Studies of a large frost-filled basin on Pluto show that this feature altered the dwarf planet's spin axis, driving tectonic activity on its surface, and hint at the presence of a subsurface ocean. See Letters p.86, p.90, p.94 & p.97
In a culture of selfies, it's difficult to imagine a more compelling picture of Pluto than the image returned by NASA's New Horizons space probe in July 2015 (Fig. 1). Pluto put its best face forward for New Horizons, showing us an area of bright, smooth terrain, in the shape of a heart. A renewed love of Pluto, which was stripped of its planethood a decade ago, swelled in the hearts of planetary scientists and schoolchildren alike. Four papers published in this issue of Nature1,2,3,4 show that the heart formed as a result of the interplay of slow deposition of frozen noxious chemicals, bitterly cold winds, cracking icy crusts, cryogenic buried oceans and planetary cartwheels. Pluto may be cute, but this is planetary science, after all.
Pluto is one of the dwarf planets in the Kuiper belt, a family of objects beyond the orbit of Neptune that are about half the size of Earth's Moon, and is composed of solid water ice and rock. On Pluto, an outer shell of water ice behaves like bedrock — the ice is shaped into mountains, fractures and faults by internal tectonic forces5, and forms impact craters when objects strike the dwarf planet6.
For decades since its discovery, astronomers have watched Pluto as it crossed the line of sight between Earth and distant stars. By measuring how the spectrum of the starlight changed during these events, researchers showed that Pluto has an atmosphere composed of nitrogen, methane and carbon monoxide7, all of which can be in the solid or the gaseous state at Plutonian temperatures. Pluto's axial tilt of 120° and eccentric orbit mean that the amount and pattern of sunlight that falls on its surface change dramatically over the course of its 90,560-day year. This variability drives volatile ices to condense in some places and sublimate (be directly converted from solid to gas) in others — these ices therefore migrate across Pluto's surface.
The left half of the heart-shaped region on Pluto is informally known as Sputnik Planitia and is unlike any other geological feature in the Solar System. It consists of a depression in Pluto's water-ice shell that is filled with the same ices that comprise the dwarf planet's atmosphere8. This deposit of atmospheric ices is about 4 kilometres thick7,9, which is similar to the average depth of Earth's oceans. Its surface is smooth and only 10 million years old6. The four new studies use data from New Horizons to explain the origins of Sputnik Planitia and its effect on Pluto.
Bertrand and Forget1 (page 86) simulate how nitrogen, methane and carbon monoxide frosts sublimate from regions that are warmed by sunlight, and condense in cold or low-lying regions. Their simulations reproduce the time-evolution of global frost deposits on Pluto, recorded by decades of telescopic observations7, and show that a low-latitude basin could easily accumulate frost that is many kilometres thick. Hamilton and colleagues2 (page 97) point out that the formation of a single frost deposit on Pluto is inevitable — as the frost begins to accumulate, that region reflects more sunlight and becomes colder, driving further deposition. Because this frost deposit is denser than the surrounding water ice, it creates its own depression.
Hamilton et al., Keane et al.3 (page 90) and Nimmo et al.4 (page 94) describe how Pluto's interior would have reacted to the frost deposit. Pluto is not perfectly spherical — the gravity of its moon, Charon, causes Pluto to be egg-shaped. Therefore, in the minimum-energy configuration of the Pluto–Charon system, the long axes of Pluto and Charon are aligned. Charon always orbits above the same spot on Pluto, as though the two bodies were joined by a rigid stick.
However, when frost accumulated in Sputnik Planitia, the system was no longer in a minimum-energy configuration. Pluto rolled over, and its ice shell was fractured by the resulting tension and compression forces, creating canyons and mountains3. The line that now joins Pluto and Charon pierces the centre of Sputnik Planitia, a configuration that is favoured only if Sputnik Planitia represents a region of excess mass, despite the fact that it is a depression. Nimmo and colleagues show that such a mass excess is possible if the ice shell is thin and underlain by a liquid ocean, which is consistent with previous findings8.
These four studies are a testament to the deductive powers of modern planetary science. On the basis of a single set of images from New Horizons, the authors have used lessons learnt from analysing other planets to unravel the mystery of Pluto's heart. With no future Pluto missions planned, are these studies verifiable? Bertrand and Forget predict a disappearance of frost in Pluto's northern hemisphere in the coming decades, which could be observed using telescopes. But the other studies offer few suggestions, apart from numerical modelling, to test their hypotheses.
However, the processes that were responsible for the formation of Sputnik Planitia also shaped other planetary bodies. For example, a similar frost migration and deposition occurred on Mars10. And the Moon11, Mars12 and Saturn's moon Enceladus13 have undergone reorientation due to a loading of material on their crusts. Perhaps, as simulations of these processes improve through further comparisons between models and data, our understanding of Pluto will be enhanced, providing support or opposition for these interpretations.Footnote 1
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