抖阴旅行射

University of Colorado Boulder researchers have discovered an atmospheric escape route for hydrogen on Mars, a mechanism that may have played a significant role in the planet鈥檚 loss of liquid water.

The findings describe a process in which water molecules rise to the middle layers of the planet鈥檚 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.

, 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.

鈥淕oing back to the 1970s, the conventional picture of Martian hydrogen loss has been one of slow, steady escape over long time scales,鈥� said Mike Chaffin, a research associate at the at 抖阴旅行射 Boulder and lead author of the new study. 鈥淲ith this work, we find that there are ways to produce much more seasonal variation than previously thought.鈥�

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

鈥淭his seems to happen every Martian year. We see efficient escape while the planet is close to the sun and less escape when it鈥檚 further away,鈥� Chaffin said. 鈥淭hat tells us that the old explanation for Martian hydrogen escape is insufficient.鈥�

Previous Martian atmospheric models held that water molecules were 鈥渃old trapped鈥� at lower levels of the atmosphere due to a gradual decline in water vapor abundance as altitude increases. A similar mechanism exists in Earth鈥檚 atmosphere. But observations from Mars Express showed that when Mars鈥櫶齦ower 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鈥檚 low gravity while oxygen is left behind.

鈥淚n 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,鈥� Chaffin said. 鈥淚t鈥檚 very satisfying as a scientist when that happens.鈥�

The researchers hope that their model will eventually be verified through combined observational evidence from the 抖阴旅行射 Boulder-led spacecraft, which studies Mars鈥檚 upper atmosphere, and the European Space Agency鈥檚 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鈥檚 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.

鈥湺兑趼眯猩� Boulder takes pride in exploring Mars鈥櫶齝limate history with the MAVEN mission. By introducing this new pathway for hydrogen escape, we鈥檙e advancing MAVEN and NASA鈥檚 core mission of understanding the evolution of Martian habitability,鈥� Chaffin said.

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