The weird long cloud on Mars is finally revealing some of its secrets is an article by Chelsea Gohd from space.com referencing ESA's Mars Express news. The Arisa Mons Elongated Cloud (AMEC) is basically an orographic cloud the lee of the volcano Arsia Mons which is approximately 20km high. The cloud is located in 46km height and 600km/h winds create a tail which is about 1000km long and about 150km wide. The cloud head grows before sunrise, then the tail quickly grows westwards, and the cloud evaporates before afternoon. Furthermore, ESA writes in its infographics:

A curious elongated cloud on Mars has been observed over 100 times by five different missions since 1976. Mars Express has provided new insights into its daily cycle.

I italized the issue I am after: Although I follow closely what happens in the field of extraterrestrial atmospheres, I have never heard about that cloud before Mars Express. Maybe somebody could point me to earlier publications on the matter which rely on other space crafts? Are there other, similar clouds on Mars?


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    $\begingroup$ There are definitely similar clouds on Mars, I've seen them in Elysium in some MARCI images I've processed. But, I suspect you would need to go to Viking to get the AMEC images simply because NASA orbiters since the late 1990s have focused on afternoon imaging of the surface, so you'd never see the AMEC with them. Hence, MEx and MOM are the ones to really provide modern data on this. $\endgroup$ Mar 15 '21 at 19:03

I am the lead author of the paper about the Arsia Mons Cloud. This is an interesting question.

First. You can check the free original text in https://arxiv.org/abs/2103.03919

The Viking image we refer to is presented in the paper in the panel 11a, you can see there the product ID in case you want to find it in the NASA archive. This image was previously noticed by some authors (see the paper text for more information) but they didn't have enough data to see the big picture: the fact that this is a really long cloud forming every sol for a long season every year following a very quick daily cycle. We could only see all these things with the aid of dozens of wide area observations by MEX/VMC together with images from other instruments.

The paper includes almost all available images before those taken in 2020 (you might want to check the list in the supporting material of the paper, available freely in the wiley site). In addition to this, a number of amateur astronomers captured the shadow of the cloud in 2020 (e.g. https://www.cloudynights.com/topic/737322-dark-shadow-near-arsia-mons/ ), and I belive that I also saw some equivalent image from previous years, but I cannot remember clearly. And some images were acquired by orbiters in 2020. The next appearance is expected to start around May-June 2022.


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