Geological Evolution of Titan's Equatorial Regions: Possible Nature and Origin of the Dune Material

Abstract

In 13 years, infrared observations from the Visual and Infrared Mapping Spectrometer onboard Cassini provided significant hints about the spectral and geological diversity of Titan's surface. The analysis of the infrared (IR) signature of spectral units enables constraining the surface composition, which is crucial for understanding possible interactions between Titan's interior, surface, and atmosphere. Here we investigate a selection of areas in the equatorial regions, imaged by Cassini's instruments, which exhibit an apparent transition from the Visual and Infrared Mapping Spectrometer IR-bright to the IR-blue and IR-brown units (from false-color composites using red: 1.57/1.27 μm, green: 2.01/1.27 μm, and blue: 1.27/1.08 μm). By applying an updated radiative transfer model, we extract the surface albedo of IR units identified in these regions. Then, we compare them with synthetic mixtures of two expected components on Titan's surface, namely, water ice and laboratory tholins. This allows us to reconnect the derived composition and grain size information to the geomorphology observed from Radio Detection and Ranging instrument (RADAR)/Synthetic Aperture Radar images. We interpret IR-bright units as hills and plains coated by organic material and incised by fluvial networks. Erosion products are transported downstream to areas where IR-blue units are seen near the IR-bright units. These units, enriched in water ice, are most likely outwash plains hosting debris from fluvial erosion. Farther away from the IR-bright units, the IR-brown units are dominantly made of organics with varied grain sizes, ranging from dust- to sand-sized particles that form the dune fields. The transition areas therefore exhibit trends in water ice content and grain size supported by geomorphological observations.