Sunsets on Mars are uniquely moody, but NASA’s Curiosity rover captured one that stands out last month. As the Sun descended over the horizon on February 2, rays of light illuminated the bank of clouds. These “sunrays” are also known as crepuscular rays, from the Latin word for “twilight”. It was the first time that the sun’s rays had been seen so clearly on Mars.
Curiosity captured the scene during the rover’s latest survey of twilight clouds, building on observations of nocturnal luminous clouds from 2021. While most Martian clouds do not rise more than 37 miles (60 kilometers) above the ground and are composed of water ice, the clouds in the latest images appear being at a higher altitude where it is particularly cold. This suggests that these clouds are made of carbon dioxide ice or dry ice.
As on Earth, clouds provide scientists with complex but essential information for understanding the weather. By looking at when and where clouds form, scientists can learn more about the composition and temperatures of Mars’ atmosphere and its winds.
The cloud survey in 2021 included multiple images by Curiosity’s black-and-white navigation cameras that provide a detailed view of the structure of the cloud as it moves. But the recent survey, which began in January and will end in mid-March, relies more often on the color Mast Camera, or Mastcam, to help scientists see how cloud particles grow over time.
In addition to the image of the sun’s rays, Curiosity captured a set of colorful feather-shaped clouds on January 27. When illuminated by sunlight, certain types of clouds can create a rainbow display called iridescence. “Where we see iridescence, that means the cloud particle sizes are the same as their neighbors in every part of the cloud,” said Mark Lemmon, an atmospheric scientist at the Space Science Institute in Boulder, Colorado. “When we look at the color gradients, we see that the size of the particles in the cloud is changing. This tells us how the cloud is evolving and how its particles change size over time.”
Curiosity captured both the sun’s rays and rainbow clouds as panoramas, each pieced together from 28 images sent back to Earth. The images have been edited to emphasize the main points.
More about the mission
Curiosity was created by NASA’s Jet Propulsion Laboratory, which is managed by Caltech in Pasadena, California. JPL is leading the mission on behalf of NASA’s Science Mission Directorate in Washington. Malin Space Science Systems in San Diego built and operates Mastcam.
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