The outer solar system is a cold place, which once meant liquids, let alone oceans, were no part of our vision for it. Yet slowly we have come round to the idea that there are seas on Titan and oceans under the surface of Europa and Enceladus. Now we might need to add Pluto to that list, making up for its demotion to dwarf planet status.
Pluto's moon Charon is more than half Pluto's diameter – by far the closest size relationship of any large orbiting objects in the solar system. This is taken as an indication that Charon formed after a large object collided with Pluto, just as our own Moon formed when a Mars-sized object smashed into the Earth, throwing huge amounts of material into space.
The heat of this event would quite likely have melted Pluto's interior, but according to a theory published in Icarus processes since may have kept the ocean liquid for billions of years, possibly even to the present day.
The claim seems astonishing – the collision was likely more than 4 billion years ago but Dr Amy Barr and Professor Geoffrey Collins of Brown University and Wheaton College respectively argue that a long-standing ocean is the most likely explanation for what we know about the Pluto-Charon system.
Over time planets and their moons become tidally locked. We see the first stage of this with our own Moon, which always keeps the same face towards the Earth, rotating at the same speed as it orbits. Pluto and Charon have taken this further, so that both keep the same face to each other, with their rotations and orbits taking 6.4 Earth days. This is called the dual synchronous state, and no other objects in the solar system have reached it yet, although eventually the Earth and the Moon will too.
This sort of dual locking is not reached quickly, and getting there depends in part on the tides each object raises in the other. Tides change an object's shape, if only slightly, and that in turn affects the angular momentum and rate of spin.
Raising and lowering tidal bulges requires energy, which comes from a slowing of rotational speed and is dissipated as heat. Barr and Collins note, “A cold viscous ('stiff') Pluto will shed angular momentum more slowly than a warm, low-viscosity Pluto. Ice is such a good insulator that, even out where temperatures are close to absolute zero it is possible for the internal heating processes to keep a planet liquid inside.
However, as Barr and Collins note, it is much easier to produce a tidal bulge in a liquid than a solid. They modeled the evolution of the system from one where Pluto was spinning much faster and Charon was in a more stretched out orbit to the one we have today. Three models of Pluto were used – one in which it is consistent all the way through, a second where Pluto has layers of rock and ice, but no liquid ocean, and a third where the heat of the collision melts some of the interior, which is then maintained by the heat dissipated from the tidal interactions.
Modeling these required estimations of Pluto's “Love Numbers”, which are not nearly as romantic as they sound, in fact being three parameters measuring the rigidity of a planetary body and its susceptibility to tidal deformation. We don't know enough about Pluto and Charon to know the Love numbers accurately, but Barr and Collins incorporated measurements from the moons of Jupiter and Saturn. They concluded that getting to the current orbital characteristics required either an ocean beneath Pluto or a lot of luck. The luck could come in the form of Charon starting with an orbit close to the right period, or the internal heat generated almost, but not quite, melting Pluto's innards.
While Pluto having had an ocean during the period where Charon's orbit was changing seems the most likely scenario under the pair's modeling, the energy source would have been turned off once the dual synchronous state was achieved. However, with a layer of ice that may be 100km thick it would take quite a while for the inner regions to freeze, so if this is a recent state the ocean could still be in existence, particularly if large quantities of ammonia are acting as antifreeze.
The New Horizons mission is on the way to Pluto and Charon, arriving July 2015. It is hoped that as it passes it will get a clear enough look to determine whether tectonic activity has wiped Pluto clean of older craters as well as leaving evidence of fault lines, as can be seen on Europa. Barr and Collins say, “If New Horizons uncovers evidence for ancient tectonic activity consistent with despinning on Pluto and/or tidal recession on Charon, the most self-consistent explanation is that Pluto had an ocean during the time period of Charon's orbital evolution”
Moreover the nature of the fault lines should be different at the poles from near the equator. By seeing what latitudes particular features take up the authors suggest we can determine how recently the internal ocean may have survived, or even if it is still existing today. “If no tectonic features are observed, this could indicate an extremely cold Pluto pre- or post-impact (and thus a very long timescale for orbital evolution) or that tectonic features have been removed or buried by other processes," they say.