Scientists Are Monitoring a Strange Giant Cloud That Just Appeared on Mars

Since 13 September 2018, the Visual Monitoring Camera (VMC) on board ESA’s Mars Express has been observing the evolution of a curious cloud formation that appears regularly in the vicinity of the 20 km-high Arsia Mons volcano, close to the planet’s equator. ESA/GCP/UPV/EHU Bilbao, CC BY-SA 3.0 IGO

Astronomers have spotted a strange, elongated cloud formation on Mars in the skies above Arsia Mons—a 12-mile-high volcano, close to the Red Planet's equator.

Using the Mars Express orbiter (MEO), researchers at the European Space Agency have been monitoring the cloud since September 13.

Even though it appears to be emanating from the crater of Arsia Mons, the cloud is not linked to volcanic activity. Instead, it is a water ice cloud driven by the influence of the volcano's leeward slope—the slope facing downwind—on the surrounding air flow. Known as an orographic or lee cloud, this feature has appeared before in the region.

In an image taken on October 10 by the MEO's Visual Monitoring Camera (VMC), the white feature can be seen extending around 930 miles westward of Arsia Mons. As a comparison, the volcano itself has a diameter of about 270 miles at its base.

It is the second largest volcano known to science in terms of volume, beaten only by its neighbor Olympus Mons. In fact, Arsia Mons has 30 times the volume of Mauna Loa in Hawaii, the largest volcano on Earth.

The northern hemisphere of the Red Planet experienced its winter solstice on October 16. In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons. For the rest of the year, its summit is covered with clouds.

However, even in the lead up to the solstice, a water ice cloud like the one shown in the image is known to form along the southwest flank of the volcano. It has previously been observed by Mars Express and other missions in 2009, 2012 and 2015.

Over the course of the Martian day, the cloud spotted by the MEO varies in appearance, growing in length until it reaches such an impressive size that it may even be visible to telescopes on Earth.

The formation of water ice clouds is sensitive to the amount of dust present in the Martian atmosphere. The latest images—which were captured after a massive dust storm that engulfed the entire planet in June and July—will help researchers to better understand the effect of dust on cloud development throughout the year.

The massive dust storm caused problems for NASA's Opportunity Rover, which has been operating on the surface of Mars for more than 14 years.

The space agency has not heard from the vehicle since June after the storm blotted out the Sun, meaning that the rover could not recharge its solar-powered batteries. This forced NASA to suspend scientific operations as the vehicle shifted to a low energy-use state, which is sufficient to keep its critical heaters running but nothing more.

NASA says it will continue with the current strategy for attempting to make contact with the Opportunity rover for the foreseeable future.