NASA’s MAVEN mission has discovered a new population of particles in Mars’s upper atmosphere. It’s also found a plume of particles escaping from the planet’s poles, confirming atmospheric loss is happening today.
Members of NASA’s MAVEN mission to Mars presented early results from the spacecraft at the American Geophysical Union meeting in San Francisco on December 15th. The craft arrived at the Red Planet in September and so has only been observing for a couple of months — and part of that time has been spent dealing with Comet Siding Spring and a big solar flare, both of which showered particles on Mars’s atmosphere in October.
Although the preliminary results are just that — preliminary — there are some fun facts emerging.
For example, the Solar Wind Ion Analyzer (SWIA) has found that a fraction of solar wind particles manage to dive fairly deep into Mars’s upper atmosphere by playing a chameleon game. The solar wind is charged, made up of ions whizzing through the solar system at a few hundred kilometers per second and carrying with them the solar magnetic field. This solar wind should be deflected around Mars by its ionosphere, the ionized component of its upper atmosphere.
But the team found that about 1⁄500 of the solar wind particles bombarding the Martian atmosphere show up about 200 km above the planet’s surface — less than 1⁄10 the altitude of the ionosphere where they’re normally stymied. Plus, they don’t show up in the region between these two layers. It’s as though they teleport between locations.
They don’t — although there is an element of poofing into and out of existence involved. SWIA instrument lead Jasper Halekas (University of Iowa) explains that upon arrival, the solar wind particles are likely stealing electrons from the uppermost atmosphere, thereby transforming to neutral particles. Unlike ionized particles, neutrals can pass through the ionosphere and any magnetic fields with impunity. They’re therefore able to drill deep into Mars’s atmosphere, sailing unencumbered until the atmosphere grows denser and there’s more stuff to run into. Then they again do a charge exchange with atmospheric particles, becoming ions again.
This explanation might seem a bit handwavy. But it does match previous theoretical work that suggested a small fraction of the solar wind should make it this low in the atmosphere. Plus, the correspondence between the solar wind and these chameleon particles is just too good, Halekas says. “Whatever the solar wind does, this population follows,” he explains. Velocity, density, energy levels — it all tracks. “And it’s not just close to the solar wind, it’s dead-on the solar wind.”
Using this population of flip-flop solar wind particles, researchers can track what the solar wind is doing high above while the spacecraft is in the atmosphere, giving them an instantaneous look at what’s going on in the solar wind, how much energy it’s dumping into the upper atmosphere, and how the atmosphere responds. That’ll be a huge help with MAVEN’s mission, which is to understand the drivers of atmospheric loss on Mars.
Plumes of Particles
Other initial results the team presented include finding that the concentration of various atoms and compounds changes over fairly small ranges in altitude. These changes likely arise because Mars’s atmospheric pressure is so low (1% that at Earth’s sea level). Dust storms can heat the atmosphere, raising it, and as the atmosphere flows over volcanoes and other big topographic features, it can develop waves that propagate upward into the upper atmosphere, where MAVEN is taking observations.
The Suprathermal and Thermal Ion Composition (STATIC) instrument also detected a plume of ions at Mars’s pole, created by ions heated and escaping from the upper atmosphere. Instrument lead Jim McFadden (University of California, Berkeley) compared the effect to a Mohawk hairdo. It’s unclear which of the various processes at work in Mars’s atmosphere is stripping the ions away, but the measurements do confirm that the loss is happening.
You can read more about the MAVEN results in NASA’s press release.
Want to know more about Mars? Read up on the latest developments in our understanding of the Red Planet in Sky & Telescope's special issue Mars: Mysteries and Marvels of the Red Planet.