Scientists revisiting a mysterious signal from the south pole of Mars have suggested a new potential explanation, and it doesn't bode well for hopes of finding liquid water on the Red Planet.
In 2018, scientists using data from the European Space Agency's Mars Express orbiter's Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument announced that they'd observed a radar signal that could be interpreted as evidence of liquid water. That signal, a strange bright reflection, came from the Martian south pole in a region known as Ultima Scopuli. Researchers investigating the reflection now suggest that the signal didn't come from the ice itself, or even from liquid water, but from underlying geological layers made of minerals and frozen carbon dioxide. In particular, it turned out that the thickness of these layers, rather than what they're made of, creates the otherworldly reflection.
On Earth, dazzling reflections like this often come from liquid water. For example, subglacial lakes like Antarctica's Lake Vostok, which has been lying under more than 2 miles (3 kilometers) of ice for millions of years, causes a bright radar signal like that found on Mars. However, just because it's possible for something like this to have also occurred on Mars doesn't guarantee the presence of liquid water.
Related: These dry ice glaciers on Mars are moving at its south pole
The research team used radar data from MARSIS, along with computer simulations, to investigate this mystery. The scientists simulated layers of ice and other substances, like basalt rock that formed after ancient volcanic eruptions on Mars, to see how these materials would react to incoming light.
Because there is an immense amount of carbon dioxide frozen in the Martian south pole, Cornell University planetary scientist and lead author of the research Dan Lalich was sure to include layers of this ice in the simulations. And one simulation in particular, with a layer of carbon dioxide ice and below the water ice, revealed that the separation and thickness of layers determined the strength of a reflection.
Previous studies Lalich worked on also found that certain minerals could also conjure a reflection like this. He believes that even layers of the Red Planet's dust-obscured ice are capable of it. Either way, no liquid water is necessary to create the reflection.
"I could have used rock layers or even particularly dusty water ice and I would have gotten similar results," he said in a statement. "The point of this paper is really that the composition of the basal layers is less important than the layer thicknesses and separations."
The new research doesn't mean that there is no chance of liquid water existing somewhere on Mars, however.
"None of the work we’ve done disproves the possible existence of liquid water down there," Lalich said. "We just think the interference hypothesis is more consistent with other observations. I'm not sure anything short of a drill could prove either side of this debate definitively right or wrong."
Whether under a glacier or deep beneath the planet's scorched reddish surface, water — and possibly traces of life or — may still be lurking somewhere.
The research is described in a paper published Sept. 28 in Nature Astronomy.
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