Last giant impact on Uranus. Constraints on oligarchic masses in the trans-Saturnian region

PARISI, M. GABRIELA

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Año: 2011 vol. 534 p. 1 - 4

0004-6361

Context. Modern models of the formation of ice giants attempt to account for the formation of Uranus and Neptune within theprotoplanetary disk lifetime. These models assume a higher initial surface density well above that of the minimum mass solar nebulamodel and/or the formation of all giant planets in an inner compact configuration. Other effects include planetesimals migration dueto gas drag and the small size of the accreted planetesimals, which accelerates the accretion rate. However, at present, none of thesemodels account for the spin properties of the ice giants.Aims. Stochastic impacts by large bodies are, at present, the usually accepted mechanisms able to account for the obliquity of the icegiants. We attempt to set constraints on giant impacts as the cause of Uranus’s current obliquity of 98 degrees and on the impactor masses.Methods. Since stochastic collisions among embryos are assumed to occur beyond oligarchy, we model the angular momentumtransfer to proto-Uranus by the last stochastic collision (GC) between the protoplanet and an oligarchic mass at the end of Uranus’sformation. We take a minimum impactor mass mi of 1 Earth mass.Results. We find that an oligarchic mass mi between 1 and 4.5 Earth masses  would be required at the GC to reproduce the present rotationalproperties of Uranus. An impact with mi > 4.5 Earth masses is not possible, unless the impact parameter of the collision is very small and/orthe angle between the spin axis of Uranus prior and after the GC is higher than 130 degrees. This result is valid if Uranus formed in situ orbetween 10 and 20 AU and does not depend on the occurrence of the GC after or during the possible migration of the planet. This resultis very similar to one obtained for Neptune from its rotational properties.Conclusions. If the stage of stochastic impacts among oligarchs has occurred and if the present rotational status of Uranus is the resultof such processes, the 4.5 Earth masses limit must be understood as an upper constraint on the oligarchic masses in the trans-Saturnianregion at the end of ice giants’ formation. This result may be used to set constraints on planetary formation scenarios.