The Mars that was and the Mars that is are two very different things. Three or so billion years ago, the Red Planet was awash with water, as now-dry riverbeds, deltas, and ocean basins reveal. But when the planet lost its magnetic field, it lost its protection from the solar wind, which stripped away much of the planet’s atmosphere and allowed most of its water to escape to space.
But the key here is “most.” There is plenty of water, in the form of ice, locked up in Mars’s polar caps—about the same amount as exists in Earth’s Greenland ice sheet. For exobiologists looking for possible life on Mars, water is essential—but only in its liquid state, so ice is kind of a dealbreaker. Now, however, a new study In Nature Astronomy suggests that in the south Martian pole at least, there may be a lake buried beneath the ice. It could measure as much as 30 km (18 mi.) across and is kept warm by geothermal heating, similar to the kind generated on Earth by radioactive isotopes or subsurface magma.
One clue to the possible presence of water beneath Mars’s south pole came from the European Space Agency’s (ESA) Mars Express orbiter, which conducted radar soundings in 2018 of a region in the ice cap known as Ultimi Scopuli. The scans revealed an area that was highly reflective in a frequency consistent with a large deposit of liquid water. On Earth, such a finding beneath an ice cap would not be at all uncommon. The overlying weight of thick ice sheets often creates enough pressure to generate heat, leading to ice liquefying deep below the surface.
But there were two problems with the Mars Express findings. For one thing, the frigid temperatures at the Martian poles ought to be enough to overcome the heating caused by pressure, keeping the ice solid all the way through. Additionally, there are other materials that reflect in a frequency similar to water’s—including metal bearing minerals and hydrated clays—suggesting that the Mars Express findings might simply be an illusion.
But Mars Express wasn’t the only Martian satellite surveying Ultimi Scopuli. So too was NASA’s Mars Global Surveyor. During the course of its mission, which lasted from 1997 through 2006, that satellite took the measure of the region using not ground-penetrating radar, but a laser altimeter—and that scan revealed something intriguing. Stretching across a 10 to 15 km (6.2 to 9.3 mi) expanse of Ultimi Scopuli was a distinct surface undulation—with the ice dipping as low as 4 m (13 ft.) below the surrounding terrain and at the other end rising 7 m (22 ft.) above it.
That up and down pattern has analogs on Earth, where buried bodies of water—like Antarctica’s subsurface Lake Vostok—display precisely the same undulating pattern in the overlying ice, caused by upstream and downstream water flow. The question of just what is going on beneath the ice in Ultimi Scopuli has lingered since the twin spacecraft took their twin sightings, but now, an international team of researchers led by physical geographer Neil Arnold of the University of Cambridge has taken on the mystery and come to the conclusion that the south pole of Mars is indeed home to a liquid lake.
The investigators based their findings not on any new sightings taken at Mars, but on computer modeling. Beginning with the data returned by the two satellites, they plugged in multiple variables—including simulations of subsurface friction that would be caused by either solid ice or a body of water; varying depths of the theoretical subsurface lake; and the amount of geothermal heat beneath the surface of Mars in recent geological history. No matter how they ran their variables, the likeliest scenario the model spit out was that there is indeed a lake beneath Ultimi Scopuli’s ice, one that has been kept warm by geothermal heating.
“The combination of the new topographic evidence, our computer model results, and the radar data make it much more likely that at least one area of subglacial liquid water exists on Mars today, and that Mars must still be geothermally active in order to keep the water beneath the ice cap liquid,” said Arnold in a statement that accompanied the study’s release.
What implications this could have for life on Mars are unknown. But multiple worlds in the solar system—especially Jupiter’s Moon Europa and Saturn’s moon Enceladus—harbor comparatively warm liquid oceans, where exobiologists believe the simple recipe of chemistry plus thermal energy plus time might have been sufficient for the basic mechanics of biology to have gotten started. In October 2024, NASA will launch the Europa Clipper spacecraft, which will make multiple flybys of the Jovian moon to gather new data on Europan geology, chemistry, and more.
No such close-up missions of the Martian poles are planned for now, but Arnold, for one, does see studies like his offering the next best thing. “The quality of data coming back from Mars, from orbital satellites as well as from the landers, is such that we can use it to answer really difficult questions about conditions on, and even under the planet’s surface,” he says. “It’s exciting to use these techniques to find out things about planets other than our own.”
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