Billions of years ago, our sister planet Mars was a lot like Earth — it had flowing water, an atmosphere, and a protective magnetic field. Today, Mars is cold and barren. Briny water barely flows in the summer, the atmosphere is only 1 percent the pressure of ours, and its magnetic deflector shields are down. Scientists working on NASA’ Mars Atmosphere and Volatile Evolution mission (MAVEN) are now reporting that even before the shields went down, the atmosphere was already being blasted away by the sun.
According to four new papers published today by the MAVEN team in the journal Science (and dozens more in Geophysical Research Letters), stripping by solar wind is one of the main processes through which Mars lost its air. The data from the MAVEN spacecraft — in orbit around Mars since 2014 — is the first direct evidence of fountains of escaping particles that are slowly bleeding the planet.
Every planet in the solar system is oppressed by solar wind. Like a solar sand blaster, electrically charged particles scream out of the sun at a million miles per hour and smash into planets’ atmospheres. Thankfully, the Earth’s magnetic field directs most of these particles away from the surface and towards the poles (resulting in brilliant auroras). Mars isn’t so lucky.
While Mars likely had a larger magnetic field a few billion years ago, its planetary dynamo — a heat-powered, tumbling molten metal core that generates a magnetic field — has since shut down. Today, Mars’ magnetic field strength is only 1/40th of Earth’s, meaning that it can no longer direct solar wind away from its atmosphere. The video above shows a NASA simulation of solar wind striking Mars, and the resulting plumes of atmospheric particles that escape.
According to data from MAVEN, Mars is currently losing about 100 grams of atmosphere per second. Three quarters of that quarter pounder of air is escaping via the “tail” in the simulation above, and the rest is shooting out of the “plume” or what looks like a planetary mohawk.
“I can’t help but imagine burgers flying out of the atmosphere,” said Dave Brain, atmospheric physicist at the University of Colorado, Boulder during a NASA press conference.
This average escape rate of a burger per second is direct evidence of a leaking Martian atmosphere, but it can’t account for how thick the atmosphere was thought to be in the early solar system…until you account for how the sun was behaving billions of years ago. When the solar system was forming solar activity was also much higher — flares and storms flung out more particles more often, and faster. The MAVEN team estimates that Martian air could have been stripped at a rate 10 to 20 times higher during a particularly bad solar storm, which MAVEN proved by observing current-day Mars during a bad solar burp. And 10-20 times higher is just a lower limit.
Physically, the Martian atmosphere leaks when the solar wind changes the charge of its air, which is then accelerated off the planet by the strong electrical and magnetic fields of the wind itself. Air particles like oxygen and carbon dioxide molecules can also just be knocked by solar wind into the universe, like breaking a pyramid of pool balls with a cue going a million miles per hour.
Though the data from MAVEN is the first identify how much the Martian atmosphere is leaking, loss via solar wind probably isn’t, and wasn’t, the only factor. But the data does give scientists another piece of the puzzle. Why didn’t Mars stay warm or wet? Solar wind stripping is the start of the explanation. When the magnetic field died, the water could have quickly followed the atmosphere. “It’s like when I step out of the shower and the water still in my hair quickly evaporates in the breeze,” said Bruce M. Jakosky, principle investigator on the MAVEN mission during NASA’s press conference. “Though that’s becoming an increasingly theoretical example.”
In other words, said Michael Meyer, lead scientist for NASA’s Mars Exploration Program, “For the answer to the question: what happened to the Martian atmosphere, I’ll quote Bob Dylan. ‘The answer my friend, is blowing in the wind.'”
FEATURED IMAGE: An artist’s depiction of a coronal mass ejection from the sun bombarding Mars and its magnetic field with a tsunami of energetic particles. (NASA Goddard Space Flight Center)