Return of the extremely elongated cloud on Mars. A mysteriously long, thin cloud has again appeared over the 20-km high Arsia Mons volcano on Mars. A recurrent feature, the cloud is made up of water ice, but despite appearances it is not a plume linked to volcanic activity. Instead, the curious stream forms as airflow is influenced by the volcano's 'leeward' slope ? the side that does not face the wind. These images of the cloud, which can reach up to 1800-km in length, were taken on 17 and 19 July by the Visual Monitoring Camera (VMC) on Mars Express, which has been studying the Red Planet from orbit for the past 16 years. "We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now," explains Jorge Hernandez-Bernal, PhD candidate at the University of the Basque Country (Spain) and lead author of the ongoing study. "This elongated cloud forms every martian year during this season around the southern solstice, and repeats for 80 days or even more, following a rapid daily cycle. However, we don't know yet if the clouds are always quite this impressive". A martian day, or sol, is slightly longer than an Earth day at 24 hours, 39 minutes and 35 seconds long. A year at the Red Planet consists of 668 sols, approximately 687 days, so the seasons last for twice as long. The southern solstice is the period of the year when the Sun is in the southernmost position in the martian skies, just like 21 December on Earth. In the early mornings during this period, this fleeting cloud grows for approximately three hours, quickly disappearing again just a few hours later. Most spacecraft in orbit around the Red Planet tend to observe in the afternoon, however Mars Express is in a privileged position to gather and provide crucial information on this unique effect. "The extent of this huge cloud can't be seeCredit: ESA / eyevine Contact eyevine for more information about using this image: T: +44 (0) 20 8709 8709 E: info@eyevine