Strange events have been observed on Mars since 13 September, when ESA’s Mars Express detected an elongated cloud formation hovering in the vicinity of the Arsia Mons. The ESA released a picture taken by its Mars Express orbiter that showed “a curious cloud formation” about a thousand miles long spilling out of Arsia Mons, a giant volcano thought to be dormant for about 50 million years.
The monster volcano, the southernmost of three volcanos on the Tharsis bulge near Mars’ equator, is 12 miles high and 270 miles wide, dwarfing Mauna Loa, Earth’s largest volcano, which is only 6.3 miles high and 75 miles wide, and mostly underwater.
But, “It’s just a cloud,” said Eldar Noe Dobrea, a scientist at the Planetary Science Institute, which is based in Tucson, Ariz. and not a volcanic event, because spacecraft would have detected a rise in methane, sulfur dioxide and other gases that spill out of eruptions.
The cloud can be seen in the view above taken on 10 October by the Visual Monitoring Camera (VMC) on Mars Express – which has imaged it hundreds of times over the past few weeks – as the white, elongated feature extending 1500 km westward of cone-shaped Arsia Mons.
The Arsia Mons phenomena this is an example of orographic lifting. “It happens on Earth a lot,” says Dobrea, The clouds form when water-laden air is pushed upward along a mountain. Cooler, thinner air cannot hold as much water, causing some of the moisture to condense and freeze, forming clouds. The air on Mars is much thinner than on Earth, but the rules of weather physics also apply there.
It is rare for there not to be clouds over Arsia Mons observed Dobrea. More than a decade ago, reports the New York Times, Dobrea analyzed observations from an earlier NASA mission, Mars Global Surveyor, trying to piece together a cloud-free picture of the Martian surface and every time the spacecraft had passed over the western flank of Arsia Mons, it was cloudy. “It turns out not a single one of the observations ever had a clear view of the surface at this point,” he said.