Scientists investigating an enormous cloud that mysteriously appears over Mars each year have uncovered evidence of an exotic physical process never before identified in a planetary atmosphere.
The new research on the Arsia Mons Elongated Cloud (AMEC) was published in a recent Nature Geoscience paper, offering new insight into one of the Red Planet’s most striking features and what strange physics may lurk on distant exoplanets.
AMEC forms every year in Mars’s southern hemisphere, during the spring and summer dust season, consisting of water ice suspended in the air near the Arsia Mons volcano. During its seasonal appearance, the cloud can stretch up to 1,800 kilometers long, forming in the early morning before dissipating later in the day. This cycle repeats over several months.
Exploring Mars’s AMEC
The Martian cloud has intrigued researchers since its discovery in 2018. Previous work has explored AMEC’s behavior, evolution, and dynamics, identifying it as an orographic cloud that forms around mountains and volcanoes on Earth.
Despite years of investigation, this is the first time researchers have produced a model that closely matches observations of AMEC formation and behavior.
“To create the AMEC in our modelling, 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,” said lead author Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, France. “Once we included this physics in our simulations, the AMEC emerged just as we hoped.”
Cloud Formation on Mars
Scientists generally expect the fundamental processes governing cloud formation to be similar on Earth and Mars. As moist air cools, water vapor condenses into droplets or freezes into ice crystals, which remain suspended in the atmosphere to form clouds. Typically, those droplets and crystals require a nucleus, such as particles of salt, pollen, or dust, around which to form. Given Mars’s dusty surface conditions, scientists have long assumed Martian clouds form around suspended dust particles.
“For the AMEC, it seems that cloud formation takes place without needing any of this ‘stuff’,” adds Jorge. “Water vapour 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.”
Such a process had previously been theorized as possible on Venus or in Earth’s upper atmosphere. For it to occur, however, conditions would have to be highly unusual, with water vapor reaching extreme levels of supersaturation. Scientists therefore suspected it would be an exceptionally rare occurrence, if it occurred at all. Before this research, such a cloud-forming process had not been identified in a planetary atmosphere.
“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.
Explaining AMEC and Exoplanet Outlook
According to the researchers, a combination of Arsia Mons’s height and the thin Martian atmosphere creates unique conditions conducive to this type of cloud formation. As winds push past the roughly 12-mile-high volcano, they lift pockets of moist air to unusually high altitudes. The rapid ascent cools the air by about 30°C in approximately 10 minutes, while relative humidity climbs sharply, allowing ice crystals to form spontaneously.
Although the model closely reproduces the cloud’s behavior, further work remains to achieve an exact match with observations. Nevertheless, the team considers the findings a major advance, particularly given that scientists have far less observational data on the Martian atmosphere than on Earth’s.
“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.”
The team’s biggest takeaway is that atmospheric processes on other planets may behave quite differently from those commonly observed on Earth, potentially requiring unusual physical conditions to explain their behavior.
“While clouds on Earth and Mars seem to be governed by the same ‘rules’, Wilson concluded, “understanding this exotic Martian cloud required exotic physics – and this may be true elsewhere in the cosmos.”
The paper, “Homogeneous Ice Nucleation from Water Vapour Suggested by Elongated Clouds on Mars,” appeared in Nature Geoscience on October 7, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.