Neither of NASA’s two Mars-based rovers nor the Reconnaissance Orbiter has sent back any especially compelling photos of the Red Planet lately, thanks to a “planet-encircling” dust storm that’s made photography hard over the last couple of weeks. To remind us that, hey, there’s more to Mars than clouds of red dust, NASA recently released a stunning photo of an unexpectedly turquoise sand dune, shot in January. Alfred McEwen, Ph.D., one of the scientists who run the High Resolution Imaging Science Experiment — HiRISE — camera that took this photo, explained to Inverse how this strange image came to be.
The photo, posted by NASA on June 20, was taken by the HiRISE on the Mars Reconnaissance Orbiter on January 24. It zooms in on the floor of Lyot Crater, where a bunch of regular sand dunes is overlaid by one huge blue sand dune, sprawling over the landscape like a giant turquoise slug. The smaller dunes are classic barchan dunes — the technical term for a crescent-shaped dune formed by gusts of wind. The big blue one, however, “appearing like turquoise blue in enhanced color, is made of finer material and/or has a different composition than the surrounding,” said NASA in a press release. McEwen, a planetary geologist and director of the Planetary Image Research Laboratory at the University of Arizona, explained the extent to which photo enhancement created the striking blue color in an email to Inverse.
“The images are given min-max stretches in each individual color image to increase contrast,” he says. “The dunes are actually grey, but appear relatively blue after such a stretch because most of Mars is red.”
The reason HiRISE photos have to undergo so much editing is because, to the naked eye (or camera lens), the surface of Mars actually looks quite bland and blurry because all you really see from above is light reflecting off the dust in the atmosphere. Even the HiRISE, with its unprecedented resolution (it can image objects on Mars as small as three feet tall), can’t get around the dust. So, to see images in fine detail, the team has to turn up the contrast — which often leads to a dramatic shift in color. The photos below, of Mars’s Nili Patera region, shows what a difference some editing can make.
In an article for the HiRISE website written in 2016, McEwan explained how the color enhancement process created the above photos.
At the top is an approximation of the natural color as seen by people with normal color vision—almost no surface detail is visible. In the middle is the standard HiRISE IRB color product, consisting of the infrared, red, and blue-green images displayed as red, green, and blue, respectively, and with a min-max stretch applied to each color. In other words, the darkest pixel in the entire image is set to black, the brightest pixel is set to white, and all others are linearly interpolated. At bottom is an enhanced color product, in which each bandpass is given a linear stretch for the local subimage, sometimes saturating a small percentage of data to black or white to give the rest of the scene more contrast, followed by color saturation enhancement. Now we can see a diversity of colors that distinguish different surface units: dust, sand, and rocks with different minerals.
The same process produced the now-iconic blue dune, shown in a color-enhanced version below.
In short, the blue dune isn’t actually blue, but if it wasn’t edited to look blue, it wouldn’t be nearly as impressive. Mars purists can rest assured: Its reputation as the Red Planet is intact for now.