MARSIS radargram

The upper image of this composite is a “radargram” from the MARSIS on board ESA’s Mars Express. It shows data from the subsurface of Mars in the water-ice-rich layered deposits that surround the south pole of the planet.

There is strong evidence for an underground lake of liquid water some 12 miles long on the planet Mars.

This underground lake was discovered using a radar instrument called MARSIS, the Mars Advanced Radar for Subsurface and Ionosphere Sounding, onboard the European Space Agency’s Mars Express spacecraft. Mars Express has been orbiting Mars for 15 years, but the evidence for this lake is based on 29 dedicated observations made between 2012 and 2015.

The main targets of the MARSIS study over the past 15 years have been the interior structures of the planet’s polar layered deposits. Both poles have these deposits, which are made up of layers of dusty water ice. MARSIS’ radar soundings have traced the layers from the surface of the ice to the underlying rock buried miles below.

The 29 observations made between 2012 and 2015 focused on Planum Australe, an area near the south pole of Mars that had not been studied before. The MARSIS team picked the Planum Australe region for this study because the surface appearance of this deposit is especially bland. When the surface is bland, it is easier to see interesting things below the ice.

MARSIS transmits radio waves at Mars using a pair of 60-foot booms on each side of the spacecraft. The radio waves bounce off the planet, and MARSIS measures the time it takes for the waves to travel there and back. The strongest reflections come from the surface of the planet.

But the long radio waves can actually penetrate the planet for several miles below the surface and reflect off subsurface boundaries between layers of different properties, such as between rock and soil, between rock and ice, or between clean ice and dusty ice. MARSIS may therefore detect multiple echoes from each radio pulse, and it pulses many times along its ground track. These data build up a “radargram,” a two-dimensional map of the subsurface.

During the 2012-2015 study of this area, MARSIS collected 29 crisscrossing radargrams. In most places the radargrams were weak and diffuse, but in a few places they were crisp and bright, indicating a very strong contrast between the layered deposits and whatever underlies them. In one location where many of the radargrams crossed, they kept hitting an especially strong and bright reflective boundary about 1 mile beneath the surface.

One radargram in this area contained a 12.5-mile-wide spot of very strong radar contrast, suggesting the presence of an unusual material that is very different from the ice above it or the rock below. This feature has been consistently present over the three-year period. This is just the type of data that would be expected if a long-existing lake were present.

Subglacial lakes are found in similar locations here on Earth. But on Mars, the temperature of this lake would be about minus 90 degrees Fahrenheit, and the water should not be liquid, but rock-hard ice. However, there is a way the water could be liquid. On Earth, sub-ice lakes exist at temperatures as low as 8 degrees. The freezing point of water in lakes under ice in Antarctica, in the presence of large amounts of salt, can be even lower.

Salts of sodium, magnesium, and calcium have been found on Mars. If concentrated enough, water containing these salts could have a freezing point as low as minus 100 degrees. It is therefore possible that a lake with water containing high concentrations of salts could be liquid at this location. According to Roberto Orosei of Italy’s National Institute of Astrophysics, a sub-ice briny lake is the most likely explanation for this unusual reflective spot.

This is not quite strong enough evidence, however, to constitute scientific proof. The MARSIS radar system barely has the resolution to find a 12-mile-long lake. Higher-resolution data from a future mission could help to confirm that such a lake exists.

The discovery of possible underground lakes on Mars is intriguing because such lakes are also found near the Earth’s poles. In recent years scientists have actually drilled deep beneath the Antarctic ice into subglacial Lake Whillans. This underground lake has been cut off from the surface for millions of years, and yet bacteria were found still living there, in complete isolation.

As water retreated on the surface of Mars, any life that was present might have moved deeper underground. With liquid water and the right chemical elements available to supply energy, a buried Martian lake could have the ingredients needed to sustain life.

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at  Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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