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It has long been thought that much of a comet's nucleus is composed of ice. The detection of frozen water in the nucleus of Comet Hale-Bopp in September 1995 is the first direct observation of ice on a comet and this supports the aforementioned theory.

Billions of years ago comets and asteroids delivered enormous quantities of organic matter to Earth just about the time life arose here. Coincidence -- or cause and effect? EACH YEAR [...]

Geologic mapping has been used for over 200 years as a technique to synthesize a complicated surface into a more simplified product, identifying similar types of surface features, and placing them into a relative stratigraphy. Geomorphologic mapping has applied those principles to other terrestrial bodies throughout the solar system and has formed an important product set to understand these surfaces, plan future exploration, and conduct different scientific endeavors. We created a geomorphologic map of the New Horizonsencounter hemisphere of Pluto's binary companion, Charon. Ten primary geomorphologic unit categories were identified, covering approximately 35% of Charon's surface, and we used lower resolution data to speculate about other regions of Charon. Over 1,000 linear features were mapped, nearly 90% of them are tectonic in nature, and we use these to provide evidence of Charon being active in its past. Additionally, we placed the mapped features into a chronostratigraphic sequence, and we present a possible surface history for the body. The northern terrain typified by large crustal blocks is the oldest, having fractured early in Charon's history, and potentially similar blocks were submerged in a cryoflow of which the now solid surface of Vulcan Planitia is the remnant today. Planetary scientists will create geologic maps of a surface to perform different kinds of studies. Geologic maps identify features of different types and help to distill many different kinds of data into an easier to use format from which scientific investigations can be done. We have created such a map for Pluto's largest companion, Charon, imaged during the flyby of New Horizonsin July 2015. This geologic map covers approximately 35% of the best‐imaged region of Charon, and we mapped 16 different geologic units, over 1,000 linear features, and a variety of albedo features. We developed a chronology that places these features in time‐order, and we provide potential interpretations of the different features that we mapped. We found that Charon has potentially one of the most convincing examples of possible large cryoflow(s) of any heretofore imaged body in the solar system. We present a new geomorphologic map of New Horizons' encounter hemisphere of Charon, showing units, linear features, and albedo featuresWe introduce a chronostratigraphic system to place features and units in a historic sequence and contextWe interpret units and features in local, regional, and global context, pulling from knowledge of other solar system bodies

Describes what the space probe Voyager 2 might find when it flies by Uranus, Neptune and Pluto based on studies of the outer solar system with the help of ground-based telescopes. Orbits of planetary satellites; Surface composition; Theories of how the solar system evolved; Moons of Uranus; Neptune and its large satellite Triton; Methane ice on Pluto.

The article focuses on the Galilean satellites, the four largest moons of Jupiter, discovered by Galileo Galilei in 1610. These satellites are known by the names suggested by astronomer Simon Mayer, which are those of Jupiter's lovers in Greco-Roman mythology. They are Io, Europa, Ganymede and Callisto. Temperature measurements are used to provide information on the structure and composition of the surfaces of the satellites. The properties of the four satellites are described.

The New Horizonsencounter with the cold classical Kuiper Belt object 2014 MU69(informally named “Ultima Thule,” hereafter Ultima) on 1 January 2019 will be the first time a spacecraft has ever closely observed one of the free‐orbiting small denizens of the Kuiper Belt. Related to but not thought to have formed in the same region of the solar system as the comets that been explored so far, it will also be the most distant, and most primitive body yet visited by spacecraft. In this letter we begin with a brief overview of cold classical Kuiper Belt objects, of which Ultima is a prime example. We give a short preview of our encounter plans. We note what is currently known about Ultima from Earth‐based observations. We then review our expectations and capabilities to evaluate Ultima's composition, surface geology, structure, near space environment, small moons, rings, and the search for activity. The letter discusses the current state‐of‐knowledge for the cold classical Kuiper Belt object 2014 MU69, the target of a close encounter by NASA's New Horizon spacecraft on 1 January 2019. We also review our encounter plans and make broad speculative predictions as to might be revealed by our observations. New Horizons will fly to within 3,500 km of 2014 MU69(informally named “Ultima Thule”), acquiring images with pixel scale resolutions significantly better than 100 m/pixel). We plan to obtain topography derived from stereo imaging and body shape. Our spectroscopic observations potentially could detect H2O, CH4, N2, CH3OH, and NH3, depending on their brightness and abundance on the surface. We will also search for evidence of outgassing, small moons, and rings. This exploration will transform Kuiper Belt and Kuiper Belt object science from a purely astronomical regime, to a geological and geophysical regime, which radically changed paradigms when the same happened to asteroids and comets in past decades. New Horizons will fly to within 3,500 km of 2014 MU69, acquiring images with pixel scale resolutions significantly better than 100 m/pixelOur spectroscopic observations potentially could detect H2O, CH4, N2, CH3OH, and NH3, depending on their brightness and abundanceWe will evaluate Ultima's composition, surface geology, structure, near space environment, and the search for small moons, rings, and activity

