In late 1976, NASA’s Viking 2 spacecraft reached Mars, where it joined its sister spacecraft, Viking 1, in an ambitious mission that would eventually map close to 97 percent of the Martian surface.
Among the thousands of images obtained by Viking 2 over the course of the next two years, some of them managed to capture a particularly unusual feature: a massive cloud extending for several hundred miles, which was unlike anything else scientists had ever seen on the Red Planet.
Today, this massive feature, known as the Arsia Mons Elongated Cloud (AMEC), is better understood thanks to decades of study by NASA, as well as its international partners like the European Space Agency and its Mars Express spacecraft.
Yet much about the cloud remains mysterious. Although researchers have learned more about its unusual behavior, recent findings suggest that the conditions responsible for its formation may differ significantly from those involved in conventional cloud formation.
Mystery at Arsia Mons
Appearing over Arsia Mons, one of the Red Planet’s massive volcanoes, the unusual cloud is typically seen during the Martian spring and summer seasons and is most prominent in the morning hours.
The cloud mainly consists of water ice crystals, which the warm seasonal winds carry across long distances, resulting in a lengthy cloud that stretches more than 900 miles across the planet’s surface.
Still, many questions remain about how this remarkable feature develops. On Earth, water vapor typically condenses or freezes onto microscopic airborne particles, including dust, allowing cloud droplets and ice crystals to form.
However, the Martian atmosphere is extremely thin, and conditions at the high altitudes where the Arsia Mons cloud forms may not provide enough dust particles to support the same process. Because of this, it remains unclear whether similar processes that lead to cloud formation on Earth could be occurring on Mars, or if something entirely different causes the massive Arsia Mons Elongated Cloud to form.
Possible Answers to a Longstanding Mystery
Based on recent research, scientists now believe the Arsia Mons cloud could form, resulting from a process that does not rely on dust particles, which would normally be responsible for similar formations when they occur on Earth.
If this is correct, it potentially reveals previously unrecognized aspects of cloud formation in planetary atmospheres—a phenomenon that might gain additional confirmation from future observations of cloud formation in planetary atmospheres that are thinner than Earth’s.
Ultimately, unraveling the mystery of the Arsia Mons Elongated Cloud could help scientists refine their understanding of atmospheric processes not only on Mars, but potentially here on Earth as well.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.