Incredible Mars Photos Show Strange 1,100-Mile-Long Cloud Above a Volcano

The European Space Agency has published incredible images of a strange "extremely elongated" cloud in the atmosphere above the Arsia Mons volcano on Mars.

The images of the 1,100-mile-long cloud were captured by the Visual Monitoring Camera on the Mars Express spacecraft, which has been studying the Red Planet for more than 15 years.

The long, thin cloud of water ice appears at certain times of the Martian year—which lasts about 687 Earth days—above the 12-mile-high Arsia Mons volcano, located just south of the planet's equator.

Although it looks like the volcano—the planet's second-largest by volume—is erupting and the cloud is emanating from the crater, the atmospheric phenomenon is not the result of volcanic activity.

The cloud actually forms because of the influence of the volcano's leeward slope—the slope facing downwind—on the surrounding air flow. Scientists call this an "orographic" or "lee" cloud.

The cloud is hard to study because of its fleeting nature and the constraints of the spacecraft observing Mars. But Jorge Hernández Bernal, a PhD candidate at the University of the Basque Country in Spain, and his colleagues have been investigating the phenomenon as part of a long-term research project using Mars Express.

Evolution of the Arsia Mons elongated cloud
A series of images of the cloud above the Arsia Mons volcano, captured by a camera on the Mars Express craft. ESA/GCP/UPV/EHU Bilbao

"To clear these hurdles, we used one of Mars Express' secret tools—the Visual Monitoring Camera," Bernal said in a statement.

Mars Express is in a beneficial position to observe the cloud with its VMC, which despite having a resolution similar to a standard webcam from the early 2000s has proved particularly helpful for the study. The camera was only installed to confirm that the Beagle 2 lander had successfully separated from the spacecraft back in 2003. Subsequently, it was switched off.

"However, recently, the VMC was reclassified as a camera for science," Bernal said. "Although it has a low spatial resolution, it has a wide field of view—essential to see the big picture at different local times of day—and is wonderful for tracking a feature's evolution over both a long period of time and in small time steps. As a result, we could study the whole cloud across numerous life cycles."

In a study published recently in the Journal of Geophysical Research, Bernal and colleagues said the cloud—which they have dubbed the Arsia Mons Elongated Cloud—is about 1,100 miles long and roughly 90 miles across.

The scientists say it is the biggest orographic cloud yet seen on Mars. And Arsia Mons is the only low-latitude location on Mars where clouds are seen.

It forms when moist air is forced up the flank of the volcano, later condensing at higher and significantly cooler altitudes.

In the period that it is visible, the cloud begins growing before sunrise on the western slope of the volcano before rapidly expanding westwards for around two-and-a-half hours at more than 370 miles per hour, at an altitude of 30 miles, according to the researchers.

Once the cloud has reached its maximum extent, it detaches from its initial location and is pushed further westwards by high-altitude winds, before evaporating in the late morning as temperatures in the region rise.

The cloud forms every Martian year "around the southern solstice, and repeats for 80 days or even more, following a rapid daily cycle," Bernal said in a statement last year.

The southern solstice is the point in the Red Planet's year when the sun is located in its southernmost position in the sky.

Extremely elongated cloud on Mars
Two images of the 1,100-mile cloud taken by the Visual Monitoring Camera on the Mars Express spacecraft. ESA/GCP/UPV/EHU Bilbao