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Did Triton Destroy Neptune’s First Moons?

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This computer-generated image shows Neptune as it would appear from near its largest moon, Triton. [NASA/JPL/USGS]

Neptune’s moon system is not what we would expect for a gas giant in our solar system. Scientists have now explored the possibility that Neptune started its life with an ordinary system of moons that was later destroyed by the capture of its current giant moon, Triton.

An Odd System

Our current understanding of giant-planet formation predicts a period of gas accretion to build up the large size of these planets. According to models, the circumplanetary gas disks that surround the planets during this time then become the birthplaces of the giant planets’ satellite systems, producing systems of co-planar and prograde (i.e., orbiting in the same direction as the planet’s rotation) satellites similar to the many-moon systems of Jupiter or Saturn.

Neptune satellite orbits

Triton’s orbit is tilted relative to the inner Neptunian satellite orbits. [NASA, ESA, and A. Feild (STScI)]

Neptune, however, is quirky. This gas giant has surprisingly few satellites — only 14 compared to, say, the nearly 70 moons of Jupiter — and most of them are extremely small. One of Neptune’s moons is an exception to this, however: Triton, which contains 99.7% of the mass of Neptune’s entire satellite system!

Triton’s orbit has a number of unusual properties. The orbit is retrograde — Triton orbits in the opposite direction as Neptune’s rotation — which is unique behavior among large moons in our solar system. Triton’s orbit is also highly inclined, and yet the moon’s path is nearly circular and lies very close to Neptune.

Triton impacts

The distribution of impact velocities in the authors’ simulations for primordial satellite interactions with Triton, in three cases of different satellite mass ratios. In the low-mass case — a third of the mass ratio of the Uranian satellite system — 88% of simulations ended with Triton surviving on its high-inclination orbit. The survival rate was only 12% in the high-mass case. [Adapted from Rufu et al. 2017]

How did this monster of a satellite get its strange properties, and why is Neptune’s system so odd compared to what we would expect for a gas giant’s satellites? Two scientists, Raluca Rufu (Weizmann Institute of Science, Israel) and Robin Canup (Southwest Research Institute), propose an explanation in which Triton long ago wreaked havoc on a former system of satellites around Neptune.

Destruction After Capture

Rufu and Canup explore the scenario in which Neptune once had an ordinary, prograde system of moons around it that resembled those of the other gas giants. Triton, the authors suggest, may have been a former Kuiper belt object that was then captured by Neptune. The ensuing interactions between retrograde Triton and Neptune’s original, prograde satellite system may have then resulted in the destruction of this original system, leaving behind only Triton and Neptune’s other current satellites.

Nereid, a small irregular moon of Neptune, orbits at an average distance of more than 15 times that of Triton. Models of Triton’s orbital evolution must also account for the preservation of satellites like this one. [NASA]

Using N-body simulations that model a newly captured Triton and a likely primordial prograde system of moons, Rufu and Canup show that if the moons have a mass ratio similar to that of Uranus’s system or smaller, Triton’s interactions with it have a substantial likelihood of reproducing the current Neptunian satellite system. They even demonstrate that the interactions decrease Triton’s initial semimajor axis quickly enough to prevent smaller, outer satellites like Nereid from being kicked out of the system.

If the authors’ picture is correct, then it neatly explains why Neptune’s satellite system looks so unusual compared to Jupiter’s or Saturn’s — which means that our models of how primordial systems of moons form around gas giants still hold strong.

Citation

Raluca Rufu and Robin M. Canup 2017 AJ 154 208. doi:10.3847/1538-3881/aa9184