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

Research output: Contribution to journalArticle

17 Citations (Scopus)

Abstract

We present models of the near-infrared (1-5μm) 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 0.1-100μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2μm absorption bands, and the expected 3μm water ice primary absorption appears as an unusually narrow dip at 2.87μm. 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 2.87μm 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 2.87μm 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 2.87μm 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.

Original languageEnglish (US)
Pages (from-to)682-694
Number of pages13
JournalIcarus
Volume215
Issue number2
DOIs
StatePublished - Oct 2011
Externally publishedYes

Fingerprint

Saturn
ice
rings
water
phase shift
Mie scattering
dipoles
dip
approximation
scattering
refractive index
heuristics
diffraction
range size
particle
unity
near infrared
power law
spectrometer
monomers

Keywords

  • Ices, ir spectroscopy
  • Planetary rings
  • Radiative transfer
  • Saturn, rings

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Vahidinia, S., Cuzzi, J. N., Hedman, M., Draine, B., Clark, R. N., Roush, T., ... Sotin, C. (2011). Saturn's F ring grains: Aggregates made of crystalline water ice. Icarus, 215(2), 682-694. https://doi.org/10.1016/j.icarus.2011.04.011

Saturn's F ring grains : Aggregates made of crystalline water ice. / Vahidinia, Sanaz; Cuzzi, Jeffrey N.; Hedman, Matt; Draine, Bruce; Clark, Roger N.; Roush, Ted; Filacchione, Gianrico; Nicholson, Philip D.; Brown, Robert H.; Buratti, Bonnie; Sotin, Christophe.

In: Icarus, Vol. 215, No. 2, 10.2011, p. 682-694.

Research output: Contribution to journalArticle

Vahidinia, S, Cuzzi, JN, Hedman, M, Draine, B, Clark, RN, Roush, T, Filacchione, G, Nicholson, PD, Brown, RH, Buratti, B & Sotin, C 2011, 'Saturn's F ring grains: Aggregates made of crystalline water ice', Icarus, vol. 215, no. 2, pp. 682-694. https://doi.org/10.1016/j.icarus.2011.04.011
Vahidinia S, Cuzzi JN, Hedman M, Draine B, Clark RN, Roush T et al. Saturn's F ring grains: Aggregates made of crystalline water ice. Icarus. 2011 Oct;215(2):682-694. https://doi.org/10.1016/j.icarus.2011.04.011
Vahidinia, Sanaz ; Cuzzi, Jeffrey N. ; Hedman, Matt ; Draine, Bruce ; Clark, Roger N. ; Roush, Ted ; Filacchione, Gianrico ; Nicholson, Philip D. ; Brown, Robert H. ; Buratti, Bonnie ; Sotin, Christophe. / Saturn's F ring grains : Aggregates made of crystalline water ice. In: Icarus. 2011 ; Vol. 215, No. 2. pp. 682-694.
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abstract = "We present models of the near-infrared (1-5μm) 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 0.1-100μm size range. These spectra are very different from those obtained at lower phase angles; they lack the familiar 1.5 and 2μm absorption bands, and the expected 3μm water ice primary absorption appears as an unusually narrow dip at 2.87μm. 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 2.87μm 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 2.87μm 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 2.87μm 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.",
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AU - Clark, Roger N.

AU - Roush, Ted

AU - Filacchione, Gianrico

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