Duluth After Dust Storm

Two images from the Mast Camera (Mastcam) on NASA's Curiosity rover depict the change in the color of light illuminating the Martian surface since a dust storm engulfed Gale Crater. The left image shows the "Duluth" drill site on Sol 2058 (May 21, 2018); the right image is from Sol 2084 (June 17). The cherry red color in the post-storm image is due to a few factors. One difference between the two images is exposure time: the dust over Curiosity creates low-lighting conditions that require longer exposure times for the cameras. The pre-storm image had an exposure time of 7.3 milliseconds, which is normal for the rover; the later image had an exposure time that was 66 milliseconds -- or nine times longer. But a primary factor is red light being filtered through the dust; very little green and essentially no blue light makes it through the dust cloud. It's not unlike the way a forest fire changes the color of light, or a red stage light filters the other colors out. Though the first pre-storm image was taken at an earlier time of day, it has a much deeper shadow than the second image. That is because the whole sky is red and illuminating the rock from all sides. Individual images are available at https://photojournal.jpl.nasa.gov/catalog/PIA22330