The Planetary Society Blog
By Emily Lakdawalla
Peeking Through the Haze: Titan's Surface, part I
Oct. 12, 2006 | 14:51 PDT | 21:51 UTC
by Brad Thomson
Charles Elachi, head of the Cassini radar team, first gave an overview of the progress to date. About 15 percent of the surface has been covered by radar measurements thus far. On each incoming pass, the radar takes three types of measurements: radiometry (where the radar just passively receives radiation), scatterometry data, and finally altimetry and SAR (synthetic aperature radar) data. It is the latter type of data that has attracted the most interest because it shows surface patterns and shapes.
One of the exciting recent results is the detection of radar-dark spots at high latitudes that may be lakes of liquid methane or ethane. Some of these lakes have intricate finger-like margins that are reminiscent of Lake Powell.
Recently, the radar has measured some of the same surface features at multiple look angles. These data are only beginning to be analyzed, but they should help us understand which effects are due to topographic configurations and which are due to composition or the physical nature of the materials. Charles showed at least one image that hinted that the radar may be actually penetrating some of the dark lake material and returning information about its interior structure. We'll have to stay tuned on that one.
Charles concluded by summarizing that Titan exhibits surprisingly Earth-like surface processes. There is evidence of fluvial modification (methane rain-fed rivers), aeolian modification (sand dunes), and tectonic processes (mountain ranges formed by crustal compression).
VIMS
R. Brown spoke next about the Cassini VIMS (Visual and Infrared Mapping Spectrometer) investigation of the surface of Titan. While the hazy atmosphere of Titan blocks most of the light, the surface can be discerned through several atmospheric windows. The large-scale albedo patterns of Titan are being mapped and named. Brown mentioned a recent paper by Jason Barnes, who will be speaking soon, identifying a possible volcanic complex named Tui Reggio. What's coming next? In two weeks, the highest optical resolution data ever obtained will be acquired.
Crater Relaxation
The next talk by Nicole Baugh considered viscoelastic relaxation of impact craters on Titan. There is an observed dearth of craters less than 20 km in diameter on the surface. Others have proposed that craters in icy targets may relax and degrade over time, and the aim of this work was to model this process on Titan. Using a finite-element code to model the behavior of ice, Nicole found that not many of these craters could be relaxed away. Only the largest crater sizes show significant relaxation on reasonable time scales, so these results suggest that other factors may be responsible for the lack of craters.
Global Spectral Diversity
Jason Barnes spoke next about the global range of surface types seen with VIMS. There are seven atmospheric windows available between 1 to 5 microns. Using these windows, it is possible to infer something about surface composition, but Tom McCord will be speaking on this topic later on. Color variations seem to be dominated by latitudinal zonation. This zonation could be haze or atmospherically driven.
Much of the equatorial regions are covered with dark brown material that contains abundant dunes evident in radar. The equatorial bright regions show channel dissection. Tui Regio, in contrast, doesn't show evidence for any channels, which suggests that it is geologically young.
Hydrocarbon Deposits on Titan: Black Tar, Texas Tea...
Oil hasn't been discovered on Titan, but Roger Clark spoke next about identifying three spectral absorption features of hydrocarbons in the dark material (a hydrocarbon, by the way, is just a 25-cent word for a compound containing carbon and hydrogen). One of these spectral features at 5.05 microns is uniquely attributable to benzene. Another feature is attributed to either methane or ethane, and there is a 5.01-micron feature that remains unidentified. Another puzzle is the non-detection of acetylene, which was expected to be more abundant than benzene. [Random fact: acetylene is a gas commonly used in welding.] The lack of acetylene may indicate that it's only transiently present on Titan.
Titan Surface Composition Using VIMS
Tom McCord talked about two methods to identify surface diversity. One method is to search for spectral diversity (method used by Jason Barnes). Another is to search for absorption bands in specific spectra windows (method used by Roger Clark). With the first method, three spectral end-members were identified on the surfaces that were spatially significant. These end members are a bright unit, a dark unit, and a blue unit, and this indicates that the surface can be successfully modeled with a few number of components.
Using method #2, for each spectral unit, Tom was able to pull out spectral end-members. Although the signal to noise ratio is low (meaning that random noise sometimes drown out the real signal), the good news for the VIMS team is that the noise appears to be Gaussian. This means it is randomly distributed and not due to instrument error. Whoo-hoo! (just kidding)
Titan's bright spots
I'd like to interrupt this narrative to announce the winner in the race to have the most accent marks appear in one's name: Máté Ádámkovics. Aside from having a linguistically superior name, Mate gave one of the last and most polished talks of this session. He spoke about ground-based infrared observations of Titan. (This was the only non-Cassini talk in this session) There was a considerable amount of work involved in the reduction of this data, but one of the key results is that the brightest regions observed on the surface in two different wavelengths don't appear to physically overlap on the surface. I'm not sure what that means, but it was a good talk.
|
[ what is this? ]