How Finding Evidence of Water on Mars Is Like Baking Chocolate Chip Cookies

Mars's volcanic past is well documented—its tallest volcano, Olympus Mons, is the largest in the entire solar system—but what about the water situation, in the past or present? Turns out Mars's volcanoes may have scattered evidence of near-surface water or ice when they were erupting. According to a newly published study in Nature Communications, it's simply a matter of knowing what to look for.

The presence or absence of water during an eruption can be seen in how the lava cools. The longer it takes magma to cool, the larger the crystals that form in the cooling magma become. Liquid volcanic rock that interacts with water cools quickly and flash-freezes to mostly form "basaltic glass." Without water, it takes longer to cool, giving the crystals time to grow.

The study's lead author, Kellie T. Wall, compared the texture of the rock to that of chocolate chip cookies, which varies depending on temperature of both the dough and the oven. "We were interested in the cookie dough part of the cookie," Wall said in a statement.

With this in mind, the international research team created an index for quantifying the texture of volcanic rock called "groundmass crystallinity." They then used an x-ray diffraction machine to analyze rock samples from Iceland, the American Northwest, New Zealand, and Italy, and compared them to rocks analyzed by the x-ray diffractometer on NASA's Curiosity rover.

"The rocks that erupted and interacted with water, which we call phreatomagmatic, all had a groundmass crystallinity as low as 8 percent and ranging up to about 35 percent," Wall said. "The rocks that erupted without interaction with water had groundmass crystallinities from about 45 percent upwards to almost totally crystalline."

The Martian soil samples they analyzed fell into the latter category, meaning they erupted without interacting with water.

Last year, the Curiosity rover found that there was water locked in Martian soil—doing so required heating up a soil sample to 1,535 degrees Fahrenheit, then using a gas chromatograph, mass spectrometer, and laser spectrometer to identify chemicals in the sample. It required both knowing what to look for and how to look for it before water's presence could be confirmed. Groundmass crystallinity is another tool in Curiosity—and its descendants'—toolbelt.

"Most of the studies searching for water have focused on either looking for sedimentary structures—large- and small-scale—for evidence of water, or looking for rocks like limestones that actually would have formed in a water-rich environment," Wall said. "But being able to determine the environment through the texture of a volcanic rock is something pretty cool and different.I think it's an interesting avenue for future research."