We're not sure what is inside Pluto and Charon — this is one of the reasons to send New Horizons to find out! But scientists can make an educated guess about the bodies' interiors by answering a few questions:
- What are their surfaces like?
- What are the common materials in the outer solar system? (Basically, rock, ice and organic matter)
- How do these materials behave under the pressures and temperatures inside planetary bodies? (Experience at Earth, Moon, Mars and the icy satellites give us some ideas)
- What is the density of Pluto and Charon? (Our best guess is close to 2 grams per cubic centimeter, or about twice the density of water)
- Are there bodies differentiated? That is, do they have dense cores surrounded by a layer of lighter material?
We know some pieces of the puzzle but other pieces are still missing or poorly described. Large uncertainties in the basic statistics of radius and density especially frustrate scientists' attempts to build models of the interiors of Pluto and Charon. This figure shows some ideas of their interiors.
Like a Rock
Given the large amount of rock and the planet's size, scientists expect that the process of the planet forming and the natural radioactivity of the rock inside will have heated the interior sufficiently to melt the ice and allow it to separate from the rock. The rock inside Pluto has likely settled into a core surrounded by a thick shell or mantle of ice, as illustrated in the figure. Pluto and Charon together are estimated to be about 65% to 70% rock (including metal) and about 30% to 35% ice or material close to ice in density, which could mean liquid water or organic matter. (The word "organic" here simply means carbon-bearing and does not imply "living.") Very volatile ices (nitrogen, methane and carbon monoxide) are concentrated at Pluto's surface, but whether they form a distinct crust is unknown.
Ocean at the Edge of Forever?
An intriguing possibility is that an ocean of liquid water underlies Pluto's icy shell. The ocean may be mixed with or overlie an organic-rich layer (not shown in the diagram). Could building blocks for life exist at this very edge of the solar system? There is evidence that the icy moons of Jupiter have hidden oceans, and our improved understanding of heat generation and transport in icy worlds makes this a real possibility for the ninth planet.
Life as we know it requires three things:
- Biogenic elements - elements to build life - such as carbon, phosphorus and sulfur, in addition to the oxygen and hydrogen in water
- A source of energy (light, heat, chemical potential) that a living organism can use
Pluto's surface is far too cold for liquid water, but its interior is probably warm and maintained that way by the slow decay of naturally occurring elements such as uranium, potassium-40 and thorium.
Enough heat is released that a water ocean may exist between the rocky core of Pluto and its thick outer layer of ice. Planetary scientists have long thought that icy satellites might possess oceanic layers underneath their surface ice layers. The discovery by the Galileo orbiter that Europa, Callisto and possibly Ganymede possess interior oceans greatly increases our expectation that Pluto also possesses an ocean. Pluto's ocean is also likely to contain biogenic elements in a solution, especially if it is in contact with an organic-rich layer.
Where Pluto probably does not pass astrobiological muster is in the matter of sufficient energy to power life. Pluto's ocean would be dark and cold - near-freezing. Even if in contact with a rock core, it is almost certainly true that this modest core is today insufficiently hot to be volcanically active or even to drive circulations. So it is difficult to argue for a deep biosphere on Pluto today. On the other hand, it is also true that Pluto's rock core was much hotter and probably active in the geological past, so it is not utter lunacy to speculate that some form of primitive, microbial life may have evolved long ago and just might have once plied the "Styxian seas" of Pluto.
Since Charon is smaller and less dense than Pluto, its internal structure is simpler to model. But our lack of knowledge of about Charon, particularly its density, makes its internal structure more uncertain. The figure above illustrates a Charon that is differentiated in a similar fashion to Pluto - separated into a mantle of ice and core of rock. But we also cannot rule out the possibility that Charon has a roughly uniform mixture of rock and ice all the way through - that is, undifferentiated — as in the lower figure.
New Horizons will help find out which (if any) of these ideas are correct by measuring the masses, densities and shapes of Pluto and Charon as well as searching for clues in the surface geology of the interior structure.