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.
AAS/Division for Planetary Sciences Meeting Abstracts; 10/2012
ABSTRACT: Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared
wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery
of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying
surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous
The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft
missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers
I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev.,
2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table1
from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our
understanding (cf. Table2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important
constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper
Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon.
In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces,
and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular
attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between
the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally
we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is
likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and
discovery associated with our exploration of our planetary neighborhood.
KeywordsComposition-Icy satellites-Infrared spectroscopy
Space Science Reviews 04/2012; 153(1):113-154. · 3.61 Impact Factor
EGU General Assembly Conference Abstracts; 04/2012
ABSTRACT: In the last few years Cassini-VIMS, the Visible and Infared Mapping
Spectrometer, returned to us a comprehensive view of the Saturn's icy
satellites and rings. After having analyzed the satellites' spectral properties
(Filacchione et al. (2007a)) and their distribution across the satellites'
hemispheres (Filacchione et al. (2010)), we proceed in this paper to
investigate the radial variability of icy satellites (principal and minor) and
main rings average spectral properties. This analysis is done by using 2,264
disk-integrated observations of the satellites and a 12x700 pixels-wide rings
radial mosaic acquired with a spatial resolution of about 125 km/pixel. The
comparative analysis of these data allows us to retrieve the amount of both
water ice and red contaminant materials distributed across Saturn's system and
the typical surface regolith grain sizes. These measurements highlight very
striking differences in the population here analyzed, which vary from the
almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to
the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and
Phoebe. Rings spectra appear more red than the icy satellites in the visible
range but show more intense 1.5-2.0 micron band depths. The correlations among
spectral slopes, band depths, visual albedo and phase permit us to cluster the
saturnian population in different spectral classes which are detected not only
among the principal satellites and rings but among co-orbital minor moons as
well. Finally, we have applied Hapke's theory to retrieve the best spectral
fits to Saturn's inner regular satellites using the same methodology applied
previously for Rhea data discussed in Ciarniello et al. (2011).
ABSTRACT: Aims. The objective of this work is to summarize the discussion of a workshop aimed at investigating the properties, origins, and evolution of the materials that are responsible for the red coloration of the small objects in the outer parts of the solar system. Because of limitations or inconsistencies in the observations and, until recently, the limited availability of laboratory data, there are still many questions on the subject. Our goal is to approach two of the main questions in a systematic way:
– Is coloring an original signature of materials that are presolar in origin (“nature”) or stems from post-formational chemical alteration, or weathering (“nurture”)? – What is the chemical signature of the material that causes spectra to be sloped towards the red in the visible? We examine evidence available both from the laboratory and from observations sampling different parts of the solar system and cir- cumstellar regions (disks).
Methods. We present a compilation of brief summaries gathered during the workshop and describe the evidence towards a primor- dial vs. evolutionary origin for the material that reddens the small objects in the outer parts of our, as well as in other, planetary systems. We proceed by first summarizing laboratory results followed by observational data collected at various distances from the Sun. Results. While laboratory experiments show clear evidence of irradiation effects, particularly from ion bombardment, the first obsta- cle often resides in the ability to unequivocally identify the organic material in the observations. The lack of extended spectral data of good quality and resolution is at the base of this problem. Furthermore, that both mechanisms, weathering and presolar, act on the icy materials in a spectroscopically indistinguishable way makes our goal of defining the impact of each mechanism challenging. Conclusions. Through a review of some of the workshop presentations and discussions, encompassing laboratory experiments as well as observational data, we infer that both “nature” and “nurture” are instrumental in the coloration of small objects in the outer parts of the solar system. While in the case of some observations it is clear that the organic reddening material originated before the solar nebula (i.e. presolar grains found in meteorites), for many other cases pointers are not as clear and indicate a concurrence of both processes.
