There may not be a huge lake of liquid water at Mars’s south pole after all. In 2018, the European Space Agency’s Mars Express spacecraft spotted bright radar reflections under the ice cap there that seemed to indicate a lake of liquid water 20 kilometres across. But a new study shows that the signal could simply indicate iron-rich volcanic rocks under the ice.
The original signal was promising, but it was difficult to understand how the Martian climate could support a long-lived lake, even under the ice cap. “We do not understand how liquid water could be there, because we wouldn’t expect to have enough energy and pressure to melt water there, even if the water is salty,” says Cyril Grima at the University of Texas at Austin.
To dig into what else the signal may be, Grima and his colleagues performed a simulation of what the entire surface of Mars would look like if, like the south pole, it were buried under 1.4 kilometres of ice. They found bright reflections like the ones that Mars Express spotted scattered everywhere across the planet, covering up to 2 per cent of its surface.
These bright areas tended to match up with the locations of volcanic plains, terrain created when iron-rich lava flowed across the surface of Mars early in its history. That indicates that the signal from beneath the ice may have come from volcanic rock, not liquid water.
Organic compounds – those containing carbon atoms – found all over Mars were produced by water interacting with volcanic rocks, according to the analysis of a Martian meteorite. These compounds are important to life, and figuring out how they are produced could help us understand Mars life if we ever find it.
It is possible for living organisms to create these organics, but the compounds can also be produced by non-biological chemical reactions. How they were formed on Mars is still under debate.
Andrew Steele at the Carnegie Institution for Science in Washington DC and his colleagues examined a Martian meteorite containing organics. The rock was ejected from the surface of Mars by a collision and later landed on Earth. They found that its organics were formed by water filtering through tiny cracks and pores in the rock and interacting with the minerals there.
The team started by taking an extremely thin slice of a rock called the Allan Hills 84001 meteorite. Then, they used sophisticated microscopes to examine the slice in detail. “A lot of the features we’re looking at are even smaller than a single bacterium,” says Steele.
They found telltale signs of two geological processes called carbonisation and serpentinisation, each of which occur when water interacts with rock. The rock then dissolves to leave behind the hydrogen and carbon dioxide that then react to create organic molecules. “Life isn’t part of this process,” says Steele.
However, even though the organics in Mars rocks seem to be created abiotically – without the help of any life – those compounds could still be important in the hunt for life on Mars.
“Understanding the carbon cycle on Mars is really important to baselining life,” says Steele. “Abiotic chemistry gives you a background, and life picks and chooses what it needs from that and concentrates those up, so to find life you look for higher concentrations of those background abiotic levels.”
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