  The Mars Reconnaissance Orbiter (MRO) spacecraft has been in orbit around
Mars since March 2006. The Context Camera (CTX) on MRO has returned over
100,000 images of the planet at approximately 5-6 meters per pixel, providing
nearly global coverage. During that time, Mars has gone through nearly 7 of its
own years, changing solar distance from 1.38 to 1.67 AU and the corresponding
solar flux by 45% due to its orbital eccentricity. Seasonal effects and
transient phenomena affect atmospheric transparency. Combined with an aging
detector, CTX images are difficult to mosaic seamlessly, for all of these
changes prevent equalizing images to create a visually smoothly illuminated
product. We have developed a method, based on previous work by other
researchers for other datasets, to mitigate almost all photometric variations
between images in order to create the appearance of an evenly illuminated,
practically seamless mosaic. We describe how the process works, which uses a
reference source to tie brightness values, and demonstrate its effects across
Mars' surface. While the workflow developed for this product is applicable to
other planetary bodies, it requires a reference source, which may not yet
exist.
