Composition, Physical State, and Distribution of Ices at the Surface of Triton

Details Composition, Physical State, and Distribution of Ices at the Surface of Triton Composition, Physical State, and Distribution of Ices at the Surface of Triton

Jun 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999icar..139..159q&link_type=abstract

Icarus, Volume 139, Issue Icarus, pp. 159-178.

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71

Scientific paper

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 CH_4, N_2, CO, and CO_2 molecules previously discovered. A detailed comparison of the frequencies of the CH_4 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 mum, respectively) were also observed. Laboratory experiments have shown that C_2H_6 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, C_2H_6 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 (C_2H_2, C_2H_4, C_3H_8, NH_3, SO_2, HC_3N, CH_3OH, NO, NO_2). 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:CH_4:CO terrain, N_2:CH_4:CO consisting of a solid solution in which N_2 is the dominant molecule, and of a H_2O+CO_2 terrain, composed of a mixture of pure crystalline H_2O and CO_2 grains. The second representation is a three-region surface composed of a N_2:CH_4:CO terrain and two geographically separated H_2O and CO_2 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 CH_4 is diluted in solid beta-N_2, giving a high degree of confidence to the conclusion that the N_2:CH_4: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 CH_4 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 CH_4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH_4 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 H_2O and CO_2 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 μm).

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