D. P. Cruikshank

Oak Ridge Associated Universities, オーク・リッジ, Tennessee, United States

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Publications (677)1983.44 Total impact

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    Full-text · Dataset · Nov 2015
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    Full-text · Dataset · Nov 2015
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    ABSTRACT: The goal of this project is to study the properties of H2O ice in the environment of the Saturn satellites and in particular to measure the relative amounts of crystalline and amorphous H2O ice in and around two craters on Rhea. The craters are remnants of cataclysmic events that, by raising the local temperature, melted the ice, which subsequently crystallized. Based on laboratory experiments it is expected that, when exposed to ion bombardment at the temperatures typical of the Saturn satellites, the crystalline structure of the ice will be broken, resulting in the disordered, amorphous phase. We therefore expect the ice in and around the craters to be partially crystalline and partially amorphous.
    No preview · Article · Oct 2015 · Icarus
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    ABSTRACT: Aims. The goal of this work is to investigate the composition of the surface of (50000) Quaoar and its spatial variability. Methods. We present new continuous spectra from the visible to near-IR (0.3-2.3 μm) obtained with the X-Shooter instrument at the VLT-ESO at four different epochs on the surface of Quaoar. The data represent the highest spectral resolution data ever obtained for this object and the first near-IR dataset acquired in a single exposure over the entire wavelength range. They are complemented by previously published photometric observations obtained in the near-IR (3.6, 4.5 μm) with the Spitzer Space Telescope, which provide an extra set of constraints in the model calculation. Spectral modelling was performed for the entire wavelength range by means of a code based on the Shkuratov radiative transfer formulation and of an updated value of albedo obtained from recent Herschel observations. Results. We obtained compositional information for different observed areas that can cover about 40% of the assumed rotational period of 8.84 h. Our analysis helps to prove the presence of CH4 and C2H6, as previously reported, along with indications of the possible presence of NH3·H2O. New evidence of N2 is inferred from the shift in the CH4 bands. The albedo at the two Spitzer bands suggests there may be CO diluted in N2, and CO2 for one of the surface locations. Conclusions. The spectral similarities indicate the overall homogeneity of the surface composition of one hemisphere of Quaoar, while some subtle variations are apparent when modelling. The presence of NH3·H2O would support the idea that Quaoar's surface may be relatively young.
    Full-text · Article · Oct 2015 · Astronomy and Astrophysics
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    ABSTRACT: Radiation processing of the surface ices of outer Solar System bodies may be an important process for the production of complex chemical species. The refractory materials resulting from radiation processing of known ices are thought to impart to them a red or brown color, as perceived in the visible spectral region. In this work, we analyzed the refractory materials produced from the 1.2-keV electron bombardment of low-temperature N2-, CH4-, and CO-containing ices (100:1:1), which simulates the radiation from the secondary electrons produced by cosmic ray bombardment of the surface ices of Pluto. Despite starting with extremely simple ices dominated by N2, electron irradiation processing results in the production of refractory material with complex oxygen- and nitrogen-bearing organic molecules. These refractory materials were studied at room temperature using multiple analytical techniques including Fourier-transform infrared spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and gas chromatography coupled with mass spectrometry (GC-MS). Infrared spectra of the refractory material suggest the presence of alcohols, carboxylic acids, ketones, aldehydes, amines, and nitriles. XANES spectra of the material indicate the presence of carboxyl groups, amides, urea, and nitriles, and are thus consistent with the IR data. Atomic abundance ratios for the bulk composition of these residues from XANES analysis show that the organic residues are extremely N-rich, having ratios of N/C ∼ 0.9 and O/C ∼ 0.2. Finally, GC-MS data reveal that the residues contain urea as well as numerous carboxylic acids, some of which are of interest for prebiotic and biological chemistries. © 2015. The American Astronomical Society. All rights reserved..
