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Powering Triton’s recent geological activity by obliquity tides: Implications for Pluto geology

Introduction

In terms of their bulk properties, Triton and Pluto are remarkably similar (see Table 1). Both are presumed to have formed as Kuiper Belt objects, although retrograde Triton was captured into Neptune orbit at some point in its history (McKinnon and Kirk, 2007). Triton has a young (<100

Myr) surface (Schenk and Zahnle, 2007), deformed by a variety of tectonic and possibly cryovolcanic (Croft et al., 1995) features, and exhibits geysers that are probably powered by solar heating (Kirk et al., 1990). It is therefore of interest to consider the question: to what extent will Pluto resemble Triton?

In this MS we lean heavily on Triton’s youthful appearance in assessing its likely interior state. With Pluto, firm predictions are elusive. However, we argue that New Horizons observations will not only clarify Pluto’s interior state, but will also determine whether our favoured hypothesis for Triton’s activity is correct.

The logic of the MS is as follows. We first demonstrate that the heat released during Triton’s orbital evolution following capture only marginally affects its present-day behavior (Section 3.1). Based on its young apparent age, we assume that Triton’s icy surface is being deformed, at least in part, by convection (c.f. Stern and McKinnon, 2000), as similarly young surfaces on Europa and Enceladus are thought to do. We then argue that surface deformation and yielding require heat fluxes much greater than Triton’s radiogenic elements can supply (Section 3.2). However, the addition of tidal heating is sufficient to permit yielding to occur, and also makes a long-lived ocean possible. As argued by Jankowski et al. (1989), Triton’s odd orbital configuration makes heating by obliquity tides unusually effective. In contrast to these authors, however, we focus on dissipation within a subsurface ocean (Section 3.3). A Triton consisting of a thick convecting ice shell overlying a long-lived, cold (and currently dissipative) ocean is energetically plausible and consistent with the meagre observational constraints.

How does this picture relate to Pluto? The main difference is that tidal heating is unlikely to operate at Pluto and, as a result, surface yielding should not be occurring currently. If our scenario regarding Triton’s extra energy source is correct, Pluto should show no signs of recent geological activity. Conversely, if Pluto’s surface does turn out to be as young as Triton’s, this suggests that processes other than tidal heating are likely responsible for the activity of both moons. One possible explanation in this case would be the presence of highly volatile species enabling geological activity powered by radiogenic heat alone.

Because of the relative paucity of observational constraints compared to e.g. the saturnian or Jovian satellites, we have favoured order-of-magnitude arguments over detailed models wherever possible. Uncertainties are generally so large that exploring parameter space with complex models is impractical, and unlikely to yield additional insight beyond the simple calculations presented here. We do, however, identify some questions which may be worth exploring in more detail.

An important clue to Triton’s present-day state is the fact that its surface is so lightly cratered, suggesting a surface age less than at most 100

Myr old (Schenk and Zahnle, 2007). There are only four other known outer Solar System bodies with comparable surface ages. Titan and Io are unsuitable analogues, because the resurfacing is due in large part to erosion/sedimentation, and prodigious silicate volcanism, respectively. Europa’s heavily deformed surface is about 50

Myr old on average (Zahnle et al., 2003), while the south polar region of Enceladus is probably even younger (Porco et al., 2006). In both cases, resurfacing is plausibly due to deformation driven by convection involving motion of the entire near-surface lid (Showman and Han, 2005, Barr, 2008, O’Neill and Nimmo, 2010). In both cases the ultimate energy source driving this motion is tidal heating. Given the abundance of plausibly tectonic features on Triton’s surface (Croft et al., 1995), we shall assume below that convection-related yielding and deformation is taking place. We note, however, the possibility that mechanisms other than ice shell convection, such as cryovolcanism (Croft et al., 1995) or diapirism driven by local density variations (Schenk and Jackson, 1993) may also contribute to Triton’s resurfacing.

While Triton is also active up to the present time in the sense that it has active geysers, we do not view this as a particularly useful constraint. Although the geysers at Enceladus are probably related to its internally active state, Triton’s geyser activity is plausibly driven by solar heating (Kirk et al., 1990) rather than endogenic geological activity.

