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Glowing ultraviolet aurora spotted on Mars

NASA's orbiting Mars probe has spotted two surprises in the Red Planet's atmosphere: an unexplained dust cloud and a deep ultraviolet aurora.

Artist's concept of MAVEN imaging the aurora. University of Colorado

The NASA Mars Atmosphere and Volatile Evolution spacecraft orbiting Mars has had a bit of a surprise -- two very unusual phenomena in the atmosphere of Mars.

The first of these might be fairly common, if not for the fact that its source is unknown. It's a cloud of dust at orbital altitudes, in the zone between 150 kilometres (93 miles) and 300 kilometres (190 miles) from the Martian surface -- and where it came from and how it got there are mysteries.

The cloud has been present for at least as long as MAVEN has been in orbit around Mars -- since September 2014 -- which is only around six months, not long enough to determine whether it's a permanent feature or just a temporary event. It's very thin, and has only been detected by one of MAVEN's instruments -- the Langmuir Probe and Waves instrument, which examines the properties on Mars' ionosphere.

As for its origin, there are several hypotheses: it could have drifted upwards from the atmosphere; it could have originated on Mars' moons, Phobos and Deimos; it could have originated beyond Mars, carried in the solar wind away from the sun; or it could be made up of comet debris that had been orbiting the sun.

"If the dust originates from the atmosphere, this suggests we are missing some fundamental process in the Martian atmosphere," said Laila Andersson of the University of Colorado's Laboratory for Atmospherics and Space Physics.

What NASA researchers do know about the dust is that no known processes on Mars -- not even the above hypotheses -- can explain how the dust got to the location where it was observed.

The other phenomenon is perhaps a little more visually interesting, although much less mysterious -- a bright, glowing ultraviolet aurora that lit up Mars' northern hemisphere for five days just before Christmas last year. The phenomenon has since been nicknamed the "Christmas lights".

The aurora, imaged by MAVEN's Imaging Ultraviolet Spectrograph, was created -- like Earth's aurora -- by charged particles such as protons and electrons entering the atmosphere from above, causing ionisation, which in turn makes the atmosphere glow. In the case of Earth, these particles emanate from the solar wind -- which explains why they're more common during solar maximum -- so the most likely source of the Martian aurora is also the sun.

Earth has a magnetic field around it -- the magnetosphere -- which protects us from solar activity on our planet's surface. However, Mars lost its magnetosphere billions of years ago, so its aurorae can appear much deeper in the atmosphere.

"What's especially surprising about the aurora we saw is how deep in the atmosphere it occurs - much deeper than at Earth or elsewhere on Mars," said University of Colorado IUVS scientist Arnaud Stiepen. "The electrons producing it must be really energetic."

IUVS map of the aurora's spread over Mars' northern hemisphere. University of Colorado