Scientists Crack Mystery Of 1,100-Mile-Long Cloud On Mars That Looks Like Eruption

But none of them form quite like the Arsia Mons Elongated Cloud, which stretches up to 1,100 miles in length.

Scientists studying the Red Planet have known of the AMEC's existence since the Viking missions in the '70s, but they've had little ability to study it much until recently — and even with some advanced hardware now in orbit, even that has proven challenging.

Given that it forms over a 20 km (12.5 mi) tall extinct volcano, it's not surprising that for a time, it was thought to be the result of an eruption. It does look like a volcanic plume, after all.

It's also that it comes and goes with surprising regularity. Unfortunately, that was one of the reasons it was so difficult to study. AMEC forms in the mornings, and only during the months around the southern hemisphere's summer. By mid-day, the cloud dissipates — which is when the cameras and instruments on the various satellites orbiting Mars start recording, because shadows are less likely to cause issues at that time of day.

The other problem is that AMEC is just so big so unless cameras have a wide field of vision, they're not going to pick it up. And of course, the fancy satellites around Mars largely have wonderful high definition cameras to pick up tiny details on the planet's surface — not those wide-field cameras.

That's right, the breakthrough came via tech that's nearly 20 years old and had spent years not even being powered. The Visual Monitoring Camera originally arrived at the Red Planet on the 2003 Mars Express mission that unsuccessfully sought to put another lander on Mars. For some time, it has acted as the unofficial "Mars Webcam," allowing somewhat grainy updates of what's happening on the planet.

"Although it has a low spatial resolution, it has a wide field of view — essential to see the big picture at different local times of day — and is wonderful for tracking a feature's evolution over both a long period of time and in small time steps," explained Jorge Hernández Bernal of the University of the Basque Country in Bilbao, Spain, in a press release.

"As a result, we could study the whole cloud across numerous life cycles."

And what they found was a cloud that formed daily for a period of several months, in the mornings.

Per the European Space Agency: "It begins growing before sunrise on the western slope of Arsia Mons before expanding westwards for two and a half hours, growing remarkably fast – at over 600 km/h – at an altitude of 45 km. It then stops expanding, detaches from its initial location, and is pulled further westwards still by high-altitude winds, before evaporating in the late morning as air temperatures increase with the rising Sun."

They're called orographic clouds, and they occur when a large topographical feature like a mountain range — or a 20 km tall volcano on Mars — forces air upwards, which cools it down and causes it to condense as a cloud.

"Although orographic clouds are commonly observed on Earth, they don’t reach such enormous lengths or show such vivid dynamics," University of Basque Country professor Agustin Sánchez-Lavega said in the press release.

However, there's still at least one more mystery to uncover: why the cloud forms at Arsia Mons and not at any of the other large volcanoes nearby.

Check out the research right here.

h/t: European Space Agency