Water Ice on Titan’s Surface Spectra were obtained of the leading and trailing hemispheres of Titan with SpeX at 0.8–5.1 microns with the IRTF and at 4.6–5.2 microns with the UKIRT. The geometric albedo of the leading hemisphere was carefully modeled taking into account the atmospheric constituents and the haze. From this model the surface albedo was determined in the regions of high atmospheric transparency at at 0.83, 0.94, 1.07, 1.28, 1.58, 2.0, 2.9, and 5.0 microns. Surprisingly, the surface albedo matches well that of the water ice covered Jovian satellite Ganymede. The SpeX observations were essential since it can cover a wide wavelength range simultaneously, thus eliminating many systematic effects that arise from other grating spectrographs. C.A. Griffith, T. Owen, T.R. Geballe, J. Rayner, P. Rannou Reference: C.A. Griffith et al. 2003, Science, 300, 628.

Geometric albedo of Titan’s leading (blue) and trailing (black) hemispheres. Calculated spectra of the trailing hemisphere where methane data exists (red).

Titan’s leading hemisphere surface albedo (red squares; atmospheric “windows”) resembles that of Ganymede’s leading hemisphere (green). The blue dashed line show a laboratory synthesis of Titan’s organic sediment.

Implications Over the lifetime of the solar system, about 800 meters of organic sediments should have formed from the photolysis of methane in the atmosphere of Titan. The fact that an icy surface is observed suggests that the organic sediments moved around on the surface in such a way as to expose the bedrock ice. Image of Titan show bright and dark areas. The bright areas possibly correspond to the icy bedrock, while the darker areas could be the organic sediments. Thus a picture is emerging of high areas that have lost the sediments and lower areas that are rich in sediments. The next slide shows where the high and low areas are located on Titan.

Maps of the leading hemisphere (top) and trailing hemisphere (bottom) of Titan derived from 8500 angstrom images obtained with HST. The leading hemisphere is brighter and appears to have more exposed “icy bedrock”. The total contrast is only plus or minus about 4% of the total light collected through the filter. The grey area at the top and bottom shows regions which could not be imaged through the haze. Yellow-white shows high terrain; red shows low terrain. Images from: