A strange incredibly long and thin cloud has appeared on Mars again, above the 20 km high shield volcano Arsia Mons. If the researchers expected it, the details of its formation still remain very mysterious.
No, this is not a plume of smoke indicating any volcanic activity. It is actually a cloud made up of water ice. Spotted on July 19 by the VMC camera of the ESA's Mars Express probe, it appears here above Arsia Mons, a shield volcano 20 kilometers high located near the Martian equator and inactive for 50 million years.
Several hundred kilometers long (this one stretches 1,800 km), these clouds systematically form along the slope of the volcano opposite to that facing to prevailing winds. And this, every year around the time of the planet's southern solstice - which is the Martian equivalent of the winter solstice on Earth. The Sun is then in the southernmost position of the Martian sky, just as it is on December 21 on Earth.
In the early morning, around this period, a fleeting cloud then develops for about three hours , before quickly dissipating. The phenomenon then repeats itself every morning for about 80 days .
As a reminder, a Martian day, or sol, is slightly longer than an Earth day, namely 24 hours, 39 minutes and 35 seconds. A year on the Red Planet consists of 668 sols, or about 687 days, so the seasons last twice as long.
Most starships orbiting the planet red are sometimes witness to this phenomenon. However, given their position in the sky, they can usually only observe it in the early afternoon, and with cameras with a narrow field of view.
For its part, Mars Express is equipped with a wide-field camera and evolves in a very elliptical orbit, which allows it to take photos covering a large area of the planet early in the morning . And therefore to be able to "catch" this cloud.
Note that this cloud was also captured in a photo in 2018. Formed at the end of September, it then spread over 1,500 km before finally disappearing at mid-October.
For now, the formation of these water ice clouds still remains mysterious. It is not known how, why, or since when they appear during this time of year.
