Geodetically improved images and mosaics of Titan via rigorous photogrammetric control of Cassini Imaging Science Subsystem data

Saturn’s moon Titan is a focus of scientific investigation and future exploration by NASA’s Dragonfly mission. Its surface is covered by lakes of liquid ethane and methane, vast dune seas, and abundant organics sourced from photochemical reactions in its dense atmosphere. Over the course of its 13-year mission, NASA’s Cassini spacecraft’s Imaging Science Subsystem (ISS) acquired near-global imaging of Titan’s surface at a wavelength of 938 nm. At this wavelength, the methane that dominates Titan’s thick atmosphere is sufficiently transparent that, after extensive image processing, surface features can be discerned. From this data we generated a global photogrammetric control network consisting of 6,896 ISS images. The network included 92,831 tie points and more than 4.5 million image measures. The images are tied to Cassini Synthetic Aperture Radar data, which establish Titan’s geodetic reference frame. Least squares bundle adjustment was used to solve for tie point locations (latitude, longitude, and radius) and spacecraft (i.e., Cassini) position. Final control point uncertainties were < 600 m in latitude and longitude and < 800 m in radius. Due to the challenges resulting from generally low image signal-to-noise ratios and Titan’s diffuse surface features, hazy atmosphere and dense clouds, the network is effectively pixel-registered, rather than sub-pixel: two-sigma (95th percentile) tie point location residuals were ~1.1 pixels. Using a subset of these photogrammetrically controlled images, we generated a global mosaic. To further improve the uniformity of the mosaic and visibility of surface features, images were weighted by incidence angle and CCD position, and areas of low incidence and emission were trimmed. A 3131-pixel high pass filter was applied to the resulting mosaic to sharpen surface features. Here we provide this mosaic in both equirectangular and polar projections at 702 m/pixel. We also release updated camera extrinsics and spacecraft ephemeris data for every image in the network as a Navigation and Ancillary Information Facility (NAIF) Spacecraft, Planet, Instrument, C-Matrix, and Events (SPICE) kernel. The control network in ISIS format, image list, and bundle adjustment statistics are also included, and these provide a starting point for additional control network development or improvements.