The highest clouds ever spotted above a planet’s surface have been detected by a European spaceprobe orbiting Mars.
Scientists say the fleeting clouds, which are probably made of carbon dioxide, lie between 50 and 60 miles up (approximately 80-100km).
They were discovered when an instrument aboard the European Space Agency’s Mars Express observed stars dimming noticeably before they disappeared behind the red planet.
Any Martians would see the clouds best after nightfall when they shine with the reflected light from the sun below the horizon, said mission scientist Franck Montmessin, of France.
They resemble rare and mysterious noctilucent clouds seen in summer from the UK which also glow in the northern sky around midnight. The height of the clouds over Mars means they are unlikely to be made of water, the scientists report in the journal Icarus.
Data from SPICAM, a spectrometer observing the atmosphere in the ultraviolet and infrared, shows that they form around microscopic grains of dust blown high into the atmosphere.
The results have important implications for future missions sent to land on Mars because they suggest the upper layers of the Martian atmosphere can be denser previously thought.
The clouds in the accompanying picture were taken by Nasa’s Pathfinder rover in August 1997 and show clouds in the Martian eastern sky before sunrise. They may be of the same type as those detected by Mars Express.