Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p72b0495m&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #P72B-0495
Other
5430 Interiors (8147), 6040 Origin And Evolution, 6055 Surfaces And Interiors, 6218 Jovian Satellites, 6280 Saturnian Satellites
Scientific paper
It has long been hypothesized that the regular satellites of giant planets accreted in a planetary subnebula, but the properties of that subnebula have remained poorly understood. Unfortunately, reliable hydrodynamical planetary collapse calculations with sufficient resolution to characterize the large surface density range of the extended circumplanetary disk are lacking. Hence at present it is still useful to be guided by ``minimum mass'' models. Since satellite migration is likely, the question remains how to distribute the mass of solids within the disk. We argue that the subnebula accretion disk for Jupiter and Saturn should be divided into two components: the first extending outside (but perhaps close to) the centrifugal radius located at ~ RH/48, and the second from there out to the location of the innermost irregular satellites at ~ RH/5, where RH is the Hill radius of the primary. Though such lengthscales have been mentioned in the literature before, their meaning has not been clarified. We will discuss heuristic estimates for both the location of the transition region between the inner and outer disks, and the radial extent of the circumplanetary disk. Given such subnebulae, it is possible to compare the satellite systems of Jupiter and Saturn to see how the findings made by the Galileo spacecraft might apply to future observations by Cassini. Measurements of Callisto's gravity by the Galileo spacecraft indicate that the distribution of mass is less centrally condensed in Callisto than in Ganymede. In light of the similarities in sizes and densities between these two satellites and Titan it is fair to ask whether Titan can be expected to be more like Ganymede or more like Callisto in its degree of differentiation. Though measured in planetary radii Titan is located between Callisto and Ganymede, in terms of Hill radii of the primary the association is much closer to Ganymede. This leads us to conclude that (unlike Callisto) Titan formed in the inner disk in < 105~years. This timescale is too short for the heat generated by accretion to be radiated away, so we expect that Titan is fully differentiated. In this regard it may be more like Ganymede. On the other hand, both Callisto and Titan are likely to have received significant amounts of volatile-rich materials from the extended portion of the disk in the late stages of their accretion. Not surprisingly, then, Titan's surface is expected to be volatile rich. In particular, we argue that the composition of Titan's surface may be quite similar to that of the bright, trailing side of Iapetus (which itself is too volatile-rich to have been formed by solids accreted directly from heliocentric orbit). Indeed, late impacts with Iapetus-like or Hyperion-like objects that migrated due to gas drag into Titan's feeding zone may be responsible for the formation of Titan's thick atmosphere.
Estrada Paul R.
Mosqueira Ignacio
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