Using Cassini's Visual and Infrared Mapping Spectrometer (VIMS) instrument, we have identified two, possibly three, regions on south of Xanadu that are spectrally distinct from the rest of Titan's surface in that they are relatively highly reflective at wavelengths near 5 microns (see Figure 1). These areas are the brightest on Titan when viewed in each of the infrared spectral windows, but are nearly twice as bright as Xanadu at 5 microns. This may, however, be due more to the unusual nature of Xanadu than that of the 5-micron bright areas.
The western 5-micron bright region is located due south of Xanadu and extends from 140W to 110W longitude and from 13S to 30S latitude. The IAU has provisionally named this area Tui Regio (see Figure 2). Its eastern portion is an arcuate shape and is brighter than the more diffuse western portion. Located within the arc of east Tui Regio is a unique sharply bounded dark area first identified by Cassini's Imaging Science Subsystem (ISS) and now named Eir Macula. VIMS shows Eir Macula to be spectrally different from surrounding dark features, its presence near Tui Regio may mean that these two unusual features are related.
The eastern region, near 80 W25 S, contains a bright arc similar in length, radius of curvature, and orientation to east Tui Regio when viewed by ISS at 0.938 microns. The arc is provisionally named Hotei Arcus. The VIMS view of the area shows a broader zone of enhanced 5-micron reflectivity extending toward the center of curvature. Analyses of this region's spectra showed that the spectral variation is indeed the result of surface markings (Barnes et al., 2005), but the identity and nature of the reflecting surface material has not yet been determined (McCord et al., submitted).
The multiple instances and varying intensities of 5-micron bright terrain imply that the process that formed them was not rare. If the southern 5-micron bright region belongs to the set, then it could represent an evolutionary continuum whereby 5-micron bright areas are formed and later regress toward the planetary norm. The regression could be the result of either erosion from or deposition onto the surface.
The two primary 5-micron bright regions show arcs hundreds of km long and are surrounded by darker than surrounding terrain that might represent a 'moat'. The southern bright region is bordered on the ENE by a linear dark area of similar character. If these neighboring dark areas are indeed associated with the 5-micron bright areas then they may either cause or be caused by large scale regional tectonics.
Research into the nature of Titan's surface is only just beginning. Our understanding of the processes that control surface formation and evolution is poor. We think that the existence and nature of Titan's 5-micron bright terrain may provide insight into those global scale questions.