    No preview · Article · Oct 2015 · The Astrophysical Journal
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    ABSTRACT: The Pluto system was recently explored by NASA’s New Horizons spacecraft, making closest approach on 14 July 2015. Pluto’s surface displays diverse landforms, terrain ages, albedos, colors, and composition gradients. Evidence is found for a water-ice crust, geologically young surface units, surface ice convection, wind streaks, volatile transport, and glacial flow. Pluto’s atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars. Pluto’s diverse surface geology and long-term activity raise fundamental questions about how small planets remain active many billions of years after formation. Pluto’s large moon Charon displays tectonics and evidence for a heterogeneous crustal composition; its north pole displays puzzling dark terrain. Small satellites Hydra and Nix have higher albedos than expected.
    Full-text · Article · Oct 2015 · Science
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    ABSTRACT: Context. During the last 30 years the surface of Pluto has been characterized, and its variability has been monitored, through continuous near-infrared spectroscopic observations. But in the visible range only few data are available. Aims. The aim of this work is to define the Pluto's relative reflectance in the visible range to characterize the different components of its surface, and to provide ground based observations in support of the New Horizons mission. Methods. We observed Pluto on six nights between May and July 2014, with the imager/spectrograph ACAM at the William Herschel Telescope (La Palma, Spain). The six spectra obtained cover a whole rotation of Pluto (Prot = 6.4 days). For all the spectra we computed the spectral slope and the depth of the absorption bands of methane ice between 0.62 and 0.90 $\mu$m. To search for shifts of the center of the methane bands, associated with dilution of CH4 in N2, we compared the bands with reflectances of pure methane ice. Results. All the new spectra show the methane ice absorption bands between 0.62 and 0.90 $\mu$m. The computation of the depth of the band at 0.62 $\mu$m in the new spectra of Pluto, and in the spectra of Makemake and Eris from the literature, allowed us to estimate the Lambert coefficient at this wavelength, at a temperature of 30 K and 40 K, never measured before. All the detected bands are blue shifted, with minimum shifts in correspondence with the regions where the abundance of methane is higher. This could be indicative of a dilution of CH4:N2 more saturated in CH4. The longitudinal and secular variations of the parameters measured in the spectra are in accordance with results previously reported in the literature and with the distribution of the dark and bright material that show the Pluto's albedo maps from New Horizons.
    No preview · Article · Sep 2015 · Astronomy and Astrophysics
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    ABSTRACT: We present an analysis of the colors available for seven trans-neptunian objects (TNOs) and three centaurs among the reddest known, aimed at characterizing their surface chemical properties. In particular we seek to obtain evidence in support of the proposed correlation between the visible coloration of the surface of TNOs and their surface compositions (Brown, M.E., Schaller, E.L., Fraser, W.C. [2011]. Astrophys. J. 739, L60).
    No preview · Article · May 2015 · Icarus
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    ABSTRACT: The surface of Pluto as it is understood on the eve of the encounter of the New Horizons spacecraft (mid-2015) consists of a spatially heterogeneous mix of solid N2, CH4, CO, C2H6, and an additional component that imparts color, and may not be an ice. The known molecular ices are detected by near-infrared spectroscopy. The N2 ice occurs in the hexagonal crystalline β-phase, stable at T > 35.6 K. Spectroscopic evidence for wavelength shifts in the CH4 bands attests to the complex mixing of CH4 and N2 in the solid state, in accordance with the phase diagram for N2 + CH4. Spectra obtained at several aspects of Pluto's surface as the planet rotates over its 6.4-day period show variability in the distribution of CH4 and N2 ices, with stronger CH4 absorption bands associated with regions of higher albedo, in correlation with the visible rotational light curve. CO and N2 ice absorptions are also strongly modulated by the rotation period; the bands are strongest on the anti-Charon hemisphere of Pluto. Longer term changes in the strengths of Pluto's absorption bands occur as the viewing geometry changes on seasonal time-scales, although a complete cycle has not been observed. The non-ice component of Pluto's surface may be a relatively refractory material produced by the UV and cosmic-ray irradiation of the surface ices and gases in the atmosphere, although UV does not generally penetrate the atmospheric CH4 to interact with the surface. Laboratory simulations indicate that a rich chemistry ensues by the irradiation of mixtures of the ices known to occur on Pluto, but specific compounds have not yet been identified in spectra of the planet. Charon's surface is characterized by spectral bands of crystalline H2O ice, and a band attributed to one or more hydrates of NH3. Amorphous H2O ice may also be present; the balance between the amorphization and crystallization processes on Charon remains to be clarified. The albedo of Charon and its generally spatially uniform neutral color indicate that a component, not yet identified, is mixed in some way with the H2O and NH3·nH2O ices. Among the many known small bodies in the transneptunian region, several share characteristics with Pluto and Charon, including the presence of CH4, N2, C2H6, H2O ices, as well as components that yield a wide variety of surface albedo and color. The New Horizons investigation of the Pluto-Charon system will generate new insight into the physical properties of the broader transneptunian population, and eventually to the corresponding bodies expected in the numerous planetary systems currently being discovered elsewhere in the Galaxy.
