Elongated Clouds of Martian Volcano Arsia Mons | Europe's Mars Express
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 is based on data from Mars Express’s High Resolution Stereo Camera (HRSC), and shows the cloud stretching out for around 1500 km.
Researchers using Mars Express now think that exotic physics might be behind this cloud’s formation—physics related to how water vapor turns into icy cloud particles.
This image was processed by Flickr user J Cowart. It uses data from Mars Express orbit 18627 on September 21, 2018.
Image Description: Tan-colored horizontal rectangle showing part of the surface of Mars imaged by an orbiting satellite. Above the curved horizon is space. To the lower right of the image is a volcano. Stretching to the left of the volcano is a long wisp of white cloud, seeming to emanate from the volcano. We also see the shadow of the cloud just below the cloud itself.
The image was taken by Mars Express’s High Resolution Stereo Camera (HRSC) on June 24, 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 United Kingdom—before quickly evaporating. This cycle repeats every morning for several months.
Since September 13, 2018, the Visual Monitoring Camera (VMC) on board the European Space Agency’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. The cloud can be seen in this VMC image taken October 10, 2018, as the white, elongated feature extending 1500 km westward of the volcano.
Mars’s oddest cloud may be even odder than we thought . . . Scientists using the European Space Agency's Mars Express, in combination with the latest meteorological model of the Red Planet, have found that there may be very exotic physics behind Mars’s most curious cloud. Each spring and summer in Mars’s southern hemisphere, during the Martian dusty season, we see a spectacular cloud emerge: the Arsia Mons Elongated Cloud (AMEC), the most visually striking cloud on the Red Planet.
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.
Mars Express first revealed the AMEC in 2018, and has viewed the recurrent cloud repeatedly since. Researchers have explored its evolution, vivid dynamics and intriguing behavior, determining it to be an orographic cloud: a type of cloud also seen on Earth that forms as wind flows past the craggy topography of a volcano or mountain.
However, when trying to model exactly how the cloud springs to life, our simulations simply did not reproduce what we see in the images—until now.
“To create the AMEC in our modeling, we found that we needed to include some exotic physics… physics that, while included in textbooks, is treated as theoretical and usually thought not to happen in nature. It certainly hasn’t been seen in action before,” says Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, France, lead author of the new study. “Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
“Wholly unexpected”: a rare process in action
On Earth and elsewhere, clouds typically form when moist air cools and water vapor condenses into liquid droplets or icy crystals. This usually happens via a process known as heterogeneous nucleation, which requires specks of other ‘stuff’ to be present in the atmosphere for vapor to cling and condense onto—salt, pollen, soot, or dust, for instance. On Mars, it is thought to be dust.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” adds Jorge. “Water vapor turns directly into icy cloud particles without any middle step. It’s akin to droplets of condensation appearing in the middle of a room, rather than on a window. We call this homogeneous nucleation, and we’ve never seen it before in a planetary atmosphere. It’s wholly unexpected.”
Scientists had suggested that this process might take place in the upper atmospheres of Earth and Venus, but it has not been spotted. Its rarity is due to it requiring exceptional circumstances, with extreme relative humidity levels of over 100,000 times those usually experienced in our daily life on Earth. “We’ve not seen these conditions on Mars before, but our finding now strongly suggests that the planet’s humidity can indeed reach these extreme levels,” says Jorge.
Jorge and colleagues found that the AMEC sits in a unique position where Mars’s thin atmosphere and the towering height of the nearby Arsia Mons volcano come together to create the conditions needed to see this rare process in action.
As winds flow past Arsia Mons, the volcano’s bulk triggers a powerful wave that lifts moist parcels of air several kilometers up into the air in just a few minutes. This cools the atmosphere rapidly, causing temperatures to drop by 30 degrees in just 10 minutes and relative humidity levels to spike. Water vapor then spontaneously freezes directly into cloud particles, forming the AMEC.
While some aspects of the modelled cloud do not exactly match the observations, “the result is remarkable,” says Jorge. “We don’t have nearly as much information about Mars’s atmosphere as we do about Earth's, so reproducing the AMEC to this degree is a big success for the model.”
The researchers based their modelling on data from the three cameras aboard Mars Express: the Visual Monitoring Camera, High Resolution Stereo Camera, and OMEGA. Mars Express can image large swathes of the Martian surface at high resolution, and it is one of the few Mars-orbiting spacecraft—alongside ESA’s ExoMars Trace Gas Orbiter—able to observe during morning hours, when the AMEC is present.
“Mars Express can also track how the cloud is changing on timescales of mere hours, which gives us an unrivalled view of short-lived phenomena on the planet,” says ESA Mars Express Project Scientist Colin Wilson. “Overall, this finding is a true accomplishment for the mission and its scientists: Mars Express discovered the AMEC, has followed up and monitored it for years, and is now helping reveal the secrets of its formation.”
Beyond furthering our understanding of atmospheric processes on Mars and elsewhere, the result highlights that we should not discount unlikely processes when exploring the planets of the Universe (including exoplanets). Colin adds: “While clouds on Earth and Mars seem to be governed by the same ‘rules’, understanding this exotic Martian cloud required exotic physics—and this may be true elsewhere in the cosmos.”
The European Space Agency's Mars Express Mission (2003-2026) was launched on June 2, 2003, from Baikonur, Kazakhstan, aboard a Russian Soyuz rocket with a Fregat upper stage. In addition to being Europe’s first mission to Mars, Mars Express is the first fully European mission to any planet. The orbiter has been successfully performing scientific measurements since early 2004, namely, high-resolution imaging and mineralogical mapping of the surface, radar sounding of the subsurface structure down to the permafrost, precise determination of the atmospheric circulation and composition, and study of the interaction of the atmosphere with the interplanetary medium. Due to the valuable science return and the highly flexible mission profile, Mars Express has been granted several mission extensions.
Credits: ESA/DLR/FU Berlin; CC BY-SA 3.0 IGO
Acknowledgements: J. Cowart
Release Date: Oct. 7, 2026
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