Titan's Plains Revealed: Evidence for a Layered Surface

Abstract Undifferentiated plains are the most common terrain type on Titan, yet their composition and geologic history remain poorly understood. To better characterize their physical properties, we combined Cassini RADAR measurements from nadir altimetry and side‐looking SAR modes. We analyzed these data using radar backscatter models, finding that the multi‐angle radar response from undifferentiated plains across Titan is remarkably consistent. This uniformity suggests globally similar properties and formation processes, permitting aggregate modeling. Our analysis reveals that canonical single‐layer scattering models fail to reproduce the observed backscatter, particularly the bright near‐nadir returns captured by altimetry, which proved critical for model discrimination and accurate parameter estimation. Instead, a two‐layer model is required to fit the data across all incidence angles. Best‐fit parameters indicate undifferentiated plains likely consist of a highly porous, low‐density surface layer (effective permittivity 1.33) that is exceptionally smooth at radar wavelengths (RMS slope 2°), overlying a higher‐density (effective permittivity >2.7) and rougher buried substrate. This surface layer is likely less than 1 m thick. The layered structure, along with the observed global uniformity and extreme flatness at multiple scales, is most consistent with long‐term atmospheric deposition of organic particles (“tholin snow”), which are subsequently densified or buried, potentially during periods of different environmental conditions. The structure of the undifferentiated plains provides insights into organic processing and transport on Titan and potentially preserves a record of past environmental conditions. Measurements made by the upcoming Dragonfly mission may provide answers to questions raised by our analysis.