Clouds are rare on Mars and usually only pop up at the coldest time of the year near the planet’s equator. One Martian year ago though, NASA noticed some clouds forming in the sky above its veteran Curiosity rover and became determined to document them the following year. In recent months, Curiosity got to work, taking some amazing photos that show us what cloudy days are like on the Red Planet and broadening our understanding of just how clouds form far, far away.
NASA refers to these unexpected clouds simply as “early” clouds at the moment. In an announcement, NASA said that the Curiosity team has already made one new discovery about the early clouds from the rover’s photos: They’re at a higher altitude than usual. Those of us who aren’t cloud experts — guilty, though in my defence I think clouds are very cool — might not find that too significant at first glance, but it’s actually an important distinction that lets us know what the clouds are made of.
As explained by NASA, most clouds on Mars are usually at an altitude of 60 kilometres in the sky and are composed of water ice. These so-called early clouds, however, are at a higher and colder altitude, which means that they are likely made of frozen carbon dioxide, or dry ice. NASA did not specify at what altitude the early clouds were in Curiosity’s photos.
“Scientists look for subtle clues to establish a cloud’s altitude, and it will take more analysis to say for sure which of Curiosity’s recent images show water-ice clouds and which show dry-ice ones,” the agency said in a news announcement.
Besides gaining new information on the so-called early clouds, NASA has also witnessed some truly beautiful clouds on Mars: noctilucent clouds and iridescent clouds.
In the photos above, taken by Curiosity’s black-and-white navigation cameras, we can see the fine, rippling structures in the clouds. Known as noctilucent clouds, which is Latin for “night shining,” or twilight clouds, these clouds “grow brighter as they fill with crystals, then darken after the Sun’s position in the sky drops below their altitude,” NASA said. Scientists use this information about the Sun’s position to calculate the altitude of noctilucent clouds.
You can really appreciate the stunning beauty of these clouds in the photos below, which makes it seem like there are waves in the Martian sky.
Curiosity didn’t just give us a look at noctilucent clouds, though. The rover, through photos taken by its MastCam, also showed us that clouds on Mars can shimmer with colour. These are called iridescent clouds, or “mother of pearl” clouds, and are some of the most colourful things you’ll see on Mars.
“If you see a cloud with a shimmery pastel set of colours in it, that’s because the cloud particles are all nearly identical in size,” Mark Lemmon, an atmospheric scientist with the Space Science Institute in Boulder, Colorado, said in the NASA announcement. “That’s usually happening just after the clouds have formed and have all grown at the same rate.”
Curiosity’s cloud photos offer us a new perspective of Mars. At a time when our rovers and helicopters send us fascinating photos of the planet’s rocky surface, Curiosity shows us that the planet can be more colourful than we imagined. It’s also another reminder that NASA’s veteran rover, overshadowed right now by the agency’s shiny new Perseverance rover and Ingenuity helicopter, may be old, but it’s still carrying out amazing science.