Triton: What Origin for this Unusual Moon?

1. Triton:What Origin forthis Unusual Moon? Jared Leisner November 18, 2004

2. Introduction • Summary of the Neptune planetary system • Satellites • Rings • Possible origins of Triton • Consequences of each origin • Needed events to evolve from each origin to the current system

3. Neptune Planetary System • Satellites • Inner satellites • Naiad, Thalassa, Despina, Galatea, and Larissa • Triton • Nereid • Rings • Arcs • Not continuous rings circling the planet

4. Neptune System: Inner Satellites Modified from Banfield and Murray (1992)

5. Neptune System: Inner Satellites • Orbital parameters • Eccentricity: Close to circular • Inclination: Close to zero • Except for innermost satellite, about 4.7° • Semi-major Axis: Confined to five Neptune radii • Prograde • Physical parameters • Diameter: 60 to 200 km, linearly increasing outward • Irregularly shaped, no sign of geological modification

6. Neptune System: Triton • Orbital parameters • Eccentricity: Close to circular • Inclination: 157.3° • Semi-major Axis: 14.4 Neptune radii • Retrograde

7. Neptune System: Triton, con't • Physical characteristics • Similar to Pluto • Density: 2.1 g cm-3 (high rock-to-ice ratio) • Diameter: 2705 km • Mass: 2.14E22 kg • Five hundred times the mass of the other moons combined • High crater asymmetry with low crater count • Craters on 30% of the surface, concentrated on the leading face

8. Neptune System: Nereid • Orbital parameters • Eccentricity: 0.7152 • Inclination: 27.6° • Semi-major axis: 223.9 Neptune radii • Prograde • Physical parameters • Diameter: 340 km • Not much else is known

9. Neptune's Rings • Instead of continuous rings like those that encircle other planets, Neptune's rings are broken into discrete arcs, pointed to in yellow. • They appear to be close, but not in, a resonance with the inner satellite Galatea, pointed to in white (Sicardy et al., 1999). From Sicardy et al. (1999)

10. Origin #1: Formed withinNeptune's Planetary System • Perhaps Triton was created out of the original planetary nebula • Consequences for Triton • Orbital direction (prograde) • Inclination (nominal) • Surface activity (similar to others?) • Consequences for planetary system • Distribution, characteristics of satellites (nominal) • Full rings instead of just arcs?

11. Origin #1: To the Present • How did Triton go from a prograde orbit to a retrograde orbit? • A collision with a passing body • Would require an Earth-sized planetsimal • Neptune was originally retrograde and switched • Nominal eccentricity and inclination do not suggest such a radical change • Why would the rest of the satellites be now-prograde? • It formed retrograde • How?! • Why the atypical distribution of satellites and their orbital parameters?

12. Origin #2: Captured via TidalFriction or Third-Body Interactions • Over a reasonable timescale, tidal effects can not dissipate enough orbital energy for Triton to be captured unless periapsis was extremely close to Neptune (McKinnon and Leith, 1995). • This would have left Triton in a more tightly bound orbit than is now observed. • Solar tides make capture more difficult by oscillating the satellite's orbit and a Pluto-assisted capture is accepted as impossible (McKinnon and Leith, 1995).

13. Origin #3: Captured via Gas Drag • McKinnon and Leith (1995) modelled a gas drag capture of Triton while there was a significant Neptune nebula (resembling that modelled for Uranus) still in existance. • This model puts Triton close to its present day situation in 103-5 years, depending on solar tides. • Triton's eccentricity would have been left at ~0.2, which would have left it open to non-trivial tidal heating.

14. Origin #3: Problems withGas Drag Capture • Gas drag acts to decrease the inclination of a body's orbit and makes said more prograde. • These effects, coupled with the observed orbit of today, place upper bounds upon the amount of gas drag that may have worked upon Triton. Those upper bounds then imply lower bounds upon the amount of orbital evolution due to tidal heating.

15. Origin #4: Capture by Impact • Before Voyager 2 reached Neptune, Goldreich et al. (1989) modelled the capture of Triton as it entered the Neptune system and struck a natural satellite with a mass a few percent of its own. • Using a somewhat crude argument of gravitational focusing, the authors calculated that more than 104 bodies of Triton's size may have passed within 10 Neptune radii which, if Neptune originally had a system akin to Uranus', would yield a chance for this Triton collision-capture of several tens of percent. • Goldreich et al. calculated that if Triton (with a k2~0.1 and Q~100) entered an elliptical orbit with a periapsis of 7 Neptune radii and semimajor axis of 103 Neptune radii, then Triton could evolve, through tidal dissipation, to its present situation in less than one billion years.

16. Origin #4: Consequences • It followed from the model proposed by Goldreich et al. (1989) that Neptune would be devoid of satellites between 5 Neptune radii and Triton's current orbit. • When Voyager arrived three months later, this was found to be precisely the case. • The calculations also correctly lead to Nereid's irregular orbit as Triton cross the former's orbit 108 times, perturbing its semi-major axis, eccentricity, and inclination a few tenths of a percent each time. • A last implication of this model would be an inner satellite shepherding the ring arcs in a slightly inclined orbital path. • While Galatea does appear to shepherd the ring arcs, it is not the one inclined.

17. Geologic Activity of Triton • The low count of impact craters implies a relatively young age for Triton's surface. As low as 100 Myr, and it is possible that it is still active (Stern and McKinnon, 1999; Ruiz, 2003). • This lower bound for the age, if accurate, implies that the moon underwent significant heating; this heating would be easy to explain with tidal dissipation as its orbit around Neptune evolved.

18. Recently Discovered Moons • From Holman et al. (2004) • Centered on Neptune. The red (blue) circle indicates stability of prograde (retrograde) satellites and the color of satellite's label indicates it's orbit. • c02N4 was lost. • Presumably captured satellites, by their orbits.

19. Conclusion • Triton's origin? A collision capture. • The predictions for Goldreich et al.'s (1989) model, save the inclined shepherd, were shown by Voyager's subsequent flyby to be true. • The orbital evolution after that capture would lead to geologic activity and the young surface that is observed. • The detection of five new irregular moons, albeit of significantly smaller proportions than Triton, that would appear to be captured satellites lends credence to the idea that this would not be impossible.

20. References • Banfield, D. and N. Murray, 1992. A dynamical history of the inner Neptunian satellites. Icarus99, 390-401. • Goldreich, P., N. Murray, P.Y. Longaretti, and D. Banfield, 1989. Neptune's story. Science245, 500-504. • Holman, M.J., J.J. Kavelaars, T. Grav, B.J. Gladman, W.C. Fraser, D. Milisavljevic, P.D. Nicholson, J.A. Burns, V. Carruba, J. Petit, P. Rousselot, O. Mousis, B.G. Marsden, and R.A. Jacobson, 2004. Discovery of five irregular moons of Neptune. Nature430, 865-867. • Marachi, S., C. Barbieri, and M. Lazzarin, 2004. Mass transfer in the satellite system of Neptune: implications for Triton's crater asymmetry. Planetary and Space Science52, 671-677. • McKinnon, W.B. And A.C. Leith, 1995. Gas drag and the orbital evolution of a captured Triton. Icarus 118, 392-413. • Ruiz, Javier, 2003. Heat flow and depth to a possible internal ocean. Icarus166, 436-439. • Stern, S.A. and W.B. McKinnon, 2000. Triton's surface age and impactor population revisited in light of Kuiper Belt fluxes: evidence for small Kuiper Belt Objects and recent geologic activity. The Astronomical Journal119, 945-952.