Mars was once home to lakes and oceans — but where all the water went to transform the planet into the desolate rock we know today has been something of a mystery.
Most of it was thought to have been lost to space, but a new study funded by NASA proposes that it did not go anywhere, but is trapped within minerals in the crust.
“We’re saying that the crust forms what we call hydrated minerals, so minerals that actually have water in their crystal structure,” said Eva Scheller, lead author of the new paper in Science.
Photo: AFP / NASA / Jet Propulsion Laboratory- California Institute of Technology / Arizona State University
Scheller’s model suggests anywhere between 30 and 99 percent of the initial water remains trapped inside these minerals.
Early Mars was thought to have enough water to cover the whole planet in about 100m to 1,500m of ocean. Because the planet lost its magnetic field early in its history, its atmosphere was progressively stripped away, and it was assumed this was how it lost its water.
However, the authors of the new study believe that while some of the water did disappear, the majority remained.
Using observations made by Mars rovers, as well as of meteorites from the planet, the team focused on hydrogen, a key component of water.
There are different kinds of hydrogen atoms. Most have just one proton in their nucleus, but a tiny fraction, about 0.02 percent, have both a proton and a neutron, making them heavier. These are known as deuterium, or “heavy” hydrogen. Because the lighter kind escapes the planet’s atmosphere at a faster rate, the loss of most of the water to space would leave relatively more deuterium behind.
However, given how much water the planet is believed to have started with, and the current rate of hydrogen escape observed by spacecraft, the current deuterium-to-hydrogen ratio cannot be explained by atmospheric loss alone.
The study’s authors instead say there was a combination of two mechanisms: the trapping of water in minerals in the planet’s crust, as well as the loss of water to the atmosphere.
“Anytime that you have a rock and it’s interacting with water, there’s a series of very complex reactions that form a hydrated mineral,” Scheller said.
This process, called “chemical weathering,” also takes place on Earth — for example, in clay, also found on Mars.
However, on our planet volcanoes recycle the water back into the atmosphere, while Mars does not have tectonic plates, making the changes permanent.
According to the teams’ simulations, the planet lost most of its water between 4 billion and 3.7 billion years ago, which means “Mars was pretty much like we see how it is today for the past 3 billion years,” Scheller said.
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