The surface of Mars today — as seen from the myriad of robotic space missions sent to explore our next-door neighbor — appears cold and desert-like, and its whisper-thin atmosphere doesn’t shield the planet from a bombardment of radiation from the Sun. But all indications point towards the Red Planet being warmer and wetter in the past, with a much thicker, protecting atmosphere.
We may now have the answer to the long-standing mystery of what brought about the change. Scientists from NASA’s MAVEN (Mars Atmosphere and Volatile Evolution Mission) spacecraft have determined that solar wind and radiation stripped away the Martian atmosphere, transforming Mars from a planet that could have supported life billions of years ago into a frigid, dry world. Their findings appear in the journal Science.
“We’ve determined that most of the gas ever present in the Mars atmosphere has been lost to space,” Bruce Jakosky, principal investigator for MAVEN, said in a press release. “The team made this determination from the latest result, which reveals that about 65 percent of the argon that was ever in the atmosphere has been lost to space.”
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Argon is a stable “noble gas” and the only process that can remove it to space is a physical process called “sputtering” where atoms are physically knocked away from the atmosphere due to impacts from energetic particles from the Sun, called solar wind.
The researchers used argon gas measurements to determine the amount of other gases that would have been lost by the same mechanism. Their findings suggest that Mars' atmosphere was probably once as thick as Earth's, but made primarily of carbon dioxide. But that thicker, life-giving atmosphere has since been lost, leading to huge changes to the Red Planet's climate since its formation.
Earth’s atmosphere and magnetic field protects our planet from the effects of solar wind. Mars, too, once had a strong magnetic field, produced by a dynamo effect from its interior heat. But as the smaller planet cooled, Mars lost its magnetic field. Jakosky and the MAVEN team tie the magnetic field loss to the atmospheric loss on Mars.
“The evidence points to Mars having had a magnetic field early in its history, but then the field shutting off around 4.1 billion years ago,” Jakosky said in an email. “The magnetic field would have caused the solar wind to stand off from the atmosphere, as it does on Earth, and thereby protect the atmosphere. I suspect that it was the turn-off of the magnetic field that allowed the turn-on of stripping of the atmosphere by the solar wind.”
The team used MAVEN’s Neutral Gas and Ion Mass Spectrometer instrument to estimate Mars’ past atmosphere. They also used measurements from the Martian surface collected by the Mars Science Laboratory (MSL) Curiosity rover.
Other data from the rover indicates that lakes and rivers were likely present for millions of years in order to create the sedimentary layers of Mount Sharp and other water-related features in Gale Crater.
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Jakosky said the two missions complement one another and help create a full picture of Mars’ past.
“Their [MSL’s] results show that liquid water was present,” he explained. “Our results tell us that the thicker atmosphere that provided the heating that allowed water to be present was lost to space. That is, the thick atmosphere was stripped away by the sun and solar wind, and the thinning of the atmosphere caused the cooling of the planet that diminished the ability of liquid water to exist.”
Of course, the biggest question is whether or not Mars ever supported life. We know that liquid water is essential for life, and multiple missions have found evidence of water on Mars. An atmosphere high in concentrations of CO2 might also have lent support to life on the Red Planet.
“There are two connections here,” Jakosky noted. “The first is that the thick CO2 atmosphere is what would have allowed liquid water to exist and thereby allowed life to exist at the surface. The second is that the high abundance and availability of CO2 would have made carbon accessible to organisms, and this is a necessary ingredient to supporting life.”
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