Solar wind probably leaches Mars’ lower atmosphere

SAN FRANCISCO — Particles blasted from the sun probably spring leaks in the lower Martian atmosphere, new research suggests.

NASA’s Mars Atmosphere and Volatile EvolutioN probe, or MAVEN, has detected high-speed particles in the solar wind penetrating deeper into the planetary atmosphere than previously thought possible, mission scientists announced December 15 at the American Geophysical Union’s annual fall meeting. The particles could give an energetic kick to atmospheric gases, causing them to escape into space and helping to strip away the planet’s atmosphere, the researchers hypothesize.

The work will help scientists better “understand how loss of the atmosphere over billions of years might have changed the ability of the surface of Mars to sustain life,” said MAVEN instrument scientist Paul Mahaffy of the NASA Goddard Space Flight Center in Greenbelt, Md.

Dry lake beds and river channels carved into the Martian landscape suggest that the planet once had a warm, wet surface and therefore a thick encasing atmosphere. Over billions of years, gases such as carbon dioxide, nitrogen and argon with lighter isotopes of their atoms escaped the atmosphere, leaving behind their heavier siblings. The exact mechanism and rate of this atmospheric thinning isn’t yet fully understood.

MAVEN, launched in 2013, has been hunting for mechanisms responsible for this atmospheric loss. Since September 21, the spacecraft has been cruising around Mars in an oval-shaped path, ranging from 150 to 2,200 kilometers above the planet’s surface. On each orbit, the probe dips into a layer of electrons and charged atoms called ions that surrounds the planet. Scientists had believed that this ionosphere acts as a protective barrier around Mars, deflecting any incoming solar wind away from the planet. MAVEN’s new measurements, however, reveal that this isn’t always the case.

The probe observed ions from the solar wind pierce much deeper into the Martian atmosphere than previously thought possible, to about 200 kilometers above the surface and well below the ionosphere. These particles move at roughly 450 kilometers per second and provide a speed boost to any atmospheric gases they crash into. If enough energy is transferred during the collision, the gas can break free from Mars’ gravitational pull.

By donning a disguise as they descend, the particles may be able to penetrate into the atmosphere, said MAVEN team member Jim McFadden, a planetary scientist at the University of California, Berkeley. Positively charged atoms in the solar wind could pick up electrons somewhere along the way and become neutral, McFadden proposes. Without a net charge, these atoms would pass through the ionosphere unabated and into the lower atmosphere where they would somehow lose the hitchhiking electrons and regain the characteristics of the initial solar wind. Once in the atmosphere, the solar wind particles could help strip away parts of the atmosphere into space.

“This is kind of remarkable,” McFadden said. “There’s some population of the solar wind energy that’s there at high altitudes, goes away at middle altitudes, and comes back at low altitudes.”

How large of a role these shape-shifting solar wind ions play in Mars’ atmospheric losses remains uncertain, McFadden said.

MAVEN’s findings should help researchers piece together Mars’ past, said Christopher Webster, a planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who works with NASA’s Curiosity rover.

“The community will be able to bring together the results from Curiosity and MAVEN to better understand the evolution of the Martian climate,” he said. “MAVEN gives us closure of how atmospheric loss is happening.”

Editor’s note: This article was updated on December 20, 2014, to correct the date MAVEN began orbiting Mars.