On September 13, the orbital station Mars Express photographed an extraordinary cloud near the equator of the Red Planet. This narrow, but long ( 930 miles as of October 10) light strip of Mars cloud could be seen even from the Earth – with the help of a telescope, of course.
An ordinary observer could have decided that a plume of smoke and ash stretches to the west from the extinct shield volcano Arsia to the west. The elongated cloud was also recorded using OMEGA infrared mapping spectrometer and high-resolution stereo camera on Mars Express.
Arsia Mons, by the way, is one of the largest volcanoes in the solar system: its height is almost 12 miles, its diameter is about 270 miles. The appearance of the cloud changes throughout the Martian day.
But the cloud did not appear at all from Arsia Mons. Scientists explain that there have been no eruptions on Mars for 50 million years, so these clouds are not the result of volcanic activity.
According to experts, an orographic cloud has formed over the mountain. This happens when the air masses overcome the heights. Humidity, which forms in areas of low temperature, condenses on dust particles, and cloud ridges form in the ascending streams.
Orographic clouds are characteristic of most of the Martian year (its duration is 687 Earth days), but with the approach of the winter solstice (October 16 this year), the clouds disappear. Scientists have already observed these seasonal “ice clouds” over Arsia in 2009, 2012 and 2015.