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The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>104 km3) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto's interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto's history.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.05.035 Byline: Dale P. Cruikshank (a), Joshua P. Emery (b), Katherine A. Kornei (c), Giancarlo Bellucci (d), Emiliano d'Aversa (d) Keywords: Io; Ices; IR spectroscopy; Satellites; Surfaces Abstract: We obtained time-resolved, near-infrared spectra of Io during the 60-90min following its reappearance from eclipse by Jupiter on five occasions in 2004. The purpose was to search for spectral changes, particularly in the well-known SO.sub.2 frost absorption bands, that would indicate surface-atmosphere exchange of gaseous SO.sub.2 induced by temperature changes during eclipse. These observations were a follow-on to eclipse spectroscopy observations in which Bellucci et al. [Bellucci et al., 2004. Icarus 172, 141-148] reported significant changes in the strengths of two strong SO.sub.2 bands in data acquired with the VIMS instrument aboard the Cassini spacecraft. One of the bands (4.07[mu]m [[nu].sub.1 + [nu].sub.3]) observed by Bellucci et al. is visible from ground-based observatories and is included in our data. We detected no changes in Io's spectrum at any of the five observed events during the approximately 60-90min during which spectra were obtained following Io's emergence from Jupiter's shadow. The areas of the three strongest SO.sub.2 bands in the region 3.5-4.15[mu]m were measured for each spectrum; the variation of the band areas with time does not exceed that which can be explained by the Io's few degrees of axial rotation during the intervals of observation, and in no case does the change in band strength approach that seen in the Cassini VIMS data. Our data are of sufficient quality and resolution to show the weak 2.198[mu]m (4549.6cm.sup.-1) 4[nu].sub.1 band of SO.sub.2 frost on Io for what we believe is the first time. At one of the events (June 22, 2004), we began the acquisition of spectra [approximately equal to]6min before Io reappeared from Jupiter's shadow, during which time it was detected through its own thermal emission. No SO.sub.2 bands were superimposed on the purely thermal spectrum on this occasion, suggesting that the upper limit to condensed SO.sub.2 in the vertical column above Io's surface was [approximately equal to]4x10.sup.-5 gcm.sup.-2. Author Affiliation: (a) NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA (b) Dept. of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr., Knoxville, TN 37996, USA (c) Dept. of Physics and Astronomy, University of California Los Angeles, Box 951547, Los Angeles, CA 90095, USA (d) INAF-IFSI, Istituto di Fisica dello Spazio Interplanetario, Area Ricerca Tor Vergata, Via Fosso del, Cavaliere 100, I-00133 Roma, Italy Article History: Received 25 September 2006; Revised 28 April 2009; Accepted 1 May 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.10.010 Byline: Galina M. Chaban, Max Bernstein, Dale P. Cruikshank Keywords: Ices; Infrared observations; Spectroscopy; Jupiter; satellites; Saturn; satellites; Satellites; surfaces Abstract: An absorption band at [approximately equal to]4.26[mu]m wavelength attributed to the asymmetric stretching mode of CO in CO.sub.2 has been found on two satellites of Jupiter and several satellites of Saturn. The wavelength of pure CO.sub.2 ice determined in the laboratory is 4.2675 [mu]m, indicating that the CO.sub.2 on the satellites occurs either trapped in a host material, or in a chemical or physical complex with other materials, resulting in a blue shift of the wavelength of the band. In frequency units, the shifts in the satellite spectra range from 3.7 to 11.3 cm.sup.-1. We have performed ab initio quantum chemical calculations of CO.sub.2 molecules chemically complexed with one, two, and more H.sub.2O molecules and molecules of CH.sub.3OH to explore the possibility that the blue shift of the band is caused by chemical complexing of CO.sub.2 with other volatile materials. Our computations of the harmonic and anharmonic vibrational frequencies using high levels of theory show a frequency shift to the blue by 5 cm.sup.-1 from pure CO.sub.2 to CO-H.sub.2O, and an additional 5 cm.sup.-1 from CO.sub.2-H.sub.2O to CO.sub.2-2H.sub.2O. Complexing with more than two H.sub.2O molecules does not increase the blue shift. Complexes of CO.sub.2 with one molecule of CH.sub.3OH and with one CH.sub.3OH plus one H.sub.2O molecule produce smaller shifts than the CO.sub.2-2H.sub.2O complex. Laboratory studies of CO.sub.2:H.sub.2O in a solid N.sub.2 matrix also show a blue shift of the asymmetric stretching mode. Author Affiliation: NASA Ames Research Center, Mail Stop T27B-1, Moffett Field, CA 94035-1000, USA Article History: Received 15 August 2006; Revised 10 October 2006

