The images from New Horizons’ flyby of Pluto have sparked a battle to understand the dwarf planet’s terrain. Pluto is clearly geologically active, with mountains and fresh surfaces untouched by impacts. One such feature, Sputnik Planum, appears to be an ocean of frozen nitrogen fed by nitrogen glaciers along the coasts. But a new analysis suggests this isn’t the dwarf planet’s only ocean.
An analysis of Pluto’s internal structure and warming indicates that there are two possible probabilities: either it has a deep ocean of liquid water, or the water on Pluto has frozen and compressed into a dense form of ice called ice II. And the analysis authors suggest that the liquid ocean makes more sense given Pluto’s surface features.
The analysis was done in the same way as Sputnik Planum: determine 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 contains some of the same radioactive isotopes that help keep Earth’s core nice and warm. Above that, however, Pluto is mostly water, with hard-to-determine fractions of things like ammonia and methane.
To model possible behavior of Pluto’s interior, the authors used a very simple model that they ran under a wide range of conditions. The model itself was one-dimensional — think of drawing a line directly from Pluto’s surface to the core, and tracking what happens as the core’s heat escapes along that line. The range of conditions includes 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 transfer heat at different efficiencies; if part of Pluto freezes prematurely, it will change the future dynamics of the model.
The authors find that, depending on the details, Pluto enters one of two states they creatively call “warm” and “cold.” In either case, an ocean formed under Pluto’s crust early in the planet’s history. In the cold case, the ocean finally froze sometime about now — 4.5 billion years after the planet’s formation. However, due to the intense cold and the enormous depth of the ice on Pluto, it does not remain the water ice we are all familiar with. Instead, the deeper ice begins to compress into ice II, a denser form of water ice. The higher density means that the same amount of ice takes up less volume, causing Pluto to contract as a whole. This should create compression features on the surface.
The alternate warm scenario sustains the ocean today, though it gradually freezes over; water is only present more than 250 kilometers down, closer to the core. Since ice is still forming and it is less dense than liquid water, this creates the opposite effect to 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 did not form.” That means the cold scenario is over and Pluto probably still has liquid water deep inside. It also means that the freezing-induced expansion of the planet’s surface should also be underway – there should be Plutoquakes.
Clearly, we won’t be dropping seismic equipment on Pluto’s surface any time soon. But we can analyze the images from New Horizons and look for the frequency of expansion and compression features on the surface. We can also analyze their relative ages. This can help us determine whether there is likely some water left on the dwarf planet, or whether it is filled with ice II.
Geophysical Survey Letters2015. DOI: 10.1002/2016GL069220 (About DOIs).