Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p43f..08m&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P43F-08
Physics
[6040] Planetary Sciences: Comets And Small Bodies / Origin And Evolution, [6207] Planetary Sciences: Solar System Objects / Comparative Planetology, [6218] Planetary Sciences: Solar System Objects / Jovian Satellites, [6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
After the Galileo mission to Jupiter, Callisto's (normalized) moment of inertia (0.355) provided a stark contrast to that of differentiated Ganymede (0.312) and reinforced the Callisto-Ganymede dichotomy. However, Titan's moment of inertia (0.34) (Rappaport et al. 2008) has made this juxtaposition somewhat obsolete; that is, in light of the intermediate size and moment of inertia of Titan, it is now more compelling to think in terms of a trend (which also includes Iapetus, as we discuss in a companion abstract). Assuming as is often done that Callisto is in hydrostatic equilibrium (which is unproven and possibly wrong), its moment of inertia can be fitted using a two-layer interior model with a ~300 km clean water-ice shell and an undifferentiated ice and rock-metal interior (Schubert et al. 2004 and references therein). Mosqueira and Estrada (2003) explain the Ganymede-Callisto dichotomy on the basis of two-component gaseous subnebulae with relatively dense inner regions out to the centrifugal radii of Jupiter and Saturn, roughly at the locations of Ganymede and Titan respectively, and extended tails out to the locations of the irregular satellites. (Note that Iapetus' large separation from Titan provides strong indirect support for the presence of tails.) In such disks it is natural to expect both that a) some regular satellites form in the outer disk (i.e., Callisto and Iapetus) and some in the inner disk (i.e., Ganymede and Titan), and b) that satellites forming in the outer disk take longer to accrete (~10^6 years) than those forming in the inner disk (10^4-10^5 years). As a result, Callisto's formation takes sufficiently long that the heat of accretion may be radiated away, resulting in incomplete differentiation (see Estrada et al. 2009 and references therein). We stress that this model does not imply special circumstances for the accretion of Callisto; rather, it ties this satellite's accretion history to the time it takes for gas drag to clear the outer disk of ~100 km satellitesimals. Here we present the accretion history of Titan as an in-between case: Titan took significantly longer to form than Ganymede and received most of the solid content in the circumplanetary disk out to the location of Iapetus. Thus, we interpret Hyperion as an outer disk satellitesimal and survivor of a collisional cascade that was captured into resonance due to gas drag inward migration. We compare the accretion histories of Ganymede, Callisto and Titan in the framework of our two component model (see Mosqueira, Estrada and Turrini 2009) and describe the implications for the internal states of these three regular satellites. This work is supported by NASA PG&G and OPR grants.
Estrada Paul R.
Mosqueira Ignacio
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