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.10.021 Byline: Max P. Bernstein, Dale P. Cruikshank, Scott A. Sandford Keywords: Ices; Infrared observations; Spectroscopy; Surfaces; planets; Surfaces; satellites Abstract: We present 1.25-19 [mu]m infrared spectra of pure solid CH.sub.4 and H.sub.2O/CH.sub.4=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH.sub.4 in solid H.sub.2O with those of pure CH.sub.4. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH.sub.4 in solid H.sub.2O are estimated. Using the two largest ([nu].sub.3+[nu].sub.4) and ([nu].sub.1+[nu].sub.4) near-IR absorptions of CH.sub.4 at 2.324 and 2.377 [mu]m (4303 and 4207 cm.sup.-1), respectively, as examples, we show that peaks of CH.sub.4 in solid H.sub.2O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH.sub.4. With increasing temperature, these peaks shift to higher frequency and become increasingly broad, but this trend is reversible on re-cooling, even though the phase transitions of H.sub.2O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH.sub.4 absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies. Author Affiliation: NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA Article History: Received 13 July 2005; Revised 17 October 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.07.009 Byline: Max P. Bernstein, Dale P. Cruikshank, Scott A. Sandford Keywords: Ices; infrared observations; Spectroscopy; Surfaces; planets; satellites Abstract: We present near-IR spectra of solid CO.sub.2 in H.sub.2O and CH.sub.3OH, and find they are significantly different from that of pure solid CO.sub.2. Peaks not present in either pure H.sub.2O or pure CO.sub.2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO.sub.2 (2[nu].sub.3) overtone near 2.134 [mu]m (4685 cm.sup.-1), that is absent from our spectrum of pure solid CO.sub.2, is prominent in the spectra of H.sub.2O/CO.sub.2=5 and 25 mixtures. Second, a 2.74-[mu]m (3650 cm.sup.-1) dangling OH feature of H.sub.2O (and a potentially related peak at 1.89 [mu]m) appear in the spectra of CO.sub.2-H.sub.2O ice mixtures, but are probably not diagnostic of the presence of CO.sub.2. Other CO.sub.2 peaks display shifts in position and increased width because of intermolecular interactions with H.sub.2O. Warming causes some peak positions and profiles in the spectrum of a H.sub.2O/CO.sub.2=5 mixture to take on the appearance of pure CO.sub.2. Absolute strengths for absorptions of CO.sub.2 in solid H.sub.2O are estimated. Similar results are observed for CO.sub.2 in solid CH.sub.3OH. Since the CO.sub.2 (2[nu].sub.3) overtone near 2.134 [mu]m (4685 cm.sup.-1) is not present in pure CO.sub.2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO.sub.2 is a pure material or intimately mixed with other molecules. These observations may be applicable to Mars polar caps as well as outer Solar System bodies. Author Affiliation: NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA Article History: Received 4 February 2005; Revised 6 June 2005

