The long cloud near Mars’s Arsia Mons volcano is the Arsia Mons Elongated Cloud (AMEC), a recurring cloud of water ice—not a volcanic plume. Arsia Mons shapes the winds above it: air rises, cools in the thin atmosphere, and water vapor condenses into ice crystals. The cloud’s head is linked to strong uplift caused by downslope winds, while the full process that creates its extraordinary westward tail is not yet settled.
How Arsia Mons helps form the cloud
AMEC forms near local sunrise on the volcano’s western flank. As winds meet Arsia Mons’s steep topography, air is forced upward. The rising air cools at high altitude, allowing water vapor to condense and form ice crystals. The volcano influences the airflow, but the cloud itself is an atmospheric feature rather than material erupted from the volcano.
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A 2022 mesoscale modeling study offers a more detailed explanation for the cloud’s head: downslope flow can develop into a windstorm, followed by a hydraulic-like jump that drives a strong updraft. In the model, this process produced as much as 30 K of cooling at 40–50 km altitude, coinciding in space and time with the observed head. That figure is a model result for the proposed mechanism, not a direct temperature measurement for every cloud appearance. Read the 2022 mesoscale modeling study.
Why the cloud becomes so long
After forming, the cloud grows and is carried westward by high-altitude winds. Observations describe a rapid morning cycle: the feature starts growing before sunrise, extends for roughly 2.5 hours, then detaches from the volcano and continues downwind. It eventually evaporates before afternoon as temperatures rise. The European Space Agency reports growth at more than 600 km/h at about 45 km altitude. In Martian Year 34, an observational study measured expansion velocities around 170 m/s near 45 km altitude and recorded a maximum length of 1,800 km. ESA reports a maximum width of about 150 km. These figures come from different reports and observation contexts, rather than a single universal measurement. ESA’s account of the cloud; the observational study of its life cycle.
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When it appears—and why it can be missed
AMEC is seasonal, not a permanent plume. The observational study places its appearances around solar longitude 220°–320°, during southern spring and summer near the southern solstice. Within that season, it follows a morning pattern. Spacecraft that usually view the region in the afternoon may therefore miss it. Repeated, wide-field observations by Mars Express’s Visual Monitoring Camera (VMC) helped reveal the cloud’s daily cycle; ESA says researchers used 63 VMC observations alongside data from Mars Express’s HRSC and OMEGA instruments and other spacecraft. ESA describes the observing campaign.
What scientists know about the tail—and what remains open
Observations establish that the cloud grows, detaches, and travels westward, while modeling links the head to topography-driven airflow and strong cooling. But the 2022 model did not reproduce the observed expansion of the long tail and produced insufficient cloud optical depth. So the mechanism that initiates the head is better explained than the detailed formation and growth of the tail.
A preprint posted on September 29, 2026, by Hernández-Bernal, Määttänen, Spiga, and Forget proposes that homogeneous nucleation may help explain AMEC’s distinctive characteristics. Homogeneous nucleation means cloud particles form directly from water vapor without pre-existing nuclei. The authors report that adding this process to a Mars meteorological model reproduced the cloud’s distinctive characteristics. Because this is a recent preprint and is not established here as peer-reviewed, it is best treated as a proposed advance rather than settled consensus. Read the preprint.
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How the cloud was identified
The case that AMEC is a recurring atmospheric cloud rests on observations from multiple missions, including Mars Express, NASA’s MAVEN and Mars Reconnaissance Orbiter, Viking 2, and India’s Mars Orbiter Mission. The cloud’s timing, ice-cloud interpretation, and motion are consistent with a feature shaped by atmospheric flow and condensation, not an eruption. ESA summarizes the evidence in its report on the return of the extremely elongated cloud.
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