Artist’s rendering of a solar storm hitting Mars and stripping ions from the planet's upper atmosphere.

Earlier today we told you about new insights into the Mars atmosphere, published today in Science. This afternoon, at a press conference at NASA headquarters in Washington, Michael Meyer, lead scientist for the Mars Exploration Program, and his colleagues discussed how these findings illuminate what happened to the once-wet, now-dry Martian atmosphere. “To quote Bob Dylan,” he said, 'The answer, my friend, is blowing in the wind.'"

Studies by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft have revealed that 100 grams of Martian atmosphere are escaping into space every second due to solar wind. According to NASA, solar winds are streams of particles and electrons that issue from the Sun’s atmosphere at 1 million miles per hour. Solar wind can grab ions from a planet and strip them away, or plow into the planet at very high speeds, knocking other things off.

Bruce Jakosky, of the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder, and MAVEN’s principal investigator, joked that the atmospheric loss of Mars is about a quarter-pound—or one hamburger per second. That’s not much; it would take billions of years for the Martian atmosphere to vanish entirely. During solar storms, however, those escape rates increase by factors of 10 to 20. The Martian atmosphere, then, didn’t stand much of a chance earlier in the history of the solar system, when solar storms were more common and much more intense.

MAVEN’s observations help explain what happened to the Martian atmosphere and thus what happened to all of its water. The findings also help planetary scientists explain how atmospheres everywhere interact with stars and planets.

The MAVEN spacecraft was designed to fly to Mars, look at its upper atmosphere and the processes at work, and answer some fundamental questions about the history of the Martian atmosphere. The upper atmosphere of Mars is of particular interest to planetary scientists because that is the conduit through which all gas has to travel in order to be removed by the solar wind and swept into space.

The results released today come from the first six months of data collected by MAVEN. The spacecraft entered orbit around Mars on September 21, 2014, and in 10 days will reach the end of its primary mission. Because the spacecraft and its instruments are working well and the mission is under budget, MAVEN will then begin an extended mission of collecting data for planetary scientists back on Earth.

Today, Mars is cold and dry, with a thin atmosphere incapable of sustaining liquid water, which would freeze or evaporate quickly. But it wasn’t always that way. Mars was once an ocean world much like our own, possibly teeming with life. (At the very least, it was hospitable to life.) As observable surface features like valleys carved by water and lake beds indicate, ancient Mars would have supported standing water for long periods of time, which suggests a wetter, thicker atmosphere that was able to sustain a warmer climate. All of that changed when Mars lost its magnetic field 3.8 billion years ago, perhaps due to a massive collision with some other celestial object.

According to Jasper Halekas of the University of Iowa, who leads the MAVEN Solar Wind Ion Analyzer, all planets are affected by the solar wind. Earth, however, is protected by a strong global magnetic field. Because Mars no longer possesses such a shield, the solar wind can have its ways with the Martian upper atmosphere, stripping it away and sweeping it into space. The Martian atmosphere is thick enough, however, to protect the planetary surface from the solar winds, deflecting it away from the planet. The findings released today largely validate models previously put forth by scientists to explain the interaction between solar winds and the Martian atmosphere.

MAVEN’s scientists, meanwhile, dismissed any chance of terraforming on Mars to bring back the lost atmosphere. A consolation prize for future human settlers, however, is that energetic particles that encounter the Martian atmosphere could create stunning auroras that illuminate the whole Martian sky.