Ancient Mars water: A deep source?

The sinuous channels carved onto the Martian surface, first seen 3 decades ago, have convinced most scientists that water once flowed on the Red Planet. But where did the water come from?

A new analysis of a Mars meteorite that fell to Earth suggests that ancient Martian magma–molten rock from deep within the Red Planet–contained significant amounts of water before it erupted on the planet’s surface. Before it solidified, the molten rock contained as much as 1.8 percent water, Timothy L. Grove of the Massachusetts Institute of Technology and Harry Y. McSween Jr. of the University of Tennessee in Knoxville and their colleagues report in the Jan. 25 Nature.

When such water-bearing magma reached the surface and cooled, water vapor would have escaped into the atmosphere, much like bubbles from a bottle of soda. The vapor could then have rained down to the surface. The analysis of the Mars meteorite, known as Shergotty, doesn’t explain why the Martian interior would have contained so much water in the first place.

At 175 million years old, Shergotty is a young meteorite. Discovered in 1865, Shergotty is believed to have originated in a volcanic region on Mars called Tharsis. Measurements show that the rock now contains only a few hundredths of a percent of water. However, the researchers found signs that the meteorite had a more water-rich past. Shergotty contains certain crystals that could only have formed if the magma that makes up the rock was 2 percent water by weight.

In their laboratory, McSween, Grove, and their colleagues created a synthetic version of Mars rocks. By subjecting the synthetic rocks to high temperature and pressure, the team was able to deduce how much water Mars rocks would have contained at the time when they were molten and just beginning to solidify.

“Degassing from magmas is a very reasonable means” for producing water on ancient Mars, notes Victor R. Baker of the University of Arizona in Tucson. What’s more, he adds, recent evidence shows that “there has been considerable volcanic action through the whole of Martian history, even up to very recent times.”

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