Evolution of the dusty rings of Uranus

We present high quality images of the uranian ring system, obtained in August 2002, October 2003, and July 2004 at 2.2 μm with the adaptive optics camera NIRC2 on the Keck II telescope. Using these data, we report the first detection in backscattered light of a ring (which we refer to as the ζ ring) interior to Uranus' known rings. This ring consists of a generally uniform sheet of dust between 37,850 and 41,350 km with an equivalent width (in 2004; or ), and extends inward to 32,600 km at a gradually decreasing brightness. This ring might be related to the Voyager ring R/1986 U 2, although both its location and extent differ. This could be attributed to a difference in observing wavelength and/or solar phase angle, or perhaps to temporal variations in the ring. Through careful modeling of the I/F of the individual rings at each ansa, we reveal the presence of narrow (few 100 km wide) sheets of dust between the δ and ε rings, and between rings 4 and α. We derived a typical anisotropy factor g≈0.7 in the scattering behavior of these particles. The spatial distribution and relative intensity of these dust sheets is different than that seen in Voyager images taken in forward scattered light, due either to a difference in observing wavelength, and/or solar phase angle or to changes over time. We may have detected the λ ring in one scan at , but other scans provided upper limits below this value. A single detection, however, would be consistent with azimuthal asymmetries known to exist in this ring. We further demonstrate the presence of azimuthal asymmetries in all rings. We confirm the eccentricity of ∼0.001 in rings 4, 5, 6, which in 2004 are ∼70 km closer to Uranus in the north (near periapse; lower I/F) than in the south. We find a global optical depth of τ∼0.3 in the main rings, and of τ=0.25±0.05 in the ε ring.     

Keck and VLT AO observations and models of the uranian rings during the 2007 ring plane crossings

We present observations of the uranian ring system at a wavelength of 2.2 μm, taken between 2003 and 2008 with NIRC2 on the W.M. Keck telescope in Hawaii, and on 15–17 August 2007 with NaCo on the Very Large Telescope (VLT) in Chile. Of particular interest are the data taken around the time of the uranian ring plane crossing with Earth on 16 August 2007, and with the Sun (equinox) on 7 December 2007. We model the data at the different viewing aspects with a Monte Carlo model to determine: (1) the normal optical depth τ₀, the location, and the radial extent of the main rings, and (2) the parameter Aτ₀ (A is the particle geometric albedo), the location, and the radial plus vertical extent of the dusty rings. Our main conclusions are: (i) The brightness of the ? ring is significantly enhanced at small phase and ring inclination angles; we suggest this extreme opposition effect to probably be dominated by a reduction in interparticle shadowing. (ii) A broad sheet of dust particles extends inwards from the λ ring almost to the planet itself. This dust sheet has a vertical extent of ∼140 km, and Aτ₀ = 2.2 × 10⁻⁶. (iii) The dusty rings between ring 4 and the α ring and between the α and β rings are vertically extended with a thickness of ∼300 km. (iv) The ζ ring extends from ∼41,350 km almost all the way inwards to the planet. The main ζ ring, centered at ∼39,500 km from the planet, is characterized by Aτ₀ = 3.7 × 10⁻⁶; this parameter decreases closer to the planet. The ζ ring has a full vertical extent of order 800–900 km, with a pronounced density enhancement in the mid-plane. (v) The η_c ring is optically thin and less than several tens of km in the vertical direction. This ring may be composed of macroscopic material, surrounded by clumps of dust.     

