Every day in late southern spring and summer, an astonishingly elongated cloud forms around a giant volcano on Mars. This cloud appears every morning, grows to unbelievable length, and then disappears before noon. This apparently happens every March year, but until recently was difficult to observe by spacecraft orbiting the Red Planet.
And that in itself is incredible, because the cloud can last up to 1800 kilometers in a matter of hours!
The cloud forms on the western flanks of the volcano Arsia Mons, the southernmost in a line of three ridiculously large volcanoes in an area of Mars called Tharsis, a huge volcanic plateau about 5000 km across (about the size of the continental United States). Arsia Mons is large, about 500 km wide and 17 km high. Mount Everest on Earth is only 9 km high by comparison.
The volcano is just south of the Martian equator. Every year, starting in late spring for a period lasting many months, the cloud begins to form just before sunrise. It starts as an approximately circular spot called the head, and it can become about 50 – 250 km wide and average about 125 km. As soon as the sun rises locally, the tail begins to grow and extend westward. It can grow at staggering speeds to 600 kilometers per hour. It continues to grow for a few hours, then “loosens” from the head and disappears until it disappears before noon.
In March 34 (the last year of Mars, which ended a few weeks ago), it grew to ridiculous lengths. 1 800 km – it can easily stretch from New York to Miami. It is usually quite narrow, also less than 200 km wide.
It’s 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, sharp cirrus clouds are common). What is likely to happen is the so-called orographic lift: air blowing west rises on the flank of the volcano and blows overhead. The sky is quite high, up to the Mars mesosphere, about 45 km above the surface. It is so cold that the water forms ice crystals that grow and create the cloud’s head.
Once the ground has warmed up enough, the wind blows the cirrus ice clouds with it to the west. The tail grows. But then, as the temperature rises further, the ice sublimates (turns into a gas again without first turning into a liquid; the air pressure is far too low on Mars to sustain liquid water) and the cloud is seen disappear.
This phenomenon only occurs in the southern summer, so it is clearly temperature related. Interestingly, this is also when dust storms can occur, and it is possible that small amounts of dust in the air also play a role here. In March 33 (June 2015 to May 2017; Mars takes about two Earths to orbit the Sun) there was a global dust storm and the cloud grew more slowly. In MY 34 there was also a worldwide dust storm, a huge storm (and so thick that it caused the downfall of the Rover Opportunity), and it apparently delayed the onset of the cloud formation. Yet there are years where there are storms affecting the cloud and years where there are not, so the connection is not clear.
Given how large the cloud is, it is surprising that it has not been studied earlier. The problem lies in timing. The cloud forms in the morning and leaves by noon. Most spacecraft on Mars are in special orbits called the solar synchronous polar orbits. As they orbit the planet in a north-south direction, their orbital plane rotates itself, so it’s always noon over the part of the planet they are passing. There are similar orbits used by Earth-observing satellites, so they always see the same illumination beneath them.
The problem there is that they never pass a place on Mars when it’s local morning, and they miss the cloud. That has recently changed with the European Space Agency’s Mars Express orbit. It is on a very elliptical orbit with which it can 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 monitor the deployment of the Beagle 2 lander (which unfortunately crashed on Mars). The VMC is more like 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 put into service, but turned on again years later so that it could be used to take ‘tourist shots’ of Mars for public outreach. Scientists have realized that it can be used for scientific purposes, and it is perfect to observe the cloud.
They could also find observations of the cloud in other missions, including MAVEN, Viking 2 (from the 1970s!), Mars Reconnaissance Orbiter and the Indian Mars Orbiter Mission. With all these observations, they were able to compile what happens while the cloud is forming.
In this first paper they discuss the observations and give an overview of the formation, but a later paper will discuss physics. I would be very interested to see it. I like clouds and live in an area (just east of the Rockies) where I see many peculiar formations, including many orographic formations. It’s hard not to be fascinated and surprised 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 many secrets to reveal.
Some of them are extremely large, yet hidden.