Pluto May Be Erupting

Two mountains that look like giant volcanoes hint at current activity on Pluto.

Nature posted some of the latest images from New Horizons that suggest ice volcanoes exist near Pluto’s south pole. These mountains, one 3-5 km high and the other 6 km high, resemble features on Neptune’s moon Triton thought to be eruption centers of icy “magma” from the interior. The mountains are about 100 miles wide.

If these are indeed eruptive centers, they raise questions about the age of Pluto.

Much of the rest of Pluto’s surface is geologically active, with towering mountains and smooth icy plains. All that activity suggests that some internal heat source — most likely the radioactive decay of elements left over from Pluto’s birth, 4.5 billion years ago — keeps things warm enough to flow.

But cryovolcanoes would require enough heat to send slush from Pluto’s depths squirting through its icy surface. On Triton, the gravitational pull of nearby Neptune flexes the moon and produces enough frictional heat to sustain the icy volcanoes, says Carly Howett, a planetary scientist at the Southwest Research Institute in Boulder, Colorado, and a New Horizons team member. It’s less clear whether Pluto has enough internal heat to sustain cryovolcanoes.

If, on the other hand, they are tectonic features like those believed to exist on Ganymede, scientists cannot take comfort. “Jupiter’s moon Ganymede also has cryovolcanic-like features that are not fully understood,” a leading planetary scientist says.

Another surprise was reported by Space.com:

The new results show that as the four moons orbit Pluto and its largest moon, Charon, some of them are spinning incredibly fast, one is spinning backward against its orbit and some are tilted on their sides. This is in stark contrast to nearly every other moon in Earth’s solar system, most of which are locked into a more rigid and unmoving orbit around their parent bodies, making Pluto’s moons the wild children of the solar system.

This presents another age conundrum. If Pluto and its moons are really as old as claimed, it would seem the moons would have come to equilibrium by now or become tidally locked to the dwarf planet.  Some rapid spins might be explained by a collision, but not for all four of them. Over time, “Typically, the gravity of a parent body would dissipate the planet’s rotational motion,” but that is not what is observed.

Can radioactive heating keep this dwarf planet going for 4.5 billion years? It sounds like special pleading. Consider that smaller Charon also shows signs of recent activity. Why doesn’t this excuse apply to every planetary body? Wouldn’t it follow that heat should be a function of diameter? Yet some larger bodies do not appear as active as Pluto. Out there alone in the cold, Pluto should have become dead long ago. For a list of reasons why Pluto appears far younger than expected, see CMI’s article “The New Pluto.”