D. P. Cruikshank

Oak Ridge Associated Universities, Oak Ridge, Tennessee, United States

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Publications (620)1542.31 Total impact

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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.
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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.
    Icarus 10/2014; 241:45–65. · 3.16 Impact Factor
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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.
    The Astrophysical Journal 05/2014; 788(2):111. · 6.73 Impact Factor
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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]
    Icarus 01/2014; 233:306–315. · 3.16 Impact Factor
  • 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.
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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.
    The Astrophysical Journal 12/2013; 781(1). · 6.73 Impact Factor
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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
    AGU Fall Meeting Abstracts. 12/2013;
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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.
    AGU Fall Meeting Abstracts. 12/2013;
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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.
    The Astrophysical Journal 10/2013; 777:127. · 6.73 Impact Factor
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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.
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    ABSTRACT: A Cassini VIMS spectrum of an active location along Baghdad Sulchus is best fit by a fissure 9 m wide at T=197 K (Goguen et al. 2013, Icarus, accepted). We show that narrower and hotter fissures are unstable due to the exponential increase of the vapor pressure of ice for T greater than 200 K. Ice at 230 K will erode 1 meter/day due to sublimation, so a narrower and warmer fissure will quickly erode to meter widths. The same strong T dependence of the vapor pressure also means that wider fissures at T ~180 K cannot supply to total mass loss rate constraint from Cassini UVIS occultation data. The mass loss rate can be supplied if a significant fraction 190 km) of the total length of the fissures is active as a 9 m wide fissure with T=197 K. The contribution of this hottest component of the fissure emission contributes only a small fraction of the total observed radiated power from the fissures which is dominated by much larger areas at lower T and is best characterized using the CIRS instrument. Copyright 2013 California Institute of Technology.
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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 ?m IRAC channels. We ask for observations at 18 longitudes (~ each 20 o). The surface of Pluto is formed by patches of CH4, N2 and CO. The differences in the visible albedo on the surface of Pluto pretty much span the range from the darkest to the brightest stuff in the solar system. Near-infrared and visible observations, performed over the last 30 years, show a dynamic and variable system, with a timescale on the order of months to years. Pluto is currently moving away from the sun and entering northern summer. Despite many model predictions that the atmosphere will collapse, it has significantly increased in density over the last ~20 years. Spitzer holds a unique place in the solar system to observe Pluto, above the Earth?s atmosphere in a stable Earth-trailing environment. By 2014 Pluto will be leaving the galactic background, allowing for better measurements than have been possible in the last 5 years. Relative differences in the albedo of Pluto in ch1 and ch2 is an effective tool to study the different mixing ratios of the materials on the surface. Both channels are sensitive to different ices. This is also promising for the search of other materials expected to be on the surface of Pluto but have not been identified in the vis/NIR, e.g CO2 that has its fundamental absorption band in the wavelength range of ch2. Finally these observations will also be used to check for secular differences on the surface by comparing those Spitzer observations of Pluto made in 2004 and the ones obtained through this program. The combination of these datasets with observations done at other wavelengths will place strong constraints on Pluto?s surface albedo, and shed some light on the local interaction between the surface and the atmosphere.
    Spitzer Proposal. 10/2013;
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    ABSTRACT: We report about Saturn's rings average spectral properties and temperature as retrieved from ten Cassini VIMS radial mosaics acquired between october 2004 and january 2010. The dataset includes observations taken with solar phase running between 12° to 136° and elevation angle between -21° to +5°. These observations, after being reduced in spectrograms, e.g. 2D arrays containing the VIS-IR spectral (0.35-5.0 μm) and spatial (from 73.500 to 141.375 km) information, allow us a direct comparison of the derived spectral properties on a common spatial scale. Significant changes in VIS reddening, water ice abundance and grain sizes are observed across different rings radial regions. When observed at high solar phases, a remarkable increase of VIS reddening and water ice band depths is found, as a consequence of the presence of a red contaminant intimately mixed within water ice grains. Ring's particles temperature is retrieved by using the wavelength of the 3.6 μm continuum peak on reflectance spectra as a marker. For pure water ice the position of the peak, as measured in laboratory, shifts towards shorter wavelengths when temperature decreases, from about 3.65 μm at 123 K to about 3.55 μm at 88 K. When applied to VIMS rings observations, this method allow us to infer the average temperature across ring regions sampled with 400 km-wide radial bins. VIMS temperature radial profiles are compared with similar CIRS measurements acquired at the same time. We have found a substantial agreement between VIMS and CIRS results for the A and B ring while VIMS measures higher temperatures than CIRS across C ring and CD as a consequence of the lower optical depth and deviation from pure water ice composition. In summary, VIMS results show that 1) across C ring and CD the 3.6 μm peak wavelength is always higher than across B and A rings: C and CD are warmer than A and B rings; 2) when the solar elevation angle decreases to 0° (equinox) the peak's position shifts at shorter wavelengths: rings become colder; 3) when both afternoon and morning ansae observations are available, we have measured higher temperature across the afternoon ansa
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    ABSTRACT: Centaurs are a population of icy bodies with probable origin in the trans-Neptunian region. They move in orbits with semimajor axes and perihelia between the orbits of Neptune and Jupiter. The visible colors of the Centaurs show a clear bimodality that divides them into the gray and the red groups. The origin of this peculiar distribution is under debate. It is not yet clear whether the color bimodality is related to their origin or if it is a result of the surface processes that affect the Centaurs. In both cases, these colors are directly related to the surface composition of these bodies, and it is suggested that is a result of variations in the ratio between silicate and organic abundances. Our hypothesis is that the color bimodality extends at wavelengths (λ) longer than 2.5 μm. The aim of this work is to test if the gray group corresponds to objects covered by mantles of silicates, while the red objects are covered, at least in part, by complex organics. The organic materials are typically characterized by an absorption at λ 3 μm that lies within the bandpass of the IRAC 3.6 μm filter. We observed 40 Centaurs with Spitzer/IRAC, at 3.6 and 4.5 μm, in cycles GO2, 4, 6 and GO8. Here, we compare these colors with reflectance values derived from synthetic models. Models of the reflectance from surfaces composed by mixtures of ices, organics and silicates show a wide range of colors at these wavelengths. Thus, measurements of colors of icy bodies beyond 2.5 μm are a very effective tool to study their surface. Of particular note, silicates and organics have very different reflectance at λ > 3 μm. We also correlate the colors in the visible and the new colors in the IR to test our hypothesis of the existence of color bimodality also in this wavelength range.
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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.
