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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. Giant icy volcanos (cryovolcanos) on Pluto are unique in the imaged solar system and provide evidence for unexpected, active geology late in Pluto's history. [ABSTRACT FROM AUTHOR]

Abstract: Material of low geometric albedo () is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn''s satellite Iapetus were obtained with the Visible–Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite''s surface of ∼65 km. The spectral region 3–3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the Cing mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to -CH2- Stertching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust. [Copyright &y& Elsevier]

Abstract: We present 1.25–19 μm infrared spectra of pure solid CH4 and H2O/CH4=87, 20, and 3 solid mixtures at temperatures from 15 to 150 K. We compare and contrast the absorptions of CH4 in solid H2O with those of pure CH4. Changes in selected peak positions, profiles, and relative strength with temperature are presented, and absolute strengths for absorptions of CH4 in solid H2O are estimated. Using the two largest () and () near-IR absorptions of CH4 at 2.324 and 2.377 μm (4303 and 4207 cm−1), respectively, as examples, we show that peaks of CH4 in solid H2O are at slightly shorter wavelength (higher frequency) and broader than those of pure solid CH4. 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 H2O are irreversible. It is to be hoped that these observations of changes in the positions, profiles, and relative intensities of CH4 absorptions with concentration and temperature will be of use in understanding spectra of icy outer Solar System bodies. [Copyright &y& Elsevier]

Abstract: We present near-IR spectra of solid CO2 in H2O and CH3OH, and find they are significantly different from that of pure solid CO2. Peaks not present in either pure H2O or pure CO2 spectra become evident when the two are mixed. First, the putative theoretically forbidden CO2 () overtone near 2.134 μm (4685 cm−1), that is absent from our spectrum of pure solid CO2, is prominent in the spectra of H2O/CO2=5 and 25 mixtures. Second, a 2.74-μm (3650 cm−1) dangling OH feature of H2O (and a potentially related peak at 1.89 μm) appear in the spectra of CO2–H2O ice mixtures, but are probably not diagnostic of the presence of CO2. Other CO2 peaks display shifts in position and increased width because of intermolecular interactions with H2O. Warming causes some peak positions and profiles in the spectrum of a H2O/CO2=5 mixture to take on the appearance of pure CO2. Absolute strengths for absorptions of CO2 in solid H2O are estimated. Similar results are observed for CO2 in solid CH3OH. Since the CO2 () overtone near 2.134 μm (4685 cm−1) is not present in pure CO2 but prominent in mixtures, it may be a good observational (spectral) indicator of whether solid CO2 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. [Copyright &y& Elsevier]

Abstract: The programs of observations of Solar System bodies conducted in the first year of the operation of the Spitzer Space Telescope as part of the Guaranteed Observing Time allocations are described. Initial results include the determination of the geometric albedos of a number of Kuiper Belt objects and Centaurs from observations of their flux densities at 24 and 70 μm, and the detection of emission bands in the spectra of several distant asteroids (Trojans) around 10 and 25 μm. The 10 Kuiper Belt objects observed to date have geometric albedos in the range 0.08–0.15, significantly higher than the earlier estimated 0.04. An additional KBO [(55565) 2002 AW197] has a geometric albedo of 0.17±0.03. The emission bands in the asteroid spectra are indicative of silicates, but specific minerals have not yet been identified. The Centaur/comet 29P/Schwassmann–Wachmann 1 has a nucleus surface geometric albedo of 0.025±0.01 and its dust production rate was calculated from the properties of the coma. Several other investigations are in progress as the incoming data are processed and analyzed. [Copyright &y& Elsevier]

Abstract: The red colors of many solid bodies in outer Solar System may be caused by tholins, which are refractory organic complexes, incorporated in their surface materials. Tholins synthesized in the laboratory are shown to match the colors of these bodies when their optical properties are used in rigorous scattering models. We review recent successes in modeling the spectra of icy outer Solar System bodies with tholins as the coloring agents. New work on the systematic laboratory synthesis and analysis of tholins made by cold plasma discharge in mixtures of gaseous CH4/N2 shows that the composition of the tholin depends strongly on the pressure in the reaction chamber, and only weakly on the mixing fraction of CH4 relative to N2. In tholins made at high pressure (e.g., 23hPa) the abundance of aliphatic hydrocarbons is greater and the abundance of aromatic hydrocarbons is less than in tholins made at low pressure (e.g., 0.13hPa). Tholins made at low deposition pressures show a greater abundance of N–H bonds. [Copyright &y& Elsevier]

