Planetary scientists announced that they have found evidence that a substantial amount of liquid water exists in one spot under the surface of Mars.

!!!

Let me be clear up front: The evidence is quite good, but not 100% conclusive. There are caveats, which I’ll explain below. But looking over their work, I have to admit this looks (ironically) solid. If I had to bet, I’d say they have indeed found a shallow but wide lake of liquid water buried deep under the Martian surface.

And that’s truly amazing.

The observations come from the European Space Agency’s Mars Express spacecraft, which has been orbiting the Sun’s fourth rock since late 2003. It’s equipped with the usual fleet of cameras and detectors to peer down at Mars, to map it optically as well as mineralogically, and investigate the near-Mars environment of space.

It also has a radar instrument called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS). This device is a 40-meter-long boom (made of two 20-meter booms that extend away from the spacecraft) that sends radar (radio wave) pulses down to the planet below. Depending on the composition of the surface and the wavelength used, the pulse reflects off the surface and is sent back to the spacecraft. The time it takes to go down and back up can be used to measure the distance to the feature; the distance to a mountaintop and back is shorter than to the bottom of a valley, so the pulse takes less time for the former and more for the latter. MARSIS can be used as an altimeter in that way to make a topological surface map of Mars.

But there’s more. Some wavelengths of radar penetrate the surface, passing right through the material there. It can be weakly or strongly reflected by some materials below the surface, allowing scientists to build a depth map of Mars down to several kilometers! The resolution isn’t great — it creates a swath on the surface about 3–5 kilometers wide — but multiple passes can be used to build up enough signal to make decent maps of what lies below.

Zoom In Penetrating radar image of Mars shows the icy surface as a bright fuzzy line at the top, and deeper down (toward the bottom) the patchier, undulating bright line that may be a signal from liquid water. Credit: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018

Here’s where things get fun. The south pole of Mars has a huge deposit of ice under the surface called the South Polar Layered Deposits (SPLD), stretching well over a thousand kilometers across. This ice is nearly pure water, mixed with dust in various amounts (roughly 10-20% in places). A part of this a few hundred kilometers across sticks out above the surface, forming the south polar ice cap, which is again mostly water ice, though it has a sheet of carbon dioxide ice (“dry ice”) a meter or so thick that covers it seasonally.

It’s been known for some time that liquid water could exist under the Martian surface. Heat from the Martian interior bubbles up, and warmth from the sunlit surface works its way down. Depending on the thermal gradient (how the temperature changes with depth, which depends on the material), this means that in some places on Mars the temperature allows for water ice to melt. In general, it’s about 1 – 2 kilometers deep.

Years ago it was thought that the SPLD was too thin to have liquid water under it, but then MARSIS mapped the region (called Planum Australe, or southern plains) and showed it’s actually up to 3.7 kilometers thick in places! Hmmmm.

Zoom In The Mars Express spacecraft has detected what might be liquid water deep below the surface of Mars. The location of the signal (left) is near the south pole, and multiple overlapping radar passes (right) show the details (blue is stronger signal). Credit: Context map: NASA/Viking; THEMIS background: NASA/JPL-Caltech/Arizona State University; MARSIS data: ESA/NASA/JPL/ASI/Univ. Rome; R. Orosei et al 2018

And now, finally (well, hopefully), paydirt. Or paywater. 29 passes of MARSIS over an area show something very interesting: A 20-km-wide deposit of material about 400 km from the south pole that reflects radar very well. From the pulse timing delays, this stuff is about 1.5 km below the surface, and distributed in a layer that’s very thin, only a few dozen centimeters thick.

There are several materials that could exist under the surface that would produce radar reflections, including warm water ice, CO 2 ice, or even liquid CO 2 (which needs high pressure to remain liquid, or else it turns directly from a solid into a gas). However, either these are not reflective enough to create the signal seen, or the physical conditions needed to create the signals are pretty unlikely.

The most likely substance by a long shot that can exist there and be so reflective is the simplest: liquid water.

Zoom In The southern polar ice cap of Mars (using infrared, green, and blue light), which is mostly water ice with a thin layer of carbon dioxide ice on top, taken by Mars Express. Credit: ESA / G. Neukum (Freie Universitaet, Berlin) / Bill Dunford

So while it’s the way to bet, it’s not a lock. It’s difficult to know exactly how well the radar penetrates the surface, which can muddy the signal and makes determining the composition of the subsurface difficult. However, once again, all things considered, liquid water does the best job explaining what’s seen.

Some people are calling it a lake, which is fair enough, but at 20 km across with only the depth of the water in a bathtub, it’s more like a thin layer.

Still. This is really exciting! We’ve known about water ice on Mars for decades (the polar ice caps make that fairly obvious) as well as ice just below the surface to surprisingly low latitudes. But liquid water has been maddeningly elusive. This new result makes it seem far, far more likely to exist.

The research authors note that the low resolution of MARSIS means that smaller liquid water layers (ponds, I suppose) could exist under the Martian surface, too, and would be invisible to the instrument. That could be almost anywhere, though most likely at high latitudes. Hopefully future generation instruments will be able to find those and map them out.

So what does this mean for life? Anything at this point would be speculation, but it’s interesting to note that Mars once had lakes, seas, and oceans on its surface, billions of years ago. These evaporated away or were absorbed by materials below as the atmosphere of the planet got sandblasted by the Sun’s solar wind, so surface water didn’t last all that long.

Long enough for life to emerge? Who knows? But if it did, and liquid water still exists below the surface, then that would be a very, very interesting place to drill and get some samples.