPLD, researchers propose that perchlorate is dissolved in the water, which [[Freezing-point depression|lowers the freezing temperature]],<ref name=":0" /><ref name=":2" /> but other explanations such as saline ice or hydrous minerals have been offered. 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.<ref name=":4">{{Cite journal |last=Sori |first=Michael M. |last2=Bramson |first2=Ali M. |date=2019-02-16 |title=Water on Mars, With a Grain of Salt: Local Heat Anomalies Are Required for Basal Melting of Ice at the South Pole Today |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2018GL080985 |journal=Geophysical Research Letters |language=en |volume=46 |issue=3 |pages=1222–1231 |doi=10.1029/2018GL080985 |issn=0094-8276}}</ref> As a result, it is disputed whether these radar detections were instead caused by other materials such as saline ice<ref name=":1">{{Cite journal |last=Bierson |first=C. J. |last2=Tulaczyk |first2=S. |last3=Courville |first3=S. W. |last4=Putzig |first4=N. E. |date=2021-07-16 |title=Strong MARSIS Radar Reflections From the Base of Martian South Polar Cap May Be Due to Conductive Ice or Minerals |url=https://onlinelibrary.wiley.com/doi/10.1029/2021GL093880 |journal=Geophysical Research Letters |language=en |volume=48 |issue=13 |doi=10.1029/2021GL093880 |issn=0094-8276}}</ref> or deposits of minerals such as clays<ref name=":1" /><ref name=":6">{{Cite journal |last=Smith |first=I. B. |last2=Lalich |first2=D. E. |last3=Rezza |first3=C. |last4=Horgan |first4=B. H. N. |last5=Whitten |first5=J. L. |last6=Nerozzi |first6=S. |last7=Holt |first7=J. W. |date=August 2021 |title=A Solid Interpretation of Bright Radar Reflectors Under the Mars South Polar Ice |url=https://onlinelibrary.wiley.com/doi/10.1029/2021GL093618 |journal=Geophysical Research Letters |language=en |volume=48 |issue=15 |doi=10.1029/2021GL093618 |issn=0094-8276}}</ref>. While lakes with salt concentrations 20 times that of the ocean pose challenges for life,<ref name=":5">{{Cite journal |last=O'Callaghan |first=Jonathan |date=2020-09-28 |title=Water on Mars: discovery of three buried lakes intrigues scientists |url=https://www.nature.com/articles/d41586-020-02751-1 |journal=Nature |language=en |doi=10.1038/d41586-020-02751-1}}</ref> 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.<ref name=":10">{{Cite web |date=2014-08-20 |title=Microbes Discovered in Subglacial Antarctic Lake May Hint at Life in Space |url=https://www.nationalgeographic.com/animals/article/140820-antarctic-microbe-lake-astrobiology-science |access-date=2022-05-31 |website=Animals |language=en}}</ref><ref>{{Cite web |last=Magazine |first=Smithsonian |last2=Thompson |first2=Helen |title=Thousands of Microbe Species Live in This Buried Antarctic Lake |url=https://www.smithsonianmag.com/science-nature/thousands-microbe-species-live-buried-antarctic-lake-180952415/ |access-date=2022-05-30 |website=Smithsonian Magazine |language=en}}</ref><ref name=":11">{{Cite journal |last=Christner |first=Brent C. |last2=Priscu |first2=John C. |last3=Achberger |first3=Amanda M. |last4=Barbante |first4=Carlo |last5=Carter |first5=Sasha P. |last6=Christianson |first6=Knut |last7=Michaud |first7=Alexander B. |last8=Mikucki |first8=Jill A. |last9=Mitchell |first9=Andrew C. |last10=Skidmore |first10=Mark L. |last11=Vick-Majors |first11=Trista J. |date=August 2014 |title=A microbial ecosystem beneath the West Antarctic ice sheet |url=https://www.nature.com/articles/nature13667 |journal=Nature |language=en |volume=512 |issue=7514 |pages=310–313 |doi=10.1038/nature13667 |issn=1476-4687}}</ref><ref name=":12">{{Cite journal |last=Mikucki |first=J. A. |last2=Lee |first2=P. A. |last3=Ghosh |first3=D. |last4=Purcell |first4=A. M. |last5=Mitchell |first5=A. C. |last6=Mankoff |first6=K. D. |last7=Fisher |first7=A. T. |last8=Tulaczyk |first8=S. |last9=Carter |first9=S. |last10=Siegfried |first10=M. R. |last11=Fricker |first11=H. A. |date=2016-01-28 |title=Subglacial Lake Whillans microbial biogeochemistry: a synthesis of current knowledge |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0290 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140290 |doi=10.1098/rsta.2014.0290}}</ref><ref name=":13">{{Cite journal |last=Siegert |first=Martin J. |last2=Priscu |first2=John C. |last3=Alekhina |first3=Irina A. |last4=Wadham |first4=Jemma L. |last5=Lyons |first5=W. Berry |date=2016-01-28 |title=Antarctic subglacial lake exploration: first results and future plans |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0466 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140466 |doi=10.1098/rsta.2014.0466 |pmc=4685969 |pmid=26667917}}</ref>
