A new model proposes that Mars’s long Arsia Mons cloud forms ice without the dust particles usually needed to start freezing. Spacecraft images provide the comparison for this explanation of a seasonal cloud stretching up to 1800 kilometres. Air lifted above the volcano cools rapidly, while westward winds draw the ice into a long tail.
Arsia Mons, a Martian volcano about 20 kilometres high, hosts a seasonal morning cloud that can stretch up to 1800 kilometres along its western side. A new study proposes that the water ice in this cloud can form without dust particles acting as seeds. The explanation comes from modelling compared with spacecraft images; ice formation was not measured directly within the cloud.[1], [2]
The European Space Agency’s Mars Express orbiter observed the cloud using cameras and a visible-infrared imaging spectrometer. The cloud appears during part of the dusty Martian season, when other clouds are less common at low latitudes. The peer-reviewed study published in Nature Geoscience on 7 October adds a dust-free ice-forming route to the usual cloud model.[1]
Air flowing over the volcano generates gravity waves that lift it several kilometres and cool it rapidly. In very cold air containing more water vapour than can remain gaseous, ice can form without an existing particle surface. Westward winds carry those ice particles into the long tail. As the cold region fades during the day, the model also produces separation between the cloud’s head and tail.[1]
The author preprint, explicitly identified as a version before peer review, records discrepancies with observations. Its simulated cloud is 30% to 50% narrower and starts about 1.5 hours late. Formation on finer dust was tested too, but did not reproduce the strongly contrasted elongated tail. Seasonal observations of that fine dust remain lacking.[1]
