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Mars’ Mysteriously Long Cloud Has Finally Been Solved

Every morning during the warmer months in the southern part of the Red Planet, an exceptionally long cloud appears over a Martian volcano. The cloud of water ice, which appears close to the Arsia Mons volcano, stretches out over a thousand kilometers before dissipating within hours of forming.

Oddly enough, the Arsia Mons is the only volcano in that area that possesses such an unusual cloud on a daily basis. And now, based on observations conducted by the Visual Monitoring Camera (also known as VMC and the “Mars Webcam”) on ESA’s Mars Express, researchers have been able to solve the mystery of the unusually long cloud.

The team was able to study the cloud’s cycles by analyzing data collected by the VMC and two other instruments from the Mars Express (OMEGA and HRSC) as well as from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), Mars Reconnaissance Orbiter (MRO), Viking 2, and the Indian Space Research Organisation’s Mars Orbiter Mission (MOM). Incredibly, they even found footage of the cloud from observations conducted by Viking 2 in the 1970s.

Two views of the Arsia Mons volcano taken by the Viking orbiter.

It is the largest “orographic” cloud that’s ever been observed on Mars, measuring about 1,800 kilometers in length (1,118 miles) and 150 kilometers across (93.2 miles). It forms when the wind is pushed up by topographic features like volcanoes or mountains.

This occurs every morning during the spring and summer months. The process begins before sunrise when the moist air moves up the western slope of the Arsia Mons volcano and expands towards the west for approximately two and a half hours. This is an extremely fast process as the cloud grows as a speed of approximately 600 km/h (373 mph) and at an altitude of about 45 kilometers (28 miles). Once it stops growing, it detaches itself from the area of the volcano and moves towards the west by the wind before finally evaporating just a couple of hours later as the planet’s air temperature heats up due to the Sun rising.

Earth also has orographic clouds but nothing like what has been seen on Mars. “Although orographic clouds are commonly observed on Earth, they don’t reach such enormous lengths or show such vivid dynamics,” explained Agustin Sánchez-Lavega, who is from the University of the Basque Country and Science Lead for the VMC as well as a co-author of the study. “Understanding this cloud gives us the exciting opportunity to try to replicate the cloud’s formation with models – models that will improve our knowledge of climatic systems on both Mars and Earth.”

Dmitrij Titov, who is the ESA’s Mars Express project scientist and another co-author of the study, weighed in on the research by stating, “These findings really demonstrate the strengths of Mars Express – its unique orbit, longevity, persistent quality, and ability to adapt as it tackles the mysteries of Mars,” adding, “Repurposing the VMC has successfully enabled us to understand this transient cloud in a way that wouldn’t have been possible otherwise. The VMC allows scientists to track clouds, monitor dust storms, probe cloud and dust structures in the Martian atmosphere, explore changes in the planet’s polar ice caps, and more.” Their study was published in the Journal of Geophysical Research.

Pictures of the elongated Martian cloud can be seen here.