Like clockwork, a strange cloud has returned high above the Martian surface.
This long, thin cloud was spotted on July 17 and 19 by the Visual Monitoring Camera (VMC) attached to Mars Express, a satellite that’s been in orbit around Mars since 2004. These images were made possible owing to Mars Express’s elliptical orbit and the VMC’s wide field view, as noted in a European Space Agency press release.
The cloud is a recurrent feature on Mars, appearing annually above Arsia Mons, a 12.4-mile-high (20-kilometer) volcano located near the Martian equator. The cloud resembles a volcanic plume, but it’s not associated with any kind of volcanic activity. Composed of water ice, the cloud arises along the volcano’s leeward slope, that is, the side not facing the prevailing winds.
The Arsia Mons Elongated Cloud, or AMEC, as it’s called, can reach about 1,110 miles (1,800 km) in length. Incredibly, it can get big enough to be spotted by telescopes on Earth, according to the ESA.
“We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now,” said Jorge Hernandez-Bernal, a PhD candidate at the University of the Basque Country in Spain, in the ESA press release.
AMEC forms each year around the time of the planet’s southern summer solstice, when the Martian southern hemisphere receives the greatest amount of daylight. The cloud forms in the early morning, builds up over the next several hours, and then quickly dissipates. This meteorological cycle of birth-and-death typically lasts for around 80 days.
Hernandez-Bernal said “we don’t know yet if the clouds are always quite this impressive,” as close-up observations of AMEC date back just 11 years.
The last AMEC cycle happened two years ago. Previous observations were conducted by Mars Express in 2015, 2012, and 2009. Seasons on Mars last twice as long as they do on Earth, as a year on Mars lasts for 687 Earth days, hence the long delays between the annual AMEC cycles.
The formation of these water ice clouds might have something to do with the amount of dust present in the Martian atmosphere, but scientists have much to learn about these gigantic elongated clouds, such as when they first appeared and why they form so early in the morning.
Mars isn’t the only planet with intriguing clouds. Jupiter’s atmospheric formations are even more striking, featuring mesmerizing swirls and twists.
A previous version of this article incorrectly compared the Martian southern summer solstice to an Earth winter solstice.