Mars may have only a thin atmosphere compared to other Solar System planets, but boy does it make the most of it. Water ice can rise high in the sky to form thin clouds. Wild winds can whip up into uncontrolled dust storms that shroud the entire planet, or create dust towers that extend almost into space.
So it should come as no surprise that NASA’s Mars Curiosity rover, beavering away in the Gale Crater, sometimes lays its electronic eyes on Martian weather phenomena – and now, it’s spotted a dust devil spinning across the rocky crater floor.
Seeing weather phenomena on Mars that we also see on Earth isn’t just interesting, though – it can also tell us a lot about seasonal atmospheric changes on the Red Planet.
It’s coming into Martian summer in the planet’s southern hemisphere, where the Gale Crater can be found, and the atmosphere in the region is heating up. Just as uneven heating of the atmosphere on Earth generates atmospheric movement, so too is the Martian atmosphere affected.
“Stronger surface heating tends to produce stronger convection and convective vortices, which consist of fast winds whipping around low pressure cores,” writes atmospheric scientist Claire Newman of Aeolis Research on the Mars Exploration blog.
“If those vortices are strong enough, they can raise dust from the surface and become visible as ‘dust devils’ that we can image with our cameras.”
Dust devils are pretty well understood, and they come about the same way on both Earth and Mars. They form best in relatively flat, dry terrain, when the air at the surface level is warmer than the air above it.
This hot surface air rises through the cooler, denser air, creating an updraft. This causes the cooler air to sink. If a horizontal wind then blows through this vertical circulation, a dust devil whips into action.
They’re extremely common on Mars, but we only know this because, as they move across the ground, they sweep up the dust in their path, leaving tracks behind them. Actually seeing them in action on the Red Planet is quite rare, since our observational capabilities are limited, and dust devils themselves are relatively short-lived.
The dust devil above, seen in the top centre of the image, was captured by Curiosity’s Navcam on Sol 2847, and covers a span of about 5 minutes, Newman says. Even though it seems ghostly, the fact that we can see it means it was pretty powerful.
“We often have to process these images, by enhancing what’s changed between them, before dust devils clearly show up,” she writes. “But this dust devil was so impressive that – if you look closely! – you can just see it moving to the right, at the border between the darker and lighter slopes, even in the raw images.”
Studying these movies can reveal a lot about dust devils on Mars – where they form, for instance, how they evolve, how long they last, the type of dust they pick up, and how they vary from location to location.
They can also reveal wind speed and duration, which, in combination with meteorological readings, can help scientists learn more about Martian weather, and how dust devils fit into it.
Curiosity is the only operational rover on Mars at the moment (InSight is a stationary lander), so whatever surface information can be gleaned on Martial dust devils is very limited. Mars also has operational orbiters, though, which cover a lot more ground.