Saturn’s F ring grains: Aggregates made of crystalline water ice     

Sanaz Vahidinia  Jeffrey N. Cuzzi  Matt Hedman  Bruce Draine  Roger N. Clark  Ted Roush  Gianrico Filacchione  Philip D. Nicholson  Robert H. Brown  Bonnie Buratti  Christophe Sotin 《Icarus》2011,215(2):682-694

We present models of the near-infrared (1-5 μm) spectra of Saturn’s F ring obtained by Cassini’s Visual and Infrared Mapping Spectrometer (VIMS) at ultra-high phase angles (177.4-178.5°). Modeling this spectrum constrains the size distribution, composition, and structure of F ring particles in the 0.1-100 μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2 μm absorption bands, and the expected 3 μm water ice primary absorption appears as an unusually narrow dip at 2.87 μm. We have modeled these data using multiple approaches. First, we use a simple Mie scattering model to constrain the size distribution and composition of the particles. The Mie model allows us to understand the overall shapes of the spectra in terms of dominance by diffraction at these ultra-high phase angles, and also to demonstrate that the 2.87 μm dip is associated with the Christiansen frequency of water ice (where the real refractive index passes unity). Second, we use a combination of Mie scattering with Effective Medium Theory to probe the effect of porous (but structureless) particles on the overall shape of the spectrum and depth of the 2.87 μm band. Such simple models are not able to capture the shape of this absorption feature well. Finally, we model each particle as an aggregate of discrete monomers, using the Discrete Dipole Approximation (DDA) model, and find a better fit for the depth of the 2.87 μm feature. The DDA models imply a slightly different overall size distribution. We present a simple heuristic model which explains the differences between the Mie and DDA model results. We conclude that the F ring contains aggregate particles with a size distribution that is distinctly narrower than a typical power law, and that the particles are predominantly crystalline water ice.  相似文献   

TITAN’S GROUND REFLECTANCE RETRIEVAL FROM CASSINI-VIMS DATA TAKEN DURING THE JULY 2ND, 2004 FLY-BY AT 2 AM UT     

A. Adriani  M. L. Moriconi  G. L. Liberti  A. Gardini  R. Orosei  E. D’Aversa  G. Filacchione  A. Coradini 《Earth, Moon, and Planets》2005,96(3-4):109-117

An attempt to evaluate the preliminary values of the Titan's surface albedo at 2 μm from the first Cassini-VIMS observations of the moon is presented. The methodology is based on the application of radiative transfer calculations and a microphysical model of the Titan atmosphere based on fractal aerosol. As a first guess, the surface has been considered flat and lambertian. The results are presented as a function of the geographical coordinates associated to the image pixels. The libRadtran package, using the radiative transfer equation solver DISORT 2.0, has been applied for the calculations. A test run to evaluate the model performances, using ground based observations of Titan as reference in the range of wavelengths 0.3-1.0 μm, has been carried out.The retrieved values of the surface albedo range between 0.03 and 0.22.  相似文献   

Mapping Titan's surface features within the visible spectrum via Cassini VIMS     

Graham Vixie  Jason W. Barnes  Jacob Bow  Stéphane Le Mouélic  Sébastien Rodriguez  Robert H. Brown  Priscilla Cerroni  Federico Tosi  Bonnie Buratti  Christophe Sotin  Gianrico Filacchione  Fabrizio Capaccioni  Angioletta Coradini 《Planetary and Space Science》2012,60(1):52-61

