Briny Underground Lakes May Be All That Remain of Martian Ocean

New data confirm the 2018 discovery of a lake under Mars’ south pole and point to three smaller water features around it

A composite image shows Mars from the side, emphasizing the southern polar ice cap
The fact that Mars' south pole holds many underground lakes suggests that they might be the last remnants of the planet's ancient oceans. NASA/JPL/Malin Space Science Systems

When scientists first found signs of a lake under Mars’ south pole in 2018, questions abounded over how such a feature could form and whether the measurements were accurate. Now, a study published this week in Nature Astronomy not only confirms the size and location of the first lake, but also shows three more, smaller bodies of water nearby.

The study adds 100 measurements to the team’s original 29 figures for a clearer picture of the region. The four lakes are hidden a mile under the surface of Mars’ icy south pole, and may be full of salt and sediments to remain liquid even in extreme cold temperatures. Some scientists not involved in the study are cautious about the research team’s conclusions, but the study authors see the discovery as an optimistic signal in the search for life on Mars.

“Here we have not just an occasional body of water, but a system,” Roma Tre University planetary scientist Elena Pettinelli, a co-author on the study, tells Nadia Drake at National Geographic. “The system was probably existing a long time ago, when the planet was very different, and this is maybe the remnant of that.”

Even if the lakes are inhospitable for life, the fact that the south pole holds multiple water features suggests that they might be the last remnants of the Red Planet’s ancient oceans. Mars is covered in the signs of erosion that suggest water once flowed across its now dry, rocky surface. Observations made by the Curiosity rover suggest that Mars used to be covered in a vast ocean, Colin Schultz wrote for Smithsonian in 2013.

“As the early Martian climate cooled, such an ocean would have frozen and eventually sublimed away,” or evaporated from solid ice into water vapor without melting first, Planetary Science Institute researcher Steve Clifford tells National Geographic.

The water vapor would have traveled in the atmosphere and fallen on Mars’ poles and created tall, expansive ice caps. When Mars had more geothermal energy, it would have melted the underside of the ice caps. That water could be stored as groundwater or permafrost, and if the water is salty enough, some may persist in the lakes observed today.

Water would need to be extremely briny to be a liquid at Mars’ south pole, which can reach negative 195 degrees Fahrenheit. Salts and sediments can prevent water from freezing by getting in between water molecules, which stops them from lining up and crystallizing. (That's why cities put salt on roads before a snowstorm to prevent dangerously slippery conditions.)

But the salts in Martian water aren’t good for seasoning dinner—these are perchlorates. As astrobiologist Dirk Schulze-Makuch writes for Air & Space magazine, the toughest Earth fungus can survive in water with up to 23 percent sodium perchlorate mixed in, while the strongest bacterium could only handle a 12 percent solution. For water to remain liquid at Mars’ extremely cold temperatures, it would need to have a different kind of salt—calcium perchlorate—dissolved in it, and that salt is even more difficult for Earth microbes to handle.

Earth has a few of its own salty lakes hidden under Antarctic ice, but they aren’t teeming with life.

“There’s not much active life in these briny pools in Antarctica,” Montana State University environmental scientist John Priscu tells Nature News’ Jonathan O’Callaghan. “They’re just pickled. And that might be the case [on Mars].”

The study’s research team first began their radar measurements of Mars’ south pole to find the cause of several reflective patches under the ice. The radar, MARSIS, shoots radio waves at Mars’ surface, and they reflect back when they hit a change in the planet’s makeup. Analyzing the reflection patterns reveals what material the waves bounced off of.

“If the bright material really is liquid water, I think it’s more likely to represent some sort of slush or sludge,” Purdue University planetary geophysicist Mike Sori tells Nature News.

Mars has other bright patches under ice that haven’t been analyzed with these techniques, but all the way at the edge of the ice cap. University of Arizona planetary scientist Jack Holt tells National Geographic that if the new study’s conclusions are correct, then there would be natural springs at the glaciers’ edge.

“This is obviously not observed,” Holt tells National Geographic in an email, adding that measurements show that the region doesn’t hold an electric charge the way saltwater would, and that the Mars Reconnaissance Orbiter hasn’t been able to replicate the MARSIS observations.

Holt tells Nature News, “I do not think there are lakes… There is not enough heat flow to support a brine here, even under the ice cap.”

A Chinese mission, Tianwen-1, will begin orbiting Mars in February 2021 and may be able to provide another perspective on the observations.

“I’m convinced something funky is going on at this site to cause a spike in the reflection,” Purdue University planetary scientist Ali Bramson tells National Geographic. “Certainly if there is some weird, super-cooled, sludgy salt solution at the base of the polar cap, that’s super cool.”

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