 == Features ==
@@ Line 11: / Line 11: @@
 === Geothermal heating and perchlorate ===
-The radar evidence can be difficult to understand due to scattering effects of the layers in the SPLD on radar reflections (according to an eLetter by Hecht et al. replying to the original publication<ref name=":0" /> along with other sources<ref name=":17">{{Cite news |last=Chang |first=Kenneth |last2=Overbye |first2=Dennis |date=2018-07-25 |title=A Large Body of Water on Mars Is Detected, Raising the Potential for Alien Life |language=en-US |work=The New York Times |url=https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |access-date=2022-06-03 |issn=0362-4331}}</ref> ). As a result, recent efforts have been focused on explaining how the freezing temperature at the base of the SPLD might be lowered due to a combination of perchlorate salt and enhanced regional geothermal flux. Following the detection of perchlorate in the northern plains of Mars by the Phoenix lander,<ref>{{Cite journal |last=Hecht |first=M. H. |last2=Kounaves |first2=S. P. |last3=Quinn |first3=R. C. |last4=West |first4=S. J. |last5=Young |first5=S. M. M. |last6=Ming |first6=D. W. |last7=Catling |first7=D. C. |last8=Clark |first8=B. C. |last9=Boynton |first9=W. V. |last10=Hoffman |first10=J. |last11=DeFlores |first11=L. P. |date=2009-07-03 |title=Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site |url=https://www.science.org/doi/10.1126/science.1172466 |journal=Science |language=en |volume=325 |issue=5936 |pages=64–67 |doi=10.1126/science.1172466 |issn=0036-8075}}</ref> it was predicted that perchlorate could allow a brine of 1-3 meters deep to exist at the base of the northern ice cap of Mars.<ref>{{Cite journal |last=Fisher |first=David A. |last2=Hecht |first2=Michael H. |last3=Kounaves |first3=Samuel P. |last4=Catling |first4=David C. |date=2010-02-16 |title=A perchlorate brine lubricated deformable bed facilitating flow of the north polar cap of Mars: Possible mechanism for water table recharging |url=http://doi.wiley.com/10.1029/2009JE003405 |journal=Journal of Geophysical Research |language=en |volume=115 |pages=E00E12 |doi=10.1029/2009JE003405 |issn=0148-0227}}</ref> Perchlorate is a salt now considered to be widespread on Mars<ref>{{Cite journal |last=Clark |first=Benton C. |last2=Kounaves |first2=Samuel P. |date=2016-10 |title=Evidence for the distribution of perchlorates on Mars |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/evidence-for-the-distribution-of-perchlorates-on-mars/0F07BB901D48C448CC6F62F6F1AE8301 |journal=International Journal of Astrobiology |language=en |volume=15 |issue=4 |pages=311–318 |doi=10.1017/S1473550415000385 |issn=1473-5504}}</ref> and is known to lower the freezing point of water. The recent 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.<ref name=":0" /><ref name=":2" />  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. This is due to a small amount of pressure melting (Mars' gravity is about a third of Earth's) that would only lower the melting point by 0.3-0.5 K and an estimated low geothermal heat flux of 14-30 mW/m<sup>2</sup>.<ref name=":4" /> A geothermal heat flux greater than 72 mW/m<sup>2</sup> would support the subglacial lake, 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.<ref name=":4" /> 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.<ref>{{Cite journal |last=Arnold |first=N. S. |last2=Conway |first2=S. J. |last3=Butcher |first3=F. E. G. |last4=Balme |first4=M. R. |date=2019-08 |title=Modeled Subglacial Water Flow Routing Supports Localized Intrusive Heating as a Possible Cause of Basal Melting of Mars' South Polar Ice Cap |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006061 |journal=Journal of Geophysical Research: Planets |language=en |volume=124 |issue=8 |pages=2101–2116 |doi=10.1029/2019JE006061 |issn=2169-9097}}</ref>
