A European spacecraft is unraveling the secrets of the weird long cloud that has been appearing again and again in the Martian sky  for years. 

The Arsia Mons Elongated Cloud is a peculiar feature on Mars: a long, bright water ice cloud stretching out over the Martian surface, extending over the planet’s Arsia Mons volcano all the way to the volcano Olympus Mons, the tallest mountain in the solar system. The strange phenomenon has been recurring every year around Mars’ southern solstice, with the cloud forming and fading daily for 80 or more days at a time on the Red Planet.