Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.2101m&link_type=abstract
American Astronomical Society, DPS meeting #38, #21.01; Bulletin of the American Astronomical Society, Vol. 38, p.518
Astronomy and Astrophysics
Astronomy
Scientific paper
Recent discoveries prompt a new assessment of the properties of large, transneptunian objects. With the system barycenter determined by Nix and Hydra and its size by stellar occulation, Charon's density is now known at 1.65 ± 0.06 g cm-3 (Buie et al., A.J. 132, 290, 2006). Structural calculations imply a 60/40 rock/ice ratio, lower than the system as a whole, but still geophysically significant. Early temperatures should have easily reached the ammonia-water ice eutectic, if not water-ice melting. While ice-rock differentiation is not assured in the presence of solid-state convection, convection is now known to be a less efficient heat transport mechanism compared with earlier parameterized convection models. If Charon managed to remain undifferentiated, it should have undergone prolonged cooling and internal conversion of ice I to II, with clear observational consequences. Bodies of the Pluto class mark the transition from solely ice-I shells (differentiation assumed) to shells with lower layers of higher-pressure ice phases (e.g., 2003 UB313); the Pluto class also marks the transition from bodies with Europa-type to Callisto-type oceans (if they have oceans), and if one ignores the possibility of intervening organic-rich layers. Although the very cold surfaces of the ice dwarfs work against the possibility of solid-state convection, the deep ice shells in Pluto-class bodies more than compensate; Pluto itself is near the tipping point for convective shut-down, a late stage in the evolution of large rock-ice bodies in which internal oceans thicken rather than thin. In contrast, the rapidly rotating 2003 EL61, if interpreted as Jacobi ellipsoid, has a density close to 2.6 g cm-3 (Rabinowitz et al., Ap. J. 639, 1238, 2006). This implies a Europa-like structure, 85/15 rock/ice by mass with an 100-km thick ice shell (though no ocean). This work supported by NASA OPR and New Horizons.
Barr Amy Courtright
McKinnon William B.
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