A photograph from a spacecraft orbiting Mars shows a long, white wisp, close to a thousand miles long, spilling out of a giant volcano.
Could the volcano, thought to be dormant for some 50 million years, be about to blow?
Planetary scientists confidently say no.
“It’s just a cloud,” said Eldar Noe Dobrea, a scientist at the Planetary Science Institute, which is based in Tucson, Ariz.
This week, the European Space Agency released a picture taken by its Mars Express orbiter that showed what it described as “a curious cloud formation” stretching from east to west near Arsia Mons, the southernmost in a string of three volcanoes.
Arsia Mons 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. (It’s not the biggest volcano on Mars, though. That’s Olympus Mons, which is more than 13 miles tall and the largest in the solar system.)
Dr. Noe Dobrea said this was clearly not a volcanic event, because spacecraft would have detected a rise in methane, sulfur dioxide and other gases that spill out of eruptions. Instead, this is an example of how topography affects weather.
Meteorologists even have a term to describe this phenomenon: orographic lifting. “It happens on Earth a lot,” Dr. Eldar Noe Dobrea said, mentioning storms that frequently break out in the Sierra Nevada mountains in California.
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.
A view of clouds on Mars taken by the Mars Orbiter Mission in 2015.CreditJustin Cowart
Last month, NASA released a false-color image based on data from the Mars Odyssey orbiter that captured more details in the ice-rich clouds blowing over Arsia Mons.
Indeed, it is rare for there not to be clouds over Arsia Mons. More than a decade ago, Dr. Noe Dobrea analyzed observations from an earlier NASA mission, Mars Global Surveyor, trying to piece together a cloud-free picture of the Martian surface. But 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.