Omacatl and Elpis Maculae as possible sources of windblown dark material, from Cassini VIMS

Two visually dark, diffuse triangular features near Fensal Aztlan (the "H") on Titan's sub-Saturnian hemisphere, are examined using the Cassini Visual and Infrared Mapping Spectrometer (VIMS). Omacatl and Elpis Maculae, dark triangular features observed by both VIMS and ISS, are spectrally similar to nearby "neutral dark" material, so-called for its shallow spectral slope in the near infrared. This type of material has been associated by other authors with widespread "dune" terrain, and is spectrally distinct from less common "blue dark" material, which is found preferentially at the margins of bright and dark terrain. Using VIMS hyperspectral data, we map the shape and extent of both Omacatl and Elpis Maculae, and establish their relative spectral characteristics. We find that the maculae are purest (i.e., in neutral dark material) at their respective apexes, which we interpret to be the origin of windblown material. A compositional gradient from neutral dark to bright material is found along a profile from the apex toward the diffuse extremities of both Omacatl and Elpis. This is contrary to other neutral dark features observed by VIMS, in which contacts with bright terrain are more abrupt.