Neptune's Story

Abstract

It is conjectured that Triton was captured from a heliocentric orbit as the result of a collision with what was then one of Neptune's regular satellites. The immediate post-capture orbit was highly eccentric with a semimajor axis a ∼ 10³R_N and a periapse distance rp that oscillated periodically above a minimum value of about 5R_N. Dissipation due to tides raised by Neptune in Triton caused Triton's orbit to evolve to its present state in ≲10⁹ years. For much of this time Triton was almost entirely molten. While its orbit was evolving, Triton cannibalized most of the regular satellites of Neptune and also perturbed Nereid, thus accounting for that satellite's highly eccentric and inclined orbit. The only regular satellites of Neptune that survived were those that formed well within 5R_N and they move on inclined orbits as the result of chaotic perturbations forced by Triton. Neptune's arcs are confined around the corotation resonances of one of these inner satellites. The widths and lengths of the arcs imply that the satellite's radius is at least 30/(sin i)^2/3 kilometers for i ≲ 1, where i is the angle of inclination.