    No preview · Article · Dec 2014 · Icarus
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    ABSTRACT: We propose Warm Spitzer/IRAC GO observations of the Pluto system, as part of a worldwide observing campaign in support of the NASA New Horizons. The aim of this proposal is to characterize the surface heterogeneity of Pluto through photometric observations at the 3.6 and 4.5 µm IRAC channels as close in time to the New Horizons encounter date of 14 July 2014 as possible. We ask for observations at 18 longitudes (~ each 20 o). The surface of Pluto, formed by patches of CH4, N2 and CO, is a dynamic and variable system, with a timescale on the order of months to years. Spitzer holds a unique place in the solar system to observe Pluto, above the Earth's atmosphere in a stable Earth-trailing environment. Relative differences in the albedo of Pluto in ch1 and ch2 is an effective tool to study the different mixing ratios of the ices on the surface. This is also promising for the search of other candidate materials that have not yet been identified in the vis/NIR, e.g CO2 that has its fundamental absorption band in the wavelength range of ch2. In 2004, under a Spitzer program during the cryogenic mission (PI. Cruikshank), low-resolution light curves were obtained at 8 different longitudes. In 2014, under a Cycle 10 program (PI. Pinilla-Alonso) we obtained data at ch1 and ch2 at 18 longitudes. The analysis of these data shows clear indications not only of surface heterogeneity, but also on possible secular variations. These data are under analysis and show a great potential for the mapping of volatiles ices all over the surface of Pluto. Two parallel observational programs are in progress involving people in this group, one of near-infrared spectroscopy (~0.9-2.5 µm) at the NASA Infrared Telescope Facility (PI. Grundy) and one of visible spectroscopy (0.4-0.95 µm) at WHT, La Palma (PI. Pinilla-Alonso). The combination of these datasets, covering different wavelength regions, provide unique and complementary information, and is very important in deriving full benefit of data from the NASA New Horizons spacecraft
    No preview · Article · Nov 2014
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    ABSTRACT: The spectral properties and thermal behavior of Saturn’s rings are determined from a dataset of ten radial mosaics acquired by Cassini–VIMS (Visual and Infrared Mapping Spectrometer) between October 29th 2004 and January 27th 2010 with phase angle ranging between 5.7° and 132.4° and elevation angles between −23.5° and 2.6°. These observations, after reduction to spectrograms, e.g. 2D arrays containing the VIS–IR (0.35–5.1 μm) spectral information versus radial distance from Saturn (from 73.500 to 141.375 km, 400 km/bin), allow us to compare the derived spectral and thermal properties of the ring particles on a common reference. Spectral properties: rings spectra are characterized by an intense reddening at visible wavelengths while they maintain a strong similarity with water ice in the infrared domain. Significant changes in VIS reddening, water ice abundance and grain sizes are observed across different radial regions resulting in correlation with optical depth and local structures. The availability of observations taken at very different phase angles allows us to examine spectrophotometric properties of the ring’s particles. When observed at high phase angles, a remarkable increase of visible reddening and water ice band depths is found, probably as a consequence of the presence of a red-colored contaminant intimately mixed within water ice grains and of multiple scattering. At low phases the analysis of the 3.2–3.6 μm range shows faint spectral signatures at 3.42–3.52 μm which are compatible with the CH2 aliphatic stretch. The 3.29 μm PAH aromatic stretch absorption is not clearly detectable on this dataset. VIMS results indicate that ring particles contain about 