Compelling Evidence Suggests There’s a Liquid Lake Beneath Mars’ Surface

But do the findings hold water?

Mars' south polar cap as it appeared to the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) on April 17, 2000. Strong evidence of an underground lake was found nearby. NASA

Researchers have long harbored suspicions of water on Mars, even finding traces of a salty ebb and flow on its surface in 2015. But in a report published today, a team of scientists led by Roberto Orosei of the Italian National Institute for Astrophysics has documented the first findings of a true, persistent body of water—something far less fickle than drops in the atmosphere or landlocked permafrost. The likely lake is estimated to live about a mile beneath the surface of the planet’s icy south pole, stretching 12 miles across and at least three feet in depth—a behemoth of a wading pool.

But scientists are far from splashing around in the shallows of this lagoon. The evidence is not even completely definitive: it comes from an instrument called MARSIS, or Mars Advanced Radar for Subsurface and Ionosphere Sounding, that emits pulses of radar from its perpetual orbit around the planet. When these radio signals hit something reflective below Mars’ surface, they bounce this info back to the Mars Express, the European Space Agency’s spacecraft that has been chauffeuring MARSIS’ joyride for the past 15 years. Also aboard the Mars Express is a high-resolution camera that has been snapping images of the planet’s surface.

Water is inherently more reflective than rock—so by comparing the signals from MARSIS, scientists can look for hotspots of activity that may indicate the presence of liquid. The same techniques have been used to identify subglacial lakes on Earth, such as those in Antarctica.

This radio wave technology, called ground-penetrating radar, is of fairly low frequency—which means the waves can penetrate deep, but the resolution of the signal sent back is not terribly high. Higher frequency signals could theoretically afford more precision but would be more likely to miss treasure buried beneath the surface. So while what MARSIS reports is still tentative, the grainy composite image it has composed represents the accumulation of over three-and-a-half years of mapping from 2012 to 2015. Orosei and his team have mentally scoured many other possible explanations for the readings—and they’re left feeling optimistic.

"After years, literally a couple of years of...discussion, debate, and let's say general head scratching, we really felt confident that any other explanation would fail," Orosei says in an interview with Joe Palca of NPR.

But scientists such as Jeffrey Plaut, who work at NASA’s Jet Propulsion Laboratory, caution that the findings await further confirmation. The stakes for this are too high to be careless.

If the lake does exist, however, it’s probably not an anomaly.

“There are other areas that seem to be similar. There’s no reason to say this is the only one,” co-author Elena Pettinelli of Italy’s Roma Tre University tells Nadia Drake at National Geographic.

If pools pattern the surface of Mars, they could point to life—life that once was; tantalizingly, perhaps even life that still is. But scientists are far from such a discovery. The first next step is to definitively confirm this lake—a process that will require drilling through the rocky surface, something researchers and their robots are not yet equipped to do. We can rest assured however, that when they are, the results could be—dare we say—groundbreaking.

In the meantime, it certainly remains possible that Mars once supported a form of recognizable life. But those days are probably long gone. The red planet is currently an inhospitable dust storm, with a thin atmosphere chock full of stifling carbon dioxide and boasting an average surface temperature of minus 80 degrees Fahrenheit. For this lake to remain liquid at all, it must be punishingly salty (the brinier the water, the lower the temperature at which it will freeze) and cold—a tough sell for even the hardiest of organisms. But it’s not impossible. Some bacteria thrive in the dark, saline subglacial lakes of Antarctica; who are we to preclude the possibility on Mars?

Even if future work confirms that this new lake is the real deal, simply finding water guarantees nothing. New evidence of a salty liquid lake buried beneath the surface of Mars could be a massive breakthrough. But a hint of water is a far leap away from any promise that life may have once been, or still is, supported on our rouge red neighbor.