Shortly after Mars Express began scanning the surface of Mars with radar, in 2005, it detected unusually bright reflections from the base of the south polar ice cap. After a decade of studying those reflections, the radar instrument team announced a likely cause: a shallow lake of briny liquid water.
Today, however, that idea has some competition. Other teams have suggested clays, saline ices, and even volcanic rock as the source of the reflections. They say that conditions below the ice cap (known as SPLD—south polar layered deposits) are much too cold for water to remain in liquid form, requiring underground heating more intense than seen anywhere else on Mars. And even if there’s more heat than expected, there aren’t enough salts to lower the freezing temperature of water to a reasonable level.
“This is certainly what makes science fun,” said David Stillman, a researcher at Southwest Research Institute (SwRI) in Boulder, Colo., who did laboratory work with the prowater research group. “You have these little disputes over the different assumptions people are making. We’re working through it to come to 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 Smith, an assistant professor at York University in Toronto and a member of a team that concluded the radar was reflecting off a type of clay.
Bright Reflections Hint at Water
The saga began when liquid water was reported in 2018 by a team headed by Roberto Orosei, a researcher at the Italian National Institute for Astrophysics and a principal investigator for MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding). The instrument uses a 40-meter antenna to beam low-frequency radio waves toward Mars and record their reflections. Different materials reflect the waves in different ways, giving researchers an idea of the composition of the surface and subsurface.
The bright patch showed up in early observations, hinting at water, and the MARSIS team obtained new raw observations during 29 passes over the polar cap. Those observations localized the bright reflections to a region 9° from the south pole, roughly 10–20 kilometers wide, below 1.5 kilometers of ice.
The liquid water interpretation was based not just on the brightness of the reflections but also on the electrical properties of the reflecting material, particularly 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 how much radar power was being reflected to MARSIS as part of its 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 polar ice sheets on Earth and reported the discovery of several other “wet spots” near the first one. The researchers suggested the water was in the form of a hypersaline perchlorate brine, which can maintain its liquid form for long periods at very low temperatures.
Casting Doubts on the Water Idea
By then, however, several other research groups had cast doubts on the liquid water hypothesis. In 2019, for 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 ice at the base of the SPLD in the present day under typical Mars conditions.” The only way to maintain liquid water, 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 doubt was cast by three studies last summer. In one, researchers combed through 15 years of MARSIS data and found dozens more bright spots below the ice cap, which would require even more salts and more subsurface heat than have been discovered anywhere on Mars. Another study said the bright reflections could be caused by contrasts in the electrical conductivity of materials near the surface, which was not considered in the original discovery study. The researchers suggested the radio waves could be reflecting from clays, metal-bearing minerals, 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 little as 1 or 2 meters thick.
In this study, Smith hydrated smectite, a common clay on Earth as well as on Mars that forms under wet conditions near volcanoes, to varying levels. He then chilled it to −50°C until he produced both permittivity and conductivity readings consistent with the MARSIS results. In addition, a colleague discovered smectite just outside the ice cap in the vicinity of the original bright spots.
“To me, smectite is the most plausible argument,” said Smith. “You don’t need liquid water, you don’t need more salt than Mars can provide, you don’t need a volcano. You don’t need anything but the stuff you can see on the surface at the south pole now.”
2022: Dueling Papers
“The main outcome is [that] liquid water at the base of the ice cap cannot be carried as a unique hypothesis for the nature of this material.”
Yet another alternative to water was proposed in January by Cyril Grima, a research associate at the Institute for 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 whether liquid water was a viable hypothesis; I wanted to test whether a nonwet material was a viable hypothesis,” Grima said. “I was looking at materials we know Mars can produce, and at least at the surface, that doesn’t include liquid water.”
Grima then simulated what the MARSIS reflections would look like if different materials found across Mars were coated with 1.5 kilometers of ice. He found several areas that should produce reflections as bright as those seen below the ice cap. Those areas appear to be volcanic in origin, coated primarily with basalt. “The main outcome is [that] liquid water at the base of the ice cap cannot be carried as a unique hypothesis for the nature of this material,” Grima said.
The original study team, however, disagreed with Grima’s assessment. “The only substance capable of explaining MARSIS data remains, at least for the time being, liquid water,” said Orosei.
“We’re no longer talking lakes. This would be a slushy material, mostly ice or mostly sediment, with brine mixed in.”
In a paper published a week after Grima’s, Orosei and his colleagues disputed the claims that clays or other materials could account for the radar reflections. As part of that work, SwRI’s Stillman studied the electrical properties of perchlorate brines under Mars-like temperatures and pressures and found that the salty liquid water was still the best match to MARSIS observations. The brine would nestle between ice grains or regolith particles at a concentration of roughly 15% by volume. “We’re no longer talking lakes,” he said. “This would be a slushy material, mostly ice or mostly sediment, with brine mixed in.”
Obviously, the saga is far from over.
“There is still much work to be done, starting with new radar measurements to confirm what we have seen previously and expanding the analysis to other areas where subsurface radar echoes similar to those that led to the identification of water have been detected,” said Orosei. “We believe, however, that there is still a treasure trove of knowledge on the geological and climatic history of Mars hidden in MARSIS data.”
“Until we can drill through the ice, it’s mostly academic,” said Smith. “Eventually, I think the community will conclude that this is not liquid water, and it’s not even the second most likely thing—it’s maybe the third or fourth most likely…. Mars throws new stuff at us all the time. It’s tempting sometimes to think we know everything, but we really don’t—we’re not even close.”
—Damond Benningfield (damond5916@att.net), Science Writer