90–95% water ice while the remaining 5–10% is consistent with different contaminants like amorphous carbon or tholins. However, we cannot exclude the presence of nanophase iron or hematite produced by iron oxidation in the rings tenuous oxygen atmosphere, intimately mixed with the ice grains. Greater pollution caused by meteoritic material is seen in the C ring and Cassini division while the low levels of aliphatic material observed by VIMS in the A and B rings particles are an evidence that they are pristine. Thermal properties: the ring-particles’ temperature is retrieved by fitting the spectral position of the 3.6 μm continuum peak observed on reflectance spectra: in case of pure water ice the position of the peak, as measured in laboratory, shifts towards shorter wavelengths when temperature decreases, moving from about 3.65 μm at 123 K to about 3.55 μm at 88 K. When applied to VIMS rings observations, this method allows us to infer the average temperature across ring regions sampled through 400 km-wide radial bins. Comparing VIMS temperature radial profiles with similar CIRS measurements acquired at the same time we have found a substantial agreement between the two instruments’ results across the A and B rings. In general VIMS measures higher temperatures than CIRS across C ring and Cassini division as a consequence of the lower optical depth and the resulting pollution that creates a deviation from pure water ice composition of these regions. VIMS results point out that across C ring and CD the 3.6 μm peak wavelength is always higher than across B and A rings and therefore C ring and CD are warmer than A and B rings. VIMS observations allow us to investigate also diurnal and seasonal effects: comparing antisolar and subsolar ansae observations we have measured higher temperature on the latter. As the solar elevation angle decreases to 0° (equinox), the peak’s position shifts at shorter wavelengths because ring’s particles becomes colder. Merging multi-wavelength data sets allow us to test different thermal models, combining the effects of particle albedo, regolith composition, grain size and thermal properties with the ring structures.
    No preview · Article · Oct 2014 · Icarus
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    ABSTRACT: 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.
    Full-text · Article · May 2014 · The Astrophysical Journal
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    ABSTRACT: We present a quantitative analysis of the hydrocarbon and other organic molecular inventory as a component of the low-albedo material of Saturn’s satellite Iapetus, based on a revision of the calibration of the Cassini VIMS instrument. Our study uses hyperspectral data from a mosaic of Iapetus’ surface (Pinilla-Alonso, N., Roush, T.L., Marzo, G.A., Cruikshank, D.P., Dalle Ore, C.M. [2011]. Icarus 215, 75–82) constructed from VIMS data on a close fly-by of the satellite. We extracted 2235 individual spectra of the low-albedo regions, and with a clustering analysis tool (Dalle Ore, C.M., Cruikshank, D.P., Clark, R.N. [2012]. Icarus 221, 735–743), separated them into two spectrally distinct groups, one concentrated on the leading hemisphere of Iapetus, and the other group on the trailing. This distribution is broadly consistent with that found from Cassini ISS data analyzed by Denk et al. (Denk, T. et al. [2010]. Science 327, 435–439). We modeled the average spectra of the two geographic regions using the materials and techniques described by Clark et al. (Clark, R.N., Cruikshank, D.P., Jaumann, R., Brown, R.H., Stephan, K., Dalle Ore, C.M., Livio, K.E., Pearson, N., Curchin, J.M., Hoefen, T.M., Buratti, B.J., Filacchione, G., Baines, K.H., Nicholson, P.D. [2012]. Icarus 218, 831–860), and after dividing the Iapetus spectrum by the model for each case, we extracted the resulting spectra in the interval 2.7–4.0 μm for analysis of the organic molecular bands. The spectra reveal the C −1 2 3 −1 3 n 2 3 2 3 2 Johnson, T.V., Lunine, J.I. [2005]
