Looking up at a cloudy sky as the Sun tries to peek through is not unusual – unless you’re on Mars that is. Due to its thin, dry atmosphere, clouds don’t often form on the Red Planet, so for Curiosity to capture any at all is a win for science. That they are “early”, particularly high, and unusually colorful for Mars has scientists pretty excited.
When they do form on Mars, clouds usually appear above the equator during the coldest time of year, when Mars is furthest from the Sun in its orbit. One Martian year ago (two Earth years), however, scientists noticed strange wispy clouds forming in the sky over Curiosity earlier in the year than expected. Determined to be ready to capture them this year, they set the veteran rover to peering at the skies from January, and sure enough, these “early” clouds appeared again.
In March, Curiosity captured the wispy clouds scattering sunlight from the setting Sun and creating shimmering, colorful displays and stunning noctilucent, or night-shining, clouds. Not only do they look stunning, but they also help researchers understand what the clouds are made from.
Water-ice clouds usually form on Mars no higher than 60 kilometers (37 miles) up, but these clouds are much higher than that, suggesting that they are made from frozen carbon dioxide, better known as dry ice.
When seen just after sunset, the ice crystals in the clouds scatter the light, appearing to glow or shine. Night-shining clouds are too thin and wispy to be seen during the day, but as the Sun dips below the horizon, its light continues to be reflected by the high-altitude clouds, illuminating the clouds against the darkening sky.
The strange rippling structure of these clouds is easier to see in Curiosity's black and white photos from its navigation cameras. They grow brighter as the Sun drops below their altitude, which helps scientists determine how high up they are.
The most attractive of all is the iridescent "mother of pearl" clouds, which can be seen shimmering with blue, red, and green.
“If you see a cloud with a shimmery pastel set of colors in it, that’s because the cloud particles are all nearly identical in size,” said Mark Lemmon, an atmospheric scientist with the Space Science Institute, in a statement. “That’s usually happening just after the clouds have formed and have all grown at the same rate.”
These clouds are essentially the most colorful thing you will see on Mars. According to Lemmon, if you were standing next to Curiosity on the Martian surface you would be able to see the colors with the naked eye, albeit faintly.
“I always marvel at the colors that show up: reds and greens and blues and purples,” Lemmon said. “It’s really cool to see something shining with lots of color on Mars.”