Titan shows its surface through many methane windows in the 1–5 $\mathrm{\mu }\mathrm{m}$ region. Windows at shorter wavelengths also exist, polluted by scattering off of atmospheric haze that reduces the surface contrast. At visible wavelengths, the surface of Titan has been observed by Voyager I, the Hubble Space Telescope, and ground-based telescopes. We present here global surface mapping of Titan using the visible wavelength channels from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). We show global maps in each of the VIMS-V channels extending from 0.35 to 1.05 $\mathrm{\mu }\mathrm{m}$. We find methane windows at 0.637, 0.681, 0.754, 0.827, 0.937, and $1.046\mathrm{\mu }\mathrm{m}$ and apply an RGB color scheme to the 0.754, 0.827 and $0.937\mathrm{\mu }\mathrm{m}$ windows to search for surface albedo variations. Our results show that Titan appears gray at visible wavelengths; hence scattering albedo is a good approximation of the Bond albedo. Maps of this genre have already been made and published using the infrared channels of VIMS. Ours are the first global maps of Titan shortward of $0.938\mathrm{\mu }\mathrm{m}$. We compare the older IR maps to the new VIMS-V maps to constrain surface composition. For instance Tui Regio and Hotei Regio, referred to as $5\text{‐}\mathrm{\mu }\mathrm{m}$ bright spots in previous papers, do not distinguish themselves at all visible wavelengths. The distinction between the dune areas and the bright albedo spots, however, such as the difference between Xanadu and Senkyo, is easily discernible. We employ an empirically derived algorithm to remove haze layers from Titan, revealing a better look at the surface contrast.  相似文献   

Erratum to “Saturn’s F ring grains: Aggregates made of crystalline water ice” [Icarus 215 (2011) 682–694]     

Sanaz Vahidinia  Jeffrey N. Cuzzi  Matt Hedman  Bruce Draine  Roger N. Clark  Ted Roush  Gianrico Filacchione  Philip D. Nicholson  Robert H. Brown  Bonnie Buratti  Christophe Sotin 《Icarus》2012,218(1):736

  相似文献   

Hydrocarbons on Saturn's satellites Iapetus and Phoebe     

Dale P. Cruikshank  Eric Wegryn  R.H. Brown  B.J. Buratti  T.B. McCord  Y.J. Pendleton  G. Filacchione  P. Cerroni  R. Jaumann  K.H. Baines  G. Bellucci  Y. Langevin  D.L. Matson  P. Drossart 《Icarus》2008,193(2):334-343

Material of low geometric albedo (pV?0.1) is found on many objects in the outer Solar System, but its distribution in the saturnian satellite system is of special interest because of its juxtaposition with high-albedo ice. In the absence of clear, diagnostic spectral features, the composition of this low-albedo (or “dark”) material is generally inferred to be carbon-rich, but the form(s) of the carbon is unknown. Near-infrared spectra of the low-albedo hemisphere of Saturn's satellite Iapetus were obtained with the Visible-Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft at the fly-by of that satellite of 31 December 2004, yielding a maximum spatial resolution on the satellite's surface of ∼65 km. The spectral region 3-3.6 μm reveals a broad absorption band, centered at 3.29 μm, and concentrated in a region comprising about 15% of the low-albedo surface area. This is identified as the CH stretching mode vibration in polycyclic aromatic hydrocarbon (PAH) molecules. Two weaker bands attributed to CH₂ stretching modes in aliphatic hydrocarbons are found in association with the aromatic band. The bands most likely arise from aromatic and aliphatic units in complex macromolecular carbonaceous material with a kerogen- or coal-like structure, similar to that in carbonaceous meteorites. VIMS spectra of Phoebe, encountered by Cassini on 11 June 2004, also show the aromatic hydrocarbon band, although somewhat weaker than on Iapetus. The origin of the PAH molecular material on these two satellites is unknown, but PAHs are found in carbonaceous meteorites, cometary dust particles, circumstellar dust, and interstellar dust.  相似文献   

Cassini VIMS observations of the Galilean satellites including the VIMS calibration procedure     

T.B. McCord  A. Coradini  F. Capaccioni  G. Filacchione  P. Cerroni  K.H. Baines  J.-P. Bibring  E. Bussoletti  D.P. Cruikshank  V. Formisano  Y. Langevin  R.M. Nelson  B. Sicardy 《Icarus》2004,172(1):104-126

