[X] My Assistant

[Guest_Sunspot_*]

Notice the thin "channel" like feature coming out of the large dark area on the left?

Attached thumbnail(s)

 

[Mongo]

I am less than convinced that the dark areas are more-or-less pure water ice. Remember that this hypothesis had just been suggested a few hours before the press conference was held, and could not have been thoroughly analysed in the time available.

Why I doubt this idea:

1) The hypothesis is based almost entirely on the 2.8 micron band image of the surface of Titan, which shows almost the entire surface of Titan (other than a few clouds near the south pole) being quite dark, which is consistent with the surface having a large percentage of water ice.

However, liquid methane (at 93 degrees Kelvon) also absorbs infrared light at 2.8 microns (see attached plot). The amount of absorption is sufficient that more than a few metres depth of liquid methane should appear quite dark. I have also looked for an infrared spectrum of liquid ethane at 90-95 degrees Kelvin, that covers 2.8 microns, but cannot find one on the Web. They all stop at 2.5 microns. If anybody knows the infrared absorption of liquid ethane at 2.8 microns, it would help decide this issue one way or the other.

2) The other support for this theory is the absence of specular reflections. If the sun had been in the correct position, these should have been visible in the returned images.

On the other hand, Earth-based radar imaging of Titan does show 'glints' that suggest specular reflections. Also, the area imaged in this flyby was one of the most dark-area-free portions of Titan's surface, according to the near-global map released recently. A specular reflection requires that the spot which would reflect the image of the sun be covered by a body of liquid, but almost all the area of Titan visible in this flyby was light-colored 'land', not dark-covered 'sea', so the probability of seeing a specular reflection--even if the dark areas are in fact 'seas'--is actually quite low.

3) There are a number of dark, 'river-like' features visible in the images, which appear to drain into the dark areas. These are most simply explained as being liquid-methane+ethane rivers, or their surrounding river valleys.

4) The atmosphere of Titan is most simply explained by assuming that open bodies of methane+ethane lie on the surface of Titan, with the atmosphere in equilibrium with them, plus the effects of solar UV.

Bill

Attached thumbnail(s)