Fully Controlled Context Camera Data for Mars: Global Mosaics and New SPICE Data

Background: The community of NASA-funded and international researchers who study Mars is large, and significant data have been returned from spacecraft observing the planet. Many types of investigations of Mars rely on studying its surface through images taken by orbiting craft. Such investigations are greatly assisted by the availability of high- quality, high-resolution mosaics of the surface. The Context Camera (CTX) aboard the Mars Reconnaissance Orbiter (MRO) has taken H5 6 m/pixel visible light images of almost the entire surface of Mars at consistent lighting and pixel scales for the past fifteen years. Despite these qualities, the CTX dataset has not yet been publicly fully controlled and a mosaic made from those controlled images.

Goals & Objectives: Our first objective is to create a global set of fully controlled SPICE data for the CTX dataset. (Fully controlled means that each image pixel is in the correct location both relative to other images' pixels and on the surface as referenced to another controlled dataset, like MOLA.) Our second objective is to create a mosaic from the fully controlled data. The first objective would allow researchers to process their own CTX images with our updated kernels, which will greatly enhance others' research that requires multiple images, such as creation of digital terrain models and detection of changes on the Martian surface from dunes, new craters, or other processes. It would also allow any other dataset taken contemporaneously (such as HiRISE and CRISM) to be better controlled because we would be updating the spacecraft kernels. The second objective would provide an important product for the Mars research community for innumerable studies, a base map for the Mars cartography and geologic mapping communities, and a fully controlled reference for even higher resolution data such as HiRISE and CaSSIS.

Approach & Methodology: We divide the planet into regions based on the historic 30- division Mars Charts (MC). Our approach to controlling data is a mixture of fully automated steps mixed with manual efforts. The automated work is done through Python scripts that drive the USGS's Integrated Software for Imagers and Spectrometers (ISIS); we use Python v. 3.6 and 3.8 for different parts, and ISIS v. 4.3.0 on Linux and 4.4.0 on MacOS. Creating a fully controlled dataset starts with creating, via automated tools in ISIS, a control network which is a system of links between pixels of the same feature on different images. Manual effort comes in to validate the network, fix mistakes, add points where needed, and tie some of the links to a ground source (MOLA and the public fully controlled THEMIS mosaic). After a network is made, ISIS tools output updated spacecraft and camera data (our first objective). The updated data are then used to project the images and create a mosaic (second objective). During this process, we correct for several known but rarely corrected photometric issues with the CTX data, including the darkening at the edges, manually adjust the mosaic order to ensure the best images are "on top," and perform a cosmetic control process we developed to produce an even- looking mosaic.