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It's just a cloud. A very long cloud.
A photograph of a spacecraft orbiting Mars shows a long white wisp, nearly a thousand kilometers long, coming out of a giant volcano.
The volcano, supposed to be dormant for about 50 million years, could it be about to blow?
Planetary scientists say with confidence that no.
"It's just a cloud," said Eldar Noe Dobrea, a scientist at the Institute of Planetary Science, who based in Tucson, Arizona.
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This week, the European Space Agency has released a photo taken by its Mars Express orbiter showing what it described as "a curious cloud formation" extending from east to west close to Arsia Mons, in the extreme south of a chain of three volcanoes.
Arsia Mons is 12 miles high and 270 miles wide. It is home to Mauna Loa, the largest volcano on the planet, which is only 6 km high and 75 km wide. (But it's not the biggest volcano on Mars, it's Olympus Mons, which is over 20 km tall and is the largest in the solar system.)
Noe Dobrea said that it was clearly not a volcanic event, because a machine would have detected an increase in methane, sulfur dioxide and other gases that were spreading after eruptions. This is an example of the topography that influences weather conditions.
Meteorologists even have a term to describe this phenomenon: orographic lifting. "It often happens on Earth," said Dr. Eldar Noe Dobrea, referring to the storms that occur frequently in California's Sierra Nevada Mountains.
Clouds form when the charged air is pushed upward along a mountain. The colder and thinner air can not hold as much water, which causes some of the moisture to condense and freeze, thus forming clouds. The air on Mars is much thinner than on Earth, but the rules of meteorological physics apply to it as well.
Indeed, it is rare that there are no clouds on Arsia Mons. More than ten years ago, Dr. Noe Dobrea analyzed observations from a previous NASA mission, Mars Global Surveyor, that attempted to reconstruct a cloudless image of the Martian surface. But every time the spacecraft passed on the west side of Arsia Mons, it was cloudy.
"It turns out that not one of the sightings had an unobstructed view of the surface at this stage," he said.
Kenneth Chang has been at the Times since 2000 and has written on physics, geology, chemistry and planets. Before becoming a science writer, he was a graduate student and his research focused on controlling chaos. @kchangnyt
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