We present models of the spectral reflectances of groups of outer Solar System objects defined primarily by their colors in the spectral region 0.4–1.2 mu;m, and which have geometric albedo ∼0.04 at wavelength 0.55 μm. Our models of the groups with the strongest reflectance gradients (reddest colors) use combinations of organic tholins. We test the hypothesis that metal-reddened igneous rock-forming minerals contribute to the red colors of Centaurs and KBOs by using the space-weathered lunar soil as one of the components of our models. We find that our models can admit the presence of moderate amounts of space-weathered (metal-reddened) minerals, but that they do not require this material to achieve the red colors of the reddest outer Solar System bodies. Our models with organic tholins are consistent with the results of other investigators.

We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3–3.6 μm, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no discernible 3-μm absorption band. Such a band would indicate the presence of OH− or H2O-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 SiO3), 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 H2O ice (30-μm 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 μm. Thus, the spectrum at 3 μm 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 H2O 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 presented here. Our models appear to demonstrate that organic solids are not required to match the red color and low albedos of D-type asteroids.

We present new spectra of the leading and trailing hemispheres of Iapetus from 2.4 to 3.8 μm. 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 μm. 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.

We discuss the spectroscopic detection of H2O ice on Triton, evidenced by the broad absorption bands in the near infrared at 1.55 and 2.04 μm. The detection of water ice on Triton reconfirms earlier preliminary studies (D. P. Cruikshank et al. 1984, Icarus58, 293–305). Although crystalline H2O ice has a distinctive spectral band at 1.65 μm, 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 μm spectral band is masked. Our spectra (1.87–2.5 μm) 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, Nature397, 219), or in Triton's surface pressure (J. L. Elliot et al. 1998, Nature393, 765–767).

A near-infrared spectrum of Neptune's satellite Nereid was obtained at the Keck Observatory on 1997 May 1. The spectrum shows two absorptions near 1.54 and 2.03 μm and is well matched by a synthetic spectrum of an intimate mix of low-temperature, particulate water ice and dark, blue-colored material. Comparisons of Nereid's spectrum with those of other objects such as the Centaurs 1997 CU26and 5145 Pholus, and Neptune's largest regular satellite Proteus, admit the possibility that Nereid is a primordial satellite of Neptune (and perhaps its only remaining one) rather than a captured object, but more data are needed to rule out one scenario or the other.

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 CH4, N2, CO, and CO2molecules previously discovered. A detailed comparison of the frequencies of the CH4bands confidently indicated that this molecule exists in a diluted state in solid β-N2. 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 C2H6isolated in solid N2fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6may 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 (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).

