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.