How Mars Lost Its Water, the Key Ingredient of Life

Mars could have lost its liquid water, the key ingredient of life, because it is small, according to a study.

Washington University researchers say that the Red Planet's size may be the key reason it lost liquid water and looks so different from the "blue marble" that is Earth, despite being so close to our planet.

The theory, detailed in a paper published in PNAS, joins several others developed to explain why liquid water no longer flows across the surface of Mars.

"Mars' fate was decided from the beginning," co-author Kun Wang, assistant professor of Earth and planetary sciences, Washington University, said in a press release from the institution.

We have known since at least 1971, when the Mariner 9 Mars orbiter captured images of dry river beds on the surface of Mars, that water once freely flowed on the Red Planet. Since then further missions have detected more dry lake beds and river basins.

In fact, California Institute of Technology (Caltech) scientists estimate that around four billion years ago there was enough water on Mars to cover the whole planet in an ocean that was as deep as 1,500 meters (4,921 feet).

The fact that water only seems to exist on the planet in the form of ice at high altitudes poses the question, what happened to the liquid water on Mars?

Where Did Mars' Water Go?

In order to reach the conclusion that Mars lost its water because of its relatively small size, Wang and the Washington University team measured a stable form, or isotope, of potassium in 20 confirmed Martian meteorites. Potassium is a good measure of compounds like water that are easily vaporized, called volatiles.

The team had previously used the same method to investigate how the moon formed. They found that Mars clung to its volatiles much longer than much smaller bodies, such as the moon and the asteroid 4-Vesta.

"There is likely a threshold on the size requirements of rocky planets to retain enough water to enable habitability and plate tectonics, with a mass exceeding that of Mars," Wang said.

This suggests that the larger a planet or other space object is, the longer it likely hangs on its its volatiles including water, the key ingredient of life.

This isn't the first theory planetary scientists have developed to explain why, despite existing in the solar system's habitable zone (the region around a star that is neither too hot nor too cold to allow water to exist as a liquid) the Martian surface is dry.

One of the most prominent theories to explain this suggests that Mars once had a much stronger magnetic field. When this field weakened because of the planet's core cooling, the Martian atmosphere was stripped away by harsh radiation from the Sun.

The radical thinning of the Martian atmosphere caused low pressure, leading to water vaporizing, according to the theory. This theory suggests the water vapor then leaked into space over the following billions of years.

Other theories have suggested that its changing distance from the Sun could have altered the Martian climate, causing water to vaporize and rise into the atmosphere from where it was lost to space.

The action of dust storms has also been presented as one of the causes of Mars liquid water depletion.

In a paper published in Nature Astronomy in August 2021, researchers from the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder suggested that regional dust storms on the Red Planet could increase the rate at which water is lost to space by as much as 10 times its normal rate.

The team suggested that dust storms that occur every summer on Mars prevent water at high altitudes from turning to ice. This then leaves those water molecules vulnerable to infrared radiation from the sun, which breaks them down into hydrogen and oxygen atoms.

Because hydrogen is the lightest element it is easily lost to space from the upper atmosphere, preventing the water molecules from reforming.

One explanation for the lack of liquid water at the surface of Mars that has emerged recently suggests that the planet did not lose its liquid water at all. Rather, liquid water on the Red Planet is confined beneath its surface, locked away in its rocks.

Is Water Still on Mars Trapped Beneath its Surface?

In a paper published earlier this year in Science, Caltech researchers suggested that while it is clear Mars has lost water to space, the majority of its liquid water could be locked up deep beneath its surface.

The paper's lead author, Ph.D. candidate Eva Scheller, said in a press release from the university at the time: "Atmospheric escape doesn't fully explain the data that we have for how much water actually once existed on Mars."

The team suggested that liquid water on the surface of Mars mixed with rock from clays and other minerals that contain water. This happens here on Earth too, but because our planet is tectonically active these layers are destroyed when they move into the mantle, thus recycling the water back to the surface.

Mars isn't as tectonically active as Earth, meaning that this water remains locked up in clays and minerals beneath the Martian surface. The Caltech team will look to samples of Mars rock and mineral collected by the Perseverance rover when they are returned to Earth to confirm this theory.

Should the theory that Mars lost its water because of its smaller size, as suggested by the University of Washington team and Kun Wang, be correct it could have implications that reach beyond Mars and even beyond the reaches of our solar system.

The finding could help astronomers and planetary scientists better understand which planets can hang on to liquid water, and thus could be the best targets in our search for life elsewhere in the Universe.

The investigation of the atmospheres of exoplanets (planets beyond the solar system) is set to intensify later this year with the launch of the James Webb Space Telescope (JWST). The JWST will be looking at these atmospheres for traces of water and other compounds, including organic compounds, which could hint at life on exoplanets.

The relationship between planet size and liquid water could help the operators of the JWST decide which exoplanets it should be trained on to increase the chances of making such a detection.

"Based on size and mass, we now know whether an exoplanet is a candidate for life because a first-order determining factor for volatile retention is size," Wang said. "The size of an exoplanet is one of the parameters that is easiest to determine."