In 2013, NASA launched a robotic probe called MAVEN — for Mars Atmosphere and Volatile Evolution — to help crack the mystery. The spacecraft, a seven-foot cube flanked by solar panels that together span 37 feet, arrived in Mars orbit the next year. It since has made more than 4,000 elliptical orbits, sniffing the Martian upper atmosphere and dipping to within 100 miles of the surface in its path around the planet.
Thursday, in a paper in the journal Science, the MAVEN team published its first major finding: Much and possibly most of the Martian atmosphere has been lost to space, violently scraped from the planet by the solar wind.
The solar wind is a steady stream of particles, mostly protons and electrons, emitted by the sun. It continues far beyond Pluto before finally tuckering out. Earth is also in its path but has a protective magnetic field, something Mars conspicuously lacks. The solar wind is deflected by Earth's magnetic field while pummeling Mars head on.
Lead author Bruce Jakosky, a University of Colorado planetary scientist who is the principal investigator for the $600 million MAVEN mission, told The Washington Post that the spacecraft used a mass spectrometer to sample two isotopes of the element argon in the upper Martian atmosphere. Argon is special (a “noble” gas) because it is nonreactive chemically. Unlike, say, carbon dioxide, it would not have reacted with the surface of the young Mars and been depleted from the atmosphere that way. Once in the atmosphere, it should stay there — unless something comes along and knocks it into space by brute force. The lighter variant of argon is more likely to be blown into space, and by studying the ratio of lighter and heavier argon, the MAVEN team could calculate the likely effects of the solar wind.
That wind, streaming through space at several hundred miles per second, creates its own magnetic field, inciting atmospheric particles to accelerate to high speed and then start careening around, slamming into things. That process, known as sputtering, blew most of the argon and other atmospheric gases into space, the MAVEN team concluded.
“It's like a break shot in pool when you send a cue ball in at high speed and everything goes every which way,” Jakosky said. This process “may have played the major role in changing the climate. That is, the bulk of the atmosphere has been lost to space.”
And it's something still happening today, “potentially in quantities sufficient to change the planet's climate,” the scientists write in the new paper.
Why doesn't Mars have a magnetic field to protect it from the solar wind and all that sputtering? The problem is right in the core of the planet. Mars had a magnetic field when it was young and its iron core was molten and convecting — which is what Earth's iron core does to this day. But Mars is smaller than Earth, and sometime about 4.2 billion years ago that molten Martian core froze up, Jakosky said.
In this scenario, turning off the magnetic field meant turning on the effects of the solar wind, and Mars began losing its atmosphere.
“On Earth, the magnetic dynamo can help divert some of these particles that cause sputtering all around,” said Paul Mahaffy, a planetary scientist at the NASA Goddard Space Flight Center in Maryland and a co-author of the new paper.
The bottom line is that, although planets are common in the universe, they need a lot of things to go right if they want to be brimming with life for billions of years. So far, the number of planets known to have all the right features is stuck at one.