A strange, long cloud has returned high above the Arsia Mons volcano on Mars.
As a recurrent feature on Mars, the cloud appears annually above the 20-km-high volcano located near the Martian equator.
It is made up of water ice, but despite resembling a volcanic plume, it is not linked to any volcanic activity. However, the cloud appears along the volcano’s leeward slope (the side that does not face the wind), as airflow is influenced by it.
The mysterious cloud was detected on July 17 and 19 by the Visual Monitoring Camera (VMC) attached to Mars Express — a satellite that has been orbiting around Mars for 16 years. The images of the cloud show that it can reach up to 1800-km in length. And according to the ESA, it can even get big enough to be spotted by telescopes on Earth.
“We have been investigating this intriguing phenomenon and were expecting to see such a cloud form around now,” said the lead author of the ongoing study, Jorge Hernandez-Bernal
The cloud forms each year around the time of the Martian southern summer solstice, which is the period of the year when the Sun is in the southernmost position in the martian skies.
The cloud appears in the early morning, grows over the next 3 hours, and then quickly dissipates just a few hours later. The process is turned into a cycle that usually lasts for around 80 days.
“This elongated cloud forms every martian year during this season around the southern solstice, and repeats for 80 days or even more, following a rapid daily cycle. However, we don’t know yet if the clouds are always quite this impressive”.
There is other spacecraft in orbit around Mars that can spot the cloud in the afternoon; however, Mars Express has such a privileged position being able to give more accurate information on the curious effect.
“The extent of this huge cloud can’t be seen if your camera only has a narrow field of view, or if you’re only observing in the afternoon,” said Eleni Ravanis, who works specifically for the VMC instrument as a Young Graduate Trainee for the Mars Express mission.
“Luckily for Mars Express, the highly elliptical orbit of the spacecraft, coupled with the wide field of view of the VMC instrument, lets us take pictures covering a wide area of the planet in the early morning. That means we can catch it!”
The Mars Express team has named the cloud the Arsia Mons Elongated Cloud, aka AMEC. With the last cycle happening 2 years ago, scientists are still very intrigued by the formation of these water ice clouds.