    No preview · Article · May 2014 · Icarus
  • Yvonne J. Pendleton · D. P. Cruikshank
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    ABSTRACT: The diffuse interstellar medium inventory of organic material (Pendleton et al. 1994, Pendleton & Allamandola 2002) was likely incorporated into the molecular cloud in which the solar nebula condensed. This provided the feedstock for the formation of the Sun, major planets, and the smaller icy bodies in the region outside Neptune's orbit (transneptunian objects, or TNOs). Saturn's satellites Phoebe, Iapetus, and Hyperion open a window to the composition of one class of TNO as revealed by the near-infrared mapping spectrometer (VIMS) on the Cassini spacecraft at Saturn. Phoebe (mean diameter 213 km) is a former TNO now orbiting Saturn. VIMS spectral maps of Phoebe's surface reveal a complex organic spectral signature consisting of prominent aromatic (CH) and aliphatic hydrocarbon (CH2, CH3) absorption bands (3.2-3.6 μm). Phoebe is the source of a huge debris ring encircling Saturn, and from which particles 5-20 μm size) spiral inward toward Saturn. They encounter Iapetus and Hyperion where they mix with and blanket the native H2O ice of those two bodies. Quantitative analysis of the hydrocarbon bands on Iapetus demonstrates that aromatic CH is ~10 times as abundant as aliphatic CH2+CH3, significantly exceeding the strength of the aromatic signature in interplanetary dust particles, comet particles, and in carbonaceous meteorites (Cruikshank et al. 2013). A similar excess of aromatics over aliphatics is seen in the qualitative analysis of Hyperion and Phoebe itself (Dalle Ore et al. 2012). The Iapetus aliphatic hydrocarbons show CH2/CH3 ~4, which is larger than the value found in the diffuse ISM 2-2.5). Insofar as Phoebe is a primitive body that formed in the outer regions of the solar nebula and has preserved some of the original nebula inventory, it can be key to understanding the content and degree of processing of that nebular material. There are other Phoebe-like TNOs that are presently beyond our ability to study in the organic spectral region, but JWST will open that possibility for a number of objects. We now need to explore and understand the connection of this organic-bearing Solar System material to the solar nebula and the inventory of ISM materials incorporated therein.
    No preview · Article · Jan 2014
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    ABSTRACT: The near-Earth object (NEO) population, which mainly consists of fragments from collisions between asteroids in the main asteroid belt, is thought to include contributions from short-period comets as well. One of the most promising NEO candidates for a cometary origin is near-Earth asteroid (3552) Don Quixote, which has never been reported to show activity. Here we present the discovery of cometary activity in Don Quixote based on thermal-infrared observations made with the Spitzer Space Telescope in its 3.6 and 4.5 {\mu}m bands. Our observations clearly show the presence of a coma and a tail in the 4.5 {\mu}m but not in the 3.6 {\mu}m band, which is consistent with molecular band emission from CO2. Thermal modeling of the combined photometric data on Don Quixote reveals a diameter of 18.4 (-0.4/+0.3) km and an albedo of 0.03 (-0.01/+0.02), which confirms Don Quixote to be the third-largest known NEO. We derive an upper limit on the dust production rate of 1.9 kg s^-1 and derive a CO2 gas production rate of (1.1+-0.1)10^26 molecules s^-1. Spitzer IRS spectroscopic observations indicate the presence of fine-grained silicates, perhaps pyroxene rich, on the surface of Don Quixote. Our discovery suggests that CO2 can be present in near-Earth space over a long time. The presence of CO2 might also explain that Don Quixote's cometary nature remained hidden for nearly three decades.
