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Composition of solids in the Saturn system, from organics to other non-ice components in the rings and icy satellites and the role of nano-particles

Wednesday, June 11, 2014

It has recently been shown that the UV to NIR spectra (0.35-5.1 microns) of the icy rings and satellites of Saturn have spectra that can be explained by combinations of water ice, CO2, nano-sized grains of metallic iron and iron oxide (Clark et al., 2012, Icarus 218, p831) and trace amounts of other compounds. Classical interpretations of the UV absorber and dark material on outer Solar System satellites have been varying amounts of tholins and carbon. Nano-grains create both Rayleigh absorption and Rayleigh scattering making unusual spectral signatures that are seen in the Saturn system. The wide range of observed UV-NIR (0.1-5 micron) spectral signatures provide strong constraints on composition and grain size distribution. Spectra of the Saturnian rings and icy satellites indicate a large range of ice grain sizes, from tens of microns to sub-micron. Sub-micron ice grains create unusual spectral properties, which are seen in the spectra of the rings and satellites of Saturn and on satellites further out in the Solar System. In the blue/UV, spectra of Saturns' rings depart from that of pure ice because of the UV absorber. Some spectra of Saturns' rings are very similar to spectra of some locations on icy satellites, indicating that common compounds are spectrally active from the rings out to Phoebe. Similar spectral properties, created by the UV absorber, are seen for other icy objects including Callisto and Ganymede in the Jupiter system, and in the Uranian system. Asteroids and meteorites also show similar UV absorption, suggesting common origins. We modeled the reflectance spectra of numerous objects from the asteroid belt and further out in the Solar System as well as those for meteorites. We find that the coloring agent on many surfaces can be explained, at least in part, by the competing effects of Rayleigh scattering by fine particles, and Rayleigh absorption by highly absorbing materials like metallic iron. Iron is more than an order of magnitude more absorbing than carbon and as a Rayleigh absorber, creates a stronger UV to visible spectral slope than does carbon. If nano-iron is pervasive in the outer Solar System as the observations imply, then either space weathering is active throughout the Solar System, and/or nano grains of Iron may be entering our system as a component of interstellar dust and/or left over the formation of the Solar System.  New laboratory work is showing the varying conditions when Rayleigh scattering versus Rayleigh absorption dominates the spectral response of a planetary/satellite surface.

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