Instead of its scarce atmospheric water being confined in Mars' lower atmosphere, a new study finds evidence that water on Mars is directly transported to the upper atmosphere, where it is converted to atomic hydrogen that escapes to space. This is a pattern more apparent during dust storms, including the 2018 global dust storm, the authors say. "The seasonal and dust storm-mediated delivery of water to the upper atmosphere could have played a substantial role in the evolution of the martian climate from its warm and wet state billions of years ago to the cold and dry planet we observe today," Shane Stone and colleagues write. This process dominates the current loss of water from Mars. Mars was once a wet planet - ancient riverbeds and relic shorelines record a time when abundant liquid water flowed freely across the surface. While water still exists on Mars, there is far less than once flowed on the surface and most is locked up in the planet's ice caps, with only trace amounts of water vapor in its atmosphere. Most of Mars' water has been slowly converted to hydrogen in the atmosphere, which is lost to space. This gradually removed the planet's water over several billion years, in a process which continues today. Models of the process suggest water is converted to molecular hydrogen at lower altitudes, before being transported to the upper atmosphere. Using in situ measurements taken by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft as it flew through the red planet's upper atmosphere, Shane Stone and colleagues found water at higher altitudes than expected. The results show that water is transported directly to the upper atmosphere and converted there into atomic hydrogen by reactions with atmospheric ions. The authors also found that water abundance in Mars' upper atmosphere varied seasonally, peaking in the southern summer and surging during regional and global dust storms.
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Journal
Science