We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0-2.5 [micro]m, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution (R = [lambda]/[DELTA][lambda]) of the data shown here is in the range 800-1000, depending on the specific instrument and configuration used; this is higher than the resolution (R = 225 at 3 [micro]m) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal [H.sub.2]O ice at 1.65 [micro]m. We model the leading hemisphere of Rhea in the wavelength range 0.3-3.6 [micro]m with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are [H.sub.2]O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3-1.0 [micro]m. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of C[O.sub.2] (a few weight percent, depending on particle size and mixing), C[H.sub.4] (same), and N[H.sub.4]OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for N[H.sub.4]OH are presented for the purpose of detection on icy bodies. These limits for C[O.sub.2], C[H.sub.4], and N[H.sub.4]OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 [micro]m. Keywords: Ices: Infrared observations: Satellites of Saturn: Spectroscopy: Surfaces, satellite: Organic chemistry

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane = 90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5-2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with -C[H.sub.3], -N[H.sub.2], and -C[equivalent to]N with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13-26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127-137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties. Keywords: Titan; Haze; Tholin; Spectroscopy; Organic chemistry; Nitrogen-containing polycyclic aromatic compounds

Haze particles exert a significant influence over the thermodynamics and radiation absorption properties of the Titan haze, as well as its complex organic chemistry. Characterization of both the molecular and the submicrometer components of the haze is therefore vital for understanding the global properties of Titan. We have carried out a Titan tholin synthesis experiment and measured the time variation of the infrared spectrum of the product as a thin film developed. Also, to examine the possibility of oxygen contamination, we compared the infrared spectrum of the tholin film with that of a tholin film exposed to dry air and laboratory air. The objective of this study is to understand the chemical processes related to how simple organic molecules are processed into more complex haze particles. The progressive development of features characteristic of amines, aromatic and aliphatic hydrocarbons, and nitriles in the experimental mixture is clear. Of particular interest is the formation of aromatic rings after only a few seconds of glow discharge, indicating that these compounds appear to be intermediates between simple haze molecules and microphysical aerosols. The early dominance of aromatic ring structures is accompanied during the later stages of the experiment by the appearance of nitrile and amine compounds. This time-dependent succession of chemical structures provides vital clues to the possible chemical formation pathways of Titan haze aerosols. Key Words: Titan; tholins; stratospheric chemistry; aromatics; amines; alkanes; hydrocarbons; aerosols.

A report of the detection of the C-H hydrocarbon band complex at 3.4/[micro]m in an asteroid spectrum, by D. P. Cruikshank and R. H. Brown (1987, Science 238, 183-184) is not confirmed by recent data of higher quality. Spectra of the same asteroid and six other low-albedo asteroids do not show this feature, which if present would indicate the presence of hydrocarbons and might link these asteroids with certain classes of carbonaceous meteorites. Key Words: asteroid composition; infrared observations; organic chemistry; spectroscopy; asteroid surfaces.

We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3-3.6 [micro]m, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-[micro]m absorption band. Such a band would indicate the presence of OH- or [H.sub.2]O-bearing silicate minerals, or macromolecular carbon-rich organic material of the kind seen on the low-albedo hemisphere of Saturn's satellite Iapetus. The absence of spectral structure is itself indicative of the absence of the nitrogen-rich tholins (which show a distinctive absorption band attributed to N-H). The successful models in this study all incorporate magnesium-rich pyroxene (Mg, Fe Si[O.sub.3]), which satisfactorily matches the red color of Hektor. Pyroxene is a mafic mineral common in terrestrial and lunar lavas, and is also identified in Main Belt asteroid spectra. An upper limit to the amount of crystalline [H.sub.2]O ice (30-[micro]m grains) in the surface layer of Hektor accessible to near-infrared remote sensing observations is 3 wt%. The upper limit for serpentine, as a representative of hydrous silicates, is much less stringent, at 40%, based on the shape of the spectral region around 3 [micro]m. Thus, the spectrum at 3 [micro]m does not preclude the presence of a few weight percent of volatile material in the uppermost surface layer of Hektor. Below this 'optical' surface that our observations probe, any amount of [H.sub.2]O ice and other volatile-rich materials might exist. All of the models we calculated require a very low-albedo, neutral color material to achieve the low geometric albedo that matches Hektor; we use elemental carbon. If elemental carbon is present on Hektor, it could be of organic or inorganic origin. By analogy, other D-type asteroids could achieve their red color, low albedo, and apparent absence of phyllosilicates from compositions similar to the models

