Thermal phase curves observed in Saturn's main rings by Cassini‐CIRS: Detection of an opposition effect?

Altobelli, Nicolás; Spilker, L. J.; Pilorz, S.; Leyrat, C.; Eddgington, S.; Wallis, B. D.; Flandes, A.

doi:10.1029/2009gl038163

Cited by 21 publications

(17 citation statements)

References 20 publications

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“…Independent support for the interparticle shadowing opposition effect is provided by the Cassini CIRS measurements, showing a pronounced opposition effect in the ring's thermal phase curves (Altobelli et al 2009). CB is ruled out, since there can be no interference between the incoming visual photons heating the particle and the infrared photons reradiating the heat.…”

Section: Discussionmentioning

confidence: 93%

The opposition and tilt effects of Saturn’s rings from HST observations

Salo

French

2010

Icarus

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The two major factors contributing to the opposition brightening of Saturn's rings are i) the intrinsic brightening of particles due to coherent backscattering and/or shadow-hiding on their surfaces, and ii) the reduced interparticle shadowing when the solar phase angle α → 0 • . We utilize the extensive set of Hubble Space Telescope observations (Cuzzi et al. 2002, Icarus 158, 199-223) for different elevation angles B and wavelengths λ to disentangle these contributions. We assume that the intrinsic contribution is independent of B, so that any B dependence of the phase curves is due to interparticle shadowing, which must also act similarly for all λ's. Our study complements that of Poulet et al. (2002, Icarus 158, 224), who used a subset of data for a single B ∼ 10 • , and the French et al. (2007b, PASP 119, 623-642) study for the B ∼ 23 • data set that included exact opposition. We construct a grid of dynamical/photometric simulation models, with the method of Salo and Karjalainen (2003, Icarus 164, 428-460), and use these simulations to fit the elevation-dependent part of opposition brightening. Eliminating the modeled interparticle component yields the intrinsic contribution to the opposition effect: for the B and A rings it is almost entirely due to coherent backscattering; for the C ring, an intraparticle shadow hiding contribution may also be present.Based on our simulations, the width of the interparticle shadowing effect is roughly proportional to B. This follows from the observation that as B decreases, the scattering is primarily from the rarefied low filling factor upper ring layers, whereas at larger B's the dense inner parts are visible. Vertical segregation of particle sizes further enhances this effect. The elevation angle dependence of interparticle shadowing also explains most arXiv:1007.0349v1 [astro-ph.EP] 2 Jul 2010 -2of the B ring tilt effect (the increase of brightness with elevation). From comparison of the magnitude of the tilt effect at different filters, we show that multiple scattering can account for at most a 10% brightness increase as B → 26 • , whereas the remaining 20% brightening is due to a variable degree of interparticle shadowing. The negative tilt effect of the middle A ring is well explained by the the same self-gravity wake models that account for the observed A ring azimuthal brightness asymmetry (Salo et al. 2004, Icarus 170, 70-90; French et al. 2007a, Icarus 189, 493-522).

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Section: Discussionmentioning

confidence: 93%

The opposition and tilt effects of Saturn’s rings from HST observations

Salo

French

2010

Icarus

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“…One puzzle from previous CIRS studies is the discrepancy between the narrow surge found by Altobelli et al (2007) and the broad surge found by Altobelli et al (2009). There are several ways to understand why these two studies did not provide the same values for the thermal surge width.…”

Section: Narrow Surge Versus Broad Surgementioning

confidence: 94%

“…• 03. However, there is still a debate on whether or not the ring thermal surge is narrow (less than a degree) or broad (several tens of degrees), see Wallis et al (2005Wallis et al ( , 2006 and Altobelli et al (2007Altobelli et al ( , 2009. This is particularly important because the thermal surge width was used in Altobelli et al (2007Altobelli et al ( , 2009) to derive ring properties.…”

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confidence: 99%

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Cassini CIRS and ISS opposition effects of Saturn’s rings – I. C ring narrow or broad surge?