+The radar evidence can be difficult to understand due to scattering effects of the layers in the SPLD on radar reflections (according to an eLetter by Hecht et al. replying to the original publication<ref name=":0" /> along with other sources<ref name=":17">{{Cite news |last=Chang |first=Kenneth |last2=Overbye |first2=Dennis |date=2018-07-25 |title=A Large Body of Water on Mars Is Detected, Raising the Potential for Alien Life |language=en-US |work=The New York Times |url=https://www.nytimes.com/2018/07/25/science/mars-liquid-alien-life.html |access-date=2022-06-03 |issn=0362-4331}}</ref> ). As a result, recent efforts have been focused on explaining how the freezing temperature at the base of the SPLD might be lowered due to a combination of perchlorate salt and enhanced regional geothermal flux. Following the detection of perchlorate in the northern plains of Mars by the Phoenix lander,<ref>{{Cite journal |last=Hecht |first=M. H. |last2=Kounaves |first2=S. P. |last3=Quinn |first3=R. C. |last4=West |first4=S. J. |last5=Young |first5=S. M. M. |last6=Ming |first6=D. W. |last7=Catling |first7=D. C. |last8=Clark |first8=B. C. |last9=Boynton |first9=W. V. |last10=Hoffman |first10=J. |last11=DeFlores |first11=L. P. |date=2009-07-03 |title=Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site |url=https://www.science.org/doi/10.1126/science.1172466 |journal=Science |language=en |volume=325 |issue=5936 |pages=64–67 |doi=10.1126/science.1172466 |issn=0036-8075}}</ref> it was predicted that perchlorate could allow a brine of 1-3 meters deep to exist at the base of the northern ice cap of Mars.<ref>{{Cite journal |last=Fisher |first=David A. |last2=Hecht |first2=Michael H. |last3=Kounaves |first3=Samuel P. |last4=Catling |first4=David C. |date=2010-02-16 |title=A perchlorate brine lubricated deformable bed facilitating flow of the north polar cap of Mars: Possible mechanism for water table recharging |url=http://doi.wiley.com/10.1029/2009JE003405 |journal=Journal of Geophysical Research |language=en |volume=115 |pages=E00E12 |doi=10.1029/2009JE003405 |issn=0148-0227}}</ref> Perchlorate is a salt now considered to be widespread on Mars<ref>{{Cite journal |last=Clark |first=Benton C. |last2=Kounaves |first2=Samuel P. |date=October 2016 |title=Evidence for the distribution of perchlorates on Mars |url=https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/abs/evidence-for-the-distribution-of-perchlorates-on-mars/0F07BB901D48C448CC6F62F6F1AE8301 |journal=International Journal of Astrobiology |language=en |volume=15 |issue=4 |pages=311–318 |doi=10.1017/S1473550415000385 |issn=1473-5504}}</ref> and is known to lower the freezing point of water. The recent 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.<ref name=":0" /><ref name=":2" />  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. This is due to a small amount of pressure melting (Mars' gravity is about a third of Earth's) that would only lower the melting point by 0.3-0.5 K and an estimated low geothermal heat flux of 14-30 mW/m<sup>2</sup>.<ref name=":4" /> A geothermal heat flux greater than 72 mW/m<sup>2</sup> would support the subglacial lake, 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.<ref name=":4" /> 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.<ref>{{Cite journal |last=Arnold |first=N. S. |last2=Conway |first2=S. J. |last3=Butcher |first3=F. E. G. |last4=Balme |first4=M. R. |date=August 2019 |title=Modeled Subglacial Water Flow Routing Supports Localized Intrusive Heating as a Possible Cause of Basal Melting of Mars' South Polar Ice Cap |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2019JE006061 |journal=Journal of Geophysical Research: Planets |language=en |volume=124 |issue=8 |pages=2101–2116 |doi=10.1029/2019JE006061 |issn=2169-9097}}</ref>
 Since magnesium and calcium perchlorate solutions can be supercooled to as low as 150 K<ref name=":2" /><ref>{{Cite journal |last=Toner |first=J. D. |last2=Catling |first2=D. C. |last3=Light |first3=B. |date=2014-05-01 |title=The formation of supercooled brines, viscous liquids, and low-temperature perchlorate glasses in aqueous solutions relevant to Mars |url=https://www.sciencedirect.com/science/article/pii/S0019103514000499 |journal=Icarus |language=en |volume=233 |pages=36–47 |doi=10.1016/j.icarus.2014.01.018 |issn=0019-1035}}</ref> and the surface temperature at the south pole is approximately 160 K<ref name=":0" />, 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.<ref name=":2" /> However, a study found high basal signatures to be widespread across the SPLD. Consequently, it is unlikely that the source of the bright reflectors was due to liquid water because the reflectors can reach close to the surface where the mean annual surface temperature would not be low enough for super-cooled perchlorate brines to exist.<ref name=":7" />
 === Surface features ===
