Saturn's rings in the thermal infrared

doi:10.1016/j.pss.2003.05.004

Planetary and Space Science

Volume 51, Issues 14–15, December 2003, Pages 929-935

https://doi.org/10.1016/j.pss.2003.05.004 Get rights and content

Abstract

This paper reviews our current knowledge of Saturn's rings’ physical properties as derived from thermal infrared observations. Ring particle composition, surface structure and spin as well as the vertical structure of the main rings can be determined. These properties are the key to understand the origin and evolution of Saturn's rings. Ring composition is mainly constrained by observations in the near-infrared but the signature of some probable contaminants present in water ice may also be found at mid-infrared wavelengths. The absence of the silicate signature limits nowadays their mass fraction to 10^−7±1. Recent measurements on the thermal inertia of the ring particle surface show it is very low, of the order of $5±2 J m^{−2} K^{−1} s^{−1/2}$ . New models and observations of the complete crossing of the planetary shadow are needed to attribute this low value either to compact regoliths covered by cracks due to collisions and thermal stresses or to large fluffy and irregular surfaces. Studies of the energy balance of ring particles show a preference for slowly spinning particles in the main rings. Supplementary observations at different phase angles, showing the temperature contrast between night and day sides of particles, and new models including finite spin and thermal inertia, are needed to constrain the actual spin distribution of ring particles. These results can then be compared to numerical simulations of ring dynamics. Many thermal models have been proposed to reproduce observations of the main rings, including alternative mono- or many-particles-thick layers or vertical heterogeneity, with no definitive answer. Observations on the lit and dark faces of rings as a function of longitude, at many incidence and emission angles, would provide prime information on the vertical thermal gradient due to interparticle shadowing from which constraints on the local vertical structure and dynamics can be produced. Future missions such as Cassini will provide new information to further constrain the ring thermal models.

Introduction

Saturn's rings were first discovered about 400 years ago. Throughout the intervening centuries, outstanding scientists like Galileo, Huygens, Cassini, Laplace, Maxwell and Poincaré have contributed to the understanding of the nature and dynamics of this fascinating astrophysical object. In the last few decades, ring systems have been discovered around the other giant planets, either from the ground or from the pioneering Voyager spacecraft.

These recent discoveries have revolutionized our concept of rings. Planetary rings are a very common phenomenon. Their structure appears amazingly complex compared to the expected picture of a disk flattened by collisions and homogenized by keplerian shear due to differential rotation. Waves, wakes, narrow rings, gaps, arcs, and irregular radial structures are present and reveal the action of gravitational forces due to nearby satellites, electromagnetic interactions or perhaps local viscous instabilities in the disk. Rings are evolving objects, first dynamically under satellite torques, collisions, or ballistic transport, second, physically by erosion due to collisions, meteoroid bombardment or thermal stresses, and third, chemically through pollution by meteoroid bombardment. Timescales might be as short as 100 Myr for the lifetime of Saturn's rings and erosion might have eliminated the smallest particles in only few thousand years. To avoid the anthropocentric bias for young rings, which become visible just when humanity is able to observe them, we have to postulate that the rings must be continuously replenished if the material is rapidly evolving. Otherwise, they are much older and slowly evolving. This question is all the more crucial for Saturn's rings, because the equivalent of a $200 km$ sized satellite would have to be broken apart every $100 Myr$ for the ring system to be maintained as we see it today.

The dynamical evolution of dense rings under the gravitational perturbations of nearby satellites depends on their kinematic viscosity, a function of the disk density and of the velocity dispersion which depends on the size distribution and on the physical properties of ring particles involved in collisions. The standard model of Goldreich and Tremaine (1980) and Borderies 1982, Borderies 1983 has provided an analytical description for the dynamic behavior of dense rings under very specific conditions, which was first explored numerically by Brahic (1977). Since the 1980s numerous numerical simulations have been run to continue this effort under more realistic hypotheses, including self-gravitation, gravitational scattering, size distribution and particle spin due to friction. In collisions, orbital energy is transferred into random motion, which is characterized by a velocity dispersion, or into rotational energy. The fraction of energy portioned into the spin of particles depends on the size distribution and the state of the surface (rough or smooth). For a standard ring, the vertical thickness, H, is directly related to the velocity dispersion, c, by the relation $H≈c/Ω$ where $Ω$ is the orbital motion at a distance r from Saturn center. Measurements of ring vertical thickness and particle spin can provide good insights into the local dynamics of dense rings and their evolution.

