  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.
