Science Gazette

Is there liquid water under the south polar cap of Mars?

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Slushy brines are suspected based on geophysical radar reflections and laboratory studies.

A scientist from the Southwest Research Institute tested the characteristics of ice-brine mixes as low as -145 degrees Fahrenheit to establish that salty water occurs between granules of ice or sediment under Mars’ south pole ice cover. Oddly intense reflections seen by the MARSIS subsurface sounding radar onboard ESA’s Mars Express mission are supported by laboratory measurements made by SwRI geophysicist Dr. David Stillman.

MARSIS flies around the earth with a 130-foot antenna, bouncing radio waves over a target region and then receiving and analyzing the echoes or reflections. Any near-surface liquid water should emit a bright, powerful signal, but ice and rock should emit a considerably weaker signal.

Many scientists have questioned the likelihood of liquid water on Mars since traditional models predict temperatures below the melting point of water near the south polar cap. Clay, hydrated salts, and saline ices have all been suggested as possible sources for the strong basal reflections. The team lead by an Italian utilized previously published data, models, and fresh laboratory observations to investigate the postulated occurrences.

Stillman, a specialist in detecting water in any format — liquid, ice, or absorbed — on planetary bodies and co-author of a paper describing these findings, said, “Lakes of liquid water actually exist beneath glaciers in Arctic and Antarctic regions, so we have Earth analogs for finding liquid water beneath ice.” “Exotic salts found on Mars have incredible ‘antifreeze’ qualities, enabling brines to stay liquid at temperatures as low as -103 degrees Fahrenheit. In our lab, we investigated these salts to see how they might react to radar.”

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Stillman has over a decade of expertise detecting and characterizing underlying ice, unfrozen water, and the possibility for life across the solar system by evaluating the characteristics of materials at freezing temperatures. Stillman used a SwRI environmental chamber to study the characteristics of perchlorate brines, which creates near-liquid-nitrogen temperatures at Mars-like pressures.

Stillman said, “My Italian colleagues contacted me to check whether my laboratory experiment findings would confirm the existence of liquid water under the Martian ice cover.” “The study found that we don’t need lakes of perchlorate and chloride brines to have a high dielectric response; instead, these brines may reside between the grains of ice or sediments. This is akin to how saltwater saturates grains of sand on the beach or how flavoring pervades a slushie, but at -103 degrees Fahrenheit under a mile of ice near Mars’ South Pole.”

Because all known life needs water, the quest for water in the universe is based in the search for possible habitability.

“In this instance, ‘following the river’ has taken us to a location where life as we know it could not thrive,” Stillman said. “But it’s still fascinating, and who knows where alien life may have evolved?”

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