Knowledge of exact ring composition is the best way to understand ring origin, either formed from the direct condensation of the protoplanetary nebula or resulting from the disruption of a parent body, such as a satellite or a comet (Harris, 1984; Dones, 1991). But as chemically evolving objects, ring particles may hide their genes under a dark, polluted regolith. The ring particles may also be fairly uniformly mixed as a result of ongoing meteoroid bombardment.

Significant progress in the knowledge of ring particle properties and disk microphysics can be gained by studying their thermal emission. It provides a necessary complement to the observations in the visible and near-IR in order to constrain the numerous model parameters. It also provides unique markers of some properties, like the particle spin for example. In the next section, past and current work on the thermal and physical properties of Saturn's rings as viewed from the infrared are reviewed. The CIRS infrared spectrometer onboard the Cassini spacecraft en route to Saturn will undoubtedly bring significant advances in this domain. Its scientific objectives and observing strategy are described in Section 3 before concluding.

Section snippets

Rings in the thermal infrared

The temperature of Saturn's rings varies between 60 and $100 K$ depending on seasons. Their thermal emission peaks in the region between 30 and $50 μm$ . The first detections of Saturn's rings in the infrared occurred in the 1970s using bolometers and wide band filters through the Q band atmospheric window $(20 μm)$ , with some incursions in the far infrared between 300 and $800 μm$ . The Pioneer infrared radiometer was the first to observe the rings at $45 μm$ during its flyby of Saturn in September 1979 when

Saturn's rings and the CIRS infrared spectrometer

Saturn's rings will be one of the main targets of the Cassini spacecraft during its four-year orbital tour that will begin in July 2004. Most instruments will obtain ring observations that will address key questions concerning ring origin and evolution. The CIRS instrument will observe both the dense main rings and diffuse rings between 7 and $1000 μm$ in three focal planes, two of them being equipped with 10 pixel-arrays. The sensitivity of the infrared spectrometer has been improved by a factor

Conclusions

Saturn's rings may be rapidly evolving objects. Numerical experiments can help us to understand how and why the rings might have evolved but observational constraints are lacking to reduce the field of exploration. The timescales of evolution have also to be confirmed by new and dedicated observations. A great deal can be learned about Saturn's ring properties from thermal infrared measurements that can be used to constrain ring evolution. The CIRS infrared spectrometer onboard the Cassini

Acknowledgements

This work was performed at JPL under contract with NASA and at CEA Saclay supported by the Programme National de Planetologie.

