Every day in late southern spring and summer, an amazing elongated cloud forms around a giant volcano on Mars. This cloud appears every morning, grows out incredible length, and then disappears before noon. This apparently happens every Mars year, but until recently was difficult to detect by spacecraft orbiting the Red Planet.
And that is great in itself, 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 in a row 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 mainland United States). 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 Mars’ equator. Every year, starting in late spring for a period lasting many months, the cloud begins to form just before sunrise. It starts as a roughly circular spot called the head, which can reach 50 – 250 km wide, with an average of about 125 km. As soon as the sun rises locally, the tail begins to grow and stretch to the west. It can grow at a dizzying speed, up to 600 kilometers per hourIt continues to grow for a few hours, then “detaches” from the head and disappears until it is gone before noon.
In Mars year 34 (the last Mars year, which ended a few weeks ago) it grew to ridiculous lengths, 1,800 km – it can easily stretch from New York City to Miami. It is usually quite narrow, also less than 200 km wide.
That’s a big cloud.
It is almost certainly made from water ice. Mars is very dry, but there is a small amount of moisture in the atmosphere (and thin, wispy cirrus clouds are common). What is likely happening is the so-called orographic lift: air blowing westward rises the volcano’s flank and blows over the top. The sky is lifted quite high, up to the mesosphere of Mars, 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.
As soon as the ground has warmed up sufficiently, the wind blows the cirrus ice clouds to the west. The tail grows. But as the temperature continues to rise, the ice sublimates (turns back into a gas without first turning into a liquid; air pressure is far too low on Mars to hold liquid water) and the cloud appears to disappear.
This phenomenon only occurs around the southern summer, so it is clearly temperature related. Interestingly, dust storms can also form there, and it’s possible that small amounts of dust in the air may play a role here as well. In the year 33 of Mars (June 2015 to May 2017; Mars takes about two Earth years to orbit the sun) there was a global dust storm and the cloud grew more slowly. In MY 34, there was also a global dust storm, an immense one (and so thick it caused the downfall of the rover Opportunity), and that appears to have delayed the onset of cloud formation. Still, there are years when there are storms affecting the cloud and years when they don’t, so the connection isn’t clear.
Considering how big the cloud is, it’s surprising that it hasn’t been explored before. The problem is the timing. The cloud forms in the morning and is gone by noon. Most spacecraft on Mars are in special orbits called midday sun synchronous polar orbits. As they circle the planet in a north-south direction, their orbital plane itself rotates, so it is always noon over the part of the planet they pass. There are similar orbits used by Earth observation satellites, so they always see the same illumination underneath.
The problem there is that they never pass a spot on Mars when it is local morning, so they miss the cloud. This recently changed 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 is more to it. Mars Express has a camera on board called the Visual Monitoring Camera, which is designed to track the deployment of the Beagle 2 lander (which unfortunately crashed on Mars). The VMC is more a webcam than a scientific instrument, with a relatively low resolution but a wide field of view. It was turned off after Beagle 2 was deployed, but turned back on years later so it could be used to take “tourist photos” of Mars for public outreach. Scientists realized it could be used for scientific purposes, and it is perfect for observing the cloud.
They were also able to find cloud sightings in other missions, including MAVEN, Viking 2 (from the 1970s!), Mars Reconnaissance Orbiter, and India’s Mars Orbiter Mission. All these observations together allowed them to summarize what is happening while the cloud is forming.
In this first article they discussed the observations and gave an overview of the formation, but a later article will go into detail about the physics. I am very interested to see that. I love clouds and live in an area (just east of the Rockies) where I see many special 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 weird little planet, with so much to see and discover. The Arsia Mons cloud can be used to help scientists understand Mars’ atmosphere, which clearly has many secrets to reveal.
Some of these are clearly huge yet hidden in plain sight.