It’s rarely a good thing when experts admit they know what something isn’t, but have no idea what it is. If we’re looking at Mars, there are plenty of candidates for that description, but today’s is one we’ve seen but not heard an answer for before – a mysterious elongated plume over the thought-to-be-long-dead Arsia Mons volcano. This mysterious plume was spotted by the European Space Agency’s Mars Express orbiter in October 2018 and was hotly debated back then by astronomers, seismologists, Mars experts and even conspiracy theorists who think it’s a real volcanic eruption that’s being covered up by the powers that be because that’s what the powers that be do. On the other hand, the rest were still arguing long after the plume disappeared. Now … it’s back again. Let the debates begin!
“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”.
Jorge Hernandez-Bernal, a PhD candidate at the University of the Basque Country in Spain and lead author of an ongoing ESA study on the plume (nice work if you can get it), says anybody with a good telescope on Earth can see the Arsia Mons Elongated Cloud (AMEC – yes, Mars doesn’t have hurricanes so the mysterious clouds get names) because it’s approaching 1,800 km (1,110 miles) in length – yes, that’s ‘length’, not height. This Martian plume stretches along the ground in a kind of visible air stream beginning from the 20 km (12.4 mile) high volcano’s leeward (downwind) side.
“A martian day, or sol, is slightly longer than an Earth day at 24 hours, 39 minutes and 35 seconds long. A year at the Red Planet consists of 668 sols, approximately 687 days, so the seasons last for twice as long.”
The ESA doesn’t just monitor the Mars Express – it can read your mind too. A Martian year is 687 Earth days, so the mysterious AMEC plume appeared right on time, just like it has in 2015, 2012, and 2009. And before that? The Mars Express wasn’t in orbit before that, but it’s estimated that Arsia Mons last erupted about 50 million years ago and hasn’t had a lava flow anywhere in at least 2 million years – which doesn’t answer the question but is an interesting factoid for your next trivia contest. All the ESA researchers know is that the plume appears around the Martian southern solstice (equivalent to our winter solstice) in the early morning, flows for about three hours and disappears until the following day – a process that’s repeated for 80 days and then abruptly ends.
So, it’s not a volcanic cloud but it’s definitely the biggest and seemingly most predictable of similar cloud appearances on Mars. There's no government cover-up -- NASA knows about it too. It‘s made of water ice, so that’s a good thing to keep in mind if and hopefully when humans reach the Red Planet. And it appears like clockwork at Arsia Mons in case the batteries die all at once on everyone’s Martian calendar watches.
Welcome back, AMEC. Yes, we know it’s a boring name but no one knows much Latin anymore.