It sounds like volcanic activity on Mars – but it's not – Astronomy Now

A long cloud extending 1,500 kilometers from the Marsian Arsia Mons volcano looks like an obvious sign of volcanic activity. But that's not it. Perhaps less exciting, it is a regular phenomenon called orographic water ice cloud, caused by the wind that pbades beyond the leeward slope of the volcano.

The Mars Express orbiter of the European Space Agency has imagined the cloud hundreds of times in recent weeks.

Mars marked the winter solstice in its northern hemisphere on October 16th. Cloud activity on the planet's large volcanoes is normally dissipating, but a "seasonal recurrent ice-water cloud is known to form on the southwestern flank of this volcano," explained European Space Agency. "It had already been observed by Mars Express and other missions in 2009, 2012 and 2015.

"The appearance of the clouds varies throughout the Martian day, stretching during the local morning downwind of the volcano, almost parallel to the equator, and reaching a size as impressive as it could be seen even by telescopes on Earth. "

The view below shows the cloud in context.