Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.0910h&link_type=abstract
American Astronomical Society, DPS meeting #42, #9.10; Bulletin of the American Astronomical Society, Vol. 42, p.956
Astronomy and Astrophysics
Astronomy
Scientific paper
The water ice on the Galilean satellites is segregated spatially from the darker non-ice materials by a well known thermal process that depends on the temperature of the different materials (for the Galilean satellites, pure water snow has temperatures under full sun of 120-130 K while the segregated dark materials reach 160 K. The Saturnian satellites with the darkest materials (1-3 % albedo), i.e., Iapetus, Phoebe, and Hyperion, also have been shown to have spatially segregated dark material which reaches 130 K. But the bright material need not be pure water ice, and in fact is mostly slightly dirty ice with an albedo of 30-50%. This is evident from the overall albedo of extended bright areas such as on the trailing hemisphere of Iapetus, and from the lack of weak bands of water ice near 1.04 and 1.25 microns in its near-infrared spectrum. These bands are the first to disappear when you mix water ice with dark substances. We will run radiative transfer models to show how much dark material needs to be mixed into the ice to remove these bands (we expect <<0.1% mass ratio) and calculate the modified area of icy material in models we have done for Phoebe using Cassini-VIMS data, where we assumed pure water snow as the icy component. Such modeling, even for these segregated terrains, is complicated because the amount of dark material in the ice can vary a lot and still achieve segregation. We will estimate the maximum dark material mixing ratio beyond which the albedo is to low for effective segregation, while nearly pure ice snow can also exist (isolated locations on Phoebe do show the short-wave ice bands, and are therefore purer ice than typical). Models of the areal fraction of icy material may have large uncertainties because of this effect.
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