Geology of the Saturnian moon, Titan: Constraining the surface composition and geological history of the main types of terrains found in the equatorial belt of Titan

Abstract

In thirteen years, infrared observations from the Visual and Infrared Mapping Spectrometer (VIMS) onboard the Cassini orbiter have provided significant hints about the spectral and geological diversity on Titan's surface, the largest moon of Saturn. The analysis of the infrared signature of spectral units enables constraining the surface composition, which is essential to understand the possible interactions between Titan's interior, surface and atmosphere and to constrain the hydrocarbon cycle existing on the moon. Here, a selection of areas are investigated in the equatorial regions of the moon, imaged by Cassini's remote sensing instruments, which exhibit an apparent transition from the VIMS IR-bright to the IR-blue and IR-brown units. These spectral units are named as such owing to their appearance in false-color composites at infrared wavelengths (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, the surface albedo was extracted for each of the infrared units identified in these regions. Surface albedo was then compared with synthetic spectra of binary mixtures of the two most expected components of Titan's surface, namely water ice and laboratory tholins. Water ice is supposed to primarily form Titan's substratum, while the tholins are analogous to the aerosols photochemically produced in the atmosphere. This compositional analysis allows to reconnect the derived surface composition and grain size information to the geomorphology retrieved from RADAR's SAR swaths. Hence, IR-bright units are interpreted as hills and plains coated by organic material and incised by fluvial networks. The erosion products are transported downstream to areas where IR-blue units are seen near the IR-bright units. These areas, enriched in water ice, are most likely outwash plains hosting icy and organic 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 dunes fields. The transition areas therefore exhibit trends in terms of water ice content and grain size supported by geomorphological observations.