The reflectivity spectrum and opposition effect of Titan's surface observed by Huygens' DISR spectrometers

We determined Titan's reflectivity spectrum near the Huygens' landing site from observations taken with the Descent Imager/Spectral Radiometer below 500 m altitude, in particular the downward-looking photometer and spectrometers. We distinguish signal coming from illumination by sunlight and the lamp onboard Huygens based on their different spectral signatures. For the sunlight data before landing, we find that spatial variations of Titan's reflectivity were only similar to 0.8%, aside from the phase angle dependence, indicating that the probed area within similar to 100 m of the landing site was very homogeneous. Only the very last spectrum taken before landing gave a 3% brighter reflectivity, which probably was caused by one bright cobble inside its footprint. The contrast of the cobble was higher at 900 nm wavelength than at 600 nm. For the data from lamp illumination, we confirm that the phase function of Titan's surface displays a strong opposition effect as found by Schroder and Keller (2009. Planetary and Space Science 57, 1963-1974). We extend the phase function to even smaller phase angles (0.02 degrees), which are among the smallest phase angles observed in the solar system. We also confirm the reflectivity spectrum of the dark terrain near the Huygens' landing site between 900 and 1600 nm wavelength by Schroder and Keller (2008. Planetary and Space Science 56, 753-769), but extend the spectrum down to 435 nm wavelength. The reflectivity at zero phase angle peaks at 0.45 +/- 0.06 around 750 nm wavelength and drops down to roughly 0.2 at both spectral ends. Our reflectivity of 0.45 is much higher than all previously reported values because our observations probe lower phase angles than others. The spectrum is very smooth except for a known absorption feature longward of 1350 nm. We did not detect any significant variation of the spectral shape along the slit for exposures after landing, probing a 25 x 4 cm(2) area. However, the recorded spectral shape was slightly different for exposures before and after landing. This difference is similar to the spectral differences seen on scales of kilometers (Keller et al., 2008. Planetary and Space Science 56, 728-752), indicating that most observations may probe spatially variable contributions from two basic materials, such as a dark soil partially covered by bright cobbles. We used the methane absorption features to constrain the methane mixing ratio near the surface to 5.0 +/- 0.3%, in agreement with the 4.92 +/- 0.24% value measured in situ by Niemann et al. (2005. Nature 438, 779-784), but smaller than their revised value of 5.65 +/- 0.18% (Niemann et al., 2010. Journal of Geophysical Research 115, E12006). Our results were made possible by an in depth review of the calibration of the spectroscopic and photometric data. (C) 2011 Elsevier Ltd. All rights reserved.