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The spatial resolution is most often limited to a couple of thousand kilometers during the studied epochs because of the large field of view (3.9 mrad). Observations follow different designs to pursue scientific objectives (Spilker et al., 2003; Flasar, 2004). Azimuthal scans at fixed Saturn distance provide diurnal temperature variations induced by the few-hours-long eclipse into the planetary shadow.
Structural and thermal properties of Saturn’s B ring and its particles are derived from orbital and seasonal temperatures variations observed by the Cassini CIRS spectrometer between 2004 and 2009 equinox. Our multiscale thermal model (Ferrari, C., Reffet, E. [2013]. Icarus 223, 28–39), which assumes negligible heat transfer by vertical motion of particles in dense rings, is adjusted to the data. Most observations were focused on the center of the B ring, at 105,000 km from Saturn. A very good fit is obtained for conductive particles embedded in a moderately conductive ring medium. Assuming a bulk composition of water ice, the thermal inertia of particles is found to be $Γ_{1} = 160 – 200 J / m^{2} / K / s^{1 / 2}$ and to vary with seasons as part of the heat transfer is radiative, then temperature-dependent. For the same reason, the thermal inertia of the ring, $Γ_{0}$ , varies with seasons, between 30 and 35 $J / m^{2} / K / s^{1 / 2}$ . It is very comparable to the thermal inertia of icy satellite surfaces. The porosity of particles $p_{1}$ found to fit this thermal inertia is very high (0.93) and may emphasize an inappropriate modeling of particles by an effective porous medium. The ring filling factor is fairly high, $D = 0.34 \pm 0.01$ , but stays typical of a compact medium and compatible with the output of numerical simulations of dense ring dynamics. The thickness of the B ring at 105,000 km from Saturn is estimated at $H_{S} = 2.2 \pm 0.2$ m. The observed correlation of its optical depth with the thermal gradient between lit and unlit sides is easily reproduced by the model if the radial variations in optical depth are due to varying thickness $H_{S} (a)$ with constant filling factor. This thickness varies between 1 and 3 m across the $B_{2}, B_{3}$ and $B_{4}$ regions. It is thinner than the neighboring C ring and Cassini Division. This can be understood as a consequence of self-gravity. The ring surface mass density $Σ = (1 - p_{1}) ρ_{0} {DH}_{S} (a)$ derived from these structural parameters is too low for a self-gravitating ring, due to the high porosity found. But its radial variations, if deduced from coupling $H_{S} (a)$ with known aspect-ratios $H / λ$ of self-gravity wakes, range between 300 in the inner B ring and most certainly exceed 1000 $kg / m^{2}$ in the $B_{3}$ ring, values compatible with a self-gravitating ring. The inferred minimum total mass of Saturn's rings is $M_{R} = 1.4 \pm 0.2 \cdot 10^{19}$ kg, close to the mass expected for a viscous disk evolving over the age of the Solar System and compatible with current higher estimates.
Cassini-VIMS observations of Saturn's main rings: I. Spectral properties and temperature radial profiles variability with phase angle and elevation
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This property, observed for the first time on Voyager’s IRIS infrared spectrometer data (Hanel et al., 1982), is a consequence of the low albedo particles in C ring and Cassini division where non-icy materials accumulate. Moreover, C ring and Cassini division higher temperature is caused by the reduced mutual shadowing among particles as a consequence of the low optical depth (Spilker et al., 2003). Conversely, lower temperatures are typical of the A–B ring particles whose composition is dominated by water ice.
The spectral properties and thermal behavior of Saturn’s rings are determined from a dataset of ten radial mosaics acquired by Cassini–VIMS (Visual and Infrared Mapping Spectrometer) between October 29th 2004 and January 27th 2010 with phase angle ranging between 5.7° and 132.4° and elevation angles between −23.5° and 2.6°. These observations, after reduction to spectrograms, e.g. 2D arrays containing the VIS–IR (0.35–5.1 μm) spectral information versus radial distance from Saturn (from 73.500 to 141.375 km, 400 km/bin), allow us to compare the derived spectral and thermal properties of the ring particles on a common reference. Spectral properties: rings spectra are characterized by an intense reddening at visible wavelengths while they maintain a strong similarity with water ice in the infrared domain. Significant changes in VIS reddening, water ice abundance and grain sizes are observed across different radial regions resulting in correlation with optical depth and local structures. The availability of observations taken at very different phase angles allows us to examine spectrophotometric properties of the ring’s particles. When observed