Astronomy and Astrophysics 09/2011; 533:A98. · 4.59 Impact Factor
ABSTRACT: We present a detailed study of an Iapetus mosaic of VIMS data with high spatial resolution (0.5 0.5° or 6.4 km/pixel). The spectra were taken in August 2007 and provide the highest VIMS spatial resolution data for this object during Cassini’s primary mission. We analyze this set of data using a statistical clus- tering approach to reduce the analysis of a large number of data (104 spectra from 0.35 to 5.10 lm) to the study of seven representative groups accounting for 99.6% of the surface covered by the original sam- ple. We analyze the spectral absorption bands in the spectra of the different clusters indicative of differ- ent composition over the observed surface. We find coherence between the distribution of the clusters and the geographical features on the surface. We give special attention to the study of the water ice and CO2 bands. We find that CO2 is widespread over the entire surface being studied, including the bright and dark areas on Iapetus’ surface, and is probably trapped at the molecular level with other materials. The strength of the CO2 band in the areas where both, H2O- and carbon-bearing materials exist, gives sup- port to the hypothesis that this volatile is formed on the surface of Iapetus as a product of irradiation of these two components. Finally, we also compare the Iapetus CO2 with that on other satellites confirming, that there are evident differences on the center, depth and width of the band on Iapetus and Phoebe, where CO2 has been suggested to be endogenous.
Icarus 01/2011; 215:75-82. · 3.38 Impact Factor
American Geophysical Union, Fall Meeting; 12/2010
EGU General Assembly Conference Abstracts; 01/2010
American Astronomical Society, DPS meeting $#42$ (abstract); 01/2010
ABSTRACT: The present status of observing Saturn's satellite Rhea by the Cassini
VIMS spectrometer will be presented showing that the derived spatial
variations of Rhea's spectral properties appear to be similar to the
neighboring satellite Dione.
ABSTRACT: We obtained thermal images and spectra of comet and Centaur object 29P/Schwassmann-Wachmann 1 in late 2003 November. Images at 8, 24, and 70 μm reveal an extensive coma. At 24 μm the coma extends at least 8' from the nucleus and exhibits a single jet. The dust production rate is estimated as 50 kg s-1. The 24 to 70 μm color temperature of the coma is 160 K. The debris trail is also detected at 24 μm and has an optical depth of ~(7 ± 3) × 10-9. Thermal models fitted to photometry at 8, 24, and 70 μm indicate a nuclear radius of 27 ± 5 km, larger than all previous size estimates, and a geometric albedo of 0.025 ± 0.01, lower than any other Centaur object, but consistent with other comets. Analysis of the jet morphology indicates a rotation period in excess of 60 days. The spectra reveal features at 11.3 and 34 μm, which are tentatively identified as emission from olivine, including forsterite. This is the first identification of the minerology of the dust emitted by a Centaur object.
The Astrophysical Journal Supplement Series 12/2008; 154(1):463. · 13.46 Impact Factor
ABSTRACT: The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus in which new absorption bands are seen in the spectra of both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice slightly darkened with tholin. The detection of CO2 in the low-albedo material on the leading hemisphere supports the contention that it is carbon-bearing material from an external source that has been swept up by the satellite's orbital motion.
The Astrophysical Journal 12/2008; 622(2):L149. · 6.02 Impact Factor
ABSTRACT: The Cassini orbiter's Visual & Infrared Mapping Spectrometer (VIMS)
observes Titan's surface intensity at wavelengths where its methane-rich
atmosphere is heavily absorbing and light is strongly scattering.
Therefore, most analyses of Titan's surface that require use of the VIMS
dataset (e.g., photoclinometry, geologic interpretation, spectral
identification of surface materials, photometry) are impeded until a
method to separate the atmospheric from the surface spectral signature
of Titan is fully developed. In a previous work (Pitman et al.