Triton’s retograde orbit indicates that it was captured. Three capture mechanisms have been proposed: aerodynamic drag (McKinnon and Leith, 1995); collision with another satellite (Goldreich et al., 1989); and exchange capture (Agnor and Hamilton, 2006). Of these, the last – in which a binary object encounters Neptune and one member of the binary (Triton) is captured – is by far the most probable. The timing of the capture event is somewhat unclear. Aerodynamic drag can only have operated during Neptune’s formation, and the probability of a collision, always low, becomes much lower once the main stage of accretion ended. Exchange capture could in theory occur at any time, but modeling by Vokrouhlicky et al. (2008) suggests that it probably happened within the first 5–10

Myr of Solar System history.

The conventional picture of Triton’s post-capture orbital evolution may be divided into two phases (Chyba et al., 1989, Ross and Schubert, 1990). In the first phase, its initially highly eccentric orbit was circularized by tidally-driven dissipation. Because of the strong positive feedback between dissipation and temperature, the majority of the circularization probably took place rapidly (<100

My). The duration of the entire circularization process depends on poorly-known rheological parameters, but was almost certainly <1000

Myr. An alternative, more rapid (∼0.1

My) mode of circularization is via interaction with a disk resulting from collisions between other pre-existing satellites (Cuk and Gladman, 2005). In either case, the end state was a body on an inclined, but essentially circular orbit.

The second phase involves more gradual evolution to the present-day situation. Tidal dissipation in a satellite damps both eccentricity and inclination, while dissipation in the primary can have the opposite effect (Murray and Dermott, 1999). For the Neptune–Triton system, it is not obvious whether dissipation in the primary or the satellite dominates (Chyba et al., 1989). However, irrespective of this issue, the inclination will damp more slowly than the eccentricity (as is evident from the current circularity of Triton’s orbit). We discuss this issue in more detail in Section 3.3 and Eq. (9) below, and demonstrate that the inclination is not expected to have damped over the age of the Solar System. The reason this issue is important is that it is Triton’s non-zero inclination which we hypothesize is the ultimate cause of present-day tidal heating (Section 3.3).

Section snippets

Structure and parameter choices

For a body consisting of two layers of uniform density, the bulk density ρb is given byρb=ρi1+(ρs-ρi)ρiRsR3where the density of the outer and inner layers are ρi and ρs, respectively, and the radial position of the interface is Rs. For Triton and Pluto, we assume the outer layer is Ice I (ρi=950kgm-3) and the inner layer is anhydrous silicates plus iron with a density similar to Io’s (ρs=3500kgm-3). The resulting radius of the rock–iron core Rs and the maximum thickness of the ice shell dmax

Primordial heating

Immediately after capture, Triton’s orbit was highly elliptical, leading to large tides. These tides in turn caused heating, reducing Triton’s effective rigidity and thus further increasing the amplitude of tidal deformation and heating. This positive feedback probably led to a brief, intense period of heating, up to ∼1

Gyr after capture, after which Triton’s orbit was essentially circular (Ross and Schubert, 1990).

The total energy released, expressed as a mean temperature change, is given by

Implications for Pluto

Fig. 2 summarizes the key results of this MS: for Triton, radiogenic heating alone is incompatible with convection-driven surface yielding. Conversely, with the addition of obliquity tidal heating in a subsurface ocean, convection-driven yielding is likely to occur if the ocean contains suitable concentrations of antifreeze and the ice shell is sufficiently thick. Triton likely possesses a convecting ice shell above a long-lived, cold, dissipative ocean, with the long-term evolution of the

Discussion

We have argued above that Triton is unusually active because of heating caused by its unusual orbit around Neptune; the corollary is that Pluto will not be active.

One potential flaw in this logic is that Triton might have been captured recently; for instance, Fig. 1b shows that a heat flux >10

m⁻² could be sustained for 1

Gyr. Recent capture could thus explain ongoing activity, but (as noted in Section 1.2) this is considered a low-probability event. Another possibility is that Triton’s young

Acknowledgments

We thank an anonymous reviewer and especially Sasawata Hier-Majumder for helpful comments and discussion.