Scientists Find a Giant Reservoir of Liquid Water Under the Surface of Mars

Radar data from the Mars Express orbiter has given us some of the biggest Red Planet news in years

Illustration of the Mars Express spacecraft superimposed on a radar cross section of the southern polar layered deposits of Mars.
Image: Davide Coero Borga/INAF/ESA
In the background is a radar cross section of the southern polar layered deposits. The white line is the surface radar echo, while the light blue spots highlight areas of very high reflectivity, interpreted as being caused by the presence of water.

NASA is usually the organization that gets to regale the world with news of the latest and greatest space discoveries, but this time, the honor belongs to its partners. Radar data acquired by the European Space Agency’s Mars Express orbiter points to the existence of a giant, stable body of liquid water sitting under a 1.5-kilometer-thick sheet of ice near Mars’s south pole. It’s the biggest discovery in years when it comes to the investigation of water on Mars. And it’s some of the most powerful evidence yet that Mars possesses the conditions necessary to be habitable to life as we know it.

The body of water exists within a 20-km-wide zone in the South Polar Layered Deposits (SPLD), a region of particularly thick glacial ice. “We’ve arrived at the conclusion that the only plausible explanation for the data is water,” says Elena Pettinelli, a researcher at Roma Tre University and a coauthor of a paper on the new findings that was published this week in Science.

“One of the most impressive parts of Mars from a radar point of view is the polar caps,” says Pettinelli. The planet exhibits frequencies between 1.8 megahertz and 5 MHz, an interval in which we can study ice that has penetrated several kilometers into the ground. “So, the idea is to use this radar to look deep into the Martian crust, especially in the polar caps which are made mainly of water-ice.”

Mars Express, in orbit around Mars since December 2003, wields an instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding, or MARSIS. The tool is “specifically designed and built to look for liquid water on Mars,” says Pettinelli. On Earth, scientists have found water below the Antarctic ice sheet using radar waves that pass through the ice itself and are reflected back at points where different materials (like ice, bedrock, or water) interface. These reflections are stronger when there’s a water interface.

MARSIS works essentially the same way, spotting the location of subsurface liquid water on Mars by beaming pulses at Mars’s surface at four different frequencies (1, 3, 4, 5, and 8 MHz). Some of the pulses immediately bounce back, while others penetrate the ground, and can be used to build an image of the subsurface composition.

So why is it only now, after 15 years, that we’re finally seeing this laborious search for water bear fruit? Pettinelli explains that there’s a number of reasons for the disappointing delay. These include the orbit of the radar and the decision to use MARSIS only at night to avoid interference with the Martian ionosphere, which resulted in the tool picking up nothing more than a few bright reflection detections. Around 2010 and 2011, the team began taking the bright reflections on the radargram more seriously. The individual data points were, according to Pettinelli, “perfect,” but there was still some inconsistency among multiple data points themselves, which the team pegged to problems in the onboard data processing on the spacecraft. The team elected to start fresh and reacquire all of the radar data again without running any onboard processing; they did so between May 2012 and December 2015.

The researchers are pretty confident they’ve found something real and groundbreaking, and much of that confidence is due to their own cautious approach. “In reality, we discussed the results over two years,” says Pettinelli, choosing to prioritize a quantitative analysis to verify the presence of liquid water versus relying too heavily on interpreting the imagery directly. She notes the results are quantitatively similar to radar observations of subsurface water in Greenland and Antarctica.

Granted, what the researchers have detected couldn’t be pure liquid water. Base temperatures at the SPLD are thought to be around -68 ˚C, and certainly wouldn’t be warmer under 1.5 km of ice. But the freezing point of water is lower in the presence of salts, and high amounts of salinity caused by sodium, magnesium, and calcium (all of which are present on Mars) could feasibly suppress the freezing point to as much as -74 ˚C.

While the results certainly raise the specter of liquid water elsewhere on Mars, MARSIS is not the tool for that job. The technique the European researchers describe can be used only to study the subglacial parts of Mars at the poles, where the radar can actually penetrate the surface. Still, the study authors note, “there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location,” and it’s a safe bet to assume these newest findings will spur more interest in finding liquid water—and possibly life—elsewhere on the Red Planet.

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