On the formation location of Uranus and Neptune as constrained by dynamical and chemical models of comets

Abstract

The D/H enrichment observed in Saturn's satellite Enceladus is remarkably similar to the values observed in the nearly-isotropic comets. Given the predicted strong variation of D/H with heliocentric distance in the solar nebula, this observation links the primordial source region of the nearly-isotropic comets with the formation location of Enceladus. That is, comets from the nearly-isotropic class were most likely fed into their current reservoir, the Oort cloud, from a source region near the formation location of Enceladus. Dynamical simulations of the formation of the Oort cloud indicate that Uranus and Neptune are, primarily, responsible for the delivery of material into the Oort cloud. In addition, Enceladus formed from material that condensed from the solar nebula near the location at which Saturn captured its gas envelope, most likely at or near Saturn's current location in the solar system. The coupling of these lines of evidence appears to require that Uranus and Neptune were, during the epoch of the formation of the Oort cloud, much closer to the current location of Saturn than they are currently. Such a configuration is consistent with the Nice model of the evolution of the outer solar system. Further measurements of the D/H enrichment in comets, particularly in ecliptic comets, will provide an excellent discriminator among various models of the formation of the outer solar system.