The Cassini-Huygens mission, which ended on September 15, 2017 after a 20-year journey, made many discoveries about Saturn, its famous rings, and its many satellites, the largest of which is Titan. A large number of familiar landscapes (seas, lakes, rivers, dunes, mountains, craters...) have been identified on the surface of Titan in an exotic geological and climatic context: a surface mainly composed of water and hydrocarbons ice, extremely low temperatures (-180°C), and a climate based on the methane cycle. During its descent into Titan's atmosphere in January 2005, the Huygens module identified a dendritic network of rivers incising a hill near the landing site, similar to the networks observed on Mars and Earth.
Like water on Earth, liquid methane incises the Titan substrate made of water and/or hydrocarbon ice to form complex river networks. This phenomenon is particularly striking in the images acquired near the equator by the Cassini probe. To better understand the processes responsible for the formation of these landscapes, an accurate digital terrain model (DTM) representing the topographic surface of this region is required. The first and, until recently, only DTM of the Huygens module landing site was produced by USGS (United States Geological Survey) by applying the technique of photogrammetry to images acquired by the DISR (Descent Imager/Spectral Radiometer) cameras. Realized in 2005 under non-optimal conditions (shooting geometries unsuitable for photogrammetric reconstruction and low number of original images), this DTM presents a number of imperfections and inconsistencies that limit its use for the precise analysis of river formation observed on this site.
IPGP researchers and their international colleagues have implemented a new approach, benefiting from recent DISR image processing that significantly improves the quality of the original images. For the reconstruction of the DTM, the scientists used MicMac, an open-source and highly configurable photogrammetry software. In order to overcome the difficulties related to the quality of the data and the complexity of the shots (unusual geometrical configuration), a specific processing chain was developed. Quantitative geomorphology concepts (e.g., river flow direction) were applied to assess the latest ambiguities on the overall orientation of the DTM.
Thanks to this new approach, the researchers obtained a DTM covering a larger area than previously with a much better spatial sampling (18 meters, the most resolved for the surface of Titan). In fact, this new DTM significantly improves the USGS DTM. It provides the scientific community with a fundamental tool, now accurate and unbiased, to continue the quantitative study of the formation of river valleys and the erosion and incision processes caused by liquid methane on Titan.
Daudon, C., Lucas, A., Rodriguez, S., Jacquemoud, S., Escalante López, A., Grieger, B., et al. (2020). A new digital terrain model of the Huygens landing site on Saturn's largest moon, Titan. Earth and Space Science, 7, e2020EA001127. DOI: 10.1029/2020EA001127