High over the peak of a long dead volcano on the Martian equator, a feathery white cloud stretches across the sky.
At first glance it looks like the plume of an eruption, but Mars is a world with a cold, dead heart. It has been for some time. So what's going on?
Researchers think they have a good idea what's really behind this fog trail, having seen its kind before.
The 1,500 kilometre (930 mile) strip of vapour over Arsia Mons appeared on images taken of Mars by the Mars Express orbiter more than a month ago on September 13.
It's hung about ever since, shifting with changes in weather and growing long enough in the Mars' morning for Earth-based telescopes to spot it, stark white against the rusty Martian soil.
While it appears to emerge from the summit of a volcano, to researchers from the European Space Agency this is simply a trick of the eye. Mars hasn't seen a single eruption for millions of years, so there's zero chance that Arsia Mons is on the verge of reawakening.
Still, even if the plume isn't volcanic in origin, the elevated peak of Arsia Mons is still responsible.
Orographic Lifting on Mars
When the wind hits a huge structure, like Arsia Mons, it is forced to go up. This is when it starts cooling and expanding because of the lower atmospheric pressure. Then, water vapor is formed and condenses and freezes into clouds. This entire chain reaction is called – Orographic lift, and we can also see it on Earth – that is why most mountain regions appear cloudy.
According to the senior scientist Dr. Eldar Noe Dobrea (the Planetary Science Institute), “it turns out not a single one of the observations ever had a clear view of the surface at this point.”
Similar clouds were seen in 2009, 2012, and 2015 during the Martian winter season. This year’s massive dust storm that has put Opportunity rover to deep sleep is also helping the tiny ice particles anchor into the dust grains and makes the clouds more visible.
As for active volcanoes, Mars is not that active anymore. However, Olympus Mons volcano, which is the tallest known mountain in the solar system (13.6 miles high) is dormant – not inactive, but the volcanic activity is so rare that it’s unlikely to be observed.