The origin of Phoebe, which is the outermost large satellite of Saturn, is of particular interest because its inclined, retrograde orbit suggests that it was gravitationally captured by Saturn, having accreted outside the region of the solar nebula in which Saturn formed. By contrast, Saturn's regular satellites (with prograde, low-inclination, circular orbits) probably accreted within the sub-nebula in which Saturn itself formed. Here we report imaging spectroscopy of Phoebe resulting from the Cassini-Huygens spacecraft encounter on 11 June 2004. We mapped ferrous-iron-bearing minerals, bound water, trapped CO2, probable phyllosilicates, organics, nitriles and cyanide compounds. Detection of these compounds on Phoebe makes it one of the most compositionally diverse objects yet observed in our Solar System. It is likely that Phoebe's surface contains primitive materials from the outer Solar System, indicating a surface of cometary origin. [ABSTRACT FROM AUTHOR]

Reflectance spectroscopy of Solar System bodies provides a rich source of information on their compositions (minerals, ices, metals, and macromolecular carbon-bearing materials). Models calculated with radiative transfer theories for the spectral distribution of diffusely scattered sunlight from planetary surfaces yield information on the compositions, abundances, physical states, layering, and particle microstructure of those surfaces. We discuss and evaluate the scattering theories of Hapke and Shkuratov that are widely used for modeling the reflectance spectra and color data for Kuiper Belt objects, Centaur objects, and other airless bodies in the Solar System. Both theories yield good models of the reflectance spectrum of Centaur 5145 Pholus using five components (ices, carbon, a silicate mineral, and a complex organic material), although the derived abundances differ widely. To cite this article: D.P. Cruikshank et al., C. R. Physique 4 (2003). [Copyright &y& Elsevier]

Telescopic data combined with data gathered by spacecraft has brought worlds as distant as Pluto and the Kuiper belt objects under increasingly close scrutiny. Of particular interest here is the progress that has been made in our understanding of the properties of the satellites of the giant planets. As such, the purpose of this chapter is to review the present understanding of the nature of icy surfaces in the outer solar system, the ongoing processes that affect the composition, distribution, and physical state of volatiles on icy surfaces, and new techniques for probing the mysteries of the origin and evolution of icy bodies in the Solar System. [ABSTRACT FROM AUTHOR]

The close encounters of the Pluto–Charon system and the Kuiper Belt object Arrokoth (formerly 2014 MU69) by NASA's New Horizons spacecraft in 2015 and 2019, respectively, have given new perspectives on the most distant planetary bodies yet explored. These bodies are key indicators of the composition, chemistry, and dynamics of the outer regions of the Solar System's nascent environment. Pluto and Charon reveal characteristics of the largest Kuiper Belt objects formed in the dynamically evolving solar nebula inward of ~30 AU, while the much smaller Arrokoth is a largely undisturbed relic of accretion at ~45 AU. The surfaces of Pluto and Charon are covered with volatile and refractory ices and organic components, and have been shaped by geological activity. On Pluto, N2, CO and CH4 are exchanged between the atmosphere and surface as gaseous and condensed phases on diurnal, seasonal and longer timescales, while Charon's surface is primarily inert H2O ice with an ammoniated component and a polar region colored with a macromolecular organic deposit. Arrokoth is revealed as a fused binary body in a relatively benign space environment where it originated and has remained for the age of the Solar System. Its surface is a mix of CH3OH ice, a red-orange pigment of presumed complex organic material, and possibly other undetected components. [ABSTRACT FROM AUTHOR]

Reports on studies of the physical properties and surface composition of bodies in the outer solar system, which can yield important clues to conditions that existed in the early solar system. Detection of water ice on the surface of Centaurs and the Kuniper belt objects (KBO), which orbit beyond Neptune; Methods used in analyzing the spectra of the Centaur 1997 CU26; Determination that the Kuniper belt is the source of the Centaurs.

