© University of Colorado
The MAVEN spacecraft arrived at Mars in September 2014 to study the planet's atmosphere.
NASA's MAVEN spacecraft has discovered a dust cloud billowing above Mars, up to 1,000 kilometres above the planet's surface. The dust does not threaten spacecraft orbiting the red planet, but the unexpected finding poses big challenges to atmospheric researchers trying to explain where the cloud came from.

"This is the first discovery of dust or debris at orbital altitudes around Mars," says Bruce Jakosky, a planetary scientist at the University of Colorado at Boulder and the mission's principal investigator. "It's hard to understand how this stuff got here."

Jakosky reported the finding on 18 March at the Lunar and Planetary Science Conference in The Woodlands, Texas. But the dust cloud was not the only new finding from MAVEN, which has also discovered a diffuse, high-altitude aurora glowing above Mars's northern hemisphere.

The spacecraft arrived at Mars last September to fly through and study the planet's upper atmosphere. One of its instruments, a Langmuir probe, measures the densities and temperatures of electrons. It has picked up small whiffs of plasma that form when dust hits the spacecraft and vaporizes, Jakosky says. These kinds of tiny impacts have been measured before by other spacecraft, such as the Voyager and Cassini probes, but have never been measured at Mars.

The dust appears to be concentrated between about 150 and 500 kilometres above the Martian surface, and more on the day side, the hemisphere lit by the Sun, than the night side of Mars. The dust has been seen ever since the spacecraft arrived at Mars in September, so it is not related to Comet Siding Spring, which whizzed by Mars in October and dumped a load of fresh dust in the Martian atmosphere.

The dust may have come from earlier comet visits or some other sort of dust arriving from outside the Martian system. Another possibility is that it is being shed from Mars's moons, Phobos and Deimos. Or, the dust may have been lofted somehow from the surface.

Separately, MAVEN has also spotted a diffuse aurora unlike anything seen before at the planet. Mars does not have a planet-wide magnetic field as Earth does, and so it does not have the standard northern and southern lights as are seen above Earth's poles. The European Space Agency's Mars Express did find auroras glowing above patches of the Martian surface that have a magnetic field1.

For five days in December, MAVEN saw a dim aurora, which the team dubbed the "Christmas lights aurora." Rather than being focused above specific magnetic features, it was distributed across the entire northern hemisphere. It appeared at the same time that a storm of charged particles was blasting from the Sun and past Mars. The particles penetrated deep into Mars's atmosphere, down to altitudes where the aurora appeared, and so the solar storm may be causing the aurora.

Jasper Halekas, a space physicist at the University of Iowa in Iowa City, says the MAVEN findings are already rewriting what scientists know about the magnetic realms around Mars. "It's not your grandfather's magnetosphere," he says.

Nature - doi:10.1038/nature.2015.17152


Bertaux, J.-L. et al. Nature 435, 790 - 794 (2005).