We present spectra of Comet Hale-Bopp (C/1995 O1) covering the range 1.4-2.5 [[micro]meter] that were recorded when the comet was 7 AU from the Sun. These spectra show broad absorption features at 1.5 and 2.05 [[micro]meter]. While we recognize that much of the light from the comet is scattered from the coma, we show that some, but not all, of the absorption features can be matched by an intimate mixture of water ice and a low-albedo material such as carbon on the nucleus. Furthermore, the absence of the 1.65-[[micro]meter] absorption feature of crystalline ice suggests that the cometary ice was probably in an amorphous state at the time of these observations. An unidentified additional component may be required to account for the downward slope at the long-wavelength end of the spectrum.

Spectroscopic observations of Pluto reveal that the nitrogen ice on Pluto has a temperature of 40 Kelvin. This is higher than the temperature of nitrogen on the satellite Triton. The study indicates that the nitrogen on the planet is concentrated at the polar caps of the planet and absent in the equatorial region.

Seven narrow absorption bands are found in the moderate-resolution reflectance spectra of Mars. The reflectance spectra are obtained using the UKIRT 3.8 meter telescope in the 2.04 to 2.44 micrometer region of the seven bands, five of which are produced by the CO2 and CO present in the atmosphere of Mars. The other two bands may be caused by nonatmospheric absorptions and may be due to the reflection of sun's light.

Excess absorption features in the infrared transmission spectra of the surface of Pluto and Triton are due to the alkanes and other similar molecules frozen in the nitrogen. This indicates that Triton and Pluto have several types of organic compounds embedded in the N2 ices found on the surface. The absorption features caused by these organic compounds occur in the region lower then 4, 450 per centimeter of the spectra.

The near-infrared spectrum of the minor planet 5145 Pholus was investigated using near-infrared photometry. The results showed that 5145 Pholus possessed unusual red visual to infrared colors. The near-infrared spectrum featured a structure in the range of 1.8 to 2.4 micrometers. This possibly indicated that surface ices mixed with yet-unknown complex organic compounds existed on Pholus' surface.

Asteroid 1992 AD or 5145 has been posited to harbor organic materials due to spectroscopic analysis. A history of the asteroid and the reasons for research into its make-up is presented. When compared to other asteroids, it is only the organic material which explains the unique red color. Tholins and irradiated methane ice are posited to be the organic materials in question. Additional research is recommended.

The reflectance spectrum of Jupiter's sixth satellite, Himalia, is featureless in the wavelength region 1.95-2.50 [micro]m as seen at a spectral resolution of 0.005 [micro]m, with no absorptions deeper than a few percent. From model calculations we establish an upper limit of 10% by weight of [H.sub.2]O (30-[micro]m grains) mixed intimately in the soil of Himalia, or alternatively 0.3% of the surface covered by exposures of [H.sub.2]O ice spatially segregated from the darker soil. For C[H.sub.4] and C[O.sub.2] ices the upper limits in spatially segregated models, are both 0.3%. Key Words: ices; satellites of Jupiter; spectroscopy; surfaces, satellite.

We present new spectra of the leading and trailing hemispheres of Iapetus from 2.4 to 3.8 [micro]m. We have combined the leading hemisphere spectra with previous observations by others to construct a composite spectrum of the dark side (leading) hemisphere from 0.3 to 3.8 [micro]m. We review attempts to deduce the composition of the dark material from previously available spectrophotometry. None of them (numbering more than 20 million!) leads to a synthetic spectrum that matches the new data. An intimate mixture of water ice, amorphous carbon, and a nitrogen-rich organic compound (modeled here as Triton tholin) can fit the entire composite dark side spectrum. Observations in this spectral region have not revealed this mix of material on any other object observed thus far. We propose that this dark material may have originated on Titan, where atmospheric photochemistry has been producing nitrogen-rich organic compounds for 4.5 GY. [C] 2001 Academic Press Key Words: Iapetus; organic chemistry; satellites of Saturn; spectrophotometry.

