Salty subglacial lakes are controversially inferred from radar measurements to exist below the South Polar Layered Deposits (SPLD) in Ultimi Scopuli 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 the 1980s. [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 radar detections of bright reflectors 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]
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]
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, further work has 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 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, computer simulations predict 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]
Liquid brine water is proposed to be plausible at the SPLD because 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] [2] In addition, it is expected that the temperature at depth for the ice would increase at a rate based on the undetermined geothermal flux and thermal properties of the SPLD. [2] However, a study found the bright radar reflectors to be widespread across the SPLD, rather than limited to the previously identified areas of the putative subglacial lakes. [20] Since the bright radar detections covered a wide variety of conditions at the SPLD (e.g., different temperatures, ice thicknesses), this presents challenges for all of the bright radar reflectors to be indicative of liquid water. [20]
Additional approaches to determining the plausibility of the subglacial lakes included a study looking for surface features induced by such lakes. [28] On Earth, examples of surface features caused by a subglacial lake include fractures or ridge features like at Pine Island Glacier in Antarctica. [29] While a study on Mars only found surface features that match CO2 and wind-related 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 possibly millions of years old, it is hard to constrain when the putative subglacial lake would have modified the surface features. [28]
In contrast with the hypothesis of subglacial water at the base of the SPLD, other suggestions include materials such as saline ice, [8] a conductive mineral deposit such as clays, [8] [9] and igneous materials. [31] Future work is necessary to resolve how these alternative hypotheses hold under Mars-like conditions using instruments like MARSIS.
While the initial study assumed negligible conductivity in their calculation of the permittivity values, [1] by accounting for conductivity, 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] another 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]
The mineralogical explanation is the most favored in follow-up studies, especially with specific hydrous minerals such as jarosite (a sulfate) [8] and smectite (a clay mineral). [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]
Another study applied computer simulations to look 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]
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 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 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 chemoautotroph microbes 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] [34] [39]
Mars Express is a space exploration mission being conducted by the European Space Agency (ESA). The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was designed and built. However, "Express" also describes the spacecraft's relatively short interplanetary voyage, a result of being launched when the orbits of Earth and Mars brought them closer than they had been in about 60,000 years.
Deep Space 2 was a NASA space probe, part of the New Millennium Program. It included two highly advanced miniature space probes that were sent to Mars aboard the Mars Polar Lander in January 1999. The probes were named "Scott" and "Amundsen", in honor of Robert Falcon Scott and Roald Amundsen, the first explorers to reach the Earth's South Pole. Intended to be the first spacecraft to penetrate below the surface of another planet, after entering the Mars atmosphere DS2 was to detach from the Mars Polar Lander mother ship and plummet to the surface using only an aeroshell impactor, with no parachute. The mission was declared a failure on March 13, 2000, after all attempts to reestablish communications following the descent went unanswered.
The possibility of life on Mars is a subject of interest in astrobiology due to the planet's proximity and similarities to Earth. To date, no proof of past or present life has been found on Mars. Cumulative evidence suggests that during the ancient Noachian time period, the surface environment of Mars had liquid water and may have been habitable for microorganisms, but habitable conditions do not necessarily indicate life.
Ganges Chasma is a deep canyon at the eastern end of the vast Valles Marineris system on Mars, an offshoot of Capri Chasma, and is in the Coprates quadrangle. It is named after the River Ganges in South Asia. Ganges Chasma is thought to have formed through a series of catastrophic discharges of water and CO2 from chaos terrains such as that preserved in Ganges Chaos at its southern margin. Most of the evidence for these discharges and the ensuing collapses is believed to have been washed away. Prior to developing an outlet that joined it to Capri Chasma and the connected outflow channels to its east, Ganges Chasma may at some point in the Noachian period have contained a lake which drained northward through partially subsurface pathways into Shalbatana Vallis.
MARSIS is a low frequency, pulse-limited radar sounder and altimeter developed by the University of Rome La Sapienza and Alenia Spazio. The Italian MARSIS instrument, which is operated by the European Space Agency, is operational and orbits Mars as an instrument for the ESA's Mars Express exploration mission.