    Full-text · Article · Dec 2013 · The Astrophysical Journal
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    ABSTRACT: Saturn's icy satellites and ring particle surfaces have long been known to be composed mostly of frozen water. However, all surfaces show an absorption due to a non-water-ice component whose identity has not been well understood. In the near infrared, water ice has strong absorptions which limit detectability of other trace components. Similarly, at wavelengths less than about 0.18 microns, water is very absorbing. However, in the ~0.2 to ~1 micron range, water ice has low absorption and trace components are readily detected. Classical interpretations of the UV absorber and dark material on outer Solar System satellites have been varying amounts of tholins and carbon. However, tholins have spectral structure not seen in the icy spectra in the Saturn System. Many silicates also have UV spectral structure that reject them from contributing significantly to the observed spectral signatures. We have constructed a new UV spectrometer and a new environment chamber for studying the spectral properties of materials from 0.1 to 15 microns. In our survey of the spectral properties of materials so far, we find that small amounts of metallic iron and iron oxides in the icy surfaces are compatible with and can explain the UV, visible and near-infrared spectra of icy surfaces in the Saturn system (0.12 to 5.1 microns) using data from the Cassini UltraViolet Imaging Spectrograph (UVIS) and the Visual and Infrared Mapping Spectrometer (VIMS). The wide range of observed UV-NIR (0.1-5 micron) spectral signatures provide strong constraints on composition and grain size distribution, including grain sizes of the ice. Spectra of the Saturnian rings and icy satellites indicate they have a large range of ice grain sizes, from tens of microns to sub-micron. Sub-micron ice grains create unusual spectral properties, which are seen in the spectra of the rings and satellites of Saturn and on satellites further out in the Solar System. Clark et al. (2012, Icarus v218, p831) showed that VIMS spectra were explained by combinations ! of water ice, CO2, nano-sized grains of metallic iron and iron oxide and trace amounts of other compounds. The new UV lab data are providing further evidence for this interpretation and placing further constraints on grain size distributions and abundances of the components.
    No preview · Article · Dec 2013
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    ABSTRACT: The spectral position of the 3.6 μm continuum peak measured on Cassini-VIMS reflectance spectra is used as a marker to infer the temperature of the regolith particles covering the surfaces of Saturn's icy satellites. Laboratory measurements indicate that for pure water ice the position of the 3.6 μm peak is temperature-dependent: it shifts towards shorter wavelengths when the ice is cooled, moving from about 3.65 μm at T=123 K to about 3.55 μm at T=88 K. Starting from this experimental evidence we have used a 4th-degree polynomial fit between 3.2 and 3.8 µm to measure the wavelength at which the peak occurs with the view toward using it as a marker to retrieve the temperatures of the satellites. This method is applied to about 240 disk-integrated observations of Saturn's regular satellites collected by VIMS between 2004 and 2011 (Filacchione et al. Icarus 220, 2012) with solar phase in the 20-40 deg range, corresponding to late morning-early afternoon local times. From these observations we have retrieved average temperatures for Mimas (~88 K), Enceladus (<<88 K), Tethys (<88 K), Dione (~100 K), Rhea (~108 K), Hyperion (~113 K), Iapetus trailing (~138K) and Iapetus leading hemisphere (>170K). For some satellites, like Tethys and Dione, for which observations on both leading and trailing hemispheres are available, we have measured average temperatures higher by about 10 K on the trailing than on the leading hemisphere. Temperatures measured by VIMS with this method are in general much higher than corresponding ones reported by CIRS: this is a consequence of the shallow skindepth (few microns) to which VIMS is sensitive while CIRS measures temperature at greater depth (few millimeters). Grain size and contaminants embedded in water ice may also play a role in the 3.6 μm peak properties and these effects have yet to be investigated. Combining VIMS and CIRS measurements will allow us to better characterize the regolith physical proper ties and heat transport mechanisms
