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What are the True Colors of the Martian Landscape?

On October 2nd, NASA/JPL released this ‘postcard‘ from the Curiosity rover showing the higher regions of Mount Sharp as viewed from the rover’s work-site in the Gale Crater. The scene seems jarringly familiar for an Earth-bound viewer like me. At first glance, you’d probably be forgiven for mistaking the above-depicted topography as coming from Egpyt’s Western Desert or perhaps the Atacama in South Africa.

Mount Sharp 'postcard'
This composite image looking toward the higher regions of Mount Sharp was taken in mid-September, 2015, by NASA’s Curiosity rover.

The blue sky and brown desert landscape are just so similar to something one would expect to find on Earth. When I think of Mars, the first mental images that come to mind are those which depict a much redder planet…with a much more orange-ish sky. So what’s going on here? Is Mars much more Earth-like than I feel I’ve been led to believe?

Let’s consider the two images below. One is a raw, original image recorded using Curiosity’s Mastcam and one is a color-corrected image. Which is the raw image and which is the adjusted image? Further, why were the adjustments made in the first place?

In the above pair of images, the image on the left has been subjected to a white balance color adjustment whereas the image on the right is the raw image data received from the rover’s imaging equipment (which is designed to produce raw images similar to the cameras/software found in a typical smartphone).

As organizations with scientific mandates and objectives, NASA/JPL (and their affiliates) frequently manipulate the colors of Martian imagery to better suit their needs. By making the aforementioned white balance adjustments, for example, researchers can better perceive patterns within the topography. There is also a some benefit to creating an Earth-like sense of illumination when it comes to making geologic comparisons between the red planet and our own. By effectively “normalizing” the illumination, geologists can better compare features present on both planets.

Now, anyone with a camera knows that the imagery captured by that camera and depicted through a digital screen (e.g. computer monitor) is never totally congruent with how the subject of that imagery is perceived with the human eye. To complicate matters, just as no two people will perceive color the same way, no two camera or monitor models will record and generate imagery the same way. Since we have yet to place a human explorer on Mars, it isn’t yet possible to know exactly how “true to form” the raw imagery sent home from Curiosity is when compared to the optical recording/processing of the human eyes and brain, but NASA/JPL think they are on the right track and often release “natural” images of the Martian landscape.

Mastcam Color Calibration
This set of images illustrates the twin cameras of the Mast Camera (Mastcam) instrument on NASA’s Curiosity Mars rover (upper left), the Mastcam calibration target (lower center), and the locations of the cameras and target on the rover. The known colors of materials on the calibration target aid researchers in adjusting images to estimate “natural” color, or approximately what the colors would look like if we were to view the scene ourselves on Mars. The design of the target is essentially the same as the color calibration target used for the Pancam instruments on the Mars Exploration Rovers Spirit and Opportunity. For Curiosity, the target was modified to include six small embedded magnets that can capture and concentrate dust from the Martian atmosphere. These are the reddish rings seen in the image of the target shown here, which was taken on Sol 69 of Curiosity’s mission on Mars (Oct. 15, 2012). Image credit: NASA/JPL-Caltech/MSSS/ASU

To achieve the “natural” color correction, Curiosity’s imaging equipment relies on a calibration target located toward the right-aft portion of the vehicle that provides a colorimetric standard against which the colors in the image can be measured. The colors in the images can then be digitally corrected to better approximate the “natural” color of the landscape. To make those comparisons and approximations even more complete, the calibration target also employs a set of magnets that capture the iron-rich dust swirling around the rover and put it in close proximity to the color standards for and even better comparison.

In the center image, NASA/JPL have taken the raw data from Curiosity’s Mastcam and rendered a ‘calibrated image’ which researchers believe provides the closest representation of how the human eyes and brain would perceive the surface of Mars using the calibration target as a standard.

Martian Landscape in 3 versions
These three versions of the same image taken by the Mast Camera (Mastcam) on NASA’s Mars rover Curiosity illustrate different choices that scientists can make in presenting the colors recorded by the camera. The left image is the raw, unprocessed color, as it is received directly from Mars (and as available on the MSL Raw Images Web site: The center rendering was produced after calibration of the image to show an estimate of “natural” color, or approximately what the colors would look like if we were to view the scene ourselves on Mars. The right image shows the result of then applying a processing method called white-balancing, which shows an estimate of the colors of the terrain as if illuminated under Earth-like, rather than Martian, lighting. The image was taken by the Mastcam on Sol 19 of Curiosity’s mission on Mars (Aug. 23, 2012), using only the camera’s red-green-blue Bayer filters. It looks south-southwest from the rover’s landing site toward Mount Sharp. Image credit: NASA/JPL-Caltech/MSSS

Ok, so we know that Mars imagery is often color corrected and that the images can be calibrated beyond the raw image data to better approximate Mars’ “natural” coloration, but why is there a color discrepancy in the first place?

The two main factors at play here appear to be time of day and the amount of particulate matter (dust) in the sky at the time of the image capture. Just like on Earth, particulate matter in the atmosphere has an effect on light which, in turn, effects how visible-spectrum based imaging gear like cameras and organic eyes perceive color. During most of the Martian day, the ample dust in the atmosphere scatters light in such a way as to make yellow to orange colors more prominent; thus creating the characteristic yellow to orange hues so commonly associated with the Martian landscape.  As the sun processes across the sky, the changing angle of sunlight through the atmosphere interacts with the variable amount of dust in the atmosphere and alters the perceived color of everything on the surface. This is what makes the color calibration instrumentation/software so indispensable.

So, while the Martian sky is almost never the Earth-like blue depicted in Curiosity’s recent postcard, there are some instances in which the Martian sky actually does appears blue naturally. During sunsets on the red planet, the same dust particles responsible for the yellow-orange sky coloration during most of the day impart blue tones onto the landscape. The below animation was compiled from images captured by Curiosity on April 15, 2015 using its Mastcam imaging equipment.

Martian Sunset
NASA’s Curiosity Mars rover recorded this sequence of views of the sun setting at the close of the mission’s 956th Martian day, or sol (April 15, 2015), from the rover’s location in Gale Crater. The four images shown in sequence here were taken over a span of 6 minutes, 51 seconds. Image credit: NASA/JPL-Caltech/MSSS/Texas A&M Univ.
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