About 9 months ago NASA’s New Horizons spacecraft flew past the binary dwarf planets Pluto and Charon more than 9 years after launch. Everyone knew they would be frigid little worlds but the great risk was that they might turn out to be geologically boring. The relief when the first images finally arrived – New Horizons’ telecoms are pretty slow – was obvious on the faces at mission control. Even non-Trekkies, such as me, will be thrilled by the first in-depth, illustrated account (Moore, J.M. and 41 others 2016. The geology of Pluto and Charon through the eyes of New Horizons. Science, v. 351, p. 1284-1293), part of a five-article summary of early findings; the other 4 are on-line and scheduled for full publication later (summaries in Science, 18 March 2016, v. 351, p. 1280-1284). A gallery of images can be seen here and an abbreviated summary of the series here.
They are astonishing places, even at a resolution of only about 1 km (270 m for some parts), and only one fully illuminated hemisphere was imaged for each because of the short duration of the fly-by. Pluto is by no means locked in stasis, for one of its largest features, Sputnik Planum, is so lightly cratered that is must be barely 10 Ma old at most. It is a pale, heart-shaped terrane dominated by smooth plains, which have a tiled or cellular appearance, with flanking mountains up to 9 km high that appear to be a broken-up chaos. Much of it is made of frozen nitrogen, carbon monoxide and methane. The dominant nitrogen ice has low strength which accounts for the large area of very low relief. The highly angular mountains are water ice that is buoyant and stronger relative to the others making up Sputnik Planum. Across the plain are areas of pitting and blades that seem to have formed by ice sublimation (solid to gas phase transitions) much like terrestrial snow or ice fields that have begun to degrade, and there are even signs of glacier-like flow.
4 Ga old cratered, upland terranes surrounding Sputnik Planum display grooved, ‘washboard’ and a variety of other surface textures reminiscent of dissection. The may have formed by long-term lateral flow (advection) of nitrogen ice and perhaps some melting. It is in this rugged part of Pluto that colour variation is spectacular, with yellows, blues and reds, probably due to deposition of hydrocarbon ‘frosts’ condensed from the atmosphere. That Pluto is still thermally active is shown by a few broad domes with central depressions that suggest volcanism, albeit with a magma made of ices. Areas of aligned ridges and troughs provide signs of tectonics, possibly extensional in nature.
Charon shows little sign of remaining active and capable of remoulding its surface. The hemisphere that has been imaged is spectacularly bisected by a 200 km wide belt of roughly parallel escarpments, ridges and troughs with a relief of about 10 km. Superimposed by large craters the extensional system probably dates back to the early history of the outer Solar System. Dominated by water ice it seems that Charon’s surface may have lost any more volatile ices by sublimation and loss to space. This suggests that superficial differences between two small worlds of similar density may be explained by Charon’s lower mass and gravitational field, resulting in the loss of its most volatile components that partly veneer the surface of Pluto.
Being hugely distant from any other sizeable body it is likely that the energy used to form cryovolcanic eruptions and deform the surface of both dwarf planets is due to internal radioactivity. Their similar mean density around 1.9 implies rocky cores that could host the required unstable isotopes. Being the only Kuiper Belt objects that have been closely examined naturally suggests that the rest of the myriad bodies that clutter it are similar. There are currently as many as 9 other sizable bodies suspected of eccentrically orbiting the Sun in the Kuiper Belt, including one that may be ten times more massive than Earth – a candidate for a ninth planet to replace Pluto, which was removed from that status following redefinition in 2006 of what constitutes a bona fide planet.