Nasa’s Perseverance rover captures strange ‘Blue Sunset’ on Mars

Images help scientists study the composition of the Martian atmosphere

Vishwam Sankaran
Monday 09 October 2023 11:24 BST
Related video: NASA Mars rover captures ghostly 1.2-mile-high dust devil

Nasa’s Perseverance Mars rover has captured a rare strange “blue sunset” over the Red Planet’s horizon, an image that sheds more light on the physics behind an alien sunset.

The rover took the snap on its 933rd Martian day with its navigation camera as the sky above the Red Planet began to go dark.

Perseverance’s image shows the Sun setting down on the Martian horizon glowing a strange, cool blueish green.

The image captured by the rover’s navigation camera, located high on the rover’s mast aiding in driving, was acquired by Nasa on Thursday, the American space agency noted.

The otherworldly nature of the sunset image also sheds light on the physics of light scattering on the Red Planet, and how it contrasts with a similar phenomenon on the Earth.

On the Earth, when different wavelengths of light from the Sun enters the atmosphere, it is scattered by tiny particles including gasses like oxygen, nitrogen, carbon dioxide, and water vapour as well as other particulate matter.

During the peak of the day, when a planet’s side is closer to the Sun, blue light – which travels in shorter waves – is scattered far and wide making the sky appear blue during this time.

However, during sunrise and sunset, the sunlight travels more distance in the atmosphere.

This causes light with shorter wavelengths, including violet and blue to be scattered away, leaving only the orange and red light to reach the eyes.

Mars’ atmosphere is very thin, about 1 per cent of the Earth’s, and the Red Planet is also 50 per cent further from the Sun than the Earth.

Light from the Sun interacts with large-sized iron-rich dust particles on Mars, compared to oxygen, nitrogen, and CO2 on the Earth.

This dust scatters the lower-frequency red light during the day, giving the Red Planet’s sky its characteristic red hue.

At sunset, when light has a longer distance to travel, red light is scattered away, painting the sky a cool blue colour.

How sunset appears different on Mars than from Earth
How sunset appears different on Mars than from Earth (Nasa/Damia Bouic)

Images of sunrise and sunset captured by rovers on the Red Planet not only show the strangeness of the alien planet but also help scientists study the composition of the Martian atmosphere.

“The colors come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently,” atmospheric scientist Mark Lemmon from the Texas A&M University in the US explained previously.

“When the blue light scatters off the dust, it stays closer to the direction of the Sun than light of other colors does. The rest of the sky is yellow to orange, as yellow and red light scatter all over the sky instead of being absorbed or staying close to the Sun,” he said.

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