Our first look at a Martian sunset can tell us a lot about the Red Planet’s atmosphere

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As part of its ongoing study of Mars’ clouds, NASA’s Curiosity rover recently captured a stunning image of a Martian sunset. As the sun dips below the horizon, the light forms into sunbeams that are visible in streaks across the sky.

Technically known as twilight rays, this is the first time the phenomenon has been imaged in such detail on Mars. And by studying the way the rays shine through the clouds, scientists can learn more about Mars’ atmosphere and weather system.

Although Mars’ atmosphere is extremely thin, only 1 percent of the density of Earth’s atmosphere, it is still active and changeable. The planet experiences strong winds of up to 60 miles per hour, which can pick up the fine dust particles covering much of the planet’s surface and whip them into global dust storms. With low atmospheric pressure and significant temperature fluctuations between day and night, dynamic events such as dust devils are regularly seen.

Technically known as twilight rays

Partly due to the thin atmosphere, there are only occasional clouds in the Martian sky. With only small amounts of water vapor in the atmosphere, the presence of clouds varies through the seasons. However, the clouds visible there are not like clouds on Earth, as they are composed of liquid water. On Mars, the low pressure means that clouds form from water ice or carbon dioxide (dry ice) instead.

The new images from Curiosity show high-altitude clouds, suggesting they are composed of carbon dioxide rather than water ice. Another image recently captured by Curiosity shows another important cloud phenomenon called iridescence. The different colors seen in the cloud can reveal information about the particles that make it up.

“Where we see iridescence, it means that the particle size of a cloud is identical to their neighbors in every part of the cloud”

“When we see iridescence, it means that the particle size of a cloud is identical to their neighbors in every part of the cloud,” Mark Lemmon, an atmospheric scientist at the Space Science Institute in Boulder, Colorado, said in a statement. “By looking at color transitions, we see the particle size change throughout the cloud. That tells us about how the cloud evolves and how the particles change size over time.”

The two images were stitched together from 28 separate images each. They were captured by Curiosity’s Mastcam instrument, which, unlike many previous cloud observations made with the rover’s black-and-white navigation cameras, can capture images in color. Curiosity has been conducting its cloud survey since January and will continue for a few more weeks.

Curiosity has previously captured other striking images of Martian weather phenomena, such as the blue sunset it captured in 2015. The color seen there is also due to the dust in the atmosphere, after a dust storm that had suspended dust in the atmosphere. This floating dust scatters different colors of light in different amounts and scatters the light in a certain direction. As a result, red light is filtered out more, so what remains is the blue color that can be seen in the Martian sky.

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