Spatially resolved cloud structure on Uranus: Implications of near-IR adaptive optics imaging

Seven-band near-IR adaptive optics imaging of Uranus by the Keck II telescope during 2004, with the assistance of selected Hubble Space Telescope images, provides new constraints on the uranian vertical cloud structure and CH₄ mixing ratio, after tuned deconvolutions are applied to remove significant limb darkening distortions. The most strongly absorbing bands approximately agree with the stratospheric haze model of Rages et al. [Rages, K., Pollack, J.B., Tomasko, M.G., Doose, L.R., 1991. Icarus 89, 359–376]. The next most absorbing bands suggest a CH₄ relative humidity of 50–60% above the 1.2-bar condensation level. Window channels imply effective cloud pressures at 12° S that vary from 9 to 3.5 bars, and reflectivity values that vary from 7 to 4%, as the assumed CH₄ mixing ratio varies from 0.75 to 4%. The shape of the center-to-limb radiance profile is in best agreement with the deep cloud being translucent, with relatively low optical depth, and is most consistent with low methane mixing ratios (0.75–1%) if the cloud particles are conservative. Non-conservative particles provide good fits over a wide range of mixing ratios. If C and S are enhanced by the same factor over solar mixing ratios, then the cloud pressures inferred from near-IR observations would be less than H₂S condensation pressures for methane mixing ratios of 1.3% or greater. The bright band at 45° S must be partly produced by increased particulate scattering at pressures 2 bars to be consistent with its absence in 1.9-μm images and its presence in 0.619-μm images. The reflectivity of the lower clouds declines to nearly negligible values in the northern hemisphere, where I/F observations beyond 50° N are nearly those of a clear atmosphere. The most surprising result is the general lack of scattering originating from the 1.2-bar region where methane is expected to condense. Exceptions occur for discrete features. A large and long-lived discrete feature at 34° S is associated with particulates near 700 mb and 4.5 bars. The highest discrete feature, near 26° N, reached pressures 200 mb and was eleven times brighter than the background atmosphere in K^′ images.     

Keck NIRC2 photometry of Uranus, uranian satellites, and Triton in August 2004

On August 11, 2004, we made adaptive optics observations of the Uranus and Neptune systems with the Keck II Near Infrared Camera. Uranus and Triton were observed in three broadband filters (J, H, and K-prime) and four narrowband filters [Hcont, FeII, He1_B, and H2(v=1-0)]. Miranda, Ariel, Umbriel, and Oberon were observed in the four narrowband filters only. To achieve the highest possible photometric accuracy, and thus the tightest possible constraints on atmospheric aerosol models and gas mixing ratios, we used aperture photometry that accounted for the dependence of point-spread functions and growth curves on the adaptive optics reference object, and accounted for recently determined phase curves of each object. The satellite albedos we determined were compared with published relative spectra to verify the relative consistency of our observations, which were subsequently used to convert published relative spectra to absolute spectra. We used these absolute spectra and synthetic photometry methods to compare published values in dissimilar photometric systems to each other and to our observations. We find our satellite albedos in best agreement with photometry from Karkoschka [Karkoschka, E., 2001. Icarus 151, 51-68], which is the best characterized and most contemporaneous data set. Our estimated absolute accuracy of 5% to 8% is consistent with these intercomparisons. We obtained the following ring-subtracted and discrete feature-free albedos of Uranus: J: (1.66±0.07)×¹⁰⁻², H: (1.09±0.05)×¹⁰⁻², K^′: (2.08±0.15)×¹⁰⁻⁴, Hcont: (3.71±0.23)×¹⁰⁻², FeII: (1.14±0.07)×¹⁰⁻³, He1_B: (2.06±0.16)×¹⁰⁻⁴, and H2: (1.27±0.10)×¹⁰⁻⁴.     

Modeling the uranian rings at 2.2 μm: Comparison with Keck AO data from July 2004

We present a Monte Carlo model of the uranian rings, and compare this model to images of the system obtained with the Keck adaptive optics system in July 2004, at a wavelength of 2.2 μm (from de Pater et al. (de Pater, I., Gibbard, S.G., Hammel, H.B. [2006a]. Icarus 180, 186-200)). We confirm the presence of the ζ ring, but show that this ring must extend inwards much further than previously thought, although with an optical depth much lower than that in the main ζ ring component. We further confirm dust rings between rings α-4 and β-α, as well as near the λ ring. In addition, we show that a broad sheet of faint material (τ₀ ∼ 10⁻³) must be present through most of the ring region, from the α ring through the λ ring.     

