Quick Look
Capability Development for Modeling
Viscously Evolving Satellite Disks,
15-9465
Principal Investigator
Robin Canup
Co-Investigator
William R. Ward
Inclusive Dates: 03/01/04 - 06/28/04
Background - We received support for the preliminary development of a new model for the viscous evolution of an impact-generated circumplanetary disk of material orbiting a giant planet. The particular application of interest is the system of large satellites of Uranus, whose origin remains a mystery. While the planet Uranus has an approximately 97-degree tilt to its rotational axis (so that it effectively rotates on its side relative to the plane of the solar system), its large satellites occupy remarkably circular and nearly co-planar orbits within the planet's equatorial plane. A strong case has been made for an impact to the planet as the cause of the planet's large tilt. The regular alignment of its large satellites within its askew equatorial plane suggests that they formed subsequent to such an event in the planet's history. A natural question is then whether the same impact responsible for the Uranian obliquity also led to the creation of its satellite system, in a similar manner as is believed responsible for the origin of the Earth's moon. A fundamental challenge to such a scenario has been reconciling the radial extent of impact-ejected orbiting material, which typically occupies initial orbits within a few planetary radii from the center of the planet, with the relatively extended Uranian satellite system whose outermost large satellite orbits at about 23 Uranian radii.
Approach - Here we conducted an initial investigation of the possible mode of evolution of an impact-generated disk of material as it viscously diffuses. An impact-generated disk would have had very high initial temperatures and be primarily vapor. In addition, the post-impact temperature of Uranus would be approximately 10,000K for an impact sufficiently large to account for the planet's current rotation state. The initially vaporized disk material would continue to be heated by its own viscosity and the planet's luminosity as it radially diffuses, while radiative cooling will become increasingly effective as the surface area of the disk grows. The balance of these processes determines the disk temperature as a function of time.
Accomplishments - We developed a self-consistent model for the radial expansion of the disk while in a primarily vapor phase and then evaluated under what circumstances the disk components might have cooled sufficiently to condense as the outer edge of the disk expanded to the scale of the current satellites (i.e., approximately 20 to 25 Uranian radii). Finally, we performed satellite accretion simulations in the resulting disk to confirm that a distribution of satellites similar to the Uranian system could result.