at high phase angles, a remarkable increase of visible reddening and water ice band depths is found, probably as a consequence of the presence of a red-colored contaminant intimately mixed within water ice grains and of multiple scattering. At low phases the analysis of the 3.2–3.6 μm range shows faint spectral signatures at 3.42–3.52 μm which are compatible with the CH₂ aliphatic stretch. The 3.29 μm PAH aromatic stretch absorption is not clearly detectable on this dataset. VIMS results indicate that ring particles contain about 90–95% water ice while the remaining 5–10% is consistent with different contaminants like amorphous carbon or tholins. However, we cannot exclude the presence of nanophase iron or hematite produced by iron oxidation in the rings tenuous oxygen atmosphere, intimately mixed with the ice grains. Greater pollution caused by meteoritic material is seen in the C ring and Cassini division while the low levels of aliphatic material observed by VIMS in the A and B rings particles are an evidence that they are pristine. Thermal properties: the ring-particles’ temperature is retrieved by fitting the spectral position of the 3.6 μm continuum peak observed on reflectance spectra: in case of pure water ice the position of the peak, as measured in laboratory, shifts towards shorter wavelengths when temperature decreases, moving from about 3.65 μm at 123 K to about 3.55 μm at 88 K. When applied to VIMS rings observations, this method allows us to infer the average temperature across ring regions sampled through 400 km-wide radial bins. Comparing VIMS temperature radial profiles with similar CIRS measurements acquired at the same time we have found a substantial agreement between the two instruments’ results across the A and B rings. In general VIMS measures higher temperatures than CIRS across C ring and Cassini division as a consequence of the lower optical depth and the resulting pollution that creates a deviation from pure water ice composition of these regions. VIMS results point out that across C ring and CD the 3.6 μm peak wavelength is always higher than across B and A rings and therefore C ring and CD are warmer than A and B rings. VIMS observations allow us to investigate also diurnal and seasonal effects: comparing antisolar and subsolar ansae observations we have measured higher temperature on the latter. As the solar elevation angle decreases to 0° (equinox), the peak’s position shifts at shorter wavelengths because ring’s particles becomes colder. Merging multi-wavelength data sets allow us to test different thermal models, combining the effects of particle albedo, regolith composition, grain size and thermal properties with the ring structures.
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Polycyclic aromatic hydrocarbons (PAHs) are organic compounds based on fused aromatic rings, and are formed in a variety of astrophysical, solar nebula and planetary processes. Polycyclic aromatic hydrocarbons are known or suspected to occur in a wide variety of planetary settings including icy satellites, Titan’s hazes, carbonaceous meteorites, comet nuclei, ring particles; and terrestrial organic-rich lithologies such as coals, asphaltites, and bituminous sands. Relatively few measurements of the visible and near-infrared spectra of PAHs exist, yet this wavelength region (350–2500 nm) is widely used for remote sensing. This study presents detailed analyses of the 350–2500 nm reflectance spectra of 47 fine-grained powders of different high-purity solid-state PAHs. Spectral properties of PAHs change with variations in the number and connectivity of linked aromatic rings and the presence and type of side-groups and heterocycles. PAH spectra are characterized by three strong features near ∼880 nm, ∼1145 nm, and ∼1687 nm due to overtones of νCH fundamental stretching vibrations. Some PAHs are amenable to remote detection due to the presence of diagnostic spectral features, including: NH stretching overtones at 1490–1515 nm in NH- and NH₂-bearing PAHs, aliphatic or saturated bond CH overtone vibrations at ∼1180–1280 nm and ∼1700–1860 nm; a broad asymmetric feature between ∼1450 nm and ∼1900 nm due to OH stretching overtones in aromatic alcohols, CH and CO combinations near ∼2000–2010 nm and ∼2060–2270 nm in acetyl and carboxyl-bearing PAHs. Other substituents such as sulphonyl, thioether ether and carboxyl heterocycles, or cyano, nitrate, and aromatic side groups, do not produce well-resolved diagnostic spectral features but do cause shifts in the positions of the aromatic CH vibrational overtone features. Fluorescence is commonly suppressed by the presence of heterocycles, side-groups and in many non-alternant PAHs. The spectral characteristics of PAHs offer the potential, under suitable circumstances, for remote characterization of the classes of PAH present and in some cases, identification of particular heterocyclic or side-group substituents.