A close look at Saturn's rings with Cassini VIMS

Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H₂O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring.     

Near-IR methane absorption in outer planet atmospheres: Improved models of temperature dependence and implications for Uranus cloud structure

Near-IR absorption of methane in the 2000-9500 cm⁻¹ spectral region plays a major role in outer planet atmospheres. However, the theoretical basis for modeling the observations of reflectivity and emission in these regions has had serious uncertainties at temperatures needed for interpreting observations of the colder outer planets. A lack of line parameter information, including ground-state energies and the absence of weak lines, limit the applicability of line-by-line calculations at low temperatures and for long path lengths, requiring the use of band models. However, prior band models have parameterized the temperature dependence in a way that cannot be accurately extrapolated to low temperatures. Here we use simulations to show how a new parameterization of temperature dependence can greatly improve band model accuracy and allow extension of band models to the much lower temperatures that are needed to interpret observations of Uranus, Neptune, Titan, and Saturn. Use of this new parameterization by Irwin et al. [Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Bowles, N., Calcutt, S.B., 2005b. Icarus. In press] has verified improved fits to laboratory observations of Strong et al. [Strong, K., Taylor, F.W., Calcutt, S.B., Remedios, J.J., Ballard, J., 1993. J. Quant. Spectrosc. Radiat. Trans. 50, 363-429] and Sihra [1998. Ph.D. Thesis, Univ. of Oxford], which cover the temperature range from 100 to 340 K. Here we compare model predictions to 77 K laboratory observations and to Uranus spectra, which show much improved agreement between observed and modeled spectral features, allowing tighter constraints on pressure levels of Uranus cloud particles, implying that most scattering contributions arise from pressures near 2 bars and 6 bars rather than expected pressures near 1.25 and 3.1 bars. Between visible and near-IR wavelengths, both cloud layers exhibit strong decreases in reflectivity that are indicative of low opacity and submicron particle sizes.     

Keck near-infrared observations of Saturn's E and G rings during Earth's ring plane crossing in August 1995

We present near-infrared (1.24-2.26 μm) images of Saturn's E and G rings which were taken with the W.M. Keck telescope in 1995 August 9-11, during the period that Earth crossed Saturn's ring plane. Our data confirm that the E ring is very blue. Its radial and vertical structure are found to be remarkably similar to that apparent in the HST ringplane crossing data at visible wavelengths, reinforcing models of the ring's peculiar narrow or very steep particle size distribution. Our data show unambiguously that the satellite Tethys is a secondary source of material for the E ring. The G ring is found to be distinctly red, similar in color to Jupiter's main ring, indicative of a (more typical) broad particle size distribution.     

Post-equinox observations of Uranus: Berg’s evolution, vertical structure, and track towards the equator

We present observations of Uranus taken with the near-infrared camera NIRC2 on the 10-m W.M. Keck II telescope, the Wide Field Planetary Camera 2 (WFPC2) and the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) from July 2007 through November 2009. In this paper we focus on a bright southern feature, referred to as the “Berg.” In Sromovsky et al. (Sromovsky, L.A., Fry, P.M., Hammel, H.B., Ahue, A.W., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M. [2009]. Icarus 203, 265-286), we reported that this feature, which oscillated between latitudes of −32° and −36° for several decades, suddenly started on a northward track in 2005. In this paper we show the complete record of observations of this feature’s track towards the equator, including its demise. After an initially slow linear drift, the feature’s drift rate accelerated at latitudes ∣θ∣ < 25°. By late 2009 the feature, very faint by then, was spotted at a latitude of −5° before disappearing from view. During its northward track, the feature’s morphology changed dramatically, and several small bright unresolved features were occasionally visible poleward of the main “streak.” These small features were sometimes visible at a wavelength of 2.2 μm, indicative that the clouds reached altitudes of ∼0.6 bar. The main part of the Berg, which is generally a long sometimes multipart streak, is estimated to be much deeper in the atmosphere, near 3.5 bars in 2004, but rising to 1.8-2.5 bars in 2007 after it began its northward drift. Through comparisons with Neptune’s Great Dark Spot and simulations of the latter, we discuss why the Berg may be tied to a vortex, an anticyclone deeper in the atmosphere that is visible only through orographic companion clouds.     

