Mechanism for Pluto's young surface

Question

According to recent reports (e.g. in Nature), Pluto is geologically active. It has some mechanism by which it has "recycled" its surface. I originally thought we had a similar case to Jupiter's moon Europa: tidal energy causes the moon to flex up and down and all that energy is then released as friction and heat, keeping the theoretical underwater ocean in Europa.

We can't use this mechanism for Pluto, because it is locked with Charon, which is also geologically active. If the dwarf planet has some internal heat, its density is way too small for it to have lasted billions of years; in that case, Mars should still be geologically active.

Does anyone have any idea what keeps Pluto active or are scientists still trying to find out?

Answer 1

Prior to the New Horizons flyby, a popular model for Pluto’s surface involved frost migration. Delsanti & Jewitt (2006) explain that at different points during the year, there should be seasonal pressure variations in the atmosphere. This then causes changes in the extent of nitrogen and methane ices on the surface, which over long periods of time - remember, Pluto has a very long period of revolution - alters local geography. The authors also mention cryovolcanism as a potential source for replenishment of methane that is destroyed by cosmic rays.

Trilling (2016) discusses cryovolcanism and two other processes as ways to resurface Pluto and remove craters:

• Viscous relaxation. Ices on the surface “flow”, removing certain surface formations.
• Convective overturn. Temperature differences in convective cells cause flows similar to those in viscous relaxation.
• Cryovolcanism. In addition to releasing methane (and potentially nitrogen), cryovolcanism can heat up parts of the surface, melting ices, which then erode features.

So far, two cryovolcano candidates have been tentatively identified, Piccard Mons and Wright Mons:

^{Image from Nature; image credit NASA/SWRI/JHUAPL.}

The cryovolcanic situation on Pluto may be closer to that of Enceladus than that of Europa. Plumes were observed on Enceladus’s south pole by Cassini, which appear to be cryovolcanic in nature:

^{False color photograph of Enceladus’s south pole jets. Image in the public domain.}

Tidal heating is thought to be a large contributor to the heat necessary to trigger the plumes. However, this is likely not enough. One explanation is the hot start model, popular for many years, in which Enceladus initially generated enormous internal heat through the decay of radioactive isotopes of aluminum and iron. Other elements that are slower to decay could then have provided more heat as the aluminum and iron stopped producing heat. The subsurface ocean that may have resulted on Enceladus could also exist on Pluto, according to Hussmann et al. (2006).

This is only one possible explanation for a heat source to power cryovolcanism, but there does need to be some sub-surface process on both Enceladus and Pluto that can cause these effects. The hot start model is a good option.

Answer 2

I'm digging up this question, because since it was asked there were many publications about New Horizons findings, proposing at least two other processes (not mentioned in the previous answer) for resurfacing Pluto.

Sublimation. Moore et al. (2017) showed how sublimation of nitrogen ice could lead to the formation of the pit fields observed in some icy plains. This is somewhat similar to the formation of penitentes on Earth.

Dunes. Although the previous study states that "there are no unambiguous aeolian landforms (e.g., dunes) observed on Pluto, which is consistent with either a lack of sand sized particles, and/or surface winds [...] capable of transporting them", Telfer et al. (2017) think otherwise. They interpreted some features as methane ice dunes. Sublimation actually provides a nice mechanism for lofting the ice grains.

These two precesses do not need internal heat to explain the recycling of the surface, but you could say that Pluto is at least "glaciologically" active!