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High-altitude water acts as atmospheric escape route for Martian hydrogen

January 30, 2017
Hydrogen in Mars’ upper atmosphere comes from water vapor in the lower atmosphere. An atmospheric water molecule can be broken apart by sunlight, releasing the two hydrogen atoms from the oxygen atom that they had been bound to. Several processes at work in Mars’ upper atmosphere may then act on the hydrogen, leading to its escape. (Courtesy NASA/GSFC; CU/LASP)

Hydrogen in Mars’ upper atmosphere comes from water vapor in the lower atmosphere. An atmospheric water molecule can be broken apart by sunlight, releasing the two hydrogen atoms from the oxygen atom that they had been bound to. Several processes at work in Mars’ upper atmosphere may then act on the hydrogen, leading to its escape. (Courtesy NASA/GSFC; CU/LASP)

LASP researchers have discovered an atmospheric escape route for hydrogen on Mars, a mechanism that may have played a significant role in the planet’s loss of liquid water.

The findings describe a process in which water molecules rise to the middle layers of the planet’s atmosphere during warmer seasons of the year and then break apart, triggering a large increase in the rate of hydrogen escape from the atmosphere to space in a span of just weeks.

The new study, which appears today in the journal Nature Geoscience, cuts against traditional models that have historically considered Martian hydrogen escape to be slower and more constant.

“Going back to the 1970s, the conventional picture of Martian hydrogen loss has been one of slow, steady escape over long time scales,” said Mike Chaffin, a research associate at LASP and lead author of the new study. “With this work, we find that there are ways to produce much more seasonal variation than previously thought.”

Atmospheric loss has controlled Mars’s habitability over time, removing most of the planet’s liquid water through the escape of atomic hydrogen and oxygen to space. In 2007, observations from NASA’s Hubble Telescope and the European Space Agency’s Mars Express spacecraft first noticed large seasonal variations in the planet’s rate of hydrogen escape. When Mars’s orbit brings it closest to the sun, hydrogen escape increases by a factor of up to 100.

“This seems to happen every Martian year. We see efficient escape while the planet is close to the sun and less escape when it’s further away,” said Chaffin. “That tells us that the old explanation for Martian hydrogen escape is insufficient.”

Previous Martian atmospheric models held that water molecules were “cold trapped” at lower levels of the atmosphere due to a gradual decline in water vapor abundance as altitude increases. A similar mechanism exists in Earth’s atmosphere. But observations from Mars Express showed that when Mars’s lower atmosphere warms during southern summer, water molecules that would normally freeze are able to rise higher than normal in the atmosphere, bypassing the cold trap.

The work released today shows that this water, now nestled in the middle altitudes and exposed to more ultraviolet light from the sun, splits apart to produce atomic oxygen and hydrogen. The atoms and molecules released from this water can then cover the large distance between the middle and upper atmosphere. Once at the highest altitudes, hydrogen (which is the lightest gas) is free to escape Mars’s low gravity while oxygen is left behind.

“In this case, we had two unexpected findings: seasonal changes in hydrogen escape and excess water in the middle atmosphere. But taken together, these two unexpected things make sense,” said Chaffin. “It’s very satisfying as a scientist when that happens.”

The researchers hope that their model will eventually be verified through combined observational evidence from the LASP-led Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, which studies Mars’ upper atmosphere, and the European Space Agency’s Trace Gas Orbiter, which will begin studying the lower altitudes in March 2018.

The large effect of this newly discovered mechanism on Martian hydrogen escape has significant implications for piecing together the planet’s transition from a warm, wet state to its current cold, desolate state.

Going forward, Chaffin said, the community of Mars scientists will need to focus on understanding how and when middle atmospheric water is present, not just to understand the climate today but to understand how the climate has evolved though time.

“The University of Colorado Boulder takes pride in exploring Mars’s climate history with the MAVEN mission. By introducing this new pathway for hydrogen escape, we’re advancing MAVEN and NASA’s core mission of understanding the evolution of Martian habitability,” said Chaffin.

The study was co-authored by LASP planetary scientsits Justin Deighan, Nick Schneider and Ian Stewart.

Citation:
Chaffin, M.S., J. Deighan, N.M. Schneider, and A.I.F. Stewart (2017), Elevated atmospheric escape of atomic hydrogen from Mars induced by high-altitude water, Nature Geoscience, doi: 10.1038/ngeo2887.