For more than a month now, a European orbiter circling Mars has been watching a long, plume-like cloud on the Red Planet.
The cloud has remained in place over a mountain called Arsia Mons near the Martian equator since Sept. 13, according to a statement released by the European Space Agency (ESA). But that location is just a coincidence, the agency adds. No volcanic process is producing the cloud — the volcano hasn’t been active in about 50 million years, scientists believe.
In this version of a Mars Express image of the strange cloud, the European Space Agency has labeled nearby geologic features.
The same spacecraft, called Mars Express, and its predecessors have spotted similar clouds on at least three previous occasions, and those structures formed around the same time in the Martian year. And that’s not a coincidence, the ESA wrote.
Since 13 September, ESA’s Mars Express has been observing the evolution of an elongated cloud formation hovering in the vicinity of the 20 km-high Arsia Mons volcano, close to the planet’s equator.
In spite of its location, this atmospheric feature is not linked to volcanic activity but is rather a water ice cloud driven by the influence of the volcano’s leeward slope on the air flow – something that scientists call an orographic or lee cloud – and a regular phenomenon in this region.
The cloud can be seen in this view taken on 10 October by the Visual Monitoring Camera (VMC) on Mars Express – which has imaged it hundreds of times over the past few weeks – as the white, elongated feature extending 1500 km westward of Arsia Mons. As a comparison, the cone-shaped volcano has a diameter of about 250 km; a view of the region with labels is provided here.
Mars just experienced its northern hemisphere winter solstice on 16 October. In the months leading up to the solstice, most cloud activity disappears over big volcanoes like Arsia Mons; its summit is covered with clouds throughout the rest of the martian year.
However, a seasonally recurrent water ice cloud, like the one shown in this image, is known to form along the southwest flank of this volcano – it was previously observed by Mars Express and other missions in 2009, 2012 and 2015.
These types of clouds are also related to dust storms on Mars. After a dust storm, the atmosphere is full of solid particles called condensation nuclei. These tiny dust grains provide a solid surface for the water ice to condense onto. The major, planet-wide dust storm that engulfed Mars last June and July provided perfect conditions for a cloud of this size to form.
As on Earth, the seasons play a role in cloud activity. October 16th is the solstice in the northern hemisphere. The summit of Arsia Mons is typically covered in clouds throughout the rest of the year, but leading up to the solstice, most of the large volcanoes lose their cloud cover. This cloud is the exception though. This seasonally recurrent ice cloud has a history of forming along the southwest flank of Arsia Mons. Mars Express and other orbiters have also spotted it in 2009, 2012 and 2015.
h/t Digital mix guy