Since September 13, 2018, ESA’s Mars Express spacecraft has been observing the evolution of a water ice cloud formation hovering in the vicinity of Arsia Mons, the southernmost in a trio of giant Martian volcanoes known collectively as Tharsis Montes.
The elongated cloud can be seen in an image taken on October 10, 2018 by Mars Express’ Visual Monitoring Camera as the white, elongated feature extending 932 miles (1,500 km) westward of the 12-mile (20 km) tall Arsia Mons volcano.
In spite of its location, this atmospheric feature is not linked to volcanic activity but is rather a water ice cloud driven by the influence of the volcano’s leeward slope on the air flow — something that scientists call an orographic or lee cloud — and a regular phenomenon in this region.
“Mars just experienced its northern hemisphere winter solstice on October 16,” ESA researchers said.
“In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons.”
“Its summit is covered with clouds throughout the rest of the Martian year.”
“However, a seasonally recurrent water ice cloud is known to form along the southwest flank of this volcano — it was previously observed by Mars Express and other missions in 2009, 2012 and 2015.”
The cloud’s appearance varies throughout the Martian day, growing in length during local morning downwind of the volcano, almost parallel to the equator, and reaching such an impressive size that could make it visible even to telescopes on Earth.
“The formation of water ice clouds is sensitive to the amount of dust present in the atmosphere,” the scientists explained.
“New images, obtained after the major dust storm that engulfed the entire planet in June and July 2018, will provide important information on the effect of dust on the cloud development and on its variability throughout the year.”