I love being surprised by a picture.

I first saw the photo above when my friend Bobak Ferdowsi tweeted a link to it, having seen it himself on the RidingWithRobots Twitter feed. When I saw it, I literally gasped. It’s a view from the Opportunity rover on Mars, showing a dust devil whipping across the floor of a huge crater.

Oh my, yes. Let me explain.

I write a lot about the Curiosity Mars rover, of course, the powerful mobile science lab currently moving across the surface of the red planet. But don’t forget about Opportunity, a smaller but no less successful and ambitious rover that is still going strong after an astonishing 12 full years on Mars.

Right now Opportunity is skirting the rim wall of Endeavour crater, a 22 kilometer impact feature. From orbit, measurements show the presence of phyllosilicates in the crater rim rocks—minerals that form when water is present; more evidence that long ago, Mars was wet. Opportunity has been poking around this area for quite some time to see for itself what it can find.

Carved out along the western rim of the crater is a notch, named Marathon Valley, which runs east-west for about 300 meters and is a few dozen meters wide. Opportunity is inside that valley, investigating the northern and southern walls.

It was commanded to make a steep ascent along one of the ridges marking the edge of the valley, but after many attempts, the wheels simply couldn’t get enough traction to move Opportunity up the ridge. Engineers commanded it to backtrack and try a different route.

Looking back down over ground it had already covered, facing toward the interior crater, Opportunity caught the scene in the photo above. You can see the disturbed surface where the six wheels dug in, and the rolling hills in the valley. Past that is the floor of the crater … and the whirling, towering dust devil.

Mars has an atmosphere, but it’s thin, less than one percent the pressure of Earth’s air at sea level. But it’s enough to move around, make wind. If a horizontal air flow passes over a warmer section of ground, a rolling vortex of air can be formed. The rising air lifts it up, making it vertical, and you get a dust devil. These look like tornadoes, but dust devils are far weaker, and form from the ground up, while tornadoes drop down from the bottoms of clouds.

Opportunity has two cameras side by side in its navigation camera, to provide stereoscopic views. Each took a shot of the dust devil, and I combined them into a flicker animation, which gives a bit of illusion of depth. You can see the topography of the shot a little better:

Dust devils are frequently seen on Mars. Besides capable of creating almost unbelievable calligraphy on the Martian surface as lighter dust is swept away, revealing darker rock underneath, dust devils have been a boon to Opportunity: They clear the accumulated dust off the rover’s solar panels, giving it a boost in power. This was unexpected when the rovers were sent, but accepted gratefully by mission engineers.

So dust devils are a kind of good luck charm for the rover team. But they’re more than that to me ...

When I read about water on Mars a billion years ago, and rovers poking at sedimentary and metamorphic rocks altered by that water, it feels a little bit like finding fossils; fascinating and fun but a reminder that Mars used to be a happening place.

But it still is. Those dust devils are a reminder that Mars isn’t some dead world, it’s still an active one. Maybe the pace has slowed over the past eon or two, but there’s still a lot to see and explore. Opportunity proves that every day.