Just as life first emerged on Earth, our neighbor Mars lost its best chance for life to take root there when it lost most of its atmosphere — and the culprit was our sun, researchers say.
Despite an early warm and wet environment supported by a carbon dioxide-dominated atmosphere that could have supported life, that opportunity was lost as the solar wind stripped the planet of most of that atmosphere, turning the Red Planet into the cold, arid planet that exists today, NASA scientists say.
That finding is supported by data from the space agency's Mars Atmosphere and Volatile Evolution mission, which shows the rate at which the remaining thin Martian atmosphere is being lost into space, they explain.
Solar wind is stripping gas away from the planet at a rate of about 100 grams, roughly a quarter of a pound, every second, Maven found.
While that may not seem like a lot, it becomes significant over the time scales involved, NASA says.
"Like the theft of a few coins from a cash register every day, the loss becomes significant over time," says Bruce Jakosky, principle investigator for Maven who is at the University of Colorado, Boulder.
That loss of atmosphere shows significant increases during solar storms, suggesting the loss was much more rapid and pronounced billions of years in the past, when our sun was younger and much more active, he says.
That also suggests it was very much involved in changing the early climate of Mars, researchers say.
"Solar-wind erosion is an important mechanism for atmospheric loss, and was important enough to account for significant change in the Martian climate," says Joe Grebowsky, Maven project scientist from NASA's Goddard Space Flight Center in Greenbelt, Md.
The Maven data suggests most of the Red Planet's atmosphere would have been lost into space by around 3.7 billion years ago; today, it stands at just one percent of the density of Earth's atmosphere at our sea level.
That coincides with the point at which Mars lost its planetary magnetic field, which had protected its atmosphere from the solar-driven stripping effect, as Earth's field does today.
Things would have happened very quickly from that point, "within a few hundred million years after the shutoff of the magnetic field," says Jakosky of the CU Laboratory for Atmospheric and Space Physics.
That may have short-circuited the best chance for Mars to develop life, NASA officials say.
"Mars appears to have had a thick atmosphere warm enough to support liquid water which is a key ingredient and medium for life as we currently know it," says John Grunsfeld, associate administrator for the NASA Science Mission Directorate in Washington. "Understanding what happened to the Mars atmosphere will inform our knowledge of the dynamics and evolution of any planetary atmosphere."