Two years after discovering a large, subterranean lake on Mars, a team of scientists has reaffirmed its original finding and identified three additional ponds using reflectance data from a spacecraft orbiting the planet. The results, published September 28 in Nature Astronomy, suggest that the red planet is home to several liquid bodies of water, kept from freezing due to their high salt content. If confirmed by further research, the south pole of Mars could become a strong target in the search for extraterrestrial life.
“Not only did we confirm the position, extent and strength of the [results] from our 2018 study, but we found three new bright areas” generated by the ponds, Elena Pettinelli, a geophysicist at Roma Tre University in Italy and a coauthor on the new study, tells BBC News.
Compared to Pettinelli and colleagues’ previous work, the new study includes a wider area, imaged with finer detail over a longer period of time. Previously, the lake was identified based on 29 observations made between 2012 and 2015. The current study includes 134 observations taken across 75,000 square kilometres from 2012 to 2019, Nature reports.
To visualize the lake and the three ponds, which are hidden more than one mile beneath the planet’s frozen surface, the team relied on technology used to study lakes buried under the ice at the Earth’s poles, according to the Associated Press. The observations were made using a radar sounder on the European Space Agency’s Mars Express orbiter called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). MARSIS bounces radio waves off the surface and subsurface of Mars. The properties of the signals that return to the satellite, such as its reflectance, tell scientists something about the nature of the terrain below.
The identification of the lake in 2018 began “a new line of inquiry that’s very exciting,” Jim Green, NASA’s chief scientist, told Nature at the time. Its confirmation now lends even greater weight to the idea that Mars can in fact support liquid water. In the newest study, the team was able to further detail the lake’s proportions—roughly 18.6 by 12.4 miles in area.
The three ponds are thought to be separated from the lake, although the resolution of the MARSIS data is not fine enough to definitively confirm. Even so, Roberto Orosei, a radioastronomer at the Istituto Nazionale di Astrofisica and the chief scientist of the study, tells BBC News that “the discovery of an entire system of lakes implies that their formation process is relatively simple and common, and that these lakes have probably existed for much of Mars’ history.” Mars was once warm and wet, similar to Earth.
Orosei and his colleagues report that the lakes could be kept liquid by their high salt content, which lowers their freezing point. While the new evidence has stirred discussion among astronomers, not everyone is sold on this explanation. Because Mars does not have an intact atmosphere like Earth, the pressure on its surface is much lower, keeping any water locked up as ice. “If the bright material really is liquid water, I think it’s more likely to represent some sort of slush or sludge,” Mike Sori, a planetary geophysicist at Purdue University who was not involved in the current research, tells Nature.
Jack Holt, a planetary scientist at the University of Arizona, similarly tells Nature that although he thinks the data are sound, he doesn’t agree with the team’s interpretation. “I do not think there are lakes. There is not enough heat flow to support a brine here, even under the ice cap.”
Previous research has shown that water saturated with certain dissolved salts, such as magnesium and calcium perchlorate, can remain liquid even at –123 °C; the surface temperature of the south pole of Mars is thought to be roughly –113 °C, getting slightly warmer nearer to the planet’s interior. Speaking to BBC News, Graziella Caprarelli, a planetary scientist at the University of Southern Queensland and coauthor on the new study, says those experiments show that brines could exist as liquids under conditions similar to those found at the poles of Mars.
The obvious question is whether such lakes or ponds could support life. Microbes have previously been found in subglacial lakes beneath Antarctic glaciers, but the salt does pose one possible issue. Life as we know it is able to persist in water up to five times the saltiness of Earth’s oceans, but beyond 20 times the level of salt found in seawater, it becomes very difficult to survive, John Priscu, an environmental scientist at Montana State University, tells Nature.
“There’s not much active life in these briny pools in Antarctica,” says Priscu, who studies microbiology in icy environments and was not involved in the current study. “They’re just pickled. And that might be the case [on Mars].”