Decoding the 1,800km-long cloud on Mars.

The cloud formed above the Arsia Mons volcano before dawn, then expanded at speeds up to 600 km/h before dissipating.

Scientists used the "Mars Webcam" on the European Space Agency's (ESA) Mars Express spacecraft to monitor the lifecycle of the Arsia Mons long cloud (AMEC) and discovered some new information, Cnet reported on March 9. The research was published in the journal JGR Planets.

A cloud 1,800km long over a Martian volcano. (Image: ESA/GCP/UPV/EHU Bilbao).

AMEC appears seasonally above the Arsia Mons volcano. This isn't the first time scientists have seen the cloud, but it's difficult to study because it dissipates quickly, and tracking it using spacecraft in Martian orbit isn't easy. Martian webcams have a wide field of view, so they're being used to study AMEC.

"Many Mars orbiters can only observe the Arsia Mons volcanic region in the afternoon due to their orbital characteristics. So this is really the first time we've studied this interesting structure in detail ," said study co-author Agustin Sánchez-Lavega, an expert at the University of Basque Country.

AMEC appears during the Martian spring and can be up to 1,800 km long and 150 km wide. "This is the largest mountain cloud ever observed on Mars. It forms when winds are forced upwards by topographic features such as mountains or volcanoes on the planet's surface ," ESA said.

AMEC appears and disappears every day, for 80 days or more. The cloud begins to form before sunrise and then grows at a very rapid rate in about two and a half hours. It can expand at speeds up to 600 km/h before stopping. Afterward, the cloud separates from its original source, spreads further, and evaporates by late morning.

Similar mountain clouds exist on Earth, but they don't reach the impressive lengths of AMEC. "If we understand this cloud, we will have an exciting opportunity to recreate cloud formations using models, thereby gaining more information about weather systems on Mars and Earth ," said Sánchez-Lavega.

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