Astrobio Top 10: Liquid water discovered on Mars

Categories: Feature Stories Mars

As 2018 comes to a close, Astrobiology Magazine is counting down our ‘Top 10’ stories for the year. At number 1: The long quest for liquid water on Mars may have finally met with success. This story was originally published on July 25, 2018.

Liquid water has been found about a mile beneath Mars south polar ice cap. Image credit: ESA/DLR/FU Berlin/Bill Dunford.

Radar echoes have revealed liquid water beneath the south polar ice cap on Mars, marking a major turning point in our exploration of the red planet as a potentially habitable world.

The water, which was detected by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on board the European Space Agency’s Mars Express spacecraft, is located 1.5 kilometers (~one mile) beneath the ice, in the form of a lake 20 kilometers (12.4 miles) across under the freezing plains of Planum Australe.

The depth of the lake is impossible to determine using radar, which returns echoes from the boundary between rock and the bottom of the ice cap, meaning that the water could be anywhere between being as deep as Earth’s Lake Vostok, to being a muddy layer just a meter thick.

For the water to remain liquid under the cold ice at low temperature and pressure, it must be a brine saturated with perchlorate salts that lower the freezing point of water. Perchlorate, first detected on the red planet in 2008 by NASA’s Phoenix lander, are widespread on Mars.

The nature of the subterranean water, however, raises a quandary when it comes to its suitability for life. Planetary scientists have been embarked on a quest to find liquid water on the red planet, because life as we know it cannot survive without water to act as a solvent in its biochemistry. Previous claims of evidence for liquid water, in the form of changes in gullies and dark streaks called recurring slope lineae attributed to flows of briny water, had turned out to be caused by dry avalanches. This new discovery suggests that there had always been liquid water on Mars, but we had been looking in the wrong place.

Cold water

However, the presence of liquid water does not necessarily mean this region of Mars is habitable. Part of the problem is that we do not know how cold this water discovered by MARSIS is. On Earth, the most cold-loving extremophile known is Synechococcus lividus, which thrives in temperatures of –20 degrees Celsius (–4 degrees Fahrenheit). Other life has adapted ways of surviving, but not thriving, in even colder temperatures. However, depending upon how briny it is, water can potentially remain liquid down to –70 degrees Celsius (–94 degrees Fahrenheit). It therefore remains an open question whether the water is warm enough for life; perhaps Martian extremophiles are even more extreme than their cold weather terrestrial counterparts.

Mars and its south polar ice cap, seen by the Hubble Space Telescope. Image credit: NASA/ESA/J. Bell (Cornell University)/M. Wolff (SSI).

The Planum Australe lake may not be the only sub-surface body of water on Mars, but MARSIS is limited by its spatial resolution, which would prevent it from finding bodies of water smaller than the Planum Australe lake. Another problem is that SHARAD, the Shallow Radar instrument on NASA’s Mars Reconnaissance Orbiter, operates at a higher frequency designed for studying ice on the surface. Higher frequency radar pulses are not able to penetrate deep enough to detect the boundary between ice and liquid beneath the polar cap, and so SHARAD has been unable to detect this lake.

Future missions currently in the planning stages are also expected to have high-frequency radar sounding systems. Roberto Orosei of the Istituto Nazionale di Astrofisica in Italy, who led the research that is published in the journal Science, is pinning his hopes on the upcoming Chinese mission to Mars, the Mars Global Remote Sensing Orbiter, Lander and Small Rover, which will have on board a radar sounding experiment that will operate at frequencies between the 5MHz of MARSIS and the 20MHz of SHARAD.

The subglacial Martian lake also brings to mind the challenge of drilling down through the ice to reach the liquid water in order to sample it, reminiscent of projects drilling into subglacial lakes on Earth such as Lake Vostok, or mission plans to go to Jupiter’s moon Europa or Saturn’s moon Enceladus and drill down to sample the oceans there. It could be that Mars’ south polar lake becomes the perfect testing ground for even more ambitious missions to the outer Solar System.