In September 2018, a mysterious, white cloud formation appeared to hover over the site of a volcano on Mars.
Since then, the cloud has continued to appear over Mars around the same time of the Martian year and yet astronomers are not sure exactly how and why it forms. However, now they are getting a closer look at this cloudy formation as the European Space Agency's (ESA) Mars Express mission captures images of this mysterious, elongated cloud that provide astronomers with a new look at the white plume.
The Mars Express spacecraft captured images of the cloud on July 17 and 19 using its Visual Monitoring Camera.
In the images, the cloud appears as a white plume hovering over the Arsia Mons volcano on Mars, near the Martian equator, which stretches over 20 kilometers high. Despite claims that the cloud was an indication of volcanic activity on Mars, astronomers have concluded that that is not the case.
Instead, although the cloud is made up of water ice, it forms as airflow due to volcano’s ‘leeward’ slope − the side that does not face the wind, according to ESA.
The science team behind Mars Express has now named the cloud the Arsia Mons Elongated Cloud, AMEC.
“We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now,” Jorge Hernandez-Bernal, Ph.D. candidate at the University of the Basque Country (Spain) and lead author of the ongoing study, said in the ESA statement.
The cloud appears during the same season every Martian year, which is the equivalent of 687 days on Earth, around the southern solstice of Mars.
The southern solstice is the time of the year on Mars when the Sun is in the southernmost position in the martian skies, the same way it is on December 21 on Earth.
The mysterious cloud then persists for 80 days or more, with a rapid daily cycle, according to the researchers. AMEC grows for approximately three hours in the early mornings before quickly disappearing again a few hours later.
Although clouds are common on Mars, this one was rather unusual. The Martian cloud is weirdly elongated, reaching up to 1800-km in length. Another aspect that adds to its mystery is how long it lasts hovering over the Red Planet, staying up there for more than three months at a time, which is longer than any other cloud spotted on Mars.
The Mars Express mission has been orbiting the Red Planet for the past 16 years, studying the interior, surface and subsurface of Mars.