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Brightness of Saturn's rings with decreasing solar elevation
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Early ground-based and spacecraft observations suggested that the temperature of Saturn's main rings (A, B and C) varied with the solar elevation angle, $B^{'}$ . Data from the composite infrared spectrometer (CIRS) on board Cassini, which has been in orbit around Saturn for more than five years, confirm this variation and have been used to derive the temperature of the main rings from a wide variety of geometries while $B^{'}$ varied from near $- 24^{\circ}$ to $0^{\circ}$ (Saturn's equinox).
Still, an unresolved issue in fully explaining this variation relates to how the ring particles are organized and whether even a simple mono-layer or multi-layer approximation describes this best. We present a set of temperature data of the main rings of Saturn that cover the $\sim 23^{\circ} —range$ of $B^{'}$ angles obtained with CIRS at low ( $α \sim 30^{\circ}$ ) and high ( $α \geq 120^{\circ}$ ) phase angles. We focus on particular regions of each ring with a radial extent $\leq 5000 km$ on their lit and unlit sides. In this broad range of $B^{'}$ , the data show that the A, B and C rings’ temperatures vary as much as 29–38, 22–34 and 18–23 K, respectively. Interestingly the unlit sides of the rings show important temperature variations with the decrease of $B^{'}$ as well. We introduce a simple analytical model based on the well known Froidevaux monolayer approximation and use the ring particles’ albedo as the only free parameter in order to fit and analyze this data and estimate the ring particle's albedo. The model considers that every particle of the ring behaves as a black body and warms up due to the direct energy coming from the Sun as well as the solar energy reflected from the atmosphere of Saturn and on its neighboring particles. Two types of shadowing functions are used. One analytical that is used in the latter model in the case of the three rings and another, numerical, that is applied in the case of the C ring alone. The model lit side albedo values at low phase are 0.59, 0.50 and 0.35–0.38 for the A, B and C rings, respectively.
A multilayer model for thermal infrared emission of Saturn's rings: Basic formulation and implications for Earth-based observations
2009, Icarus
Citation Excerpt :
The difficulty, which complicates modeling of the thermal emission, is that the thermal response to heat sources (e.g., the Sun), unlike responses in other wavelengths, can not be modeled only with an instantaneous snapshot of a ring, as the thermal energy is transported by the azimuthal, vertical, and spin motion of particles. Therefore, thermal modeling must be directly coupled with dynamics of rings (Cuzzi et al., 1984; Spilker et al., 2003). The spin frequency of ring particles is a very important factor.
We present our new model for the thermal infrared emission of Saturn's rings based on a multilayer approximation. In our model, (1) the equation of classical radiative transfer is solved directly for both visible and infrared light, (2) the vertical heterogeneity of spin frequencies of ring particles is taken into account, and (3) the heat transport due to particles motion in the vertical and azimuthal directions is taken into account. We adopt a bimodal size distribution, in which rapidly spinning small particles (whose spin periods are shorter than the thermal relaxation time) with large orbital inclinations have spherically symmetric temperatures, whereas non-spinning large particles (conventionally called slow rotators) with small orbital inclinations are heated up only on their illuminated sides. The most important physical parameters, which control ring temperatures, are the albedo in visible light, the fraction of fast rotators ( $f_{fast}$ ) in the optical depth, and the thermal inertia. In the present paper, we apply the model to Earth-based observations. Our model can well reproduce the observed temperature for all the main rings (A, B, and C rings), although we cannot determine exact values of the physical parameters due to degeneracy among them. Nevertheless, the range of the estimated albedo is limited to $0 – 0.52 \pm 0.05$ , $0.55 \pm 0.07 – 0.74 \pm 0.03$ , and $0.51 \pm 0.07 – 0.74 \pm 0.06$ for the C, B, and A rings, respectively. These lower and upper limits are obtained assuming all ring particles to be either fast and slow rotators, respectively. For the C ring, at least some fraction of slow rotators is necessary ( $f_{fast} ⩽ 0.9$ ) in order for the fitted albedo to be positive. For the A and B rings, non-zero fraction of fast rotators ( $f_{fast} ⩾ 0.1 – 0.2$ ) is favorable, since the increase of the brightness temperature with increasing solar elevation angle is enhanced with some fraction of fast rotators.

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