A curious cloud reappeared in the atmosphere of Mars, over the volcano Arsia Mons. The phenomenon is recurrent, but what intrigues scientists is that the plume, made up of water ice, is not linked to volcanic activity. Instead, the long, narrow cloud forms with the wind on the steepest side of the volcano, which is 20 km high.
The images taken by the Mars Express mission of the European Space Agency (ESA), between 17 and 19 July, show the 1,800 km long cloud. “It is formed every Martian year at this time and is repeated for 80 days or more, after a fast daily cycle. However, we still don’t know if the clouds are always so impressive”, explains astronomer Jorge Hernandez-Bernal, from the University from Basque Pas (Spain), which has been studying the phenomenon.
We are at the time of the southern solstice on Mars, the time of year when the Sun is in the southernmost position in the Martian skies (as well as on December 21 on Earth). Early in the morning, during this period, the cloud grows for approximately three hours, disappearing just a few hours later.
Tharsis Plateau, with the Arsia Mons volcano in the center. Image: Nasa / JPL / USGS
A Martian day is 24 hours, 39 minutes and 35 seconds long, while a year on the Red Planet is 668 days – that is, the stations last twice as long. In the months leading up to the solstice, most cloud activity disappears in large volcanoes like Arsia Mons, whose cloud-covered summit for the rest of the Martian year.
The ice cloud was registered by Mars Express and other missions in 2009, 2012, 2015 and 2018. Depending on the size, it may be visible even to telescopes on Earth. The formation of ice clouds in the water depends on the amount of dust present in the atmosphere.
Located south of the Mars equator, Arsia Mons is part of a trio of volcanoes known as Tharsis Montes. NASA research suggests that the last volcanic activity in the region ceased about 50 million years ago – at the time of Cretaceous-Palegene’s extinction on Earth.
“We estimate that the peak of activity at the summit of Arsia Mons probably occurred approximately 150 million years ago – the late Jewish period on Earth – and then disappeared at the same time as the dinosaurs,” explains Jacob Richardson of the Goddard Space Flight Center.