The pictures that came from New Horizons' flyby of Pluto have set off a scramble to make sense of the dwarf planet's terrain. Pluto's clearly geologically active, with mountains and fresh surfaces that haven't been pummeled by impacts yet. One of those features, Sputnik Planum, appears to be an ocean of frozen nitrogen, fed by nitrogen glaciers that line its shores. But a new analysis suggests that this isn't the only ocean on the dwarf planet.

An analysis of the internal structure and heating of Pluto indicates that there are two likely probabilities: either it has a deep ocean of liquid water, or the water on Pluto has frozen solid and compacted into a dense form of ice called ice II. And the authors of the analysis suggest that the liquid ocean makes more sense given Pluto's surface features.

The analysis was done in a similar manner to the ones that tackled Sputnik Planum: figure out Pluto's composition and its heat budget and trace the effects of the heat as it escapes to the surface. The heat itself comes from Pluto's rocky core, which carries some of the same radioactive isotopes that help keep the Earth's core nice and toasty. Above that, however, Pluto is mostly water, with difficult-to-determine fractions of things like ammonia and methane.

To model possible behaviors of Pluto's interior, the authors used a very simple model that they ran under a large range of conditions. The model itself was one-dimensional—think of drawing a line straight from Pluto's surface to its core, and tracking what happens as the heat from the core escapes along that line. The range of conditions include things like the amount of ammonia present, how efficiently the core transfers heat, and so on.

The model is then run forward in time, and the system is allowed to evolve as conditions change. For example, ice and water transmit heat with different efficiencies; if part of Pluto freezes early, then that will alter the future dynamics of the model.

The authors find that, depending on the details, Pluto enters one of two states that they creatively call "warm" and "cold." In both cases, an ocean formed beneath Pluto's crust early in the planet's history. In the cold case, the ocean finally froze over sometime right about now—4.5 billion years after the planet's formation. Due to the intense cold and the huge depth of the ice on Pluto, however, it doesn't remain as the water ice we're all familiar with. Instead, the deeper ice starts to compress into ice II, a denser form of water ice. The higher density means the same amount of ice takes up less volume, which causes the entirety of Pluto to contract. This should create compression features on the surface.

The alternative warm scenario keeps the ocean around in the present day, although it's gradually freezing over; water is only present over 250 kilometers down, closer to the core. Since ice is still forming, and it's less dense than liquid water, this creates the opposite effect as the cold Pluto scenario: the planet is experiencing an expansion, and the surface features should reflect that.

"Since there is no strong evidence of compressional tectonic activity," the authors write, "we conclude that ice II has not formed." That means that the cold scenario is out, and Pluto probably still has liquid water deep in its interior. It also means that the freezing-driven extension of the planet's surface should be ongoing as well—there should be Plutoquakes.

Obviously, we're not going to be dropping seismic equipment on Pluto's surface any time soon. But we can analyze New Horizons' images and look for the frequency of extension and compression features on its surface. We can also analyze their relative age. This could help us determine whether there's likely to still be some water left on the dwarf planet—or if it's filled with ice II.

Geophysical Research Letters, 2015. DOI: 10.1002/2016GL069220 (About DOIs).