Empirical Brightness Control and Equalization of Mars Context Camera Images

Abstract

AbstractThe Mars Reconnaissance Orbiter (MRO) spacecraft has been in orbit around Mars since March 2006. The Context Camera (CTX) on MRO has returned over 115,000 images of the planet at approximately 5–6 m per pixel, providing nearly global coverage. During that time, Mars has gone through nearly seven 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. These changes, along with an aging detector, prevent easily equalizing images to create a visually smoothly illuminated product from CTX images. We have developed a method, based on previous work by other researchers for other data sets, to mitigate almost all brightness 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.