Discussion on the Dark Terrain: Part II
With ISS and Keck having already shown that widespread liquids in the dark terrain was unlikely and Arecibo and Cassini RADAR perhaps finding a few isolated areas of surface liquid, it was up to Huygens to perhaps resolve the mystery of the dark material. Luckily, Huygens landed near a boundary between dark and bright material so that both types of terrain can be seen up close for easy and unambiguous comparison. Even better, Huygens landed in a dry patch of dark terrain. Features seen in the dark material from altitude such as dark streams between small patches of bright terrain and streamlined shapes to the larger bright patches within the dark terrain suggest that although the dark material at the landing site was dry, fluid had at one time flowed in the area. This is supported by the observation of channels in the bright terrain terminating at the boundary between dark and bright regions and by the eroded shapes of ice rocks in the vicinity of the Huygens. Images from the surface using the medium-resolution imager were affected by the lamp but suggest that the close up texture of the surface is akin to gravel or sand.
In addition to imaging, the GCMS instrument, the penetrometer on the SSP instrument, and the spectrometer on the DISR instrument examined the composition and texture of the dark material. Spectra from altitude by DISR shows that the terrain is mostly made of water ice with some kind of darkening agent. The data from the penetrometer on the Surface Science Package has been interpreted as showing that that dark material at the landing site was covered in a thin crust of material with icy sand below. This is consistent with the DISR-MRI view of the surface. Finally, the GCMS instrument, with its heated inlet, found methane percolated into the instrument after being heated up to boiling.
Putting together these data, it appears that the area where Hugyens landed as modified by fluids, possibly from rainfall during a rainy season but that's still an early hypothesis, especially since we have yet to see clouds at that latitude, perhaps since clouds have only been observed for a few years (though a large cloud may have been see at northern mid-latitudes by HST in 1995). However, the views from above and from the surface both suggest that the terrain has been modified by fluid flow and erosion. The GCMS, SSP penetrometer, and MRI images seem to indicate a dry initial surface layer of ice sand with liquid methane 15-30 cm below the surface mixed in with the ice sand in a kind of methane slushee.