Icarus 139, 159–178 (1999) Article ID icar.1999.6111, available online at http://www.idealibrary.com on Composition, Physical State, and Distribution of Ices at the Surface of Triton Eric Quirico, 1 Sylvain Dout´ e, and Bernard Schmitt Laboratoire de Glaciologie et G´ eophysique de l’Environnement, CNRS 54, rue Moli` ere, Domaine Universitaire, BP 96 38402 St. Martin d’H` eres cedex, France E-mail: quirico@ias.fr Catherine de Bergh 2 Observatoire de Paris-Meudon, DESPA 5, place Jules Janssen, 92195 Meudon cedex, France Dale P. Cruikshank 2 NASA Ames Research Center, Space Sciences Division, Moffett Field, California 94035-1000 Tobias C. Owen 2 Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822 Thomas R. Geballe Joint Astronomy Centre, 660 N. A’ohoku Place, Hilo, Hawaii 96720 and Ted L. Roush NASA Ames Research Center, Space Sciences Division, Moffett Field, California 94035-1000 Received July 20, 1998; revised February 4, 1999 This paperpresents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September7, with the cooled grating spectrometerCGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules compos- ing Triton’s surface by comparing the observations with laboratory transmission spectra (direct spectral analysis);this also gives infor- mation 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 de- tailed comparison of the frequencies of the CH 4 bands confidently indicated that this molecule exists in a diluted state in solid β -N 2 . Three new bands peaking at 5717, 5943, and 6480 cm −1 (1.749, 1.683, and 1.543 µm, respectively) were also observed. Laboratory experiments have shown that C 2 H 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 2 H 6 may exist 1 Current address: Institut d’Astrophysique Spatiale, Bˆ atiment 121, Universit´ e Paris-Sud, 91405 Orsay cedex, France. 2 Guest observers, United Kingdom Infrared Telescope. in anotherphysical state, and more experiments are necessary. No plausible candidate was found for these three bands when compar- ing with the spectra of nine molecules (C 2 H 2 ,C 2 H 4 ,C 3 H 8 , NH 3 , SO 2 , HC 3 N, CH 3 OH, 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 pos- sible 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 consist- ing of a solid solution in which N 2 is the dominant molecule, and of a H 2 O + CO 2 terrain, composed of a mixture of pure crystalline H 2 O 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 2 O and CO 2 terrains. The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Dout´ e 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 β -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 ter- rain, namely the size of the grains, the geographical abundance, and the CH 4 and COconcentrations. The large grain size (around 10 cm) would mean that the texture of this terrain is a compact crystalline solid ratherthan granular, which is in agreement with calculations 159 0019-1035/99 $30.00 Copyright c  1999 by Academic Press All rights of reproduction in any form reserved.