Vertical cloud structure of Uranus from UKIRT/UIST observations and changes seen during Uranus’ northern spring equinox from 2006 to 2008

Long-slit spectroscopy observations of Uranus by the United Kingdom Infrared Telescope UIST instrument in 2006, 2007 and 2008 have been used to monitor the change in Uranus’ vertical and latitudinal cloud structure through the planet’s northern spring equinox in December 2007.The observed reflectance spectra in the Long J (1.17-1.31 μm) and H (1.45-1.65 μm) bands, obtained with the slit aligned along Uranus’ central meridian, have been fitted with an optimal estimation retrieval model to determine the vertical cloud profile from 0.1 to 6-8 bar over a wide range of latitudes. Context images in a number of spectral bands were used to discriminate general zonal cloud structural changes from passing discrete clouds. From 2006 to 2007 reflection from deep clouds at pressures between 2 and 6-8 bar increased at all latitudes, although there is some systematic uncertainty in the absolute pressure levels resulting from extrapolating the methane coefficients of Irwin et al. (Irwin, P.G.J., Sromovsky, L.A., Strong, E.K., Sihra, K., Teanby, N.A., Bowles, N., Calcutt, S.B., Remedios, J.J. [2006] Icarus, 181, 309-319) at pressures greater than 1 bar, as noted by Tomasko et al. and Karkoschka and Tomasko (Tomasko, M.G., Bezard, B., Doose, L., Engel, S., Karkoschka, E. [2008] Planet. Space Sci., 56, 624-647; Karkoschka, E., Tomasko, M. [2009] Icarus). However, from 2007 to 2008 reflection from these clouds throughout the southern hemisphere and from both northern and southern mid-latitudes (30° N,S) diminished. As a result, the southern polar collar at 45°S has diminished in brightness relative to mid-latitudes, a similar collar at 45°N has become more prominent (e.g. Rages, K.A., Hammel, H.B., Sromovsky, L. [2007] Bull. Am. Astron. Soc., 39, 425; Sromovsky, L.A., Fry, P.M., Ahue, W.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R., van Dam, M.A. [2008] vol. 40 of AAS/Division for Planetary Sciences Meeting Abstracts, pp. 488-489; Sromovsky, L.A., Ahue, W.K.M., Fry, P.M., Hammel, H.B., de Pater, I., Rages, K.A., Showalter, M.R. [2009] Icarus), and the lowering reflectivity from mid-latitudes has left a noticeable brighter cloud zone at the equator (e.g. Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557;Karkoschka, E., Tomasko, M. [2009] Icarus). For such substantial cloud changes to have occurred in just two years suggests that the circulation of Uranus’ atmosphere is much more vigorous and/or efficient than is commonly thought. The composition of the main observed cloud decks between 2 and 6-8 bar is unclear, but the absence of the expected methane cloud at 1.2-1.3 bar (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987] J. Geophys. Res., 92, 14987-15001) is striking (as previously noted by, among others, Sromovsky, L.A., Irwin, P.G.J., Fry, P.M. [2006] Icarus, 182, 577-593; Sromovsky, L.A., Fry, P.M. [2007] Icarus, 192, 527-557; Sromovsky, L.A., Fry, P.M. [2008] Icarus, 193, 252-266; Karkoschka, E., Tomasko, M. [2009] Icarus) and suggests that cloud particles may be considerably different from pure condensates and may be linked with stratospheric haze particles drizzling down from above, or that tropospheric hazes are generated near the methane condensation level and then drizzle down to deep pressures as suggested by Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus).The retrieved cloud structures were also tested for different assumptions of the deep methane mole fraction, which Karkoschka and Tomasko (Karkoschka, E., Tomasko, M. [2009] Icarus) find may vary from ∼1-2% in polar regions to perhaps as much as 4% equatorwards of 45°N,S. We found that such variations did not significantly affect our conclusions.     

