On Wednesday, a team of European scientists announced it had found strong evidence of a vast reservoir of liquid water hiding under the ice cap on the south pole of Mars.
As detailed in Science, the buried lake was detected using ice-penetrating radar from the European Space Agency’s Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) satellite. MARSIS has been in orbit around the Red Planet since 2003 and uses high frequency radio pulses to map Mars’ subsurface geology.
When the satellite sends out its radio pulse toward Mars, it can penetrate up to around two miles below the surface before reflecting back toward the satellite’s sensors. By studying the way the radio pulse is distorted when it is reflected back toward the spacecraft, researchers can learn a lot about the type of material buried beneath the surface.
A few years after MARSIS was parked in orbit around the Red Planet, it began to receive anomalous radar echoes over various areas of the south pole. These radar echoes were too “small and bright” to indicate rock or ice beneath the surface. If anything, they seemed to indicate the presence of liquid water.
The researchers on the MARSIS mission assumed the data was probably a glitch in the spacecraft’s system because these small, bright echoes weren’t seen in every pass over the same area of the south pole. Several years later, however, the researchers learned that the MARSIS computer system was averaging across pixels to reduce the amount of data being sent back to Earth. In these cases, the bright spots would effectively be erased from the data.
So the European Space Agency researchers began to store some raw data on another orbiter, the Mars Express, over the course of three years to ensure the data wouldn’t be altered. The data stored on Mars Express was unambiguous. The bright spots were seen in 29 passes over the south pole. The largest spot is about a mile under the ice and spans over 12 miles.
Some bright reflections alone aren’t enough to confirm liquid water, however. Water is able to store energy in electric fields better than rock or ice— a quality known as permittivity. Calculating permittivity for the patches requires knowing the signal power when they reflect. This could only be estimated by the researchers, however, because the power of the MARSIS antenna couldn’t be calculated from the ground. Nevertheless, the estimated permittivity was higher on the bright patches than anywhere else on the planet.
The discovery is a major boon for those hoping to find evidence of life on Earth’s barren neighbor. As far as we know, liquid water is a prerequisite for the emergence of life and some think that vast liquid oceans may once have coated the Martian surface. So far, however, the only water discovered on Mars has been locked away in ice sheets.
If lakes of liquid water exist beneath Martian glaciers, they probably are our best shot of finding extraterrestrial life on Mars. It has already been demonstrated that microbial life is capable of surviving in subglacial lakes in Antarctica, and similar extremophiles may exist on the Red Planet.
Still, not everyone is so certain that the ESA researchers found water. Jeffrey Plaut, a MARSIS researcher at NASA’s Jet Propulsion Laboratory who was not involved with the new research told Science Magazine that “the interpretation is plausible, but it’s not quite a slam dunk yet.” Plaut cited the difficulty of explaining how liquid water could exist at the Martian poles, given that there’s almost no heat flowing from Mars’ interior to keep the water in subglacial lakes in liquid form as happens on Earth.
Still, the new research offers the strongest evidence of liquid water on Mars yet. Further data is needed, but two planned Mars missions launching in two years—NASA’s Mars2020 and the European Space Agency’s ExoMars—will be able to shed some light on the issue by better characterizing the subsurface of the Red Planet.