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 is almost jarringly familiar for an Earth-bound viewer like me. At first glance, I could almost mistake it for Egpyt’s Western Desert or perhaps the Atacama in South Africa.
The blue sky and brown desert landscape are just so similar to something I could 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).
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 to facilitate 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 what they capture with their camera (and what is ultimately depicted on a digital screen) is never totally congruent with how the 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.
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.
Ok, so we know that Mars imagery is often color corrected and that the images can be calibrated 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 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.