[X] My Assistant

[Guest_AlexBlackwell_*]

The above-titled paper by Barnes et al. is being published online today in Geophysical Research Letters.

[remcook]

The slope in the above paper, and all other papers except the mention of the slope of the two sub-windows near 2.8 um in McCord et al., are the slopes from one window to the next. For VIMS for instance, the "surface spectrum" is basically defined by 6 or 7 points, each at the position of a different methane window. The relative intesities of these points are then compared to spectra of e.g. water or tholin.

The Negrao et al. paper has much higher spectral resolution, but these are disc-averaged spectra. Also, they cannot yet fit the fine structure in the windows, but their point about the unidentified component refers to one very low point in the spectra for Titan's trailing side.

I think the story so far is (surprisingly):
dark material = water + perhaps little tholin
light material (Xanadu etc) = ???

About looking for different materials, here's a bit from McCord et al.:

"We also checked for matches with other material spectra. Obvious candidate materials are simple organics and cyanide molecules. We note, however, that many of these show considerable spectral structure in the 5-μm window region that is not seen in the Titan spectra (Fig. 12). We ran a comparison check of these Titan reflectances at the specific window wavelengths with the USGS spectral library (Clark et al., 2003 and Clark et al., 2005), including over 2000 spectra. Some interesting compounds that provide partial matches to Titan's normbrit and ddark spectra, when combined with a neutral absorber, are given in Table 8, including pyrene, decane, paraffin, and KCN. The long wavelength reflectances match Titan well, including the 2.79/2.71 μm reflectance ratio, but the 1.28-μm reflectance deviates from that derived here for Titan."