28.06

Gas Drag Effects on Planetesimals Evolving Under the Influence of Jupiter and Saturn

S. J. Kortenkamp, G. W. Wetherill (Carnegie Institution of Washington - DTM)

The ``core-mantle'' accretion model for the formation of giant gaseous planets requires about tex2html_wrap_inline17 - tex2html_wrap_inline19 years for Jupiter and Saturn to attain their present masses. By this time accretion in the inner Solar System is thought to have been well under way, with perhaps a few dozen lunar to martian sized bodies distributed in the terrestrial planet region. However, an alternative model of giant planet formation, involving gravitational instability of the solar nebula, suggests that Jupiter and Saturn may have formed as quickly as tex2html_wrap_inline21 years (Boss, Science 1997, in press). If this was the case then the evolution of small planetesimals in the inner Solar System would have proceeded while enveloped in the solar nebula and under the gravitational influence of Jupiter and Saturn. We have begun investigating just such a scenario in order to determine whether the presence of Jupiter and Saturn helps or hinders terrestrial planet formation.

We use an initial population of test particles on orbits near 1 AU ( tex2html_wrap_inline23 AU) with low eccentricity and inclination tex2html_wrap_inline25 and randomly distributed arguments of pericenter and longitudes of node. We find that the secular perturbations of Jupiter and Saturn force the eccentricities and inclinations to values as high as tex2html_wrap_inline27 and tex2html_wrap_inline29 and impose a common argument of pericenter and longitude of node on all of the orbits. While gas drag does not effectively reduce the forced e and I due to Jupiter and Saturn it does act to reduce the proper e and I due to the initial distribution. After decaying 0.05 AU in semi-major axis the mutual inclination of the individual orbits with respect to each other drops from the initial range tex2html_wrap_inline39 to tex2html_wrap_inline41 and the dispersion in eccentricity drops by about a factor of two. The encounter velocities of bodies traveling along these nearly coplaner and concentric orbits remain low (a few meters per second) despite the considerable forced eccentricities. Our preliminary finding is that gas drag can prevent encounter velocities from growing to kilometer per second levels, even in the presence of significant perturbations from Jupiter and Saturn.