NASA’s MAVEN spacecraft detects massive water loss from ancient Mars

The dust storms triggered the loss of huge amounts of water from above into the atmosphere of Mars, according to a new analysis of data collected by NASA’s MAVEN spacecraft. For the first time, water has been traced to the upper atmosphere of Mars, say the authors of a new article in the journal Science.

Data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission shows that the abundance of water at these altitudes increases dramatically when there is a sandstorm – the dust allows water to be transported to higher altitudes, Bruce Jakosky, Principal Investigator of MAVEN and a planetary scientist at the University of Colorado at Boulder, he told me.

The greater abundance of water at these altitudes leads directly to a higher escape rate of hydrogen, Jakosky says. It’s possible that much of the hydrogen leaking into space occurs during dust storms that occur every few years, rather than spreading throughout the year, he says.

“Understanding the loss of hydrogen is the key to fixing the history of water on Mars, so this is a significant new achievement,” Jakosky said.

Water only lasts for about four hours in Mars’ upper atmosphere, which means it is destroyed about 10 times faster in the upper atmosphere than if it stopped in the central atmosphere, Shane Stone, a planetary scientist at the Lunar and University of Arizona Planetary Laboratory and the lead author of the article told me.

Stone and his team linked this new water runoff mechanism to seasonal variations in Mars’ atmosphere. Mars is warmer when it’s closest to the Sun during the summer in its southern hemisphere, he says. That’s when these regional and global dust storms occur. During such periods, the hygro pause (a cold layer in the atmosphere where water condenses from vapor to liquid, producing clouds) is heated and weakened. This, in turn, allows water to enter the upper atmosphere.

During these warm periods, water ice in the polar ice caps of Mars is converted from ice to vapor, says Stone. The water is subsequently carried upward after a weak hypogropause and into the upper atmosphere, he says. Once there, it is dissociated into its constituent hydrogen and oxygen atoms.

These measurements show that H2O vapor is being transported to higher altitudes than previously thought, as far as the MAVEN spacecraft, about 150 km in altitude, Jakosky says. From there, it’s much easier to break up the H2O and then the hydrogen to escape into space, he says.

The team estimates that this high-velocity loss of water to the upper atmosphere is likely a process that has been going on for at least a billion years. And just in the last billion years, Mars may have lost as much as a global water body about 17 inches deep due to this process.

Over its 4.5 billion-year history, however, the authors estimate that Mars has likely lost enough water to cover the planet’s surface with an ocean tens to hundreds of meters deep, and that the rates of H2O loss must have been even higher in the past.

We still don’t know exactly when this mechanism started working, says Stone. Extrapolating atmospheric escape rates back in time to more than 4.5 billion years ago was already an incredibly difficult thing to do, he says. “This new mechanism just makes the calculation more difficult,” Stone said.