Mars is constantly losing part of its atmosphere to space and pressure from solar wind pulses is a significant cause.NASA
— Moving traffic jams of solar particles create extra powerful pulses that rip away Martian air.
— It happens because Mars has no planetary magnetic field, like Earth has.
— Mars has limited ways of replenishing its lost air.
Scientists have identified a sort of double-whammy solar super wave that is responsible for blowing away air from Mars and keeping its atmosphere thin, frigid and downright inhospitable for any possible future travelers.
The waves happen when one stream of solar wind is overrun and amped up by another, faster gale of solar particles. That creates a flying traffic jam of particles that slam into Mars as one large pulse.
"People have known for some time that Mars loses atmosphere," said Niklas Edberg of the Swedish Institute of Space Physics and the University of Leicester. The reason is that, unlike Earth, Mars has no planet-wide magnetic field to redirect the atmosphere-eroding solar wind around and away from the planet.
Now, by using actual measurements from the Mars Express spacecraft of particles being kicked into interplanetary space, Edberg and his colleagues, have zeroed-in on the cause and effect of these pulses of solar wind.
First Edberg and his colleagues identified 41 doubled solar particle waves and solar powerful particle waves from what are called coronal mass ejections from 2007 to 2008 detected by the Advanced Composition Explorer (ACE) spacecraft that monitors the space weather near Earth. Then they identified 36 of the same events hitting Mars in Mars Express data.
When Edberg and his colleagues compared these events at Mars to the flow of heavier atoms blowing past Mars Express, they discovered that fully a third of Martian air loss happens during the 15 percent of the time when doubled-up solar wind pulses hit the planet.
As for how these two differing speeds of solar wind are created, they come from different parts of the sun, Edberg told discovery News.
The higher solar latitudes produce faster winds of particles and the more equatorial zones produce slower winds. When these two winds overlap and create pulses, they are called corotating interaction regions (CIRs).
"They found that yes, more atoms escape during these events," agreed Mars researcher Dave Brain of the University of California at Berkeley, who works on an entirely different mechanism that could account for further losses of Martian air.
Brain looks at the remnants of Mars' magnetic fields — little islands of magnetism in the Martian crust. The pull of these fields extend all the way into space, and have generally been thought of as protective domes that keep air in those places from being stripped away.
But it's not always the case.
"When the solar wind blows by Mars the domes stretch like a wind socks into space," Brain explained. Eventually the wind socks are stretched so long downstream in the solar wind that they break off and take away chunks of Martian air with them.
All of which begs the question: If Mars is so regularly losing atmosphere, how does it manage to have any atmosphere at all?
The answer, it turns out, is that Mars depends on comet impacts, polar dry ice and perhaps underground water that de-gasses over time to keep the red planet from being entirely airless, Brain explained.