-Additional approaches to determining the plausibility of the subglacial lakes included a study looking for related surface features.<ref name=":18">{{Cite journal |last=Landis |first=M. E. |last2=Whitten |first2=J. L. |date=2022-05-28 |title=Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars |url=https://onlinelibrary.wiley.com/doi/10.1029/2022GL098724 |journal=Geophysical Research Letters |language=en |volume=49 |issue=10 |doi=10.1029/2022GL098724 |issn=0094-8276}}</ref> On Earth, examples of surface feature expressing a subglacial lake below can include fractures or ridge features like at [[Pine Island Glacier]] in Antarctica.<ref>{{Cite journal |last=Vaughan |first=David G. |last2=Corr |first2=Hugh F. J. |last3=Bindschadler |first3=Robert A. |last4=Dutrieux |first4=Pierre |last5=Gudmundsson |first5=G. Hilmar |last6=Jenkins |first6=Adrian |last7=Newman |first7=Thomas |last8=Vornberger |first8=Patricia |last9=Wingham |first9=Duncan J. |date=2012-08-03 |title=Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica |url=http://dx.doi.org/10.1029/2012jf002360 |journal=Journal of Geophysical Research: Earth Surface |volume=117 |issue=F3 |pages=n/a–n/a |doi=10.1029/2012jf002360 |issn=0148-0227}}</ref> While the study only found surface features that match CO<sub>2</sub> and aeolian processes and none corresponding to the putative subglacial lakes, the lack of surface features also do not rule out the possibility of the subglacial lake.<ref name=":8">{{Cite journal |last=Landis |first=Margaret |last2=Whitten |first2=Jennifer |date=2021 |title=GEOLOGIC CONTEXT OF THE BRIGHT MARSIS REFLECTOR IN ULTIMI SCOPULI, SOUTH POLAR LAYERED DEPOSITS, MARS |url=http://dx.doi.org/10.1130/abs/2021am-366811 |journal=Geological Society of America Abstracts with Programs |publisher=Geological Society of America |doi=10.1130/abs/2021am-366811}}</ref> This is because while the surface of the SPLD is expected to be at least thousands of years old and possibily millions of years old, it is hard to constrain when the putative subglacial lake would have modified the surface features.<ref name=":18" />
+Additional approaches to determining the plausibility of the subglacial lakes included a study looking for related surface features.<ref name=":18">{{Cite journal |last=Landis |first=M. E. |last2=Whitten |first2=J. L. |date=2022-05-28 |title=Geologic Context of the Bright MARSIS Reflectors in Ultimi Scopuli, South Polar Layered Deposits, Mars |url=https://onlinelibrary.wiley.com/doi/10.1029/2022GL098724 |journal=Geophysical Research Letters |language=en |volume=49 |issue=10 |doi=10.1029/2022GL098724 |issn=0094-8276}}</ref> On Earth, examples of surface feature expressing a subglacial lake below can include fractures or ridge features like at [[Pine Island Glacier]] in Antarctica.<ref>{{Cite journal |last=Vaughan |first=David G. |last2=Corr |first2=Hugh F. J. |last3=Bindschadler |first3=Robert A. |last4=Dutrieux |first4=Pierre |last5=Gudmundsson |first5=G. Hilmar |last6=Jenkins |first6=Adrian |last7=Newman |first7=Thomas |last8=Vornberger |first8=Patricia |last9=Wingham |first9=Duncan J. |date=2012-08-03 |title=Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica |url=http://dx.doi.org/10.1029/2012jf002360 |journal=Journal of Geophysical Research: Earth Surface |volume=117 |issue=F3 |pages=n/a–n/a |doi=10.1029/2012jf002360 |issn=0148-0227}}</ref> While the study only found surface features that match CO<sub>2</sub> and aeolian processes and none corresponding to the putative subglacial lakes, the lack of surface features also do not rule out the possibility of the subglacial lake.<ref name=":8">{{Cite journal |last=Landis |first=Margaret |last2=Whitten |first2=Jennifer |date=2021 |title=Geologic Context of the Bright Marsis Reflector in Ultimi Scopuli, South Polar Layered Deposits, Mars |url=http://dx.doi.org/10.1130/abs/2021am-366811 |journal=Geological Society of America Abstracts with Programs |publisher=Geological Society of America |doi=10.1130/abs/2021am-366811}}</ref> This is because while the surface of the SPLD is expected to be at least thousands of years old and possibily millions of years old, it is hard to constrain when the putative subglacial lake would have modified the surface features.<ref name=":18" />
 == Alternative hypotheses ==
@@ Line 36: / Line 36: @@
 == Terrestrial analogue sites and habitability ==
 [[File:Lake Vostok.jpg|thumb|347x347px|Lake Vostok and other subglacial rivers and lakes in Antarctica.<ref name=":16" /> (Image credit: NASA.)]]
