The long cloud over Mars’s Arsia Mons is a water-ice cloud formed when the volcano’s slopes push moist air upward. The air cools at altitude, water vapor condenses into ice, and high-altitude winds stretch the cloud westward. It is not smoke or a plume from an eruption.
How Arsia Mons forms the cloud
The feature is called the Arsia Mons Elongated Cloud, or AMEC. It is an orographic cloud: a cloud created when airflow encounters elevated terrain. Air moving across Arsia Mons is forced up the volcano’s slopes. As it rises into colder air, water vapor condenses into ice crystals, forming the bright cloud head near the volcano’s western flank. Winds at higher altitudes then carry the ice cloud west, drawing out its long tail. The 2021 study describes this as a proposed formation sequence based on observations, rather than a direct measurement of every step. Hernández-Bernal et al., Journal of Geophysical Research: Planets (2021); European Space Agency (2021)
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Why it stretches west and disappears by late morning
The cloud’s head forms over Arsia Mons, but the prevailing high-altitude flow transports and elongates it toward the west. ESA’s account of the observed daily cycle says the cloud begins growing before sunrise on the volcano’s western slope. It expands westward for about two and a half hours, detaches from the slope, and continues farther west before evaporating in the late morning as sunlight warms the air. The study places head formation around sunrise and detachment at roughly 8.5 local true solar time. ESA (2021); Hernández-Bernal et al. (2021)
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When the Arsia Mons cloud appears
The study reports recurring formation during solar longitude (Ls) 220°–320°, a span that includes southern solstice. ESA describes the morning cycles as occurring over several months of Martian spring and summer. This seasonal timing is notable: the paper says relatively little cloud activity is generally observed at Mars’s large volcanoes during this dusty-season interval. The observations associate AMEC occurrence with a combination of winds, temperatures, pressure, and water-vapor content; they do not establish that dust alone causes it. Hernández-Bernal et al. (2021); ESA (2021)
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How large it can get
Reported dimensions vary with the observation and with whether a measurement describes a maximum or a typical width. The figures below are reported observational values, not dimensions reached in every appearance.
| Measurement | Reported value | Source and qualification |
|---|---|---|
| Maximum length | About 1,800 km | ESA (2021) and Hernández-Bernal et al. (2021); reported maximum extent. |
| Width | About 150 km at its largest in ESA’s account; typically below 200 km in the study | ESA (2021); Hernández-Bernal et al. (2021). The descriptions use different width qualifications. |
| Initial westward expansion | More than 600 km/h | ESA (2021); rate during the initial growth period. |
| Altitude | About 45 km in ESA’s account; head maxima of 40–50 km in the study | ESA (2021); Hernández-Bernal et al. (2021), reporting maximum head altitudes. |
| Cloud-head diameter | Average about 125 km | Hernández-Bernal et al. (2021). |
It is not an eruption plume—or NASA’s spiral dust cloud
Despite its position above a volcano, AMEC is identified as a water-ice orographic or lee cloud, not a feature linked to volcanic activity. ESA (2021)
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It is also distinct from a recurring spiral dust cloud NASA has described over Arsia Mons. NASA’s account attributes that spiral to dust lifted by slope-heated air converging over the caldera; the same images show separate thin water-ice clouds. The shared location does not make the dust spiral and AMEC the same phenomenon. NASA
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How scientists tracked a brief morning feature
AMEC is easy to miss if observations are made at the wrong time of day. The 2021 study combined images from ESA’s Mars Express, NASA’s MAVEN and Mars Reconnaissance Orbiter, Viking 2, and India’s Mars Orbiter Mission. ESA reports that the life-cycle analysis used 63 Mars Express Visual Monitoring Camera observations, supplemented by the spacecraft’s HRSC and OMEGA instruments and observations from other orbiters. The paper notes that many spacecraft observations favor afternoon local times, which can leave a cloud that grows and detaches in the morning under-observed. Hernández-Bernal et al. (2021); ESA (2021)
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