Salt deposits on Mars contain clues to ancient water sources

For centuries, miners have dug into the earth in search of salt, deposited in thick layers of ancient oceans long evaporated. When scientists discovered huge salt deposits on Mars, they immediately wondered if it meant that Mars had also had giant oceans. Yet the significance of these deposits on the climate of the red planet remains unclear.

A new study by UChicago researchers disrupts the image of Martian salt and suggests new ways to test the appearance of Mars water.

"They are not in the right places to mark the death of the oceans, but they date from the transition of the climate of Mars, from the beginning of the time of the rivers and overflowing lakes to the cold and desert planet we see today. "," Said the author of the study, Edwin Kite. , an assistant professor of geophysical science at the University of Chicago and an expert in the history of Mars and other-world climates. "So, these salt deposits could tell us how and why Mars has dried up."

The salt contained in the Martian deposits is not the same as that of the Earth's oceans. It actually looks more like Epsom salt, consisting of two ingredients: magnesium and sulfuric acid. By determining how these two chemicals combined can give us information on the climate of Mars.

One possibility is that Mars water has circulated deep in the soil, carrying magnesium to the surface where it reacted with sulfuric acid. This means that the planet would have been hot enough to allow the groundwater to run off.

The other option is that the magnesium was simply blown into the form of dirt. In this case, the climate could be as cold as the coast of Antarctica.

The Kite team focused on the groundwater scenario, building models to see if it would be realistic. The researchers' analysis emphasized that there was so much Martian salt that it could not be deposited as a spot dewatering: the water had to recover the salts several times, evaporate , turn in liquid water and repeat the cycle. Whenever this happened, as the water flowed into the soil, it would have resulted in a little carbon dioxide from the atmosphere with it.

The problem is that too much carbon dioxide in the atmosphere warms the planet – as we discover on Earth – too little will freeze it. If too much carbon was trapped in the soil and the resulting atmosphere was too thin to keep Mars warm, the movement of groundwater would stop when the planet was frozen. And the analysis revealed that the cycle was blocking a lot of carbon.

This does not look promising for the groundwater scenario, said Kite, but that does not refute it. "Most of our models use poor groundwater, but we have also discovered some" loopholes "that could allow Mars to keep enough carbon in the atmosphere," he said.

Fortunately, there would be signals that NASA's Curiosity rover (currently on Mars) could test when it arrives at a salt deposit, hopefully by 2020.

"Curiosity has an excellent set of instruments, so it's possible to get very interesting data," said Mohit Melwani Daswani, co-author of the study, former postdoctoral researcher. at UChicago, today at NASA Jet Propulsion Laboratory.