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    ABSTRACT: The width and temperature of the active fissures on Saturn's satellite Enceladus provide key observable constraints on physical models of these geyser-like eruptions. We analyze a sequence of high spatial resolution near-infrared spectra acquired with VIMS at 0.025 s intervals during a 74 km altitude flyover of the South Pole of Enceladus by the Cassini spacecraft on 14 April 2012 UTC. A thermal-emission spectrum covering 3- to 5-μm wavelengths was detected as the field of view crossed one of the four major fissures, Baghdad Sulcus, within 1 km of 82.36S latitude and 28.24W longitude. We interpret this spectrum as thermal emission from a linear fissure with temperature 197 ± 20 K and width 9 m. At the above wavelengths, the spectrum is dominated by the warmest temperature component. Looking downward into the fissure at only 13° from the vertical, we conclude that our results measure the temperature of the interior fissure walls (and the H2O vapor) at depths within 40 m of the surface.
    Icarus 09/2013; 226(1):1128-1137. · 3.16 Impact Factor
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    ABSTRACT: Over the last eight years, the Visual and Infrared Mapping Spectrometer (VIMS) aboard the Cassini orbiter has returned hyperspectral images in the 0.35-5.1 micron range of the icy satellites and rings of Saturn. These very different objects show significant variations in surface composition, roughness and regolith grain size as a result of their evolutionary histories, endogenic processes and interactions with exogenic particles. The distributions of surface water ice and chromophores, i.e. organic and non-icy materials, across the saturnian system, are traced using specific spectral indicators (spectral slopes and absorption band depths) obtained from rings mosaics and disk-integrated satellites observations by VIMS.
    The Astrophysical Journal 01/2013; 766(2). · 6.73 Impact Factor
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    ABSTRACT: Trans-neptunian objects (TNOs) are a population of small objects orbiting the Sun beyond Neptune. Because of their distance they are difficult to observe spectroscopically, but a large body of photometric observations is available and growing. TNOs are important tracers of the evolution of the outer Solar System and key when testing current dynamical evolution theories. Previous statistical studies of the colors of TNOs have yielded useful but limited results regarding the chemical history and evolution of these bodies. With the aim at obtaining compositional information on the small and distant TNOs we introduce a statistical cluster analysis (labelled albedo) based on colors and published albedos of TNOs. We compare it to a previous taxonomy, to illustrate the significance of including the albedo information when determining the composition of the objects. When the albedo contribution is removed from the data, the new taxonomy (now labelled classical) is in general agreement with the published ones, supporting the applicability of our approach. Making use of modeled reflectance spectra of a variety of plausible mixtures found on the surface of TNOs, we extract the average surface composition of each taxon, for both the classical and the albedo taxonomy, in a statistically consistent fashion. Differently from previous and classical, the albedo taxonomy establishes a direct link between the colors and albedos of the objects and their surface composition, allowing, for the first time, a quick assessment of the chemical history of TNOs. In fact, under closer examination the taxa show trends in composition that might be evolutionary in nature. If a simple ‘snow lines’ model is adopted, we can infer that albedo taxa relate the current objects’ locations to their original ones, prior to the migration of the outer planets. We regard the large population that characterizes the darkest classes spread at a variety of semi-major axis distances as one of the intriguing results of this work.
    Icarus 01/2013; 222(1):307-322. · 3.16 Impact Factor
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    ABSTRACT: Transneptunian objects (TNOs) and Centaurs are small bodies orbiting the Sun in the cold outer regions of the Solar System. TNOs include Pluto and its satellite Charon, and Neptune's large satellite Triton is thought to have been captured from the TNO population. Visible and near-infrared spectroscopy of a number of the brightest of these bodies shows surface ices of H2O, CH4, N2, CH3OH, C2H6, CO, CO2, NH3•nH2O, and possibly HCN, in various combinations; water ice is by far the most common. Silicate minerals and solid complex carbonaceous materials are thought to occur on these bodies, but their spectral signatures have not yet been positively identified. The pronounced red color of several TNOs and Centaurs is presumed to result from the presence of carbonaceous materials. In all, the TNOs and Centaurs are thought to be primitive bodies in the sense that they have undergone relatively little modification by heating and by the space environment since their condensation in the volatile-rich outer regions of the solar nebula. As such, they hold the potential to yield important information on the chemical and physical conditions of the solar nebula. Continued and expanded studies of TNOs and Centaurs require additional basic laboratory data on the physical and the optical properties of the ices already identified and those candidate materials that have not yet been confirmed. New sky surveys and large telescopes projected for operation in the near future will reveal many more objects in the outer Solar System for detailed study.
    The Science of Solar System Ices. 01/2013;
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    ABSTRACT: The radial distribution of surface water ice and chromophores across Saturn's rings, regular and minor satellites is traced using Cassini-VIMS (Visual and Infrared Mapping Spectrometer) data. Reflectance spectra of these different objects are analyzed and clustered in spectral classes using specific VIS-IR indicators applied to VIMS datasets (Filacchione et al., 2012). Specifically, we report about the results retrieved from the analysis of about 3,000 disk-integrated observations of the icy satellites and ten ring's radial mosaics. Surface compositions and regolith properties are inferred through the comparison with laboratory and synthetic spectra of analogue icy materials. For each target it is essential to process as many observations taken at different illumination conditions as possible, in order to decouple the phase response from spectral analysis for both spectral slopes and band depth: we have observed infact a significant reduction in band depth at low phases caused by multiple scattering, which dominates single scattering on the continuum wings. Rings spectra appear more red than the icy satellites in the visible range but show more intense 1.5-2.0 micron band depths (Cuzzi et al., 2009, 2010). Chromophores mixed in ice are constrained thanks to their characteristic reddening shown at visible wavelengths (Nicholson et al., 2008). With the exclusion of Phoebe and the dark material coating Iapetus' leading hemisphere (Clark et al., 2012), VIMS data display that the water ice radial distribution, traced using the 1.5-2.0 micron band depths, is almost constant across the entire saturnian system and reaches maximum abundances on A-B rings and Calypso. The maximum visual reddening is measured across the A-B rings and on Rhea and Hyperion. Moreover our analysis allows us to recognize several other specific effects characterizing the saturnian population, like: 1) the dichotomy between regular satellites leading and trailing hemispheres caused by the accumulation of exogenic material and by interaction with magnetospheric particles; 2) the low reddening seen in the spectra of the satellites orbiting within the E-ring environment (from Mimas to Tethys) caused by the layering of Enceladus' plumes particles; 3) the spectral similarities seen among Prometheus, Pandora and A-B ring particles which point to a possible common origin; 4) the spectral differences observed between Tethys' lagrangian moons, with Calypso much more water ice-rich than Telesto; 5) Similarly Helene, one of Dione's lagrangian moons, appears bluer and hence more water ice-rich than Dione; 6) carbon dioxide ice and organics are mainly identified on the three outermost satellites, Hyperion, Iapetus, Phoebe; 7) faint absorption bands caused by aliphatic stretch of CH2 in the 3.42-3.52 micron interval are detected across A-B rings. Such comparative analysis and radial trends could help us to decipher the origins, histories and evolutionary processes of rings and satellites orbiting in Saturn's system. This research is supported by an Italian Space Agency (ASI) grant.
    AGU Fall Meeting Abstracts. 12/2012;

