Gas Drag and the Orbital Evolution of a Captured Triton

Abstract

Gas drag is a possible mechanism for capturing Triton. We examine the energetics of gas drag capture, compare the role of gas drag in subsequent orbital evolution with that of tides, and evaluate whether a Triton captured by gas drag could have avoided spiraling into Neptune. On its own, gas drag causes the inclination of an orbit to become more prograde, and Triton's retrograde status potentially places an upper limit on the orbital energy lost due to gas drag and hence a lower limit on the amount of tidal evolution and concomitant heating. A putative Neptune nebula is modeled after a minimum mass Uranus nebula and is assumed to be isothermal and cool. Triton's initial apocenter is set at the radius of Neptune's Hill sphere and its initial pericenter within the radial range of the nebula. Despite strong evolution of both semimajor axis and eccentricity, Triton's inclination only changes by a few degrees due to gas drag, when uncoupled to inclination variations due to solar-torque-induced precession. Results are insensitive to variations in radial surface density distribution or temperature, for fixed nebular mass. Thus, Triton could have evolved to its present retrograde circular orbit by gas drag alone, and massive tidal heating is not an inevitable consequence of capture. Nontrivial tidal heating is still likely, though, because Triton's post-gas-drag eccentricity, i.e., that remaining when Triton's present-day angular momentum was reached, is likely to have exceeded ∼0.2.Despite Triton's mass, single-pass capture is shown to be energetically favorable for our nominal nebula. Capture is also energetically possible for (i) nebulae that are less massive by an order of magnitude, if Triton's heliocentric orbit was energetically close to a temporary gravitational capture, and (ii) the solar gas flowing into Neptune's accretion sphere at an earlier stage of formation. Survival of Triton against gas drag in the last case is unlikely, but the low-mass case may apply to the compact H- and He-depleted nebulae that form around planets such as Uranus and Neptune in various accretion scenarios. Once captured, evolution is rapid in the absence of solar perturbations, with the orbital angular momentum reduced to Triton's present value in under 103 years for the case of the nominal nebula. Solar tides, however, cause Triton's orbit to oscillate in and out of the protasatellite nebula and can extend the gas drag time scale, for certain initial conditions, to ∼104-105 years. Therefore Triton could have survived a short-lived (∼103 years), hot, turbulent nebula that may have formed by a giant impact with Neptune. It might have also survived a cooler, less turbulent, but longer-lived (∼106 years) low-mass nebula: the mass and angular momentum of Triton and that of a low-mass (Hand He-depleted) nebula are comparable, so orbital evolution for such a capture may "self-terminate." In particular, Triton may clear an annular region in the nebula after multiple passes through it, and for sufficiently slow radial redistribution of nebular mass, cease evolving by gas drag entirely.