Paris (AFP)

Scientists are beginning to understand the origin of a curious cloud some 1,800 km long detected in the 70s on the planet Mars, where it appears daily for several months, from one of the largest volcanoes on the red planet.

Spotted in September 2018 by the instruments of the European probe Mars Express, the long plume that a Russian probe had photographed in the 1970s intrigued scientists, recalls a press release from the European Space Agency (ESA) on Tuesday.

But its observation was made difficult by its location, in a place on the planet where the probe's instruments could only observe it for very short periods.

The team studying the clouds of Mars found the parade using "a secret tool from Mars Express", a visual surveillance camera called "VMC", explains Jorge Hernandez Bernal, of the University of the Basque Country in Bilbao, in the ESA press release.

With a resolution equivalent to that of a vulgar webcam, it had served briefly in 2003, shortly after the launch of Mars Express, before being turned off and used sporadically for educational operations.

The probe has been orbiting Mars since the end of 2003.

But unlike the much more sophisticated scientific instruments of the probe, the VMC camera has "a large field of view (...) and is well suited to monitoring the evolution of a phenomenon", according to Mr. Bernal, who co-authored a study on the subject in the latest issue of the Journal of Geophysical Research.

The observation revealed that during several of the twelve Earth months that the Martian spring and summer last, the cloud forms every morning on the inner flank of the Arsia Mons volcano, before sunrise.

Formed by ice water vapor, it then rises with the first rays to an altitude of about forty kilometers, well above the approximately 17 km where the ancient volcano culminates.

There, high winds stretch it westward at speeds of up to 600 km / h, up to 1,800 km away.

The phenomenon does not last more than two and a half hours, before disappearing under the sun's rays.

The observations of the VMC were quickly relayed by those of the high-resolution stereoscopic camera (HRSC) and by the French spectro-imager OMEGA.

The latter "made it possible to give the composition of the cloud: water," Brigitte Gondet, from the Institute of Space Astrophysics at the University of Paris-Saclay, told AFP.

The small VMC, it took the lead in the exercise and has since been promoted to the rank of scientific instrument.

© 2021 AFP