An image of Mars' southern polar ice cap, captured by the European Mars Express spacecraft. (ESA/DLR/FU-Berlin/Bill Dunford)
In 2018, a highly sophisticated instrument probing the surface of Mars called MARSIS (Mars Advanced Radar for Subsurface and Ionospheric Sounding) detected radar echoes from an area deep beneath the dry, frigid surface of the planet’s southern polar region.
A group of researchers analyzing that data from the European Mars Express spacecraft were excited by a tempting possibility: the radar pings could have reflected off a lake of liquid water laying hidden below the surface of the planet, a protected underworld.
Later, the researchers identified several more similar reflections nearby. And just last month, two scientists from NASA’s Jet Propulsion Laboratory reported finding dozens of the unusual formations, deepening the mystery further.
The JPL scientists, examining 15 years of MARSIS observations, found the lake-like, radar-reflecting bodies were actually spread across an area much broader than suggested by the original findings from back in 2018, and at greater range of depths below the planet's surface. The region surrounds Mars' South Pole.
All of this research raises a tantalizing question: Are there lakes hiding on Mars?
Ghost Lakes, or Something Else?
If the MARSIS radar reflections are caused by pools of actual, liquid water, researchers will have found another place in the solar system with the potential to harbor a life-friendly environment; a possible gold mine for astrobiologists searching for life beyond Earth.
But, there’s reason to be skeptical.
Some of the so-called lakes are within a mile of Mars’ polar surface, where temperatures, as low as -63 degrees Celsius, should freeze water solid. Even briney water with a colder freezing point has little chance of remaining liquid under these frigid conditions.
Could heat flowing outward from deep within Mars keep temperatures near the planet's surface warm enough to thaw ice? Might there be active volcanism going on down there?
Researchers have considered these possibilities, but find them unlikely. The flow of heat from Mars’ interior would need to be double what scientists understand the planet’s internal thermal dynamics are capable of.
And they haven’t identified any strong evidence of current or recent volcanic activity at the South Pole, which throws a brick of frozen ice on that idea.
So, lakes? Yes or no? At this time, the answer is not certain one way or the other.
The dozens of mysterious radar reflections spread around Mars’ South Pole are like a swarm of ghosts: we have sensed their presence, but so far their true nature eludes explanation.
Clues to Mars’ Past?
Whatever the unusual radar reflections detected by MARSIS turn out to be, their discovery and further investigation may yield more clues to secrets of Mars’ past.
Scientists are interested in what Mars’ southern polar region can tell us about the planet’s climate history.
Over the last billion or so years, Mars’ climate shifted between warmer, wetter conditions and cold, dry spells. Layer upon layer of water-ice, frozen carbon dioxide, and dust have built up to form a vast ice cap. By studying the layers, scientists can learn about the planet’s past, similar to how paleoclimatologists on Earth track ancient climate by studying the growth rings of trees or the surface of granite boulders embedded in glacial moraines.
The Hunt For Water on Mars
Mars exploration is all about the search for liquid water, and the possibly life-friendly environments it could nurture. Scientists are thirsty to find Martian water, wherever and whenever it may have existed: flowing down crater walls in seasonal bursts; underground, away from the dry, frigid conditions on the planet's surface; or deep in Mars’ distant past.
That Mars long ago possessed copious amounts of liquid surface water is an almost indisputable fact, based on many observations and measurements by orbital spacecraft, landers and rovers.
The Curiosity rover is currently crawling up the slope of a mountain in a 90-mile wide crater that once held a vast lake. The newer Perseverance robot is prospecting the dry lakebed of Jezero Crater, as well as a complex of sediments deposited by a river that once flowed into it.
Both rovers are probing mineral and geological clues left behind by water now long dried up, piecing together a window into Mars' past environment.
But can the life-giving liquid be found anywhere on Mars today?
Only further exploration can answer this.
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