We present moderate-resolution (λ/Δλ = 300 to 370) reflectance spectra of Mars from 2.04 to 2.44 μm that were obtained at UKIRT during the 1993 opposition. Seven narrow absorption features were detected and found to have a Mars origin. By comparison with solar and Mars atmospheric spectra, five of these features were attributed all or in part to Mars atmospheric CO2or CO (2.052 ± 0.003, 2.114 ± 0.002, 2.150 ± 0.003, 2.37,1 ± 0.001, and 2.357 ± 0.002 ±m). Two of the bands (2,331 ± 0.001 and 2.357± 0.002 μm) appear to have widths and depths that are consistent with additional, nonatmospheric absorptions, although a solar contribution cannot be entirely ruled out. Two other weak hands centered at 2.278 ± 0.002 and 2.296 ± 0.002 μm may be at least partially mineralogic in origin. The data provide no conclusive identification of the mineralogy responsible for these absorption features. However, examination of terrestrial spectral libraries and previous moderate spectral resolution mineral studies indicates that the most likely origin of these features is either (bi)carbonate or (bi)sulfate anions in framework silicates or (Fe, Mg)-OH bends in sheet silicates. If the bands are caused by phyllosilicate minerals, then an explanation must be found for the extremely narrow widths of the cation-OH features in the Mars spectra as compared to terrestrial minerals.

We present near-infrared photometry which confirms that asteroid 5145 Pholus has unusually red visual to infrared colors. Its near-infrared spectrum exhibits structure between 1.8 and 2.4 µm which may indicate the presence of surface ices mixed with complex organic compounds (e.g., tholins) of some as yet unknown type. Copyright 1993, 1999 Academic Press

The spectral geometric albedos of Charon and Pluto are derived at near-infrared wavelengths (1.4–2.5 μm) from measurements obtained in 1987. Comparisons of these to theoretical calculations are used to place constraints on the identity and relative abundances of surface ices on Charon. These comparisons suggest that widespread regions of pure CH4ice do not occur on Charon and that if CH4is abundant on Charon then it is large grained (≈5 mm) and is likely mixed at the granular level with H2O ice, and possibly CO2ice.

The infrared transmission spectra and photochemical behavior of various organic compounds isolated in solid N2ices, appropriate for applications to Triton and Pluto, are presented. It is shown that excess absorption in the surface spectra of Triton and Pluto, i.e., absorption not explained by present models incorporating molecules already identified on these bodies (N2, CH4, CO, and CO2), that starts near 4450 cm-1(2.25 μm) and extends to lower frequencies, may be due to alkanes (CnH2n+2) and related molecules frozen in the nitrogen. Branched and linear alkanes may be responsible. Experiments in which the photochemistry of N2:CH4and N2:CH4:CO ices was explored demonstrate that the surface ices of Triton and Pluto may contain a wide variety of additional species containing H, C, O, and N. Of these, the reactive molecule diazomethane, CH2N2, is particularly important since it may be largely responsible for the synthesis of larger alkanes from CH4and other small allkanes. Diazomethane would also be expected to drive chemical reactions involving organics in the surface ices of Triton and Pluto toward saturation, i.e., to reduce multiple CC bonds.

Previous work by K. A. Tryka et al. (Science 261, 751-754, 1993) has shown that the profile of the 2.148-µm band of solid nitrogen can be used as a "thermometer" and determined the temperature of nitrogen ice on Triton to be 38⁺²_-1. Here we reevaluate that data and refine the temperature value to 38 ± 1 K. Applying the same technique to Plato we determine that the temperature of the N₂ ice on that body is 40 ± 2 K. Using this result we have created a nonisothermal flux model of the Pluto-Charon system. The model treats Pluto as a body with symmetric N₂ polar caps and an equatorial region devoid of N₂. Comparison with the infrared and millimeter flux measurements shows that the published fluxes are consistent with models incorporating extensive N₂ polar caps (down to ±15° or ±20° latitude) and an equatorial region with a bolometric albedo ≤0.2. Copyright 1994, 1999 Academic Press

This paper reviews the physical nature of the satellites of the planets, excluding the Moon but including the rings of Saturn. Emphasis is placed on the best studied objects: Titan, Phobos and Deimos, the four Galilean satellites (Io, Europa, Ganymede, and Callisto), and the rings of Saturn.

New spectra of Jupiter in the region 0.93–1.63 µ are presented. Laboratory comparisons of spectra of NH3 and CH4 permit estimates of the absorbing pathlength for various bands of these two gases. Abundances in a single transmission through the Jupiter atmosphere, above the mean reflecting level, vary from 10 to 100 m-atm for CH4 and from 0.2–5 m-atm for NH3, depending on the bands considered. Upper limits for other gases are derived from new laboratory spectra and comparison with the Jupiter spectra presented herein. These are as follows: C2H2<2 m-atm, H2S<0.25 m-atm, HCN<0.05 m-atm, CH3NH2<0.02 m-atm. A table summarizing the chemical composition of Jupiter's atmosphere is presented.

