An in-situ formation for Triton and Nereid

All giant planets in the solar system have two types of moons as defined by their orbits and mode of origin. The first type, referred to as the regular moons, has tight circular orbits close to the equatorial plane of the host, implying primordial accretion in the circum-planetary disc. The second type, called the irregular moons, in contrast, is characterised by wide, highly-eccentric and -inclined orbits and are believed to be captured by their host from heliocentric orbits through some form of dissipation.

 

However, the Neptunian moon Triton, 3000 km across, does not neatly fit in any of the two categories — it is orbiting the host rather close-in but in a direction opposite to the spin of Neptune. The obvious incompatibility between its retrograde orbit and an in-situ accretion origin suggests that it was captured by Neptune, for example, as a component of a binary asteroid pair. Another moon in the system, Nereid, is a distant irregular satellite. It is the largest of its kind and at the same time features the tightest and the most eccentric orbit for an irregular moon.

 

Here we explore an in-situ formation formation for these two moons. We assume that both initially formed as regular satellites at Neptune. Then a planetary encounter triggers an evolutionary sequence of events for these two moons towards their observed orbits. Such an encounter cannot happen in the present solar system. But rather in the early solar system, there is an instability period as envisioned by the Nice scenario. Specifically in a later version of the Nice scenario where three gas giants (IG) are initially orbiting the Sun; during the instability period the additional IG gains a significant orbital eccentricity, allowing it to encounter other planets until finally ejected. Here, we model such an encounter between a moon-bearing Neptune and an IG.

 

We find that during the encounter, about half of the pre-existing Neptunian moons are ejected and the surviving moons are highly excited. Among the survivors, a few per cent gain retrograde orbits (Triton analogues, TAs) while a similar fraction acquire wide, eccentric orbit (Nereid analogues, NAs). While the NAs orbits match that of Nereid quite well, those of the TAs are highly eccentric. Often the orbit of the TA intersect that of the NA; then the latter will be removed due to scattering or collision within a Myr. How can the NA survive then? A further issue is after the Neptune-IG encounter, some of the other moons may also survive. Why are these additional moons not observed today?

 

We find that if these moons are small, collisions between them and the TA would eliminate the former without endangering the latter. Collisions also shrink the orbit of the TA, decouple from that of NA and hence NA is protected. Finally, tides takes control and circularise TA’s orbit on Gyr timescale.

An illustration of our model is shown in Figure 1 and an example from the numerical simulations in Figure 2. Depending on how stringently we define a TA and NA, our model has an efficiency of 10^-5 - 10^-3.

In this in-situ formation model, Triton and Nereid accrete in the circum-planetary disk (see also, Harrington & Van Flandern, 1979, Icarus, 39, 131; Li et al. 2020, A&A, in press, doi: 10.1051/0004-6361/201936672) whereas the conventional capture model (e.g., Agnor & Hamilton, 2006, Nature, 441, 192; Nesvorny et al., 2007, AJ, 133, 1962) predicts that the two form in the circum-stellar disk. The environment, e.g., the temperature, in the two disks could be rather different, potentially leading to different compositional properties for example the fraction of volatiles. Hence, further observations as well as space missions would be helpful to constrain the formation path of the two moons.

 

Full details can be found in Li & Christou (2020, AJ, 159, 184).

 

The authors thank Dr. Craig B. Agnor for direct contributions to this work. DL acknowledges financial support from Knut and Alice Wallenberg Foundation (2014.0017 and 2012.0150) and from Vetenskapsrådet (2017-04945). The authors also thank the Royal Physiographic Society of Lund. Astronomical research at the Armagh Observatory and Planetarium is funded by the Northern Ireland Department for Communities (DfC).