The rotational temperature and column density of H3 in Uranus

H₃⁺ emission from Uranus has been observed repeatedly for over a decade. However, the details of the emission mechanisms are still poorly understood. In this paper, we discuss our findings from the observations we made in September 2000 and September 2001. The spectrum of Uranus was recorded at the NASA Infrared Telescope Facility using the SpeX instrument between 3 and 5 μm, with a resolving power of 1000. The 3.4–4.1 μm range permits a determination of both the H₃⁺ column density and its rotational temperature. The H₃⁺ emission, measured at 3.986 μm in the 0.8×3.7 arcsec aperture, was 0.031 Jy in September 2000 and 0.053 Jy in September 2001. The rotational temperature was found to be 560±40 K and 640±40 K in 2000 and 2001 respectively, with corresponding column densities of 5.1 (+3.2,−1.4) 10¹¹ and 4.0 (+1.8,−1.0) 10¹¹ cm⁻². These results extend the baseline for the variability study of the H₃⁺ emission (Astrophys. J. 524 (1999) 1059). Previous observations between 1992 and 1998 seemed to indicate a correlation between the H₃⁺ intensity and the solar cycle. The current data for 2000 and 2001 appear to be consistent with this general tendency.     

The size distribution of Jupiter's main ring from Galileo imaging and spectroscopy

Galileo's Solid State Imaging experiment (SSI) obtained 36 visible wavelength images of Jupiter's ring system during the nominal mission (Ockert-Bell et al., 1999, Icarus 138, 188-213) and another 21 during the extended mission. The Near Infrared Mapping Spectrometer (NIMS) recorded an observation of Jupiter's main ring during orbit C3 at wavelengths from 0.7 to 5.2 μm; a second observation was attempted during orbit E4. We analyze the high phase angle NIMS and SSI observations to constrain the size distribution of the main ring's micron-sized dust population. This portion of the population is best constrained at high phase angles, as the light scattering behavior of small dust grains dominates at these geometries and contributions from larger ring particles are negligible. High phase angle images of the main ring obtained by the Voyager spacecraft covered phase angles between 173.8° and 176.9° (Showalter et al., 1987, Icarus 69, 458-498). Galileo images extend this range up to 178.6°. We model the Galileo phase curve and the ring spectra from the C3 NIMS ring observation as the combination of two power law distributions. Our analysis of the main ring phase curve and the NIMS spectra suggests the size distribution of the smallest ring particles is a power law with an index of 2.0±0.3 below a size of ∼15 μm that transitions to a power law with an index of 5.0±1.5 at larger sizes. This combined power law distribution, or “broken power law” distribution, yields a better fit to the NIMS data than do the power law distributions that have previously been fit to the Voyager imaging data (Showalter et al., 1987, Icarus 69, 458-498). The broken power law distribution reconciles the results of Showalter et al. (1987, Icarus 69, 458-498) and McMuldroch et al. (2000, Icarus 146, 1-11), who also analyzed the NIMS data, and can be considered as an obvious extension of a simple power law. This more complex size distribution could indicate that ring particle production rates and/or lifetimes vary with size and may relate to the physical processes that control their evolution. The significant near arm/far arm asymmetry reported elsewhere (see Showalter et al., 1987, Icarus 69, 458-498; Ockert-Bell et al., 1999, Icarus 138, 188-213) persists in the data even after the main ring is isolated in the SSI images. However, the sense of the asymmetry seen in Galileo images differs from that seen in Voyager images. We interpret this asymmetry as a broad-scale, azimuthal brightness variation. No consistent association with the magnetic field of Jupiter has been observed. It is possible that these longitudinal variations may be similar to the random brightness fluctuations observed in Saturn's F ring by Voyager (Smith et al., 1982, Science 215, 504-537) and during the 1995 ring plane crossings (Nicholson et al., 1996, Science 272, 509-515; Bosh and Rivkin, 1996, Science 272, 518-521; Poulet et al., 2000, Icarus 144, 135-148). Stochastic events may thus play a significant role in the evolution of the jovian main ring.     