We report the first detection and profile characterization of thermal emission from the surface of Venus at 0.85 and 0.90 [micro]m, observed in the first planetary spectrum acquired by the Visual-Infrared Mapping Spectrometer onboard the Cassini spacecraft en route to the Saturn system. The strength and shape of these two newly observed nightside emissions agree with theoretical predictions based on the strength of the strong emission observed at 1.01 [micro]m. These emissions, together with previously-reported surface emission features at 1.01, 1.10, and 1.18 [micro]m, potentially provide a new technique for remotely mapping the mineralogical composition of the venusian surface. [C] 2000 Academic Press Key Words: Venus; spectrophotometry; surfaces, Planets; Venus, surface; minerology.

We discuss the spectroscopic detection of [H.sub.2]O ice on Triton, evidenced by the broad absorption bands in the near infrared at 1.55 and 2.04 [micro]m. The detection of water ice on Triton reconfirms earlier preliminary studies (D. P. Cruikshank et al 1984, Icarus 58, 293-305). Although crystalline [H.sub.2]O ice has a distinctive spectral band at 1.65 [micro]m, and our new models slightly favor the presence of this phase, we cannot unambiguously determine whether Triton's water ice is crystalline or amorphous. Both phases might be present, and special conditions in the surface microstructure may affect the spectroscopic signature of water ice in such a way that crystalline ice is present and its 1.65 [micro]m spectral band is masked. Our spectra (1.87-2.5 [micro]m) taken at an interval of nearly 3.5 years do not show any significant changes that might relate to reports of changes in Triton's spectral reflectance (B. Buratti et al. 1999, Nature 397,219), or in Triton's surface pressure (J. L. Elliot et al. 1998, Nature 393, 765-767). [C] 2000 Academic Press Key Words: Triton; spectroscopy; ices; infrared observations; satellites of Neptune; surfaces, satellite.

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.sub.4], [N.sub.2], CO, and C[O.sub.2] molecules previously discovered. A detailed comparison of the frequencies of the [CH.sub.4] bands confidently indicated that this molecule exists in a diluted state in solid [Beta]-[N.sub.2]. Three new bands peaking at 5717, 5943, and 6480 [cm.sup.-1] (1.749, 1.683, and 1.543 [[micro]meter], respectively) were also observed. Laboratory experiments have shown that [C.sub.2][H.sub.6] isolated in solid [N.sub.2] fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, [C.sub.2][H.sub.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.sub.2][H.sub.2], [C.sub.2][H.sub.4], [C.sub.3][H.sub.8], N[H.sub.3], S[O.sub.2], [HC.sub.3]N, [CH.sub.3]OH, NO, N[O.sub.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.sub.2]: [CH.sub.4]: CO terrain, [N.sub.2]: [CH.sub.4] :CO consisting of a solid solution in which [N.sub.2] is the dominant molecule, and of a [H.sub.2]O + C[O.sub.2] terrain, composed of a mixture of pure crystalline [H.sub.2]O and C[O.sub.2] grains. The second representation is a three-region surface composed of a [N.sub.2] : [CH.sub.4]: CO terrain and two geographically separated [H.sub.2]O and C[O.sub.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.sub.4] is diluted in solid [Beta]-[N.sub.2], giving a high degree of confidence to the conclusion that the [N.sub.2]: [CH.sub.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.sub.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.sub.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.sub.4] ice can be present at the surface of Triton, thus explaining the high atmospheric [CH.sub.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.sub.2]O and C[O.sub.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.sup.-1] (1.749, 1.683, and 1.543 [[micro]meter]). Key Words: Triton; infrared spectroscopy; ices; surfaces; satellites.