-The putative subglacial lakes are of interest for the possibility of supporting life.<ref>{{Cite web |last=Rothery |first=David |title=Mars: mounting evidence for subglacial lakes, but could they really host life? |url=http://theconversation.com/mars-mounting-evidence-for-subglacial-lakes-but-could-they-really-host-life-146732 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref> If physical conditions allowed one location of subglacial liquid water on Mars to exist, then this might extend to other subsurface biospheres on the planet.<ref>{{Cite web |last=Billings |first=Lee |date=July 25, 2018 |title=Deep within Mars, Liquid Water Offers Hope for Life |url=https://www.scientificamerican.com/article/deep-within-mars-liquid-water-offers-hope-for-life/ |access-date=2022-05-31 |website=Scientific American}}</ref> On Earth, subglacial lakes exist below hundreds of meters of ice in both the Arctic<ref name=":15">{{Cite journal |last=Rutishauser |first=Anja |last2=Blankenship |first2=Donald D. |last3=Sharp |first3=Martin |last4=Skidmore |first4=Mark L. |last5=Greenbaum |first5=Jamin S. |last6=Grima |first6=Cyril |last7=Schroeder |first7=Dustin M. |last8=Dowdeswell |first8=Julian A. |last9=Young |first9=Duncan A. |date=2018-04-06 |title=Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic |url=https://www.science.org/doi/10.1126/sciadv.aar4353 |journal=Science Advances |language=en |volume=4 |issue=4 |pages=eaar4353 |doi=10.1126/sciadv.aar4353 |issn=2375-2548 |pmc=PMC5895444 |pmid=29651462}}</ref> and Antarctic<ref name=":13" /><ref>{{Cite journal |last=Wright |first=Andrew |last2=Siegert |first2=Martin |date=2012-12 |title=A fourth inventory of Antarctic subglacial lakes |url=https://www.cambridge.org/core/journals/antarctic-science/article/abs/fourth-inventory-of-antarctic-subglacial-lakes/81B35C31B0DFCE1B3A0705B779D3AF58 |journal=Antarctic Science |language=en |volume=24 |issue=6 |pages=659–664 |doi=10.1017/S095410201200048X |issn=0954-1020}}</ref> and act as a planetary analog for both the potential subglacial lakes on Mars and liquid oceans below icy shells of moons like [[Europa (moon)|Europa]].<ref name=":15" /> To study the life in subglacial lakes on Earth, ice core drilling is used to reach the water, but contamination is commonly considered to have compromised attempts to sample from the water of both [[Lake Vostok]] and [[Lake Ellsworth (Antarctica)|Lake Ellsworth]].<ref name=":13" /> However, microbes have been sampled from the accretion ice (frozen lake water) of Lake Vostok.<ref>{{Cite journal |last=Bulat |first=Sergey A. |date=2016-01-28 |title=Microbiology of the subglacial Lake Vostok: first results of borehole-frozen lake water analysis and prospects for searching for lake inhabitants |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0292 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140292 |doi=10.1098/rsta.2014.0292}}</ref> Also, [[Lake Whillans]] was a successful sampling endeavor from under 800 m of ice, where over 4000 species of [[Chemotroph|chemoautotrophs]] have been identified.<ref name=":10" /><ref name=":11" /><ref name=":12" /> <ref name=":13" /> Whether similar microbes could survive in the putative salty subglacial lakes on Mars is still unknown, but if liquid water is present, it could preserve inactive microbial life.<ref name=":17" /><ref>{{Cite web |last=Rothery |first=David |title=Mars: mounting evidence for subglacial lakes, but could they really host life? |url=http://theconversation.com/mars-mounting-evidence-for-subglacial-lakes-but-could-they-really-host-life-146732 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref><ref>{{Cite web |last=Hallsworth |first=John E. |title=What on Earth could live in a salt water lake on Mars? An expert explains |url=http://theconversation.com/what-on-earth-could-live-in-a-salt-water-lake-on-mars-an-expert-explains-101148 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref>
