Billions of years ago, scientists think Mars was much warmer and wetter than it is now, so where did all that water go? New research suggests much of it is actually locked inside the Martian rocks, which have soaked up the liquid water like a giant sponge.
That teases an interesting addition to the commonly held hypothesis that the planet dried out as its atmosphere was stripped away by solar winds.
Using computer modelling techniques and data we've collected on rocks here on Earth, the international team of scientists reckon that basalt rocks on Mars can hold up to 25 percent more water than the equivalent rocks on our own planet, and that could help explain where all the water disappeared to.
"People have thought about this question for a long time, but never tested the theory of the water being absorbed as a result of simple rock reactions," says lead researcher Jon Wade from the University of Oxford in the UK.
Thanks to differences in temperature, pressure, and the chemical make-up of the rocks themselves, water on Mars could've been sucked up by the rocky surface while Earth kept its lakes and oceans, the researchers say.
Martian rocks can also hold water down to a greater depth than the rocks on Earth can, according to the simulations.
"The Earth's current system of plate tectonics prevents drastic changes in surface water levels, with wet rocks efficiently dehydrating before they enter the Earth's relatively dry mantle," explains Wade.
In the early days of the Earth and Mars, however, this wouldn't have been the case, the researchers suggest. Volcanic lava layers would have changed the make-up of the rocks at the surface and could have made them more absorbent.
"On Mars, water reacting with the freshly erupted lavas that form its basaltic crust, resulted in a sponge-like effect," says Wade. "The planet's water then reacted with the rocks to form a variety of water-bearing minerals.
"This water-rock reaction changed the rock mineralogy and caused the planetary surface to dry and become inhospitable to life."
Even small differences in the iron content of the rocks on Earth and Mars, for example, can add up to significant changes in the way water gets sucked up, the research says. Plus, Mars is a much smaller planet, which would also have been a factor.
The team agrees that solar winds are likely to have stripped away some of the water on Mars, but argues that much more of it could be locked away inside the Red Planet than previously thought – very handy once we get to set up base there.
Experts also think Mars is hiding big reserves of water in the form of underground ice. But until we can take more readings and samples from the surface, it's all just educated guesswork for the time being.
Now the researchers want to use the same principles to study the possibility of finding water locked away in other planets, based on the composition of their rocks and tectonic activity – and where there's water, there might be life.
"When looking for life on other planets it is not just about having the right bulk chemistry, but also very subtle things like the way the planet is put together, which may have big effects on whether water stays on the surface," says Wade.
"These effects and their implications for other planets have not really been explored."
The research has been published in Nature.