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| Titel |
Spectroscopic identification of Dione' and Rhea' terrain units using Cassini VIMS data |
| VerfasserIn |
F. Scipioni, F. Tosi, F. Capaccioni, P. Cerroni, G. Filacchione, C. Federico |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250065566
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| Zusammenfassung |
| Since July 2004, the Cassini spacecraft performed several observations of Saturn’s icy
satellites, allowing a better insight of their compositional and physical characteristics. The
Visual and Infrared Mapping Spectrometer (VIMS) is a spectrometer onboard the Cassini
Orbiter.
In this work, we have selected 76 VIMS cubes of Dione and 166 cubes of Rhea in the infrared
range 0.85-5.1 μm. These data are characterized by a phase angle smaller than 50° and a
good S/N ratio. After normalizing all data at λ=2.232 μm to minimize photometric effects,
we apply the supervised Spectral Angle Mapper (SAM) clustering technique to emphasize
the existence of spectral units. Initially, for each satellite we define two end-members,
respectively represented by one spectrum (one pixel) of a low-albedo terrain and one
spectrum (one pixel) of a high-albedo terrain as seen at high spatial resolution. In the SAM
method applied to remote sensing data, each spectrum is represented by a vector in the
n-dimensional coordinate system, where n is the number of spectral channels. To
compare the spectrum of each pixel of the target with the reference spectra selected a
priori, the algorithm evaluates an angle θ that represents the separation between
the vector of the reference spectrum (reference pixel) and the vector representing
another pixel’s spectrum in the data space. Small values of θ are indicative of a
higher degree of similarity between the data. We set θ=0.1o as the maximum allowed
angle value. In the cases detailed here, because two reference spectra/pixels show
up to be not representative of the entire surface of the satellites, further a priori
end-members are added until the whole surface as imaged by VIMS is properly
classified. In particular, we select 9 and 12 different terrain types for Dione and Rhea,
respectively.
For both satellites, the infrared spectrum is dominated by the prominent signatures of H2O
ice / OH bands at 1.5, 2.0 and 3.0 μm. For Rhea the spectral signatures due to water ice at
1.04 and 1.25 μm are observed on the overall surface, while for Dione these features are
present just on a few percentage of the surface.
A classification method applied to VIMS hyperspectral data is crucial to understand
geochemical processes taking place on the surface of the icy satellites. The goal of this work
is to investigate the possible presence on the surface of Dione and Rhea of non-water ice
material, such as methane and ammonia. From the classification we find several spectral
units on the two satellites characterized by different values of the spectral indices
(e.g., water ice bands’ depth, reflectance of the 3.6 μm peak). Finally, some classes
show also a peculiar trend with respect to the phase angle, possibly related to the
physical structure of the surface constituents (e.g, average grain size of the surface
regolith). |
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