Billions of years ago, the planet Mars was awash with water. What happened to it is not yet fully understood.

Most of the water was believed to have been lost to space. However, a new study funded by NASA suggests that much of the planet's water could be trapped within minerals in its crust. 

What happened to the water?

"We're saying that the crust forms what we call hydrated minerals, so minerals that actually have water in their crystal structure," Eva Scheller, lead author of the paper in Science magazine told AFP news agency. 

Mars is believed to to have once had enough water to cover the whole planet, to a depth of anything from around 100 to 1,500 meters (330 to 44,920 feet). As the planet lost its magnetic field, the atmosphere was stripped away and led to the loss of water.

The study suggests that anywhere between 30 to 99 percent of the arid planet's initial water could be stored in the crust. 

Much longer term, if space travel were to advance considerably, hydrogen deposits on stellar bodies could theoretically be extracted and harnessed to create abundant fuel sources.

The research team, consisting of geologists and atmospheric scientists used observations from the Mars rover as well as other meteorites from the planet. Their simulations suggest that Mars lost its water between four to 3.7 billion years ago. 

Study focused on hydrogen

The study focused on hydrogen, the key component of water along with oxygen. While most hydrogen atoms have only one proton in the nucleus, around 0.02% of hydrogen atoms have both a proton and a neutron, making them heavier. These are called deuterium or heavy hydrogen.

Had the water been lost to space, scientists would then expect relatively more deuterium to be left behind. But the current deuterium-to-hydrogen ratio cannot be explained by atmospheric loss alone. The study theorizes that the water was trapped in the crust as well as lost 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," said Scheller. 

This process occurs on Earth as well. It is called "chemical weathering" and volcanoes recycle the water back into the earth. Since Mars has no tectonic plates, the changes on the Red Planet are permanent. 

Scheller said she was excited about what NASA's Perseverance rover, which landed last month for a multi-year science mission, might be able to contribute to the area of research.