Every day, in late spring and summer in the south, an amazing elongated cloud forms around a giant volcano on Mars. This cloud appears every morning, it grows up to amazing of length and dissipates before noon. Apparently, this happens every Martian year, but until recently it was difficult to observe when navigating around the red planet.
And that in itself is amazing, because the cloud can reach 1,800 kilometers in a few hours.
The cloud forms on the western flanks of 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 in diameter (roughly the size of the United States). continental). Arsia Mons is huge, about 500 km wide and 17 km high. Mount Everest on Earth is only 9 km high, by comparison.
The volcano is located just south of the Martian equator. Every year, from late spring for a period lasting many months, the cloud begins to form just before sunrise. It starts as an approximately circular patch called a head, which can be up to 50 to 250 km in diameter, with an average of about 125 km. Once the Sun rises locally, the tail begins to grow and extends westward. It can grow at an astonishing rate 600 kilometers per hour. It continues to grow for a few hours, then “detaches” from the head and dissipates until it has disappeared before noon.
The Martian year 34 (the last Martian year, which ended a few weeks ago) grew to ridiculous, 1,800 km – could be easily stretched from New York to Miami. It is usually quite narrow, less than 200 km wide.
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, heavy cirrus clouds are common). What probably happens is what is called an orographic elevator: the air that blows west rises up the side of the volcano and blows over it. The air is elevated to the Martian mesosphere about 45 km above the surface. It is so cold there that the water forms ice crystals that grow creating the head of the cloud.
Once the ground warms up enough, the wind blows the cirrus ice clouds along with it to the west. The tail grows. But as temperatures rise further, the ice sublimes (it turns back into gas without becoming liquid; the air pressure is too low on Mars to keep the water liquid) and it is seen that the cloud dissipates.
This phenomenon only occurs south of summer, so it is clearly related to temperature. Interestingly, it is also when dust storms can form and there may also be small amounts of dust in the air. In March 33 (June 2015 to May 2017; Mars takes about two years from Earth to orbit the Sun) there was a global dust storm and the cloud grew more slowly. At MY 34 there was also a global dust storm, immense (and so thick that it caused the disappearance of the Opportunity rover), and it seems to have delayed the appearance of cloud formation. There are still years when there are storms affecting the cloud and years when there aren’t, so the connection is unclear.
Given the enormous size of the cloud, it’s surprising that it hasn’t been studied before. The problem lies in time. The cloud forms in the morning and has left at noon. Most spacecraft on Mars are in special orbits called synchronous polar orbits in the afternoon sun. As they circle in a north-south direction over the planet, their own orbital plane rotates, so it is always afternoon over the part of the planet they pass through. 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 pass over a point on Mars when it is morning, so they miss the cloud. This changed recently with the European Space Agency’s Mars Express orbiter. It is in a very elliptical orbit that allows it to see areas at different local times of the day.
But there are more things. Mars Express has an onboard camera called the Visual Monitoring Camera, which was designed to monitor the deployment of the Beagle 2 lander (which unfortunately collided with Mars). The VMC looks more like a webcam than a scientific instrument, with a relatively low resolution but a wide field of view. It was turned off after the deployment of Beagle 2, but years later it was turned back on so that it could be used to make “tourist features” of Mars for public broadcasting. Scientists 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 (from the 1970s!), Mars Reconnaissance Orbiter, and the Mars Orbiter Mission of India. All these observations together allowed them to gather what happens as the cloud forms.
In this first article they discussed the observations and gave an overview of the formation, but a later article will be devoted to details about physics. I will be very interested to see it. I love the clouds and live in an area (just east of the Rocky Mountains) where I see many quirky formations, including many orographic formations. It’s hard not to be fascinated and impressed by the beautiful cloud structures here.
… And there. Mars is a strange little planet, with much to see and explore. The Arsia Mons cloud can be used to help scientists understand the Martian atmosphere, which clearly has many secrets to reveal.
Some of which are obviously huge but hidden from view.