Water on Mars: The Red Planet is mostly bone-dry today (Image: GETTY)
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If Mars s water escaped into space through the atmosphere, the researchers would expect to find a very specific ratio of hydrogen to deuterium.
Instead, their findings suggest the planet s water was lost through atmospheric escape as well as being locked up in the crust.
On Earth, water that seeps into the crust forms mineral with water in their structure.
The minerals are melted down and the water works its way back into the atmosphere through plate tectonics and volcanism.
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Mars, however, does not have any plate tectonics so when the planet dried up, the drying was permanent.