Mars

Every year during Mars’ summer solstice, a cloud of water ice forms on the leeward side of Arsia Mons, one of Mars’ largest extinct volcanoes. The cloud can grow to be up to 1800 km (1120 miles) long. It forms each morning, then disappears the same day, only to reappear the next morning. Researchers have named it the Arsia Mons Elongated Cloud (AMEC).

The massive extinct volcano that is home to the cloud is Arsia Mons, a 20 km high (12.4 miles) landmark on Mars’ surface. Mars is too cold for liquid water, so the cloud is made of water ice. Though it looks like it could be a plume from the volcano, it’s not. All of Mars’ volcanic activity ceased a long time ago.