Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errorsThe long cloud near Mars’s Arsia Mons is the Arsia Mons Elongated Cloud (AMEC): a recurring water-ice cloud formed when the volcano’s topography and surrounding winds lift air to high, cold altitudes, where water vapor condenses into ice crystals. It is not a volcanic plume. Winds then carry the cloud westward, creating its remarkable tail. Scientists have modeled how the cloud’s head forms, but the tail’s full explanation remains unsettled.
How Arsia Mons helps make the cloud
AMEC starts on the volcano’s western flank near local sunrise. Arsia Mons disrupts the atmosphere: air moving over and around its steep terrain is lifted, then cools as it rises. When the air reaches sufficiently cold altitudes, water vapor condenses into ice crystals. The result is an atmospheric cloud shaped by the volcano, not material erupting from it. The European Space Agency describes the feature as a water-ice cloud produced by airflow and condensation.
What modeling says about the cloud’s head
A 2022 mesoscale modeling study links the cloud head to a downslope windstorm followed by a hydraulic-like jump—a sharp change in airflow that can drive air upward. In the model, this updraft caused cooling of as much as 30 K at altitudes of 40–50 km, coinciding in space and time with the observed head. That figure is a model result for the proposed head-forming process, not a direct temperature measurement of every cloud event. The study is published in the Journal of Geophysical Research: Planets.
Why the cloud grows so long and then disappears
AMEC is a morning event, not a permanent plume. Observations show it beginning before sunrise, extending westward for hours, detaching from the volcano, and continuing downwind before evaporating as temperatures rise later in the day. ESA reports growth at more than 600 km/h at about 45 km altitude. A 2021 observational study measured expansion velocities around 170 m/s in Martian Year 34 and recorded a maximum length of 1,800 km. These are source-reported observations, not a claim that every appearance reaches that size or speed. ESA’s account and the 2021 lifecycle study describe its rapid daily evolution.
Free tools Windows power users keep installed
One-click scans. No signup required.
#1 Best Overall
The cloud is also seasonal. The observational study places appearances around solar longitude 220°–320°, spanning the southern spring and summer around southern solstice. Its morning schedule helps explain why spacecraft that typically view the region in the afternoon can miss it. Repeated, wide-field observations from Mars Express’s Visual Monitoring Camera (VMC) were particularly useful: ESA reports 63 VMC observations considered alongside Mars Express’s HRSC and OMEGA instruments and data from other Mars missions, including MAVEN, Mars Reconnaissance Orbiter, Viking 2, and India’s Mars Orbiter Mission.
What is known about the tail—and what is still open
The 2022 model explains important behavior of the cloud head, but its authors did not reproduce the observed expansion of the long tail and found that the modeled cloud optical depth was insufficient. So topographic lifting and the windstorm mechanism help explain how the cloud begins; they do not, by themselves, settle every detail of how its spectacular tail develops.
A preprint posted on September 29, 2026, by Hernández-Bernal, Määttänen, Spiga, and Forget proposes an additional explanation: homogeneous nucleation, in which cloud formation begins directly from water vapor rather than on pre-existing particles. The authors report that adding this process to a Mars meteorological model reproduced the cloud’s distinctive characteristics, and interpret the result as evidence of homogeneous nucleation in a planetary atmosphere. This is a recent preprint, not an established peer-reviewed consensus. Read the authors’ preprint.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the cloud is easy to mistake for volcanic activity
Its close position to a giant volcano and its dramatic, plume-like shape make the comparison understandable. But the cited observations and atmospheric studies describe condensed water ice carried by airflow—not ash, gas, or other material from an eruption. The volcano’s role is to shape the atmosphere and help create the conditions for condensation. ESA also identifies AMEC as a cloud rather than a volcanic plume in its account of the cloud’s return.
Recommended Free Tools
Quick Recap
Best Value
Rank #4
Rank #3
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
