Mars was once a wet planet, but it has lost most of its water through reactions that produce hydrogen. In standard models, molecular hydrogen produced from water in the lower atmosphere diffuses into the upper atmosphere where it is dissociated, producing atomic hydrogen, which is lost. Using NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, a team of researchers has now found that water can reach higher altitudes than previously thought, especially during global or regional dust storms on the planet.
This artist’s concept depicts the dry environment seen at Mars today versus the early Martian environment. Image credit: Simon Fraser University.
“We know that billions of years ago, there was liquid water on the surface of Mars,” said lead author Shane Stone, a graduate student in the Lunar and Planetary Laboratory at the University of Arizona.
“There must have been a thicker atmosphere, so we know that Mars somehow lost the majority of its atmosphere to space.”
“MAVEN is trying to characterize the processes responsible for this loss, and one portion of that is understanding exactly how Mars lost its water.”
Using data from MAVEN’s NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument, Stone and colleagues found that when Mars is nearest the Sun, the planet warms, and more water — found on the surface in the form of ice — moves from the surface to the upper atmosphere where it is lost to space.
This happens once every Martian year or about every two Earth years.
The regional dust storms that occur on Mars every Martian year and the global dust storms that occur across the planet about once every 10 years lead to further heating of the atmosphere and a surge in the upward movement of water.
“The processes that make this cyclical movement possible contradict the classical picture of water escape from Mars, showing it is incomplete,” Stone said.
According to the classical process, water ice is converted to a gas and is destroyed by the Sun’s rays in the lower atmosphere.
This process, however, would play out as a slow, steady trickle, unaffected by the seasons or dust storms, which doesn’t mesh with current observations.
Two 2001 images from the Mars Orbiter Camera on NASA’s Mars Global Surveyor show a dramatic change in the planet’s appearance when haze raised by dust-storm activity in the south became globally distributed. The images were taken about a month apart. Image credit: NASA / JPL-Caltech / MSSS.
“This is important because we didn’t expect to see any water in the upper atmosphere of Mars at all,” Stone said.
“If we compare Mars to Earth, water on Earth is confined close to the surface because of something called the hygropause.”
“It’s just a layer in the atmosphere that’s cold enough to condense (and therefore stop) any water vapor traveling upward.”
The researchers argue that water is moving past what should be Mars’ hygropause, which is likely too warm to stop the water vapor.
Once in the upper atmosphere, water molecules are broken apart by ions very quickly — within four hours, they calculate — and the byproducts are then lost to space.
“The loss of its atmosphere and water to space is a major reason Mars is cold and dry compared to warm and wet Earth,” Stone said.
“The new data from MAVEN reveal one process by which this loss is still occurring today.”
When the scientists extrapolated their findings back 1 billion years, they found that this process can account for the loss of a global ocean about 43 cm (17 inches) deep.
“If we took water and spread it evenly over the entire surface of Mars, that ocean of water lost to space due to the new process we describe would be over 43 cm deep,” Stone said.
“An additional 17 cm (6.7 inches) would be lost due solely to the effects of global dust storms.”
This close-up color image of a small-scale dust storm on Mars was acquired by the HRSC instrument on ESA’s Mars Express in April 2018. Image credit: ESA / DLR / FU Berlin / CC BY-SA 3.0 IGO.
During global dust storms, 20 times more water can be transported to the upper atmosphere.
For example, one global dust storm lasting 45 days releases the same amount of water to space as Mars would lose during a calm Martian year, or 687 Earth days.
“This process likely didn’t work the same before that, because Mars might have had a stronger hygropause long ago,” Stone said.
“Before the process we describe began to operate, there must have been a significant amount of atmospheric escape to space already.”
“We still need to nail down the impact of this process and when it began to operate.”
The findings were published in the journal Science.
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Shane W. Stone et al. 2020. Hydrogen escape from Mars is driven by seasonal and dust storm transport of water. Science 370 (6518): 824-831; doi: 10.1126/science.aba5229