The Visual and Infrared Mapping Spectrometer (VIMS) observed the Galilean satellites during the Cassini spacecraft's 2000/2001 flyby of Jupiter, providing compositional and thermal information about their surfaces. The Cassini spacecraft approached the jovian system no closer than about 126 Jupiter radii, about 9 million kilometers, at a phase angle of <90°, resulting in only sub-pixel observations by VIMS of the Galilean satellites. Nevertheless, most of the spectral features discovered by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during more than four years of observations have been identified in the VIMS data analyzed so far, including a possible ¹³C absorption. In addition, VIMS made observations in the visible part of the spectrum and at several new phase angles for all the Galilean satellites and the calculated phase functions are presented. In the process of analyzing these data, the VIMS radiometric and spectral calibrations were better determined in preparation for entry into the Saturn system. Treatment of these data is presented as an example of the VIMS data reduction, calibration and analysis process and a detailed explanation is given of the calibration process applied to the Jupiter data.  相似文献   

Identification of spectral units on Phoebe     

A. Coradini  F. Tosi  F. Capaccioni  G. Filacchione  R.H. Brown  V. Formisano  J.I. Lunine  K.H. Baines  B.J. Buratti  D.P. Cruikshank  P. Drossart  Y. Langevin  T.B. McCord  R.M. Nelson  B. Sicardy  M.M. Hedman  C.A. Hibbitts  C. Griffith 《Icarus》2008,193(1):233-251

We apply a multivariate statistical method to the Phoebe spectra collected by the VIMS experiment onboard the Cassini spacecraft during the flyby of June 2004. The G-mode clustering method, which permits identification of the most important features in a spectrum, is used on a small subset of data, characterized by medium and high spatial resolution, to perform a raw spectral classification of the surface of Phoebe. The combination of statistics and comparative analysis of the different areas using both the VIMS and ISS data is explored in order to highlight possible correlations with the surface geology. In general, the results by Clark et al. [Clark, R.N., Brown, R.H., Jaumann, R., Cruikshank, D.P., Nelson, R.M., Buratti, B.J., McCord, T.B., Lunine, J., Hoefen, T., Curchin, J.M., Hansen, G., Hibbitts, K., Matz, K.-D., Baines, K.H., Bellucci, G., Bibring, J.-P., Capaccioni, F., Cerroni, P., Coradini, A., Formisano, V., Langevin, Y., Matson, D.L., Mennella, V., Nicholson, P.D., Sicardy, B., Sotin, C., 2005. Nature 435, 66-69] are confirmed; but we also identify new signatures not reported before, such as the aliphatic CH stretch at 3.53 μm and the ∼4.4 μm feature possibly related to cyanide compounds. On the basis of the band strengths computed for several absorption features and for the homogeneous spectral types isolated by the G-mode, a strong correlation of CO₂ and aromatic hydrocarbons with exposed water ice, where the uniform layer covering Phoebe has been removed, is established. On the other hand, an anti-correlation of cyanide compounds with CO₂ is suggested at a medium resolution scale.  相似文献   

G-MODE CLASSIFICATION OF SPECTROSCOPIC DATA     

F. Tosi  A. Coradini  A. I. Gavrishin  A. Adriani  F. Capaccioni  P. Cerroni  G. Filacchione  R. H. Brown 《Earth, Moon, and Planets》2005,96(3-4):165-197

We present the results of the application of the G-mode method to the spectral classification of the icy satellites of the giant planets. G-mode is a multivariate statistical technique for the classification of samples depending on many variables. Here this method is tested on the infrared spectra acquired by the Cassini/VIMS instrument onboard the Cassini spacecraft. This work demonstrates the suitability of automatic spectral classification methods for the study of fair resolution spectra, such as those from VIMS. Our data set is composed by two different kinds of data: observations of point targets (Galilean satellites data) and observations with medium spatial resolution (Phoebe data). In both situations, the G-mode classification performed well. In the first case, of a large number of subpixel observations of the Galilean satellites, through the G-mode it was possible to find statistically meaningful spectral groups of observations. In the case of Phoebe, of some spatially resolved observations, the G-mode classification of␣the infrared spectra of the surface led to several types, dominated by the different illumination geometry of the pixels, because, due to the irregular shape of the satellite, a proper illumination correction was not trivial to apply. Nevertheless, the decrease of the confidence level of the test as well as the re-application of the G-mode on the main type found, led to further types, whose statistical distance can be related to different chemical abundances. We plan to use the G-mode also on the data coming from ongoing and future observations of the icy Saturnian satellites.In the helioseismology literature, G-modes are gravity wave modes of the frequencies of oscillations of the Sun. Here we are dealing with a clustering method, which is essentially different.*E-mail: federico.tosi@rm.iasf.cnr.it  相似文献   

