The Mars Express orbiter has kept an eye on an elongated cloud above Arsia Mons volcano for the last few years. The cloud is reoccurring, grows incredibly fast and is "made up of water ice," according to the European Space Agency (ESA).

Credit: Space.com | imagery courtesy: ESA/GCP/UPV/EHU Bilbao, [CC BY-SA 3.0 IGO] Creative Commons/licenses | produced & edited by Steve Spaleta