2007,AAS-DPS meeting #39), we presented a fully-functional
plane-parallel radiative transfer (RT) correction method with core
components extended from Mars surface-atmospheric separation models that
can be used for modeling and removing Titan's atmosphere for VIMS
observations which are far from the limb. This "Mars/Titan" hybrid
plane-parallel RT correction model includes inputs from Cassini-Huygens
c. 2007, allows for vertical variation of major atmospheric properties,
and incorporates newly released methane absorption coefficients and haze
scattering properties derived from in situ measurements by the Huygens
DISR team. In this work, we attempt to resolve the issue of atmospheric
variation as a function of Titan's geographic coordinates by utilizing a
spherical-shell radiative transfer model, originally used by Cassini
engineers to model radiation flow through Titan's atmosphere and used by
other Cassini teams for atmospheric correction as well. Trade-offs on
when and where to use which type of model will be discussed. Work
performed under contract to NASA and under appointment to the NASA
Postdoctoral Program (ORAU).
AGU Fall Meeting Abstracts. 11/2007; -1:1357.
ABSTRACT: From the standpoint of their composition, Kuiper Belt Objects (KBOs) are
still a puzzle: some of them contain water ice, some methane and ethane
ice, some are rich in pyroxenes and/or olivine, and some contain a
reddening agent that, we think, has to be organic in nature. Overall it
is still a puzzle whose pieces are slowly coming together with the
improvements in telescopes and instrumentation, including space-based
observatories and spacecraft.
We have measured reflectances of a sample of KBOs, Centaurs, and low
albedo Trojan asteroids with the infrared array camera (IRAC) on the
Spitzer Space Telescope. IRAC measures broadband fluxes at 3.6, 4.5,
5.8, and 8.0 μm. Reflectance is measured for all 30 objects at 3.6
and 4.5 μm. Only a handful of TNOs and Centaurs are bright enough for
the detection of reflected flux at 5.8 μm, and none are detected at
8.0 μm. For the low albedo Trojan asteroids, the flux at 5.8 μm is
a combination of reflection and emission, whereas the 8.0 μm flux is
dominated by thermal emission. Ground-based spectra have previously been
published from the visible to 2.5 μm for all objects in the target
list. In some cases, particularly for the featureless spectra, spectral
models admit a range of possible compositions. Reflectances in the IRAC
bands allow discrimination between these possible spectral models,
thereby constraining surface compositions. For objects whose vis-NIR
spectra show specific absorption bands (e.g., H2O), the IRAC
data permit refinement of the spectral interpretations. For example, the
IRAC data of 90377 Sedna show strong absorptions that require not only
methane to properly model, but also H2O. We will present new
data and implications for the compositions of the individual objects
thus far observed.
ABSTRACT: Hyperion, Saturn's eighth largest icy satellite, is a body of irregular shape in a state of chaotic rotation. The surface is segregated into two distinct units. A spatially dominant high-albedo unit having the strong signature of H2O ice contrasts with a unit that is about a factor of four lower in albedo and is found mostly in the bottoms of cup-like craters. Here we report observations of Hyperion's surface in the ultraviolet and near-infrared spectral regions with two optical remote sensing instruments on the Cassini spacecraft at closest approach during a fly-by on 25-26 September 2005. The close fly-by afforded us the opportunity to obtain separate reflectance spectra of the high- and low-albedo surface components. The low-albedo material has spectral similarities and compositional signatures that link it with the surface of Phoebe and a hemisphere-wide superficial coating on Iapetus.
Nature 08/2007; 448(7149):54-6. · 36.28 Impact Factor
ABSTRACT: Hubble Space Telescope observations of Uranus- and Neptune-crossing object (65489) Ceto/Phorcys (provisionally designated 2003 FX128) reveal it to be a close binary system. The mutual orbit has a period of 9.554 +/- 0.011 days and a semimajor axis of 1840 +/- 48 km. These values enable computation of a system mass of (5.41 +/- 0.42) 10^18 kg. Spitzer Space Telescope observations of thermal emission at 24 and 70 microns are combined with visible photometry to constrain the system's effective radius (109 +10/-11 km) and geometric albedo (0.084 +0.021/-0.014). We estimate the average bulk density to be 1.37 +0.66/-0.32 g cm^-3, consistent with ice plus rocky and/or carbonaceous materials. This density contrasts with lower densities recently measured with the same technique for three other comparably-sized outer Solar System binaries (617) Patroclus, (26308) 1998 SM165, and (47171) 1999 TC36, and is closer to the density of the saturnian irregular satellite Phoebe. The mutual orbit of Ceto and Phorcys is nearly circular, with an eccentricity <= 0.015. This observation is consistent with calculations suggesting that the system should tidally evolve on a timescale shorter than the age of the solar system.
