It is widely believed that water once flowed across the plant's surface, as shown by dried-up deltas and riverbanks. 

Now, Mars is a barren desert. 

Where did all those waters go? 

Water on Mars

For decades, scientists have been attempting to understand the situation, trying to learn why Mars became an arid wasteland while Earth preserved its water and became a biological paradise.

A team of geologists and atmospheric scientists has produced a modern vision of Mars' history by plugging observations of the red planet into new models: Many of the planet's ancient water may have been frozen in crystals in the crust, where it may still be found today.

Where Did They Go?

According to previous studies, the majority of Mars' water fled into space when the planet's atmosphere was washed away by the sun's radiation. However, according to a recent report published today in the journal Science and discussed practically at this year's Lunar and Planetary Science Conference, Mars' water underwent both an atmospheric evacuation and a geologic entrapment.

Early Mars was believed to have had enough water to flood the whole world in an ocean ranging in depth from 100 to 1,500 meters (330 to 4,920 feet) - about half the Earth's Atlantic Ocean size. Although some of this water is undeniably lost on Mars due to atmospheric escape, the recent results presented in Science suggest that this does not account for most of its water loss.

Lead author and Caltech Ph.D. candidate Eva Scheller, along with co-authors Bethany Ehlmann, professor of planetary science at Caltech and associate director for the Keck Institute for Space Studies; and Yuk Yung, professor of planetary science at Caltech and senior research scientist at NASA's Jet Propulsion Laboratory, presented the findings at the 52nd Lunar and Planetary Science Conference (LPSC).

Related Article: Mars Dust Particles Equal to Lightning in Earth, Study Finds

Atmospheric Escape

"Atmospheric escape doesn't entirely justify the evidence we have on how much water occurred on Mars in the past," Scheller said.

The study team combined data from several NASA Mars Exploration Program missions and meteorite lab work using a wealth of cross-mission data archived in NASA's Planetary Data System (PDS). The team looked at the amount of water on Mars over time in all ways (vapor, liquid, and ice) as well as the chemical composition of the planet's present atmosphere and surface, focusing on the deuterium to hydrogen (D/H) ratio.

Although hydrogen and oxygen make up water, not all hydrogen atoms are produced equal. The vast majority of hydrogen atoms contain only one proton in their nucleus, while deuterium, or so-called "heavy" hydrogen, contains both a proton and a neutron. The lighter hydrogen is much faster to overcome the planet's gravity into orbit than its denser equivalent. As a result, the depletion of a planet's water from the upper atmosphere will leave a telling indication in the planet's atmosphere's deuterium-to-hydrogen ratio: a significant volume of deuterium would be left behind.

Did Atmospheric Escape Drain Out All the Water?

However, water leakage in the atmosphere alone cannot account for the detected deuterium-to-hydrogen signal in the Martian atmosphere as well as massive quantities of water in the past. Instead, the research argues that the detected deuterium-to-hydrogen signal in the Martian atmosphere can be interpreted by a mixture of two mechanisms: the trapping of water in minerals in the planet's crust and the leakage of water to the atmosphere.

Chemical Weathering

Chemical weathering occurs as water reacts with rock, creating clays and other hydrous minerals with water as part of their mineral composition. This is a mechanism that happens both on Earth and on Mars. Volcanism recycles water and other molecules back into the atmosphere by melting old crust into the mantle and forming new crust at plate boundaries. However, since Mars lacks tectonic plates, the "drying" of the atmosphere is irreversible until it occurs.

The paper's study and results illustrate the valuable contributions of early-career scientists in furthering our solar system knowledge. Similarly, the study, which used evidence from meteorites, balloons, satellite surveys, and samples analyzed by Mars rovers, stresses the importance of finding several ways to explore the Red Planet.

ALSO READ: Earth vs. Mars: How Similar Are They?

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