Sunset on Mars: Curiosity rover snaps first colour pictures of red planet’s blue sunset

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The Mars Curiosity rover has taken stunning pictures of sunset on the planet, the first colour images of the red planet’s blue evening light.

The images were taken on the rover’s 956th Martian day, or sol, on the planet — April 15, 2015. They will be used by scientists to study how dust is distributed across Mars’s atmosphere.

The blue colour of the images comes from the dust that sits across the planet’s atmosphere. Though the images were taken between dust storms, some of the material stays suspended in the atmosphere and gives the distinctive blue tinge to the pictures.

"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," said Mark Lemmon, the Curiosity science-team member who planned the observations. "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."

In the same way that Earth sunsets pick out the reds and oranges of normal light on the planet, Martian ones emphasise the blue of the sky.

Though the rover has now been on the planet for years, it is the first time pictures have been taken of the sunset in colour. It did so using its Mast Camera, or Mastcam — the same sensor that has been used in the past for the rover to take selfies.

The images were actually received from the rover in black and white. But they are sent using a pattern called a “Bayer matrix”, which can then be decoded into colours.

The rover’s camera, as well as many of those owned by normal people on Earth, work using a range of pixels that sense either red, green or blue light. The amount of colour seen by each can then be used to work out how they would appear to the human eye.

But Curiosity isn’t able to interpret those pictures on its own — like digital cameras can — instead sending them back to Earth. Scientists then re-assemble the pictures to make sense of the colours.

The blue sunset was spotted by the Opportunity rover — though in much less clear detail — in 2010.