ABSTRACT: We report measurements of reflectances of 90377 Sedna at λ > 2.5 µm using the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Sedna orbits well beyond even the Kuiper Belt, with a perihelion distance of 76 AU, and is therefore very faint as viewed from Earth, despite its relatively large size. Previously published near-infrared spectra show possible signatures of CH 4 and N 2 at ∼2.3 and ∼2.15 µm, respectively. These and other ices also exhibit much stronger absorptions at λ > 2.5 µm, providing the motivation for the present work. We detected flux from Sedna at 3.6 and 4.5 µm, but not at 5.8 or 8.0 µm. The measured IRAC fluxes are converted to geometric albedos and combined with previous measurements of the visible and near-infrared spectra. Strong absorption at both 3.6 and 4.5 µm (relative to the 2.0–2.5 µm region) is readily apparent, confirming the presence of ices on the surface of Sedna. Spectral modeling of the full wavelength range (0.4–4.5 µm) provides further constraints. We find that CH 4 is required to fit the new data points, but that these new data points can not be adequately described with models containing CH 4 and N 2 as the only ices. We suggest that H 2 O ice is also present. Several characteristics of the spectrum of Sedna suggest an absence of atmospheric volatile transport, in contrast to the large objects Eris and 2005 FY 9 .
A&A. 01/2007; 466:395-398.
ABSTRACT: The programs of observations of Solar System bodies conducted in the
first year of the operation of the Spitzer Space Telescope as part of
the Guaranteed Observing Time allocations are described. Initial
results include the determination of the albedos of a number of Kuiper
Belt objects and Centaurs from observations of their flux densities at
24 and 70 μm, and the detection of emission bands in the spectra of
several distant asteroids (Trojans) around 10 and 25 μm. The 10
Kuiper Belt objects observed to date have albedos in the range 0.08 --
0.15, significantly higher than the earlier estimated 0.04. An
additional KBO [(55565) 2002 AW197] has an albedo of 0.17
± 0.03. The emission bands in the asteroid spectra are
indicative of silicates, but specific minerals have not yet been
identified. The Centaur/comet 29P/Schwassmann-Wachmann 1 has a nucleus
surface albedo of 0.025 ± 0.01, and its dust production rate was
calculated from the properties of the coma. Several other
investigations are in progress as the incoming data are processed and
ABSTRACT: We report the first identification of compounds on Titan's surface using
remote sensing methods through Titan's atmosphere, and show that these
materials are preferentially associated with Titan's lower albedo
terrains. Specifically, we identify benzene (C6H6), an aromatic
hydrocarbon, using an absorption feature at 5.05 microns observed in
data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS).
The benzene maps in channels, "lakes," and boundaries between bright and
dark regions. Acetylene (C2H2), which is expected on Titan in higher
abundances than benzene, has not been detected in surface deposits by
VIMS. Benzene abundance appears higher than predicted by current models
of formation via photolysis products. This observation, and the high
benzene abundance measured by the Cassini INMS instrument, indicates
either more chemical processing on Titan than previously believed or
that somehow acetylene is hidden from detection. One explanation for
the apparently high benzene/acetylene ratio may be that the surface
organics have been chemically processed more than previously considered
and subsequently concentrated by erosional processes on Titan's surface.
We also report the detection of two additional absorptions in spectra of
Titan's surface. A feature at 4.97 microns appears to match simple
alkanes, or aliphatic hydrocarbons, particularly solid or liquid methane
and ethane, with possible contributions from higher-order alkanes. The
third and weakest absorption at 5.01 microns is currently not
identified, although solid acetonitrile (CH3CN) has a spectral band that
is close to matching Titan's, suggesting that other nitriles might be
AGU Fall Meeting Abstracts. 11/2006; -1:03.