Discussion on the dark terrain: Part I

Over the next three days, I'd like to present my thoughts on Titan's dark terrain, both in the views prior to Huygens and with Huygens data now in hand. In Part I, this post, I will present the pre-Huygens view and what data was used to form those hypotheses. In Part II, I will present the new Huygens data in context with what we knew before. In Part III, I will present a few models to explain the dark terrain as seen by Huygens (think post-Voyager Io with methane instead of sulfur, ammonia-water cryovolcanism instead of basaltic volcanism, and water volcanism instead of komatite volcanism (okay that may be going too far...)

On January 14, the Huygens probe landed on the surface of Titan within the large equatorial belt of dark material in anti-Saturnian hemisphere. The nature of the dark material on Titan has been important question for geologists ever since the first disk-resolved images of surface taken by HST were released in the mid-1990s. One of the more popular explainations prior to the Cassini mission was that the dark material consisted of seas of liquid methane and ethane. This hypothesis was supported by speckle imaging in the late 1990s by a group out of the Lawrence Livermore Lab using the Keck I telescope showing that the dark material had an albedo at 2 microns of around 2%.

A more conclusive remote test for liquids is to search for specular
reflections. A specular reflection can be seen when looking at the sun's reflection on a body of water while on a day at the beach. All that is required is that the surface be smooth at the wavelength of the light source. In this case, five seperate data sets can be brought to bear on this problem, four of which are known to this blogger. First, Campbell et al. used the Arecibo Radio Telecope in Puerto Rico to look for a specular reflection at a wavelength of 13 cm. They found specular reflections in some places, but not everywhere. Finding a specular reflection at 13 cm is not necessarily diagnostic of liquids, but it does mean that the surface in the areas where a specular reflection was seen had to be smooth at the scales of 13 cm. The RADAR experiment on Cassini also observed Titan at a different wavelength in late October. While this is not aware of any specular reflection seen by that experiment, it did see a region of very low radar reflectivity (nicknamed Cici's Halloween Cat) that might be a lake of liquid methane but the jury is still out on that. More diagnostic is the Keck and Cassini ISS data sets. ISS does have higher resolution, but Keck has better longitudinal coverage at the present time. In neither data set has a specular reflection been found, including in the dark terrain.