Sen— An orbiting space probe, NASA's Mars Reconnaissance Orbiter, has begun observing the outbreak of one of the most dramatic events on the Red Planet - a huge dust storm.
Mars, a rocky world which like the Earth has an atmosphere, experiences its own weather and prevailing wind systems. Mini whirlwinds called dust devils were often seen by the rovers Spirit and Opportunity.
And the winds that blow on Mars occasionally whip up much bigger storms which can fill much of the atmosphere with dust for months on end. In 2001 and 2007 dust storms blew up that engulfed the whole planet.
They are believed to be seasonal events on the planet, where a year is around twice as long as on Earth and scientists are keen to know why they arise in some martian years and not in others.
Rich Zurek, chief Mars scientist at NASA's Jet Propulsion Laboratory, in California, said: "This is now a regional dust storm. It has covered a fairly extensive region with its dust haze, and it is in a part of the planet where some regional storms in the past have grown into global dust hazes.
How Mars appeared from the Hubble Space Telescope when covered by a global dust storm in 2001. Credit: NASA
"For the first time since the Viking missions of the 1970s, we are studying a regional dust storm both from orbit and with a weather station on the surface.
"One thing we want to learn is why do some Martian dust storms get to this size and stop growing, while others this size keep growing and go global."
The MRO probe began tracking the storm early last week after it was seen to blow up in the martian southern hemisphere where spring is now beginning. Its effects have also been observed by Opportunity and NASA's newer rover on the other side of Mars, Curiosity.
MRO's Mars Color Imager instrument began recording the storm on 10 November as it scanned the planet in slices, producing the image above. White arrows mark the edge of the dust storm. Scientists do not expect it to become a global event this time as it already appears to be weakening.
Though it came no closer than 1,347 km to Opportunity, and that rover carries no weather station, it was able to record a slight drop in the clarity of the air as it became more hazy.
An artist's impression of Mars Reconnaissance Orbiter above the Red Planet. Credit: NASA
Curiosity - the Mars Science Laboratory - is better equipped to observe the changes in the martian atmosphere that are related to the dust storm. A drop in air pressure and small rise in the overnight low temperature have been recoded by its Spanish-built Rover Environmental Monitoring Station.
MRO's Mars Climate Sounder instrument detected a warming of the atmosphere about 25 km above the storm from 16 November. Since then, the air in the region has warmed by about 25C due to dust being lifted high in the atmosphere and absorbing sunlight.
Dust in the air would affect Opportunity more than its big cousin Curiosity. If it covered Opportunity's solar panels it would reduce that robot's power supply and so restrict day-to-day operations. Curiosity is powered by its own "nuclear power station", a radioisotope thermoelectric generator, so is much less vulnerable.