    No preview · Article · Dec 2013
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    ABSTRACT: This paper describes the spectral modeling of the surface of Phobos in the wavelength range between 0.25 and 4.0 μm. We use complementary data to cover this spectral range: the OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System on board the ESA Rosetta spacecraft) reflectance spectrum that Pajola et al. merged with the VSK-KRFM-ISM (Videospectrometric Camera (VSK)–Combined Radiometer and Photometer for Mars (KRFM)–Imaging Spectrometer for Mars (ISM) on board the USSR Phobos 2 spacecraft) spectra by Murchie & Erard and the IRTF (NASA Infrared Telescope Facility, Hawaii, USA) spectra published by Rivkin et al. The OSIRIS data allow the characterization of an area of Phobos covering from 86.8 N to 90 S in latitude and from 126◦ W to 286◦ W in longitude. This corresponds chiefly to the trailing hemisphere, but with a small sampling of the leading hemisphere as well. We compared the OSIRIS results with the Trojan D-type asteroid 624 Hektor and show that the overall slope and curvature of the two bodies over the common wavelength range are very similar. This favors Phobos being a captured D-type asteroid as previously suggested. We modeled the OSIRIS data using two models, the first one with a composition that includes organic carbonaceous material, serpentine, olivine, and basalt glass, and the second one consisting of Tagish Lake meteorite and magnesium-rich pyroxene glass. The results of these models were extended to longer wavelengths to compare the VSK-KRFM-ISM and IRTF data. The overall shape of the second model spectrum between 0.25 and 4.0 μm shows curvature and an albedo level that match both the OSIRIS and Murchie & Erard data and the Rivkin et al. data much better than the first model. The large interval fit is encouraging and adds weight to this model, making it our most promising fit for Phobos. Since Tagish Lake is commonly used as a spectral analog for D-type asteroids, this provides additional support for compositional similarities between Phobos and D-type asteroids.
    No preview · Article · Oct 2013 · The Astrophysical Journal
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    ABSTRACT: We present the results of a systematic analysis of the surface composition of nine of the reddest trans-neptunian objects (TNOs) with a view to investigate their initial chemical compositions and the evolution of that composition since their formation. The objects are mostly in the Classical and Resonant dynamical groups, with the exception of three Centaurs. The Classical and Resonant objects are expected to be similar in composition, while the surfaces of the three Centaurs could have been significantly modified as their orbits evolved. The available data consist of broad-band photometric measurements in the wavelength range between 0.3 and 4.5 μm. The photometric measurements are scaled to the albedo at 0.55 μm to yield an approximation of the spectral continuum of each object that is then compared to a library of synthetic spectra of mixtures of materials known to be present on the surfaces of TNOs. For each object we obtain a range of compositions that match their spectral distribution. This yields the likelihood for the various materials to be present on the surface as well as a measure of the error of the estimate. Ices are grouped into ‘stable’ (H2O), ‘partially stable’ (CH3OH, CO2), and ‘volatile’ (CH4, CO, N2). Our preliminary results show some difference in the amount of ‘volatile’ and ‘partially volatile’ ices among the Classical and Resonant objects. A trend in the sense of less ice present on closer and smaller objects is apparent, possibly related to the objects’ ability to retain those ices and to the ices available in the solar nebula at those distances at the time of formation. On the other hand Pholus, one of the Centaurs, exhibits loss of ‘volatile’ ices and enhancement of organic material with respect to the Classical and Resonant objects. Since Centaurs are believed to originate from TNOs captured into fairly short-lived orbits closer to the Sun, our findings are consistent with the idea that Pholus has recently lost to sublimation some of its ‘volatile’ ice reservoir, exposing more of its native organic material.
