Rabe Crater - False Color

The THEMIS VIS camera contains 5 filters. The data from different filters can be combined in multiple ways to create a false color image. These false color images may reveal subtle variations of the surface not easily identified in a single band image. Today's false color image shows part of the floor of Rabe Crater. Located in Noachis Terra, Rabe Crater is 108 km (67 miles) across. As the dunes are created by wind action the forms of the dunes record the wind direction. Dunes will have a long low angle component and a short high angle side. The steep side is called the slip face. The wind blows up the long side of the dune. In this VIS image the slip faces are less illuminated than the longer side. In this part of the crater the winds were generally moving from the lower right corner of the image towards the upper left. Craters of similar size as Rabe Crater often have flat floors. Rabe Crater has some areas of flat floor, but also has a large complex pit occupying a substantial part of the floor (visible in this image). The interior fill of the crater is thought to be layered sediments created by wind and or water action. The pit is eroded into this material. The eroded materials appear to have stayed within the crater forming a large sand sheet with surface dune forms as well as individual dunes where the crater floor is visible. Several other craters in this region have complex floors with pits.

The THEMIS VIS camera is capable of capturing color images of the Martian surface using five different color filters. In this mode of operation, the spatial resolution and coverage of the image must be reduced to accommodate the additional data volume produced from using multiple filters. To make a color image, three of the five filter images (each in grayscale) are selected. Each is contrast enhanced and then converted to a red, green, or blue intensity image. These three images are then combined to produce a full color, single image. Because the THEMIS color filters don't span the full range of colors seen by the human eye, a color THEMIS image does not represent true color. Also, because each single-filter image is contrast enhanced before inclusion in the three-color image, the apparent color variation of the scene is exaggerated. Nevertheless, the color variation that does appear is representative of some change in color, however subtle, in the actual scene. Note that the long edges of THEMIS color images typically contain color artifacts that do not represent surface variation.