Details near-infrared spectroscopic observations of the Kuiper belt object 1993SC, and the implications that these observations have for its surface composition. The finding of strongly red continuum reflectance and several prominent infrared absorption features; The suggestion of the presence of a simple hydrocarbon ice on 1993SC; The possible presence of more complex hydrocarbons.

Highlights • Deuterium and carbon 13 are detected in the rings and on satellite surfaces in the Saturn system. • New methods are presented for deriving isotopic ratios from reflectance spectra of solids. • The D/H ratio of the water in Saturn's rings and icy satellites except Phoebe is close to terrestrial bulk Earth values. • Phoebe's D/H is the highest value yet measured in the Solar system implying an origin in the cold outer Solar System beyond Saturn. • Phoebe also has a high 13C/12C much greater than terrestrial, consistent with an origin in the cold outer Solar System. Abstract Isotopic ratios have long been used to learn about physical processes acting over a wide range of geological environments, and in constraining the origin and/or evolution of planetary bodies. We report the spectroscopic detection of deuterium in Saturn's rings and satellites, and use these measurements to determine the (D/H) ratios in their near-surface regions. Saturn's moons, Phoebe and Iapetus, show a strong signature of CO 2 and the 13C component of this molecule is detected and quantified. Large averages of spectra obtained by the Cassini Visual and Infrared Mapping Spectrometer, VIMS, were computed for the rings and icy satellites. The observed intensities of the infrared absorptions in H 2 O and CO 2 and their isotopes were calibrated using laboratory data and radiative transfer models to derive the D/H and 13C/12C ratios. We find that the D/H in Saturn's rings and satellites is close to the Vienna Standard Mean Ocean Water (VSMOW) and bulk Earth (4% lower than VSMOW) value except for Phoebe, which is 8.3 times the VSMOW value. This is the highest value for any Solar-System surface yet measured, and suggests that Phoebe formed from material with a different D/H ratio than the other satellites in the Saturn system. Phoebe's 13C/12C ratio is also unusual: 4.7 times greater than terrestrial, and greater than values measured for the interstellar medium and the galactic center. The high 13C abundance in the CO 2 suggests that Phoebe was never warm enough for the large D/H ratio in its surface to have originated by evaporative fractionation of its water ice (e.g., from heating in the inner Solar System before its eventual capture by Saturn). We also report the detection of a probable O-D stretch absorption due to OD in minerals on Phoebe at 3.62 μm. This absorption is not detected on other Saturnian satellites. Stronger signatures of bound water absorptions are found in the dark material of Iapetus and we report a new detection of bound water at 1.9 μm. The position of this absorption matches that seen in spectra of hydrated iron oxides but does not match absorptions seen in spectra of tholins. Despite the strong bound water signature in the Iapetus dark material, no 3.62 μm OD absorption is seen in the spectra, further indicating the high deuterium level on Phoebe is unusual. As such, it is likely that Phoebe originated in a colder part of the outer Solar System, relative to the prevailing temperatures at Saturn's distance from the Sun. [ABSTRACT FROM AUTHOR]

Radiation processing of the surface ices of outer solar system bodies may result in the production of new chemical species even at low temperatures. Many of the smaller, more volatile molecules that are likely produced by the photolysis of these ices have been well characterized by laboratory experiments. However, the more complex refractory material formed in these experiments remains largely uncharacterized. In this work, we present a series of laboratory experiments in which low-temperature (15-20 K) N2:CH4:CO ices in relative proportions 100:1:1 are subjected to UV irradiation, and the resulting materials are studied with a variety of analytical techniques including infrared spectroscopy, X-ray absorption near-edge structure spectroscopy, gas chromatography coupled with mass spectrometry, and high-resolution mass spectroscopy. Despite the simplicity of the reactants, these experiments result in the production of a highly complex mixture of molecules from relatively low-mass volatiles (tens of daltons) to high-mass refractory materials (hundreds of daltons). These products include various carboxylic acids, nitriles, and urea, which are also expected to be present on the surface of outer solar system bodies, including Pluto and other transneptunian objects. If these compounds occur in sufficient concentrations in the ices of outer solar system bodies, their characteristic bands may be detectable in the near-infrared spectra of these objects. [ABSTRACT FROM AUTHOR]