Every day, in late spring and summer, a stunning elongated cloud forms around a giant volcano on Mars. This cloud appears every morning, growing up incredible length, and then dissipates before noon. This seems to happen every Martian year, but it was difficult to notice until recently by spacecraft orbiting the red planet.
And that in itself is amazing, because the cloud can reach up to 1,800 kilometers in just a few hours!
The cloud forms on the western flanks of the Arsia Mons volcano, the southernmost of a line of three ridiculously large volcanoes in an area of Mars called Tharsis, a huge volcanic plateau about 5,000 km wide (about the size of the American continent). . Arsia Mons is huge, about 500 km long and 17 km high. Mount Everest on Earth is only 9 km high for comparison.
The volcano is just south of the Martian equator. Every year, starting with the end of spring for a period of many months, the cloud begins to form just before sunrise. It starts as an approximately circular patch called the head, which can increase up to 50 – 250 km wide, on average about 125 km. Once the sun rises locally, the tail begins to grow, extending westward. It can grow at an astonishing rate, up to 600 kilometers per hour. It continues to grow for a few hours, then “detaches” from the head and dissipates until it disappears before noon.
In the year of March 34 (the last year of March, which ended a few weeks ago), it has grown to ridiculous lengths, 1,800 km – could easily stretch from New York to Miami. It is usually quite narrow, with a width of less than 200 km.
This is a big cloud.
It is almost certainly made of water ice. Mars is very dry, but there is a small amount of moisture in the atmosphere (and thin, rocky cirrus clouds are common). What is probably happening is what is called an orographic elevator: the air blowing west rises on the side of the volcano and blows above. The air is raised quite high, to the Martian mesosphere, about 45 km above the surface. There it is so cold that the water forms ice crystals that grow, creating the head of the cloud.
Once the ground warms up enough, the wind blows cirrus clouds with it to the west. The tail grows. But as temperatures rise further, the ice sublimates (it turns back into a gas without first turning into a liquid; the air pressure is too low on Mars to support the liquid water) and the cloud he sees that it is dissipating.
This phenomenon occurs only in southern summer, so it is clearly related to temperature. Interestingly, even then dust storms can form and small amounts of dust in the air may play a role. In March 33 (June 2015 to May 2017; Mars takes about two years from Earth to orbit the Sun) a global dust storm occurred, and the cloud grew more slowly. In the MY 34 there was also a global dust storm, a huge one (and so thick that it caused the Opportunity rover to disappear) and which seems to have delayed the onset of cloud formation. However, there are years when there are storms that affect the cloud and years when there are not, so the connection is not clear.
Given the size of the cloud, it is surprising that it was not studied earlier. The problem is synchronization. The cloud forms in the morning and disappeared by noon. Most spacecraft on Mars are in special orbits called afternoon solar synchronous polar orbits. As they rotate north-south over the planet, their orbital plane rotates in itself, so it is always afternoon over the part of the planet they pass. There are similar orbits used by Earth observation satellites, so they always see the same illumination beneath them.
The problem is that then they never cross a place on Mars when it’s local morning, so they miss the cloud. This has recently changed with the European Space Agency’s Mars Express orbiter. It is in a very elliptical orbit, which allows it to see areas at different local times of the day.
But there are many more. Mars Express has a camera on board called the Visual Monitoring Camera, which was designed to monitor the deployment of the Beagle 2 landing device (which unfortunately crashed on Mars). VMC looks more like a webcam than a scientific tool, with a relatively low resolution but a wide field of view. It was shut down after the Beagle 2 was deployed, but was restarted years later so that it could be used to take “tourist photos” of Mars for public viewing. Scientists have realized that it could be used for scientific purposes and it is perfect to observe the cloud.
They were also able to find observations of the cloud in other missions, including MAVEN, Viking 2 (since the 1970s!), Mars Reconnaissance Orbiter and Mars Orbiter Mission in India. All these observations allowed them to share what is happening as the cloud forms.
In this first paper they discussed the observations and provided an overview of the formation, but a later paper will go into detail about physics. I will be very interested to see this. I like clouds and I live in an area (just east of the Rocky Mountains) where I see a lot of special formations, including a lot of orographic formations. It’s hard not to be fascinated and amazed by the beautiful cloud structures here.
… and there. Mars is a strange little planet, with so much to see and explore. The Arsia Mons cloud can be used to help scientists understand the Martian atmosphere, which clearly has a lot of secrets to reveal.
Some of them are obviously huge, but hidden from view.