Already a desert planet, Mars is still losing the water it has left at a far greater speed than astronomers expected. Researchers from the French National Centre for Scientific Research think they know what’s happening.
Mars was once home to vast oceans, rivers, and streams. Over the course of billions of years, these bodies of water were lost to space, transforming the red planet into the desert world we see today.
Despite this, some water remains on Mars, hidden beneath the surface, and at the ice caps which bookend the planet. This water is still being lost to space, and a new study shows the rate at which this already-parched world is shedding its remaining water is significantly greater than astronomers originally believed.
Sunlight heats the Martian poles, causing ice to sublimate (turn directly from solid to gas, like dry ice on Earth), and enter the atmosphere. This moisture is carried to higher, colder altitudes by wind. When these molecules encounter dust, they can condense into clouds, preventing further lifting.
However, if there is not enough dust in the Martian atmosphere, water can collect in layers where clouds would naturally form.
“Measurements showed that large atmospheric pockets are even in a state of supersaturation, with the atmosphere containing 10 to 100 times more water vapor than its temperature should theoretically allow,” researchers from the French National Centre for Scientific Research (CNRS) state.
Under these conditions, water molecules continue to rise into the upper atmosphere, where they encounter ultraviolet light from the Sun, breaking the molecules into hydrogen and oxygen. These molecules, especially hydrogen, can easily escape to space, resulting in a net loss of water from the Red Planet.
We had Mars, we had Fun, we had Seasons in the Sun…
“There is every reason to think that in the coming years Mars and its mysteries will become increasingly familiar to the inhabitants of the Planet Earth.”
― Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space
Like Earth, Mars experiences seasons, which create cycles in the rate at which water rises into the atmosphere. Previously, astronomers believed the processes by which water was lost to space from the upper atmosphere of Mars took centuries to complete, negating seasonal effects. However, new research shows this loss can take place over a period of just months, radically altering our understanding of the rate at which Mars loses its remaining water to space.
“The observed short-term variability of the hydrogen atoms populating the exosphere could be caused by direct deposition of water molecules at altitudes high enough to expose them to sunlight, which subsequently triggers a rapid enhancement of hydrogen atoms in the exosphere,” researchers from the French National Centre for Scientific Research (CNRS) report in Science.
Another factor driving the rate at which water is driven from Mars is the distance at which the Red Planet orbits the Sun. Temperature differences resulting from moving between perihelion (closest approach to the Sun) and appellation (the greatest distance) are greater on Mars than on Earth, giving Mars a more complex seasonal cycle than our home world. These odd seasons of Mars are also known to play a role in driving dust storms on that planet, like the one that silenced NASA’s Opportunity rover.
“Our results also show that water access to high altitude is affected by the seasonal changes around perihelion. Although planetary-scale dust storms appear in this period, those irregular events have a lesser impact than does seasonal change,” investigators describe.
Data utilized in this study was recorded by the Trace Gas Orbiter, part of the ExoMars program, managed by the European Space Agency (ESA) and the Russian space agency, Roscosmos.
The year 2020 will be an exciting year for our understanding of Mars, as several missions will launch to the Red Planet this year.
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