Saturn’s icy satellites investigated by Cassini-VIMS: II. Results at the end of nominal mission     

G. Filacchione  F. Capaccioni  J.N. Cuzzi  A. Coradini  P.D. Nicholson  R.H. Brown  F. Tosi  R. Jaumann 《Icarus》2010,206(2):507-523

We report the detailed analysis of the spectrophotometric properties of Saturn’s icy satellites as derived by full-disk observations obtained by visual and infrared mapping spectrometer (VIMS) experiment aboard Cassini. In this paper, we have extended the coverage until the end of the Cassini’s nominal mission (June 1st 2008), while a previous paper (Filacchione, G., and 28 colleagues [2007]. Icarus 186, 259-290, hereby referred to as Paper I) reported the preliminary results of this study.During the four years of nominal mission, VIMS has observed the entire population of Saturn’s icy satellites allowing us to make a comparative analysis of the VIS-NIR spectral properties of the major satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus) and irregular moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso, Phoebe). The results we discuss here are derived from the entire dataset available at June 2008 which consists of 1417 full-disk observations acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage. The most important spectrophotometric indicators (as defined in Paper I: I/F continua at 0.55 μm, 1.822 μm and 3.547 μm, visible spectral slopes, water and carbon dioxide bands depths and positions) are calculated for each observation in order to investigate the disk-integrated composition of the satellites, the distribution of water ice respect to “contaminants” abundances and typical regolith grain properties. These quantities vary from the almost pure water ice surfaces of Enceladus and Calypso to the organic and carbon dioxide rich Hyperion, Iapetus and Phoebe. Janus visible colors are intermediate between these two classes having a slightly positive spectral slope. These results could help to decipher the origins and evolutionary history of the minor moons of the Saturn’s system. We introduce a polar representation of the spectrophotometric parameters as function of the solar phase angle (along radial distance) and of the effective longitude interval illuminated by the Sun and covered by VIMS during the observation (in azimuth) to better investigate the spatial distribution of the spectrophotometric quantities across the regular satellites hemispheres. Finally, we report the observed spectral positions of the 4.26 μm band of the carbon dioxide present in the surface material of three outermost moons Hyperion, Iapetus and Phoebe.  相似文献   

Saturn's Titan: Surface change, ammonia, and implications for atmospheric and tectonic activity     

R.M. Nelson  L.W. Kamp  P.G.J. Irwin  M.D. Boryta  R. Jaumann  C. Sotin  D.P. Cruikshank  J.C. Pearl  J. Lunine  G. Bellucci  F. Capaccioni  A. Coradini  G. Filacchione  T.B. McCord  P.D. Nicholson 《Icarus》2009,199(2):429-441

Titan is known to have a young surface. Here we present evidence from the Cassini Visual and Infrared Mapping Spectrometer that it is currently geologically active. We report that changes in the near-infrared reflectance of a 73,000 km² area on Titan (latitude 26° S, longitude 78° W) occurred between July 2004 and March of 2006. The reflectance of the area increased by a factor of two between July 2004 and March-April 2005; it then returned to the July 2004 level by November 2005. By late December 2005 the reflectance had surged upward again, establishing a new maximum. Thereafter, it trended downward for the next three months. Detailed spectrophotometric analyses suggest these changes happen at or very near the surface. The spectral differences between the region and its surroundings rule out changes in the distribution of the ices of reasonably expected materials such as H₂O, CO₂, and CH₄ as possible causes. Remarkably, the change is spectrally consistent with the deposition and removal of NH₃ frost over a water ice substrate. NH₃ has been proposed as a constituent of Titan's interior and has never been reported on the surface. The detection of NH₃ frost on the surface might possibly be explained by episodic effusive events occur which bring juvenile ammonia from the interior to the surface. If so, its decomposition would feed nitrogen to the atmosphere now and in the future. The lateral extent of the region exceeds that of active areas on the Earth (Hawaii) or Io (Loki).  相似文献