A little after Mars-Express began scanning the surface of Mars with radar, in 2005 it detected unusually bright reflections from the base of the southern polar cap. After a decade of studying these reflections, the radar instrument team announced a probable cause: a shallow lake of brackish liquid water.
Today, however, this idea has some competition. Other teams have suggested clays, salt ices and even volcanic rocks as the source of the reflections. They say conditions beneath the ice sheet (known as SPLDs – South Polar Layered Deposits) are far too cold for water to remain in liquid form, requiring more intense underground heating than anywhere else on Mars. And even if there is more heat than expected, there are not enough salts to lower the freezing temperature of the water to a reasonable level.
“That’s definitely what makes science fun,” said David Stillman, a researcher at the Southwest Research Institute (SwRI) in Boulder, Colorado, who has done laboratory work with the prowater research group. “You have these little arguments about the different assumptions that people make. We are working on it to find the right answer.
“Water is plausible, but there are three or four other ways to explain it, and all of them make sense on Mars,” said isaac smithassistant professor at York University in Toronto and member of a team that concluded that the radar reflected off a type of clay.
Light reflections allude to water
The saga began when liquid water was reported in 2018 by a team led by Roberto Oroseiresearcher at the Italian National Institute of Astrophysics and principal researcher for MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding). The instrument uses a 40-meter antenna to send low-frequency radio waves to Mars and record their reflections. Different materials reflect the waves in different ways, giving researchers an idea of the composition of the surface and underground.
The bright spot appeared in early observations, hinting at water, and the MARSIS team obtained new raw observations during 29 passes over the polar cap. These observations located the light reflections in a region 9° from the south pole, about 10 to 20 kilometers wide, under 1.5 kilometers of ice.
The interpretation of liquid water was based not only on the luminosity of the reflections but also on the electrical properties of the reflective material, in particular its dielectric permittivity, a quality that determines the intensity of the radar echo. Liquid water has a higher permittivity than other materials, although the research team had to estimate the amount of radar power reflected back to MARSIS as part of their calculations.
In 2020, the team extended its work with more radar profiles of the study area and new signal processing techniques like those used to study the polar caps on Earth and reported the discovery of several more “wet spots” near the first. The researchers suggested that the water came in the form of a hypersaline perchlorate brine, which can retain its liquid form for long periods at very low temperatures.
Casting doubts on the idea of water
By then, however, several other research groups had cast doubt on the liquid water hypothesis. In 2019for example, Michael Sori and Ali Bramson of the Lunar and Planetary Laboratory at the University of Arizona reported that according to their models of the Martian surface, “no concentration of salt is sufficient to melt the ice at the base of the SPLD in the present day under conditions typical of Mars.The only way to keep the water liquid, they suggested, was to heat it with a magma chamber within 10 kilometers of the surface that had been in place for hundreds of thousands of years.
Further doubts were cast by three studies last summer. Inresearchers combed through 15 years of MARSIS data and found dozens of additional bright spots beneath the ice sheet, which would require even more salts and more underground heat than have been discovered anywhere on Mars . Another study said the light reflections could be caused by contrasts in the electrical conductivity of materials near the surface, which was not considered in the original discovery study. Researchers have suggested that radio waves could be reflected off clays, minerals containing metals, or even salt-rich ice.
the third study combined modeling, laboratory experiments and observations of the Martian surface just outside the ice cap to conclude that the reflections were produced by a layer of smectite, a type of clay, as shallow as 1 or 2 meters thick.
In this study, Smith hydrated smectite, a clay common on Earth as well as Mars that forms in moist conditions near volcanoes, at varying levels. He then cooled it to -50°C until it produced permittivity and conductivity readings consistent with the MARSIS results. Additionally, a colleague discovered smectite just outside the ice cap near the original bright spots.
“To me, smectite is the most plausible argument,” Smith said. “You don’t need liquid water, you don’t need more salt than Mars can supply, you don’t need a volcano. You don’t need anything else. than what you can see on the surface at the south pole now.
2022: Duel Documents
Another alternative to water was propose in January by Cyril Grima, research associate at the Institute of Geophysics at the University of Texas at Austin. Grima looked at radar measurements of the entire Martian surface, not just the south polar region. “I didn’t want to test if liquid water was a viable hypothesis; I wanted to test if a non-wet material was a viable hypothesis,” Grima said. “I was looking at materials that we know Mars can produce, and at least on the surface, that don’t include liquid water.”
Grima then simulated what reflections from MARSIS would look like if different materials found on Mars were covered in 1.5 kilometers of ice. He found several areas that should produce reflections as bright as those seen under the ice sheet. These areas appear to be of volcanic origin, covered mainly by basalt. “The main result is [that] liquid water at the base of the ice sheet cannot be considered a unique hypothesis about the nature of this material,” Grima said.
The original study team, however, disagreed with Grima’s assessment. “The only substance capable of explaining the MARSIS data remains, at least for now, liquid water,” Orosei said.
In a document published a week after Grima’s, Orosei and his colleagues challenged claims that clays or other materials could explain radar reflections. In this work, Stillman of SwRI studied the electrical properties of perchlorate brines under Mars-like temperatures and pressures and found that salty liquid water was always the best match with MARSIS observations. The brine would nest between ice grains or regolith particles at a concentration of about 15% by volume. “We’re not talking about lakes anymore,” he said. “It would be muddy material, mostly ice or mostly sediment, with brine mixed in.”
Obviously, the saga is far from over.
“There is still a lot of work to do, starting with new radar measurements to confirm what we have seen previously and expanding the analysis to other areas where underground radar returns similar to those that led to the water identification were detected,” Orosei said. . “We believe, however, that there is still a treasure trove of knowledge about the geological and climatic history of Mars hidden in the MARSIS data.”
“Until we can drill through the ice, it’s mostly academic,” Smith said. “Eventually, I think the community will conclude that it’s not liquid water, and that’s not even the second most likely – it’s maybe the third or fourth most likely…. Mars throws new things at us all the time. It’s sometimes tempting to think we know everything, but we really don’t – we’re not even close.
—Damond Benningfield ([email protected]), Science Editor