Estelle Deau

The opposition effect in the outer Solar system: A comparative study of the phase function morphology

Abstract In this paper, we characterize the morphology of the disk-integrated phase functions of satellites and rings around the giant planets of our solar system. We find that the shape of the phase function is accurately represented by a logarithmic model [Bobrov, M.S., 1970. Physical properties of Saturns rings. In: Dollfus, A. (Ed.), Surfaces and Interiors of Planets and Satellites. Academic, New York, pp. 376–461]. For practical purposes, we also parametrize the phase curves by a linear-exponential model [Kaasalainen, S., Muinonen, K., Piironen, J., 2001. Comparative study on opposition effect of icy solar system objects. Journal of Quantitative Spectroscopy and Radiative Transfer 70, 529–543] and a simple linear-by-parts model [Lumme, K., Irvine, W.M., 1976. Photometry of Saturns rings. Astronomical Journal 81, 865–893], which provides three morphological parameters: the amplitude A and the half-width at half-maximum (HWHM) of the opposition surge, and the slope S of the linear part of the phase function at larger phase angles. Our analysis demonstrates that all of these morphological parameters are correlated with the single-scattering albedos of the surfaces. By taking more accurately into consideration the finite angular size of the Sun, we find that the Galilean, Saturnian, Uranian and Neptunian satellites have similar HWHMs ( ≲ 0 . 5 ∘ ), whereas they have a wide range of amplitudes A. The Moon has the largest HWHM ( ∼ 2 ∘ ). We interpret that as a consequence of the “solar size bias”, via the finite angular size of the Sun which varies dramatically from the Earth to Neptune. By applying a new method that attempts to morphologically deconvolve the phase function to the solar angular size, we find that icy and young surfaces, with active resurfacing, have the smallest values of A and HWHM, whereas dark objects (and perhaps older surfaces) such as the Moon, Nereid and Saturns C ring have the largest A and HWHM. Comparison between multiple objects also shows that solar system objects belonging to the same planet have comparable opposition surges. This can be interpreted as a “planetary environmental effect” that acts to locally modify the regolith and the surface properties of objects which are in the same environment.

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

Abstract At 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−2 K−1 s−1/2 in the middle A ring whereas it is ∼10 J m−2 K−1 s−1/2 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 (<108 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.