Composition, physical state, and distribution of ices at the surface of Triton

This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4,N-2,CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid beta-N-2 Three new bands peaking at 5717, 5943, and 6480 cm(-1) (1.749, 1.683, and 1.543 mu m, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N-2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2). In view of the results of D. P. Cruikshank et al. (1993, Science 261, 742; in preparation), the work presented here leads to two possible representations of the surface of Triton. First, a two-region surface composed of a N-2 : CH4 : CO terrain, N-2 : CH4 : CO consisting of a solid solution in which N-2 is the dominant molecule, and of a H2O + C-2 terrain, composed of a mixture of pure crystalline H2O and CO2 grains. The second representation is a three-region surface composed of a N-2 : CH4 : CO terrain and two geographically separated H2O and CO2 terrains. The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Doute and B. Schmitt 1998, J. Geophys. Res, Planets 103, 31367). The modeling first confirms the direct spectral analysis in that CH4 is diluted in solid beta-N-2, giving a high degree of confidence to the conclusion that the N-2 : CH4 :CO terrain is in fact a solid solution. It also provides numerical information on this terrain, namely the size of the grains, the geographical abundance, and the CH4 and CO concentrations. The large grain size(around 10 cm) would mean that the texture of this terrain is a compact crystalline solid rather than granular, which is in agreement with calculations from J. Eluszkiewicz (1991, J. Geophys. Res. 96, 19,217). In addition, an accurate modeling of the N-2 band could suggest that the temperature is greater or equal to 35.6 K. Although undistinguishable in the spectra, a maximum of 10% surface area of pure CH4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH4 abundance observed by Voyager 2. Finally, the modeling showed that none of the two- or three-region representations was able to fit simultaneously the K and H regions of the spectrum of Triton. The origin of this misfit is not yet elucidated, but an instrumental effect is suspected. Some questions about the physical state of the H2O and CO2 molecules are thus raised, but unfortunately observational constraints are missing. New near-infrared observations could partly provide these missing constraints, and would be important for detecting new molecules on Triton's surface. Such new data would be especially useful to identify the three bands at 5717, 5943, and 6480 cm(-1) (1.749, 1.683, and 1.543 mu m). (C) 1999 Academic Press.