Near-infrared spectrophotometry of the satellites and rings of Uranus

New spectrophotometry from 1.5 to 2.5 μm is reported for the Uranian satellites Titania, Oberon, and Umbriel. A spectrum of the rings of Uranus from 2.0 to 2.4 μm is also reported. No evidence is found for frost covering the surface of the ring material, consistent with the low albedo of the rings (P_K = 0.03) previously reported by Nicholson and Jones (1980). The surfaces of the satellites are found to be covered by dirty water frost. Assuming albedos of the frost and gray components covering the Uranian satellites to be the same as the light and dark faces of Iapetus, radii are derived that are roughly twice those inferred from the assumption of a visual albedo of 0.5.     

Long-term variations in the microwave brightness temperature of the Uranus atmosphere

We present a self-consistent, 36-year record of the disk-averaged radio brightness of Uranus at wavelengths near 3.5 cm. It covers nearly half a uranian year, and includes both equatorial and polar viewing geometries (corresponding to equinox and solstice, respectively). We find large (greater than 30 K) changes over this time span. In agreement with analyses made of more limited microwave data sets, our observations suggest the changes are not caused by geometric effects alone, and that temporal variations may exist in the deep uranian troposphere down to pressures of tens of bars. Our data also support an earlier suggestion that a rapid, planetary-scale change may have occurred in late 1993 and early 1994. The seasonal record presented here will be useful for constraining dynamical models of the deep atmosphere, and for interpreting observations made during Uranus' 2007 equinox passage. As part of a multi-wavelength observing campaign for this event, the Goldstone-Apple Valley Radio Telescope (GAVRT) project will continue to make frequent, single-dish observations near 3.5 cm.     

Structure of self-gravity wakes in Saturn's A ring as measured by Cassini CIRS

The CIRS infrared spectrometer onboard the Cassini spacecraft has scanned Saturn's A ring azimuthally from several viewing angles since its orbit insertion in 2004. A quadrupolar asymmetry has been detected in this ring at spacecraft elevations ranging between 16° to 37°. Its fractional amplitude decreases from 22% to 8% from 20° to 37° elevations. The patterns observed in two almost complete azimuthal scans at elevations 20° and 36° strongly favor the self-gravity wakes as the origin of the asymmetry. The elliptical, infinite cylinder model of Hedman et al. [Hedman, M.M., Nicholson, P.D., Salo, H., Wallis, B.D., Buratti, B.J., Baines, K.H., Brown, R.H., Clark, R.N., 2007. Astron. J. 133, 2624-2629] can reproduce the CIRS observations well. Such wakes are found to have an average height-to-spacing ratio H/λ=0.1607±0.0002, a width-over-spacing W/λ=0.3833±0.0008. Gaps between wakes, which are filled with particles, have an optical depth τG=0.1231±0.0005. The wakes mean pitch angle ΦW is 70.70°±0.07°, relative to the radial direction. The comparison of ground-based visible data with CIRS observations constrains the A ring to be a monolayer. For a surface mass density of 40 g cm⁻² [Tiscarino, M.S., Burns, J.A., Nicholson, P.D., Hedman, M.M., Porco, C.C., 2007. Icarus 189, 14-34], the expected spacing of wakes is λ≈60 m. Their height and width would then be H≈10 m and W≈24 m, values that match the maximum size of particles in this ring as determined from ground-based stellar occultations [French, R.G., Nicholson, P.D., 2000. Icarus 145, 502-523].     