    No preview · Article · Oct 2013
  • Dale P. Cruikshank · C. M. Dalle Ore · Y. J. Pendleton · R. N. Clark
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    ABSTRACT: We present a revised quantitative analysis of the hydrocarbon and other organic molecular inventory in the low-albedo material of Saturn’s satellite Iapetus, based on a revision of the calibration of the Cassini VIMS instrument. Our study uses hyperspectral data from a mosaic of Iapetus’ surface (Pinilla-Alonso et al. 2012, Icarus 215, 75-82) constructed from VIMS data on close fly-bys of the satellite. We extracted >2000 individual spectra of the low-albedo regions, and with a clustering analysis tool (Dalle Ore et al. 2012, Icarus 221, 735-743) separated them into two spectrally distinct groups, one concentrated on the leading hemisphere of Iapetus, and the other on the trailing. This distribution is broadly consistent with that found from Cassini ISS data analyzed by Denk et al. (2010, Science 327, 435-439). We modeled the average spectra of the two geographic regions using the materials and techniques described by Clark et al. (2012, Icarus 218, 831-860), and extracted the residual (Iapetus/model) in the interval 2.7-4.0 µm for analysis of the organic molecular bands that occur in this spectral region. These bands are the C-H stretching modes of aromatic hydrocarbons at ~3.28 μm 3050 cm-1), plus four blended bands of aliphatic -CH2- and -CH3 in the range ~3.36-3.52 μm 2980-2840 cm-1). In these data, the aromatic band, probably indicating the presence of polycyclic aromatic hydrocarbons (PAH), is unusually strong in comparison to the aliphatic bands, as was found for Hyperion (Dalton et al. 2012, Icarus 220, 752-776; Dalle Ore et al. 2012 op. cit.) and Phoebe (Dalle Ore et al. 2012 op. cit.). Our Gaussian decomposition of the organic band region suggests the presence of molecular bands in addition to those noted above, specifically bands attributable to cycloalkanes, olefinic compounds, CH3OH, and N-substituted PAHs. Insofar as the superficial layer of low-albedo material on Iapetus originated in the interior of Phoebe and was transported to Iapetus (and Hyperion) via the Phoebe dust ring (Tamayo et al. 2011, Icarus 215, 260-278), the organic inventory we observe is likely representative of a body that formed in the transneptunian region prior to its capture by Saturn.
    No preview · Article · Oct 2013

Publication Stats

11k Citations
1,983.44 Total Impact Points


  • 2015
    • Oak Ridge Associated Universities
      オーク・リッジ, Tennessee, United States
  • 1980-2015
    • NASA
      • • Earth Sciences Division
      • • Ames Research Center
      Вашингтон, West Virginia, United States
  • 2008
    • Liverpool John Moores University
      Liverpool, England, United Kingdom
  • 2007
    • Mountain View College
      Mountain View, California, United States
  • 2000-2007
    • SETI Institute
      Mountain View, California, United States
  • 2006
    • National Institute of Geophysics and Volcanology
      Roma, Latium, Italy
    • University of Kansas
      Lawrence, Kansas, United States
  • 2004-2006
    • Cornell University
      • Department of Astronomy
      Ithaca, NY, United States
    • University of Nantes
      • Nantes Planetology and Geo-dynamics Lab
      Naoned, Pays de la Loire, France
  • 2005
    • Université Paris-Sud 11
      • Institut d'Astrophysique Spatiale
      Paris, Ile-de-France, France
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1993-2005
    • The University of Arizona
      • Department of Planetary Sciences
      Tucson, Arizona, United States
  • 1998
    • Santa Clara University
      Santa Clara, California, United States
  • 1997
    • Cornell College
      Cornell, Wisconsin, United States
  • 1995
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1980-1988
    • California Institute of Technology
      • Jet Propulsion Laboratory
      Pasadena, CA, United States
  • 1972-1988
    • Honolulu University
      Honolulu, Hawaii, United States
  • 1987
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, Hawaii, United States
  • 1978-1986
    • Planetary Science Institute
      Tucson, Arizona, United States
  • 1985
    • University of Groningen
      Groningen, Groningen, Netherlands