Pluto: Evidence for a Liquid Internal Ocean

What accounts for Pluto’s interesting landscape? As we accumulate more and more data from New Horizons, we’re seeing a wide range of geologic activity on the surface, most of it involving such volatile ices as nitrogen, carbon dioxide and methane. But look at the troughs and scarps — some of them hundreds of kilometers long and several kilometers deep — and you’re seeing what are thought to be extensional faults. These are faults associated with the stretching of the dwarf planet’s crust, and in the New Horizons imagery, they appear geologically young.

We could look toward tidal interactions with Charon for an answer to what is driving tectonic activity on Pluto, but the Pluto/Charon system has reached what a new paper on the matter calls “the end point of its tidal evolution,” with the two objects locked into a synchronous state that makes the prospect unlikely. But changes in the ice shell are another matter, and as Noah Hammond (Brown University) and his fellow researchers are learning, the sinuous faults on Pluto’s surface provide evidence for a subsurface ocean that remains liquid today, though one that is probably in the process of re-freezing.

Image: The New Horizons spacecraft spied extensional faults on Pluto, a sign that the dwarf planet has undergone a global expansion possibly due to the slow freezing of a subsurface ocean. A new analysis by Brown University scientists bolsters that idea, and suggests that ocean is likely still there today. NASA/JHUAPL/SwRI.

What Hammond, a Brown graduate student, produced with colleagues Edgar Parmentier (also at Brown) and Amy Barr (Planetary Science Institute) was a model of thermal evolution that could be fed New Horizons’ data on Pluto’s diameter and density to study its interior. The energy to melt Pluto’s internal ice could have come from radioactive elements within its core, producing an ocean that would, in the frigid conditions of the Kuiper Belt, gradually begin to refreeze. An ocean that was frozen or in the process of freezing would produce the kind of extensional tectonics we see on Pluto today, assuming it were made up of normal water ice.

But different forms of ice are the crux of the argument. For low temperatures and high pressure deep within Pluto would, in a solidly frozen ocean, produce not normal ice (Ice I h , or ice phase one) but a rhombohedral crystalline form of ice with a highly ordered structure called Ice II. The compact structure of Ice II creates a frozen ocean of smaller volume — Ice II is 25 percent more dense than normal ice — and results in contraction rather than expansion.

We have a good deal of New Horizons imagery, but find no evidence on Pluto’s surface of such global contraction — no compressional tectonic features — leading Hammond’s team to conclude that Ice II has not formed, and that the ocean is therefore not completely frozen.

Critical to the argument is the thickness of the dwarf planet’s ice shell, because Ice II should form only if the shell is 260 kilometers thick or more. Anything less produces an ocean that can freeze without forming Ice II, and hence a frozen ocean that does not cause contraction. But the authors’ thermal model, adjusting the physical properties of both the silicate core and the ice shell, produces a shell that is closer to 300 kilometers thick. Moreover:

…the influence of volatile ices such as nitrogen and methane may be more effective at insulating the ocean than shown in our model. We assume volatiles are concentrated in the top 10 km of Pluto’s ice shell, but if methane clathrates are abundant in the entire ice shell, its thermal conductivity may be significantly reduced…, increasing the likelihood that the ocean will survive. We find that if the ice shell has a constant thermal conductivity of 3 W/m/K, a subsurface ocean survives even if the thermal conductivity of the core is high. The likelihood of ocean survival further increases when considering that as the ocean begins to freeze, impurities are excluded from the ice shell and ammonia and salt concentrations in the ocean will increase, further reducing the melting temperature.

A still liquid ocean gradually re-freezing within Pluto would generate continuing global expansion, producing extensional tectonic activity and young surface features. What we do not see are the kind of compressional tectonic features that would indicate the formation of Ice II. The paper’s conclusion, then, is that there are two possibilities: Either Pluto has an ocean today or it has an ice shell thinner than 260 kilometers. The latter could imply a frozen ocean (Ice I h ), but we still have to account for Pluto’s geologically young faults. A search of New Horizons data for evidence of current extensional tectonics is perhaps the best way to proceed.

Phase changes in ice can, according to this work, produce tectonic changes on the surface even for objects at the edge of the Solar System where energy is sparse. If this kind of tectonic activity can occur on Pluto, it should be possible on other Kuiper Belt objects, suggestive of continuing activity and perhaps a huge inventory of water in icy moons and KBOs.

The paper is Hammond, Barr and Parmentier, “Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell,” in press at Geophysical Research Letters (abstract / preprint).