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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.

Cometary bodies are acknowledged to contain some of the most pristine matter in the Solar System, including ices and minerals. Certain number of previously unidentified spectral emission features detected in comets can be explained as emission by hydrocarbon molecules enclosed in a Shpolskii matrix and forming frozen hydrocarbon particles. UV-induced photoluminescence spectra of several self-organized molecules exhibit emission lines coincident with unidentified cometary lines, and open the possibility of the presence of this complex organic as components of the pristine organic inventory of comets. Complex organic was detected also in three satellites of Saturn. We describe in this paper results of our investigation of complex organic of the small bodies and present new approaches and hypotheses.

Detecting heat from minor planets in the outer solar system is challenging,yet it is the most efficient means for constraining the albedos and sizes ofKuiper Belt Objects (KBOs) and their progeny, the Centaur objects. Thesephysical parameters are critical, e.g., for interpreting spectroscopic data,deriving densities from the masses of binary systems, and predictingoccultation tracks. Here we summarize Spitzer Space Telescope observations of47 KBOs and Centaurs at wavelengths near 24 and 70 microns. We interpret themeasurements using a variation of the Standard Thermal Model (STM) to derivethe physical properties (albedo and diameter) of the targets. We also summarizethe results of other efforts to measure the albedos and sizes of KBOs andCentaurs. The three or four largest KBOs appear to constitute a distinct classin terms of their albedos. From our Spitzer results, we find that the geometricalbedo of KBOs and Centaurs is correlated with perihelion distance (darkerobjects having smaller perihelia), and that the albedos of KBOs (but notCentaurs) are correlated with size (larger KBOs having higher albedos). We alsofind hints that albedo may be correlated with with visible color (forCentaurs). Interestingly, if the color correlation is real, redder Centaursappear to have higher albedos. Finally, we briefly discuss the prospects forfuture thermal observations of these primitive outer solar system objects.

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

Author(s): Roger N. Clark (corresponding author) [1]; Robert H. Brown [2]; Ralf Jaumann [3]; Dale P. Cruikshank [4]; Robert M. Nelson [5]; Bonnie J. Buratti [5]; Thomas B. McCord [6]; [...]

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.