Stone, along with 5 co-authors, reported in late 2020 that Mars has lost most of its once-abundant water to space, leaving the planet cold and dry. 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 observations from the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution spacecraft, the authors demonstrated that water is instead transported directly to the upper atmosphere, then dissociated by ions to produce atomic hydrogen. The water abundance in the upper atmosphere varied seasonally, peaking in southern summer, and surged during dust storms, including the 2018 global dust storm. The authors calculated that this transport of water dominates the present-day loss of atomic hydrogen to space and influenced the evolution of Mars’ climate.
Wood (2020) expanded on the authors' abstract by indicating that new measurements from MAVEN have added new details to the most recent chapter in the Martian water exodus. Since the late 1960s, planetary scientists had assumed that Mars’s surviving stores of H2O were relatively safe. Blocks of ice on or near the surface would steadily transform into molecules of water vapor, which would rise into the thin atmosphere until it got too cold and they condensed into clouds. The same barrier of chilly air—the “hygropause”—protects water on Earth by trapping it into clouds and preventing the hydrogen from floating away.
Then came MAVEN, which dips into the edge of the atmosphere with every orbit and directly samples the ions that come from Martian water. Reconstructing the original molecules, Stone and his colleagues were surprised to find that plenty of H2O was wafting about at more than 90 miles above the surface—way above the hygropause. Water that high up in the atmosphere is destined to be smashed into oxygen. The oxygen continues to break down, while the hydrogen is light enough to flit away from the planet forever. By analyzing how the upper atmosphere’s water content changes over time, the team also uncovered two hints as to why Mars has such a lousy hygropause.
First, MAVEN detected an atmosphere that got drier in the winter and damper in the summer. Second, the spacecraft has been orbiting Mars for long enough to bear witness to a couple of regional dust storms, during which the water content jumped. The orbiter also happened to be operating during a dust storm in June of 2018 (a once-in-a-decade event so ferocious it killed the Opportunity rover), which caused moisture in the upper atmosphere to leap to roughly twenty times its normal levels. These trends, Stone says, strongly suggest that Mars’s hygropause regularly breaks down because the atmosphere gets too warm, such as when the planet draws closer to the sun during the summer months, or when the atmosphere swirls with dust.
Independent temperature readings from another spacecraft confirmed that the atmosphere’s wet periods line up with its relative heat waves.
While the Martian hygropause was never supposed to form a perfect seal, MAVEN’s measurements suggest that it leaks far more than predicted. Over the last billion years, seasonable warming, annual regional dust storms, and decadal superstorms have caused Mars to lose enough water that could cover the planet in a global ocean two feet deep, the researchers estimated. That’s just a few percent of the water Mars has lost over its entire history (which would be enough to flood the planet in an ocean many dozens to hundreds of feet deep), Stone says, but it’s the main way the planet continues to dry out today.
While the dehydration of Mars would have devastated any life clinging to its surface, terrestrial organisms can breathe easy knowing that we won’t suffer quite the same fate. Earth does constantly lose H2O to “atmospheric escape,” but at a rate that’s far too slow to be of any concern, according to Stone. Rather, Earth has one to two billion years left of being blue, after which a brighter sun will evaporate our oceans, cranking up the thermostat to a few hundred degrees.[By that time, humans (and a supply of necessary critters and plants) will have moved on to New Earth somewhere in our galaxy]
Managed & Built by I2M Consulting, LLC | © 2010-
