Empirical Photometric Control of Mars Context Camera Images

TL;DR

This paper presents an empirical method to photometrically normalize Mars images from the CTX camera, enabling seamless mosaicking despite atmospheric, seasonal, and aging effects.

Contribution

It introduces a novel empirical photometric correction technique that accounts for various environmental and instrumental factors affecting Mars images.

Findings

Effective mitigation of photometric variations across images

Creation of visually seamless mosaics of Mars surface

Method applicable to other planetary imaging datasets

Abstract

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.