This week, the European Space Agency released a curious photo captured by the Mars Express orbiter of a 1497km-long cloud formation spilling out of the massive, 19km-high Arsia Mons volcano on Mars — a phenomena that has been observed for weeks straight. It certainly gave every superficial appearance that the volcano was about to blow, though that would be curious, as the estimated date of its last eruption was about 50 million years ago.
Turns out there's a much simpler explanation than Arsia Mons roaring back to life, per the New York Times: a routine meteorological phenomenon called orographic lifting. That's when wind hitting a massive structure such as a mountain — Arsia Mons in this case — is forced upwards, cooling and expanding due to lower atmospheric pressure.
As a result, water vapour contained within can condense and freeze into clouds. (Orographic lift is one of the reasons why on Earth, mountainous regions tend to be particularly cloudy.)
The Martian atmosphere may be far less dense than Earthlings would be used to, and contains such a tiny amount of water that it has been estimated it would only be 20 microns deep if it were spread evenly on the planet's surface, but it still has water-ice clouds.
Dr. Eldar Noe Dobrea, a senior scientist with the Planetary Science Institute, told the Times there was no chance that orbiting spacecraft would have failed to detect the telltale signs of a forthcoming eruption and that there is nothing unusual about the cloud formations on the western side of the volcano:
Dr. Noe Dobrea said this was clearly not a volcanic event, because spacecraft would have detected a rise in methane, sulphur dioxide and other gases that spill out of eruptions. Instead, this is an example of how topography affects weather.
... Indeed, it is rare for there not to be clouds over Arsia Mons. More than a decade ago, Dr. Noe Dobrea analysed observations from an earlier NASA mission, Mars Global Surveyor, trying to piece together a cloud-free picture of the Martian surface. But every time the spacecraft had passed over the western flank of Arsia Mons, it was cloudy.
"It turns out not a single one of the observations ever had a clear view of the surface at this point," he said.
As Motherboard noted, scientists detected similar clouds in 2009, 2012, and 2015, all during the Martian winter season — which is also occurring right now, accompanied by recent dust storms that could have made the clouds more visible. (Those dust storms throw tiny grains far up into the atmosphere, providing an ideal anchor for freezing ice.)
Mars used to be a much more geologically active world, with recent evidence indicating that if there is active volcanism on the planet, it's very limited in scope compared to the distant past. For example, its massive Olympus Mons volcano — the tallest known planetary mountain in the solar system at an estimated 23km high — may be dormant rather than totally inactive.
In general though, the Conversation wrote earlier this year, the planet has so much less mass than Earth and is believed to have lost so much of its heat energy that any remaining volcanic activity would be so rare as to make it very unlikely to observe any, at least on as fleeting a timescale as a human lifetime.
It could be millions of years before the planet's biggest volcanoes manage to squeeze out any more molten rock.
So, no, sorry, these clouds are not an indication that Mars is about to blow. But do feel free to briefly entertain some of the weirder conspiracy theories, such as that this is all a NASA cover-up orchestrated to hide that they really screwed up Arsia Mons while constructing a secret base in cave networks below.