DDA 36th Meeting, 10-14 April 2005
Session 11 Orbits and Orbit Evolution II: Moons
Oral, Wednesday, April 13, 2005, 9:35am-12:15pm

Previous   |   Session 11   |   Next

[11.04] Constraints on Triton's Orbital Evolution

D.P. Hamilton, K. Zhang (U. Maryland), C. Agnor (UCSC)

Three models have been proposed for the capture origin of Triton: Collision with a preexisting satellite (Goldreich 1989), Gas drag (McKinnon 1990), and three-body exchange (Agnor and Hamilton 2004). All three scenarios put Triton onto a highly elongated orbit which is subsequently circularized by satellite tides. Our goal here is to use the current state of the Neptunian system to constrain these capture scenarios.

Triton strongly affects inner satellites (or an inner disk) directly via close pericenter passages before its orbit circularizes. Since satellite tides nearly conserve angular momentum, a simple tidal model puts Triton's minimum pericenter distance at aT/2 ~7RN, where aT is its current semimajor axis. Our initial simulations show that some satellites orbiting outside Proteus (the outermost of the inner satellites at a=4.67RN) can survive these Triton passages.

So why are there no known moonlets beyond 4.67RN? Seeking answers, we have integrated Triton's orbit backwards in time with a more sophisticated model that includes J2, solar perturbations, and satellite tides. We find that Triton's pericenter smoothly descends toward 7RN, as in the simple tidal model, but with superimposed oscillations at i) 1/2 Neptune's orbital period and ii) the nodal and apsidal precession periods. At a ~94RN Triton encounters a Kozai-like resonance between these precession periods which causes its pericenter to dip to ~4.2RN - well within the current orbit of Proteus. If Triton's orbit were ever this large, then the early inner satellite system must have been much smaller than it is today.

Additional apsidal and nodal resonances between an early Triton on a highly elliptical orbit and the small inner satellites (with resonant arguments like 2nT - 2\Omegasat) are strong enough to drive moonlet inclinations up to several degrees. We are using the stengths and locations of these resonances to further limit possible capture and evolution scenarios and will report on the status of these investigations.

Previous   |   Session 11   |   Next

Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.