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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.09.001 Byline: C. Ferrari (a)(b), S. Brooks (c), S. Edgington (c), C. Leyrat (c), S. Pilorz (c), L. Spilker (c) Keywords: Planetary rings; Infrared observations; Saturn; rings; Disks Abstract: 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[degrees] to 37[degrees]. Its fractional amplitude decreases from 22% to 8% from 20[degrees] to 37[degrees] elevations. The patterns observed in two almost complete azimuthal scans at elevations 20[degrees] and 36[degrees] 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/I'=0.1607[+ or -]0.0002, a width-over-spacing W/I'=0.3833[+ or -]0.0008. Gaps between wakes, which are filled with particles, have an optical depth I.sub.G=0.1231[+ or -]0.0005. The wakes mean pitch angle I[bar].sub.W is 70.70[degrees][+ or -]0.07[degrees], 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 gacm.sup.-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 I'[approximately equal to]60 m. Their height and width would then be H[approximately equal to]10 m and W[approximately equal to]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]. Author Affiliation: (a) CEA, IRFU, AIM, F-91191 Gif-sur-Yvette, France (b) Universite Paris Diderot Paris 7, Laboratoire AIM, F-75205 Paris cedex 13, France (c) Jet Propulsion Laboratory/NASA, Pasadena, CA 91109, USA Article History: Received 4 February 2008; Revised 8 September 2008; Accepted 12 September 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.06.014 Byline: N. Altobelli (a), L. Spilker (a), S. Pilorz (a), S. Brooks (a), S. Edgington (a), B. Wallis (a), M. Flasar (b) Keywords: Planetary rings; Saturn, rings; Spectroscopy Abstract: We analyze data sets obtained with the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft after the Saturn Orbit Insertion (SOI). Using the mid-IR interferometer's FP3 channel (600-1100 cm.sup.-1), we derive radial temperature profiles for the C ring with a spatial resolution never achieved before. For the first time, the C ring's plateaus and ringlets can be clearly separated from the optically thinner background and their thermal behavior is studied separately for different viewing geometries. In particular, thermal phase curves derived for the plateaus reveal an interesting surge near 0[degrees] phase, not observed in the background. We show that mutual shadowing in the plateaus can explain the existence of the surge but is not sufficient to model the phase curves in detail. By analogy with thermal emission of asteroid surfaces we discuss the possible influence of small scale and large scale roughness of the ring structure itself. Because infrared emissivity cannot be derived without being deconvolved from the 'structural' filling factor, we examine temperature and filling factors measurements at opposition where the filling factor is most constrained. The occurrence of higher temperatures in the plateaus than in the background near opposition likely arises from enhanced mutual heating between particles, multiple scattering and surface roughness combined with a higher single-scattering albedo. Author Affiliation: (a) NASA-JPL, 4800 Oak Grove Drive, Mail Stop 230-205, Pasadena, CA 91109, USA (b) Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 10 May 2006; Revised 31 May 2007

We measured the change in reflectance and in circular polarization ratio with respect to phase angle of a suite of well-sorted, highly reflective, aluminum oxide powders for the purpose of understanding the contribution of the coherent backscattering phenomenon to the reflectance phase curves of planetary regoliths. The goniometric photopolarimeter that we used measured reflectance and circular polarization ratio over the range of phase angles from 0.05 [degrees] to 5 [degrees] where it has been suggested that coherent backscattering becomes important. The particle size of the samples varied from 0.1 to 30 [micro]m. The wavelength of the illuminating radiation from a HeNe laser was 0.633 [micro]m. We find that the reflectance at 0.05 [degrees] is weakly dependent on particle size. We measured to good approximation the half-width half-maximum (HWHM) of the phase curves, and we find that there is only a weak qualitative agreement between our experimental results and the theoretical estimates of the HWHM for particles of comparable index of refraction and packing density as predicted by M. Mishchenko (1992, Astrophys. Space Sci. 194, 327-333). For particle sizes that are close to the wavelength of the illuminating radiation we find that Mishchenko's model overestimates the HWHM of the phase curve by an order of magnitude. For particle sizes that are much larger or much smaller than the wavelength of the incident radiation the theoretical model underestimates the HWHM of the phase curve by more than an order of magnitude for the smallest particle sizes and factors of several for the large particle sizes. This disagreement between our experimental data and the predictions from theoretical modeling is not unexpected given the assumptions made in the theoretical models that the particles scatter light like isolated, perfect uniform spheres and that the variation in particle size distribution used by Mishchenko was remarkably small, smaller than that of the well-sieved particle sizes used in our experiment. For all samples the circular polarization ratio increased with decreasing phase angle consistent with the hypothesis that the coherent backscattering process is the principal contributor to the reflectance phase curve near 0 [degrees] for highly reflective, multiply scattering, particulate media. We confirm our previously reported significant opposition surges for powders with sizes 50 times larger and 6 times smaller than the wavelength of the incident light (R. Nelson et al. 1998, Proc. Lurar Planet. Sci. Conf. 29th, Abstract 1146). This suggests that for cases where the wavelength of the incident radiation is much larger than the candidate regolith particle size, the photons behave as if they were interacting with ensembles of particles of size comparable to the wavelength of the incident radiation. For cases where the particle size is much larger than the wavelength of the incident light, the photons appear to be interacting primarily with wavelength-sized cracks, surface asperities, or other irregularities in the regolith particles rather than with the particle as a whole. This work, using sorted powders as planetary surface regolith analogues, has applicability for remote sensing studies of highly reflective planetary regoliths such as might be expected to be found on many the icy satellites of the outer Solar System. Many of these bodies exhibit sharp opposition surges that are most likely due to coherent backscattering. [C] 2000 Academic Press