How plausible is a subsurface ocean on Pluto?

Question

According to this article on physorg.com, it's likely that Pluto has a subsurface ocean of liquid water. It suggests that the ocean would be about 165 km deep, under an equally deep crust of solid ice, topped by a thinner layer of solid nitrogen.

The heat to keep the water liquid would be provided by the decay of radioactive potassium. The article doesn't explicitly say so, but I presume that would be potassium-40, which has a half-life of about 1.25 billion years (which suggests that there would have been a lot more of it a few billion years ago).

Measurements of meteorites believed to have formed in the early Solar System show 10 times as much potassium as needed.

The New Horizons spacecraft, when it arrives at Pluto in 2015, should be able to see surface features that will indicate whether this hypothesis is correct.

I haven't seen any mentions of this other than that article (and references to it).

My question is, how plausible is this idea? Has anyone else crunched the numbers and confirmed or denied the possibility? Is there some fatal flaw in the argument?

(And a followup: How freakin' awesome would this be??!!)

Answer 1

I also share Brightblades excitement if this turns out to have any validity. :)

As to your actual question, there is very little written about this idea that doesn't seem to be an exact (or nearly so) duplicate of the phsyorg.com article. Space.com, New Scientist, and Universe Today being the more popular.

All seem to agree on (from the New Scientist) some of the calculations used, however the validity must await the arrival of the probe.

Density measurements suggest a rocky core fills 40 per cent of the dwarf planet's volume. If the core contains potassium at a concentration of 75 parts per billion, its decay could produce enough heat to melt some of the overlying ice, which is made of a mixture of nitrogen and water.

He points out that Earth, which probably formed with less of the volatile element due to its closer distance to the sun, has 10 times that concentration in its core.

This paper (PDF) does run some of the math for you, but again concludes with that we need to observe the body before making a final determination.

This paper (PDF) goes into A LOT more detail. They conclude with:

Our results may be summarized as follows. Whether or not Pluto develops an ocean depends on the rate of heat transfer across the ice shell compared with the heat produced by radiogenic elements (McKinnon, 2006). We find that, for the nominal potassium abundance, there is a critical reference viscosity above which an ocean develops. For reference viscosities in the range 10^13 - 10^15 Pa s vigorous convection occurs and an ocean never forms. For reference viscosities >4.16 x 10^15 Pa s, convection is either sluggish or absent, and present-day Pluto possesses an ocean of roughly 165 km thickness beneath a conductive ice shell 165 km thick. If the core is sufficiently depleted in potassium, an ocean never forms irrespective of the reference viscosity. Without an ocean during Pluto’s past, surface tectonics will be predominantly extensional (perhaps with minor recent compression) and a fossil bulge may be present. For cases which possess an ocean, there is no fossil bulge, and compressional surface stresses should dominate (perhaps with minor recent extension).

This study does not include the presence of volatiles such as ammonia, which are likely present and may have an effect on Pluto’s thermal evolution. The main effect of incorporating ammonia is to produce colder ocean temperatures and thus generate ice shells which are more likely to be conductive. Ammonia thus increases the likelihood that Pluto currently possesses a subsurface ocean.

Yep, freakin' awesome! But keep in mind - we really don't know what the little guy is made of, so all this is still quite a bit of conjecture. 2015 will be the telling time when Horizons gets there.