Properties and dynamics of Jupiter's gossamer rings from Galileo, Voyager, Hubble and Keck images

We present a comprehensive examination of Jupiter's “gossamer” rings based on images from Voyager, Galileo, the Hubble Space Telescope and the W.M. Keck Telescope. We compare our results to the simple dynamical model of Burns et al. [Burns, J.A., Showalter, M.R., Hamilton, D.P., Nicholson, P.D., de Pater, I., Ockert-Bell, M., Thomas, P., 1999. Science 284, 1146-1150] in which dust is ejected from Amalthea and Thebe and then evolves inward under Poynting-Robertson drag. The ring follows many predictions of the model rather well, including a linear reduction in thickness with decreasing radius. However, some deviations from the model are noted. For example, additional material appears to be concentrated just interior to the orbits of the two moons. At least in the case of Amalthea's ring, that material is in the same orbital plane as Amalthea's inclined orbit and may be trapped at the Lagrange points. Thebe's ring shows much larger vertical excursions from the model, which may be related to perturbations by several strong Lorentz resonances. Photometry is consistent with the dust obeying a relatively flat power-law size distribution, very similar to dust in the main ring. However, the very low backscatter reflectivity of the ring, and the flat phase curve of the ring at low phase angles, require that the ring be composed of distinctly non-spherical particles.     

Thermal response of Iapetus to an eclipse by Saturn's rings

Measurements of Iapetus as seen at 20 and 2.2 μm in the shadow of Saturn's ring are given, providing the thermal response to a rapidly varying heat input. The 20 μm thermal emission follows the 2.2 μm flux input closely. The observations, plus a simple diffusion calculation, imply that the surface of Iapetus is made of material having a very small thermal inertia, probably .     

The dynamic neptunian ring arcs: evidence for a gradual disappearance of Liberté and resonant jump of courage

We present Adaptive Optics observations of Neptune's ring system at 1.6 and 2.2 μm, taken with the 10-m W.M. Keck II telescope in July 2002 and October 2003. We recovered the full Adams and Le Verrier rings for the first time since the Voyager era (1989), and show that the overall appearance of these rings did not change much, except for the ring arcs. Both the location and intensity of all arcs changed drastically relative to trailing arc Fraternité, which has a mean orbital motion of 820.1118 ± 0.0001 deg/day, equal to that of Nicholson et al.'s (1995, Icarus 113, 295-330) solution 2. Our data suggest that all arcs may have decayed over the last decade, while Liberté, in 2003, may be on the verge of disappearing completely. The observed changes in the relative intensities and locations of all arcs further indicate that material is migrating between resonance sites; leading arc Courage, for example, has jumped ∼8°, or, when adopting Namouni and Porco's (2002, Nature 417, 45-47) CER (corotation eccentricity resonance) theory, it advanced by one full corotation potential maximum. Overall, our observations reveal a system that is surprisingly dynamic, and no comprehensive theory exists as of yet that can explain all the observed intricacies.     

The discovery of faint irregular satellites of Uranus

We report the discovery of four new uranian irregular satellites in our deep, m_R∼25.4, optical search around that planet. The orbital properties of these satellites are diverse. There is some grouping of inclinations and one of the satellites appears to be inside the Kozai resonant zone of Uranus. Further, we find that the differential size distribution of satellites is rather shallow compared to objects in the asteroid and Kuiper belts, going as ∼r^−2.4. We also report a strong coupling between semi-major axis and orbital eccentricity. We comment on the apparent paradox between the inclination grouping, shallow size distribution, and orbital correlation as they relate to the likelihood of a collisional origin for the uranian irregulars. The currently observed irregulars appear to be consistent with a disruptive formation process and a collisional origin for Uranus' obliquity.     

Saturn's rings in the thermal infrared

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 Jm⁻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.