In Mars' thin, dry atmosphere, cloudy days are uncommon. Clouds are most common around the planet's equator during the coldest part of the year when Mars' oval-shaped orbit takes it the furthest from the Sun. Scientists detected clouds gathering above NASA's Curiosity rover one full Martian year ago-two Earth years-earlier than planned.

They were ready to start photographing these "early" clouds when they first emerged in late January this year. Images of airy puffs filled with ice crystals diffused light from the setting sun, some of which shimmered with color resulted. Such photographs help scientists understand how clouds grow on Mars and why these latest ones are unusual.

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Curiosity Team's Discovery

Curiosity's crew has already found one unique discovery: the early-arrival clouds are higher in height than usual. The majority of Martian clouds are made up of water ice and hang no higher than 37 miles (60 kilometers) in the sky.

Curiosity's observed clouds, on the other hand, are at a greater altitude, where it's extremely cold, implying that they're formed of frozen carbon dioxide or dry ice. To determine a cloud's height, scientists seek minor signals. It will take an additional study to determine which of Curiosity's latest photographs indicate water-ice clouds and show dry-ice clouds.

Taken Images

Images from Curiosity's black-and-white navigation cameras make the tiny, rippling features of these clouds more visible. The color photographs from the rover's Mast Camera, or Mastcam, glow the brightest-literally. Their ice crystals capture the departing light and shine against the lowering sky when viewed just after sunset. These noctilucent clouds, often known as "night gleaming" clouds, become brighter as they fill with crystals, then darken when the Sun's position in the sky descends below their height. This is only one of the many indicators scientists use to figure out how high they are.

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Mother of Pearl

Iridescent clouds, sometimes known as "mother of pearl" clouds, are even more magnificent. "If you see a cloud with a sparkly pastel mix of hues in it, that's because the cloud particles are all virtually comparable in size," said Mark Lemmon of the Space Science Institute in Boulder, Colorado. "That normally happens shortly after the clouds start and all expand at the same rate."

Colorful Clouds

He went on to say that these clouds are among the planet's most colorful features. The colors might be seen with the naked eye if you were sky-gazing near to Curiosity, albeit they would be dim.

"The hues that show up always amaze me: reds and greens, blues and purples," Lemmon added. "It's incredible to see anything bright and colorful on Mars."

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