Sunset on Mars

[Josh]

Pretty

The Curiosity rover has been sending back data and photos since it landed on Mars nearly three years ago. Unsurprisingly, there are a lot of rocks in those images.

But finally, for the first time, on Curiosity's 956th solar Mars day -- April 15, 2015, to all you Earthlings out there -- its Mastcam high-resolution, colour camera captured the setting sun on the Red Planet. (A solar Mars day, or sol, by the way, lasts just over 24 hours and 39 minutes.)

Mars' sunset of cool blues looks starkly different from our warm-hued equivalent. This is because of the thinner Martian atmosphere and the dust in it. The images of the sunset were taken in between dust storms, but with plenty of dust still in the air. This allowed researchers to analyse the vertical distribution of dust in the Martian atmosphere.

"The colours come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently," Curiosity science team member Mark Lemmon of Texas A&M University, who planned the observations, said Thursday in a statement from NASA.

"When the blue light scatters off the dust, it stays closer to the direction of the sun than light of other colours 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."

That's the opposite of Earth sunsets, where the sky grows red and yellow as the sun dips into the horizon, with the rest remaining blue.

The images were returned from Curiosity in black and white. If you look at them carefully, you'll see a sort of hashed texture to the image. This pattern is called the Bayer matrix, and it's how the Mastcam (and other digital cameras) codes colour into images.

The Bayer filter is a series of colour filters painted over the Mastcam's detector. Each pixel is painted either red, green or blue, and when the Mastcam's shutter opens, light enters into each of these pixels according to how the human eye would see them.

However, the sensor itself can't interpret the colours on its own. Digital cameras have on-board software to decode the greyscale matrix that emerges, but some of the Bayer data coming back from Curiosity needs to be decoded on Earth. This is how we are able to extrapolate colour data from what looks like a black and white image.

These images, taken over a period of about 6 minutes, 51 seconds, were decoded by space imaging enthusiast Damia Bouic, who said in her blog last week:

"These images came in black and white, with a Bayer matrix. I had to process them through Gimp and G'MIC in order to rebuild the colours. A little processing in Pixelmator to remove these ugly white stripes due to pixels overloading. A little bit of denoising to remove Bayer matrix artifacts, et voilà!"

Although it's in glorious high-resolution, and the first view from Curiosity, this is not the first Mars sunset we've ever seen. In 2010, NASA's Jet Propulsion Laboratory released a video of the Martian sunset as observed by the Opportunity rover on November 4 and 5, 2010.