A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where gravitational pressure decreases the pressure melting point of ice. Over time, the overlying ice gradually melts at a rate of a few millimeters per year. Meltwater flows from regions of high to low hydraulic pressure under the ice and pools, creating a body of liquid water that can be isolated from the external environment for millions of years.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) was a visible-infrared spectrometer aboard the Mars Reconnaissance Orbiter searching for mineralogic indications of past and present water on Mars. The CRISM instrument team comprised scientists from over ten universities and was led by principal investigator Scott Murchie. CRISM was designed, built, and tested by the Johns Hopkins University Applied Physics Laboratory.
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SHARAD is a subsurface sounding radar embarked on the Mars Reconnaissance Orbiter (MRO) probe. It complements the MARSIS radar on Mars Express orbiter, providing lower penetration capabilities but much finer resolution.
Extraterrestrial liquid water is water in its liquid state that naturally occurs outside Earth. It is a subject of wide interest because it is recognized as one of the key prerequisites for life as we know it and thus surmised as essential for extraterrestrial life.
The presence of water on the terrestrial planets of the Solar System varies with each planetary body, with the exact origins remaining unclear. Additionally, the terrestrial dwarf planet Ceres is known to have water ice on its surface.
Mars is the fourth planet from the Sun. The surface of Mars is orange-red because it is covered in iron(III) oxide dust, giving it the nickname "the Red Planet". Mars is among the brightest objects in Earth's sky and its high-contrast albedo features have made it a common subject for telescope viewing. It is classified as a terrestrial planet and is the second smallest of the Solar System's planets with a diameter of 6,779 km (4,212 mi). In terms of orbital motion, a Martian solar day (sol) is equal to 24.5 hours and a Martian solar year is equal to 1.88 Earth years. Mars has two natural satellites that are small and irregular in shape: Phobos and Deimos.
The Medusae Fossae Formation is a large geological formation of probable volcanic origin on the planet Mars. It is named for the Medusa of Greek mythology. "Fossae" is Latin for "trenches". The formation is a collection of soft, easily eroded deposits that extends discontinuously for more than 5,000 km along the equator of Mars. Its roughly-shaped regions extend from just south of Olympus Mons to Apollinaris Patera, with a smaller additional region closer to Gale Crater.
Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere. What was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae, may be grains of flowing sand and dust slipping downhill to make dark streaks. While most water ice is buried, it is exposed at the surface across several locations on Mars. In the mid-latitudes, it is exposed by impact craters, steep scarps and gullies. Additionally, water ice is also visible at the surface at the north polar ice cap. Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian south pole. More than 5 million km3 of ice have been detected at or near the surface of Mars, enough to cover the whole planet to a depth of 35 meters (115 ft). Even more ice might be locked away in the deep subsurface.
The planet Mars has two permanent polar ice caps. During a pole's winter, it lies in continuous darkness, chilling the surface and causing the deposition of 25–30% of the atmosphere into slabs of CO2 ice (dry ice). When the poles are again exposed to sunlight, the frozen CO2 sublimes. These seasonal actions transport large amounts of dust and water vapor, giving rise to Earth-like frost and large cirrus clouds.
The Dorsa Argentea Formation (DAF) is thought to be a large system of eskers that were under an ancient ice cap in the south polar region of Mars. The ancient ice cap was at least twice the size of the present ice cap and may have been 1500–2000 meters thick. Later research suggests that the area of this polar ice sheet is believed to have covered about 1.5 million square kilometers, roughly twice the size of France or the American state of Texas. This group of ridges extends from 270–100 E and 70–90 S, around the south pole of Mars. It sits under the Late Amazonian South Polar Layered Deposits (SPLD), in the Mare Australe quadrangle.
Mars habitability analogue environments on Earth are environments that share potentially relevant astrobiological conditions with Mars. These include sites that are analogues of potential subsurface habitats, and deep subsurface habitats.
Rebecca Woodgate is a professor at the University of Washington known for her work on ocean circulation in polar regions.
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