Several billion years ago, liquid water shaped the landscape of Mars, sheltered by a beautiful atmosphere. All that water has since disappeared from the surface. The question is: where has it gone? Much of it is probably hiding underground, according to researchers at JPL and Caltech.
About four billion years ago, Mars looked a lot like Earth. Studies suggest that the planet was home to an ocean in its northern hemisphere, which at the time contained as much water as half the Atlantic. But take a billion year “fast forward” and you just end up with a cold, incredibly dry desert. From this wet past, only scars remain today that our rovers and orbiters are responsible for studying.
So where has all this water gone? The generally accepted answer is that she escaped into space.
No magnetic field, no liquid water
Compared to the Earth, Mars is indeed not very massive, which means that its gravitational field is quite weak. As a result, the nitrogen present in its primitive atmosphere escaped fairly quickly. The planet also cooled rapidly, which had the effect of solidifying its once liquid core. And without a liquid core, there is no magnetic field.
Then follows a chain reaction. Deprived of its magnetic field, Mars found itself at the mercy of the solar wind which eroded its atmosphere. As the planet’s volcanism has also decreased, its atmosphere has therefore had more and more difficulty in renewing itself. In fact, atmospheric pressure has decreased, and we know that the lower the pressure, the more difficult the liquid state has to maintain.
Gradually, Martian water then turned into vapor, before finally escaping into space.
That said, while some of the Martian water certainly escaped in this way, a team of researchers argues that this process cannot explain all of this loss. According to their calculations, much of this water is integrated into minerals in the earth’s crust.
At least 30% of the original water still there
As part of this work, the authors examined the amount of water in all its forms lining the Martian landscape throughout its history. They also took into account the chemical composition of its atmosphere and crust.
The team then focused on the ratio of “light” hydrogen to “heavy” hydrogen (or deuterium). Deuterium is a rarer and heavier isotope of hydrogen with an extra neutron in its nucleus. In fact, it is less likely to be lost in space than the lighter form.
This means that over time, if water vapor were to escape into space, it would leave behind a higher ratio of deuterium to ordinary hydrogen in the atmosphere.
However, after examining this report, they realized that something was wrong. Given the amount of water the planet was believed to contain, and given the rate of hydrogen leakage observed by spacecraft, the current deuterium / hydrogen ratio cannot be explained by atmospheric loss alone. Instead, they suggest that much – from 30 to 99% – has infiltrated the minerals in the soil.
“Every time you have a rock and it interacts with water, there is a series of very complex reactions that form a hydrated mineral”, explains Eva Scheller, who co-signs this work. This process, called “chemical weathering”, also takes place on Earth (clay is an example).
On Earth, this absorbed water is then recycled into the atmosphere through the interplay of plate tectonics and volcanism. But Mars is not subject to these geological vagaries. Also, if water did get trapped in the soil, it is probably still there.
Details of the study are published in the journal Science.