Déau

Dones

Spilker

et al. 2019

Monthly Notices of the Royal Astronomical Society

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We focus on the thermal and optical opposition effects of Saturn’s C ring seen by Cassini CIRS (Composite InfraRed Spectrometer) at 15.7 ${\mu}$m and ISS (Imaging Science Subsystem) at 0.6 ${\mu}$m. The opposition surge is a brightness peak observed at low phase angle (α → 0°). Saturn rings’ opposition surge was recently observed in reflected light and thermal infrared emission by Cassini. There is debate on whether the C ring’s thermal opposition surge width is narrow (≲1°) or broad (≳30°). This surge is important because its width was used to define the scale of ring properties driving the thermal peak. We parametrize the CIRS and ISS phase curves with several morphological models to fit the surge shape. For five of the largest C ring’s plateaus, we find that their thermal surge is 10 times wider than the optical surge and that the thermal surge width (∼4°) is neither narrow, nor broad. We compare radial differences between CIRS and ISS surge morphologies with the optical depth τ (from UVIS, UltraViolet Imaging Spectrograph) and water ice band depth (from VIMS, Visual and Infrared Mapping Spectrometer) profiles. We find that: water ice band depths (microscopic ring signatures) and τ (macroscopic ring signatures) show respectively little and large contrasts between the background and the plateaus. The thermal surge amplitude and τ are correlated, and we found no band depth dependence, contrary to the optical surge amplitude, which shows no correlation with τ. These correlations suggest a macroscopic scale dominance in controlling the C ring’s thermal opposition effect.

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“…It is more appropriate to calculate the mean temperature by integrating the thermal flux over the solid angle as done in Pilorz et al (2014) for the B ring. Nevertheless, since the A ring shows relatively weak phase dependences (Altobelli et al, 2009), this simple removal of low phase data has only a minor effect. For |B ′ | > 2 • , we also remove data near Saturn's shadow as the temperatures in the shadow are much lower than those around the noon.…”

Section: Data Selectionmentioning

confidence: 99%

Incomplete cooling down of Saturn’s A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles

Morishima

Spilker

Brooks

et al. 2016

Icarus

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a b s t r a c tAt the solar equinox in August 2009, the Composite Infrared Spectrometer (CIRS) onboard Cassini showed the lowest Saturn's ring temperatures ever observed. Detailed radiative transfer models show that the observed equinox temperatures of Saturn's A ring are much higher than model predictions as long as only the flux from Saturn is taken into account. In addition, the post-equinox temperatures are lower than the pre-equinox temperatures at the same absolute solar elevation angle. These facts indicate that the A ring was not completely cooled down at the equinox and that it is possible to give constraints on the size and seasonal thermal inertia of ring particles using seasonal temperature variations around the equinox. We develop a simple seasonal model for ring temperatures and first assume that the internal density and the thermal inertia of a ring particle are uniform with depth. The particle size is estimated to be 1-2 m. The seasonal thermal inertia is found to be 30-50 J m À2 K À1 s À1/2 in the middle A ring whereas it is $10 J m À2 K À1 s À1/2 or as low as the diurnal thermal inertia in the inner and outermost regions of the A ring. An additional internal structure model, in which a particle has a high density core surrounded by a fluffy regolith mantle, shows that the core radius relative to the particle radius is about 0.9 for the middle A ring and is much less for the inner and outer regions of the A ring. This means that the radial variation of the internal density of ring particles exists across the A ring. Some mechanisms may be confining dense particles in the middle A ring against viscous diffusion. Alternatively, the (middle) A ring might have recently formed (<10 8 yr) by destruction of an icy satellite, so that dense particles have not yet diffused over the A ring and regolith mantles of particles have not grown thick. Our model results also indicate that the composition of the core is predominantly water ice, not rock.

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Thermal phase curves observed in Saturn's main rings by Cassini‐CIRS: Detection of an opposition effect?

Cited by 21 publications

References 20 publications

The opposition and tilt effects of Saturn’s rings from HST observations

The opposition and tilt effects of Saturn’s rings from HST observations

Cassini CIRS and ISS opposition effects of Saturn’s rings – I. C ring narrow or broad surge?

Incomplete cooling down of Saturn’s A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles

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