Is There a Briny Lake on Mars? MARSIS Picks Up Signals Similar to Subterranean Liquid Water

Is There a Briny Lake on Mars? MARSIS Picks Up Signals Similar to Subterranean Liquid Water.
(Photo: Wikimedia Commons/ESA & MPS for OSIRIS Team )

European Space Agency's Mars Express orbiter used its MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument to identify what appeared to be a 20-kilometer lake of liquid water, and here's what they found out.

MARSIS Picks Up A Water Echo on Mars?

MARSIS received a radar echo similar to underground liquid water on Mars. However, liquid water cannot remain too close to the surface of Mars due to the planet's extreme temperature and low air pressure. Brine lakes could survive near the foot of the south polar ice cover due to the temperature and pressure circumstances there and a small amount of natural antifreeze.

This antifreeze may be calcium and magnesium perchlorate, a chemical molecule discovered by NASA's Phoenix mission in 2008 on the surface of Mars. When dissolved in water, magnesium, and calcium perchlorate would reduce the freezing point to a minimum of minus 68 and minus 75 degrees Celsius (minus 92 and 103 degrees Fahrenheit), respectively. These temperatures are extremely close to the minus 68 degrees Celsius (minus 90 degrees Fahrenheit) predicted to be at the base of the ice cap. As such, it is not inconceivable to conceive localized temperatures, pressures, and perchlorate concentrations that work together to allow for sizable liquid water pools on Mars.

Measuring the surface ice's undulations provided additional support for the existence of these lakes since liquid water reduces friction between ice sheets and the bedrock below them, accelerating the ice sheets' movement over the bedrock. Planum Australe exhibits precisely the troughs and peaks in the surface ice caused by this increase in flow rate.

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Radar Echo Not From a Body of Water

In a new study, a team of scientists from Cornell University have fed the flames of this skepticism with new findings that provide an alternative explanation for the radar echoes. According to Daniel Lalich, lead author of the study, it's not impossible that there's liquid on Mars. However, he isn't convinced that the echo picked up by MARSIS was from a body of water on the Red Planet, noting that there were a lot of factors to consider to determine the echo picked up by the radar.

Because lakes are so flat, they are excellent at reflecting radar to their source. On Earth, intense radar reflections found by MARSIS would almost probably indicate liquid water, just as the water pockets beneath Antarctica found in places like Lake Vostok. Planetary scientists must exercise caution when extrapolating Earth's conditions to other planets, as they differ greatly.

Lalich's team conducted hundreds of simulations to determine if several densely packed layers of ice might replicate a lake's radar signal. Each simulation included different ice layer thicknesses and compositions, or how dirty the layers were.

They discovered that densely packed ice layers that were crushed by the weight of the ice sheet and deposited a long time ago can, in certain cases, produce brilliant radar reflections that resemble those picked up by MARSIS.

The radar waves' "constructive interference" is the trick. The radar equipment on MARSIS has poor spatial resolution, making it unable to discern ice layers that are too thin. Since the layers are packed so closely together, some of the radar beams would be reflected by each layer. As a result, the radar echoes overlap and merge, intensifying and appearing brighter.

Although the existence of a briny lake beneath the south polar cap is still a mystery, Lalich contends that the simulations offer a far more straightforward and likely explanation than a lake.

"The idea that there would be liquid water even somewhat near the surface would have been really exciting," said Lalich. "I just don't think it's there."