Mars is dead – plain and simple – but billions of years ago the bone-dry planet resembled a young Earth, with a lush atmosphere and surface waters. For the longest time, scientists assumed Mars’s water escaped into the atmosphere and into space due to the planet’s weak gravity. But a team of researchers has now found between 30 and 99 percent of that water never escaped and could be lurking just beneath the surface.
According to Eva Scheller, the study’s lead author and a PhD candidate at the California Institute of Technology (Caltech), the previously accepted theory simply does not account for how much water Mars has lost.
About four billion years ago, a young Mars is believed to have once had enough water to cover the whole planet in an ocean 330 to 4,920ft (100 to 1,500m) deep.
Compared to Earth, this is about half of the volume of the Atlantic Ocean, although it is worth remembering the Red Planet is about half the size of Earth.
Some of that water would have escaped into the atmosphere but not enough to explain why Mars is so dry today.
The study’s results were published in the latest issue of Science.
The authors also presented their findings on Tuesday at the 52nd Lunar and Planetary Science Conference (LPSC), held online because of the coronavirus pandemic.
The Caltech/NASA team studied how water behaves on Mars in various forms – vapour, liquid and ice – over time.
The researchers also looked at the chemical composition of the planet’s crust, looking at the ratio of deuterium to hydrogen.
Deuterium or heavy hydrogen is a heavier isotope of hydrogen that holds one proton and one neutron at its core, unlike regular hydrogen with a single proton.
The chemical formula of water is H2O or two hydrogens and a single oxygen atom.
Within that mix, about 0.02 percent of the hydrogen atoms are deuterium isotopes.
Because regular hydrogen is lighter, escaping the clutches of Mars’s gravity is much easier.
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.
Mars, however, does not have any plate tectonics so when the planet dried up, the drying was permanent.
Michael Meyer, the lead scientist of NASA’s Mars Exploration Program, said: “The hydrated materials on our own planet are being continually recycled through plate tectonics.
“Because we have measurements from multiple spacecraft, we can see that Mars doesn’t recycle, and so water is now locked up in the crust or been lost to space.”
Ms Scheller added: “All of this water was sequestered fairly early on, and then never cycled back out.”