Publication Stats

6k Citations
1,542.31 Total Impact Points


  • 2014
    • Oak Ridge Associated Universities
      Oak Ridge, Tennessee, United States
  • 1998–2012
    • NASA
      Washington, West Virginia, United States
    • Santa Clara University
      Santa Clara, California, United States
    • Cornell University
      Ithaca, New York, United States
  • 2008
    • Hokkaido University of Education
  • 2007
    • Mountain View College
      Mountain View, California, United States
  • 2005–2007
    • SETI Institute
      Mountain View, California, United States
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
    • Université Paris-Sud 11
      • Institut d'Astrophysique Spatiale
      Paris, Ile-de-France, France
  • 1965–2007
    • The University of Arizona
      • • Department of Astronomy
      • • Department of Planetary Sciences
      Tucson, Arizona, United States
  • 1997
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1995
    • San Francisco State University
      San Francisco, California, United States
  • 1992
    • Susquehanna University
      Palo Alto, California, United States
  • 1978–1992
    • University of Hawaiʻi at Hilo
      Hilo, Hawaii, United States
  • 1984–1987
    • University of Hawaiʻi at Mānoa
      • Institute of Astronomy
      Honolulu, Hawaii, United States
  • 1972–1987
    • Honolulu University
      Honolulu, Hawaii, United States
  • 1983–1986
    • Planetary Science Institute
      Tucson, Arizona, United States
  • 1980
    • California Institute of Technology
      Pasadena, California, United States