Arsia Mons on Mars is among the tallest volcanos in the solar system – over two times Mount Everest – and it is the theatre of a very peculiar weather phenomenon. Every morning during Spring, a long and wide cloud of water ice is seen emerging from the top of the extinct volcano. And now, scientists have finally unlocked its secrets.
Published in the Journal of Geophysical Research, the cloud has been estimated to be 1,800 kilometers (1,120 miles) in length and 150 kilometers (93 miles) across. Observations have shown that the cloud itself is orographic, forming due to the volcano.
With its 20 kilometers (63,360 feet) of height, Arsia Mons plays a huge role in the local weather. Even a planet as dry as Mars has a little bit of moisture in the air. This is channeled up the flanks of the volcanos, condensing into a cloud at higher and cooler altitudes.
This phenomenon happens every morning for several months. Just before sunrise, the cloud will begin to form and expand westward at about 600 kilometers (375 miles) per hour at an altitude of 45 kilometers (27 miles). As it reaches its maximum length, it detaches, floating westward where it evaporates before the afternoon.
“Although orographic clouds are commonly observed on Earth, they don’t reach such enormous lengths or show such vivid dynamics,” co-author Agustin Sánchez-Lavega, of the University of the Basque Country, said in a statement. “Understanding this cloud gives us the exciting opportunity to try to replicate the cloud’s formation with models – models that will improve our knowledge of climatic systems on both Mars and Earth.”
Sánchez-Lavega is the science lead on Visual Monitoring Camera, or VMC, one of the instruments onboard the European Space Agency’s Mars Express. The instrument, nicknamed “the Webcam”, is a wide-field low-res camera that was used originally to confirm the separation between Mars Express and the Beagle 2 lander back in 2003 and afterward used for public outreach.
“However, recently, the VMC was reclassified as a camera for science,” added lead author Jorge Hernández Bernal, also from the University of the Basque Country. “Although it has a low spatial resolution, it has a wide field of view – essential to see the big picture at different local times of day – and is wonderful for tracking a feature’s evolution over both a long period of time and in small time steps. As a result, we could study the whole cloud across numerous life cycles.”
VMC, together with other instruments from Mars Express and missions from NASA and the Indian Space Research Organisation, has now been able to characterize how this cloud forms, changes, and disappears.