Mars Express views the most striking cloud on Mars

This image from the European Space Agency’s Mars Express shows the Arsia Mons Elongated Cloud: the most visually striking cloud on the Red Planet.

The image was taken by Mars Express’s High Resolution Stereo Camera on 24 June 2024; it shows a region of Mars just under 1250 km across (an area of 780 000 square km), with the cloud stretching out for around 980 km.

This white wisp of water ice emerges downwind of the 20-km-tall Arsia Mons volcano. It forms, grows, and fades on a daily basis, stretching out for up to 1800 km – nearly twice the length of the UK – before quickly evaporating. This cycle repeats every morning for several months.

Researchers now think that exotic physics (known as homogeneous nucleation) might be behind this cloud’s formation – physics related to how water vapour turns into icy cloud particles.

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[Image description: Tan-coloured horizontal rectangle showing part of the surface of Mars imaged by an orbiting satellite. On the right is a ring of concentric circles that indicate a volcano. Stretching to the left of the volcano is a long wisp of white cloud, which seems to emanate from the volcano. We also see the shadow of the cloud just above the cloud itself.]