A long cloud stretching 1,500 kilometres (930 miles) away from the martian volcano Arsia Mons looks like a clear sign of volcanic activity. But it’s not. Less exciting, perhaps, it is a regular phenomenon known as an orographic water ice cloud caused by wind moving past the volcano’s leeward slope.
The European Space Agency’s Mars Express orbiter has imaged the cloud hundreds of times over the past few weeks.
Mars marked winter solstice in its northern hemisphere on 16 October. Cloud activity over Mars big volcanoes normally dissipates by then, but a “seasonally recurrent water ice cloud is known to form along the southwest flank of this volcano,” the European Space Agency said in an explanation. “It was previously observed by Mars Express and other missions in 2009, 2012 and 2015.
“The cloud’s appearance varies throughout the martian day, growing in length during local morning downwind of the volcano, almost parallel to the equator, and reaching such an impressive size that could make it visible even to telescopes on Earth.”
The view below shows the cloud in context.