+The putative subglacial lakes are of interest for the possibility of supporting life.<ref>{{Cite web |last=Rothery |first=David |title=Mars: mounting evidence for subglacial lakes, but could they really host life? |url=http://theconversation.com/mars-mounting-evidence-for-subglacial-lakes-but-could-they-really-host-life-146732 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref> If physical conditions allowed one location of subglacial liquid water on Mars to exist, then this might extend to other subsurface biospheres on the planet.<ref>{{Cite web |last=Billings |first=Lee |date=July 25, 2018 |title=Deep within Mars, Liquid Water Offers Hope for Life |url=https://www.scientificamerican.com/article/deep-within-mars-liquid-water-offers-hope-for-life/ |access-date=2022-05-31 |website=Scientific American}}</ref> On Earth, subglacial lakes exist below hundreds of meters of ice in both the Arctic<ref name=":15">{{Cite journal |last=Rutishauser |first=Anja |last2=Blankenship |first2=Donald D. |last3=Sharp |first3=Martin |last4=Skidmore |first4=Mark L. |last5=Greenbaum |first5=Jamin S. |last6=Grima |first6=Cyril |last7=Schroeder |first7=Dustin M. |last8=Dowdeswell |first8=Julian A. |last9=Young |first9=Duncan A. |date=2018-04-06 |title=Discovery of a hypersaline subglacial lake complex beneath Devon Ice Cap, Canadian Arctic |url=https://www.science.org/doi/10.1126/sciadv.aar4353 |journal=Science Advances |language=en |volume=4 |issue=4 |pages=eaar4353 |doi=10.1126/sciadv.aar4353 |issn=2375-2548 |pmc=5895444 |pmid=29651462}}</ref> and Antarctic<ref name=":13" /><ref>{{Cite journal |last=Wright |first=Andrew |last2=Siegert |first2=Martin |date=December 2012 |title=A fourth inventory of Antarctic subglacial lakes |url=https://www.cambridge.org/core/journals/antarctic-science/article/abs/fourth-inventory-of-antarctic-subglacial-lakes/81B35C31B0DFCE1B3A0705B779D3AF58 |journal=Antarctic Science |language=en |volume=24 |issue=6 |pages=659–664 |doi=10.1017/S095410201200048X |issn=0954-1020}}</ref> and act as a planetary analog for both the potential subglacial lakes on Mars and liquid oceans below icy shells of moons like [[Europa (moon)|Europa]].<ref name=":15" /> To study the life in subglacial lakes on Earth, ice core drilling is used to reach the water, but contamination is commonly considered to have compromised attempts to sample from the water of both [[Lake Vostok]] and [[Lake Ellsworth (Antarctica)|Lake Ellsworth]].<ref name=":13" /> However, microbes have been sampled from the accretion ice (frozen lake water) of Lake Vostok.<ref>{{Cite journal |last=Bulat |first=Sergey A. |date=2016-01-28 |title=Microbiology of the subglacial Lake Vostok: first results of borehole-frozen lake water analysis and prospects for searching for lake inhabitants |url=https://royalsocietypublishing.org/doi/full/10.1098/rsta.2014.0292 |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=374 |issue=2059 |pages=20140292 |doi=10.1098/rsta.2014.0292}}</ref> Also, [[Lake Whillans]] was a successful sampling endeavor from under 800 m of ice, where over 4000 species of [[Chemotroph|chemoautotrophs]] have been identified.<ref name=":10" /><ref name=":11" /><ref name=":12" /> <ref name=":13" /> Whether similar microbes could survive in the putative salty subglacial lakes on Mars is still unknown, but if liquid water is present, it could preserve inactive microbial life.<ref name=":17" /><ref>{{Cite web |last=Rothery |first=David |title=Mars: mounting evidence for subglacial lakes, but could they really host life? |url=http://theconversation.com/mars-mounting-evidence-for-subglacial-lakes-but-could-they-really-host-life-146732 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref><ref>{{Cite web |last=Hallsworth |first=John E. |title=What on Earth could live in a salt water lake on Mars? An expert explains |url=http://theconversation.com/what-on-earth-could-live-in-a-salt-water-lake-on-mars-an-expert-explains-101148 |access-date=2022-05-31 |website=The Conversation |language=en}}</ref>
 {{clear}}