Halley's comet (1982 i) is now bright enough for broadband photometry by large infrared telescopes1–3. We report here on photometry of the comet in the B (0.44 µm), J (0.55 µm), V (1.25 µm) and K (2.2 µm) broadband filters during a time when the coma was very weak and presumed to contribute negligibly to the broadband photometry. The V – J and J – K colours suggest that the colour of the nucleus of Halley's comet is similar to that of the D-type asteroids, which in turn suggests that the surface of the nucleus is of albedo < 0.1.

The five known satellites of Uranus are too small and distant to permit direct measurement of their diameters. Consequently, products of the masses of the satellites, and estimates of their bulk densities and surface albedos have been used in discussions of their probable dimensions in the absence of measurements. The presence of water ice on the surfaces of Ariel, Umbriel, Titania and Oberon has been established spectrophotometrically1–4and the brightnesses of the satellites have been measured photoelectrically5. Determination of the masses of the uranian satellites depends on observations of perturbations of the satellite orbits and results chiefly in upper limits to the products of the masses of various pairs of satellites6,7. The new diameter measurements reported here were made using the photometric/radiometric technique of diameter determination. This widely used technique has enabled the measurement of the diameters and albedos of approximately 250 asteroids and many planetary satellites. A recent recalibration of the technique using independent measurements of the diameters of three Solar System objects results in absolute accuracies of better than 5%. Our new albedo measurements show that Umbriel, Titania and Oberon are similar in albedo to J4 Callisto while Ariel is very similar in albedo to S7 Hyperion; the diameters of all four satellites are similar to those of the large, icy saturnian satellites S5 Rhea and S8 Iapetus.

Triton, the major satellite of Neptune, and Pluto, the outermost planet, are too small and distant to permit direct measurement of their size, and indirect arguments must be used to estimate diameter and albedo. These two objects invite a comparative study because they have essentially the same brightness (absolute visual magnitude V(1, 0)≃ −1) and current distance from the Sun (30 AU). In addition, the presence of methane frost on the surface, and of a possible methane atmosphere, have been detected for both objects from IR spectroscopy1–4. On the basis of these similarities, Triton and Pluto were selected as targets for possible detection of thermal IR emission. We report here new upper limits to the thermal IR emission from these objects that permit us to set significant upper limits on their diameters and to demonstrate that both are high albedo objects. These results clearly exclude the possibility that Triton is the largest planetary satellite, and they are consistent with the small size of Pluto deduced from other data.

A response by Dale P. Cruikshank and David Morrison to a letter to the editor about their article "The Galilean Satellites of Jupiter" in the May 1976 issue is presented.

The current series of mutual eclipses between Pluto and its satellite, Charon, provides a very powerful means of probing the most distant known planet in our Solar System. Observations from 1985 and 1986 have already dramatically improved our knowledge of the sizes, albedos, and the orbital parameters of the system1–3. One experiment that we had been waiting to perform is to observe Pluto during a total eclipse of its satellite. This geometry provides a direct means to study the planet without contamination from the satellite. Once the spectrum of the planet is known, it is then possible to subtract it from the spectrum of the planet plus satellite and thus discern the properties of the satellite. Here we present new spectra of the Pluto–Charon system taken just before and during a total eclipse of the satellite. From these data we have extracted the spectrum of the satellite, Charon, which reveals the spectral signature of water ice. There is no evidence for any methane or ammonia frost on the surface of Charon. This observation places important constraints on the composition and origin of this planetary system.

A GROWING body of evidence leads to the conclusion that luminescence effects occur on the lunar surface. Large intensity fluctuations on the Moon have been observed photographically by Kopal and Rackham1, spectroscopically by Spinrad2, and photometrically and polarimetrically by Gehrels et al.3. If the luminescence phenomena are induced by the solar wind (see Nash4), we might do well to look to other bodies in the solar system, which are similarly unprotected by an atmosphere, for evidence of luminescence.