New astronomical and laboratory data show that the abundances of the two dominant ices, nitrogen and methane, on the surfaces of the Solar System's two largest dwarf planets are surprisingly similar — raising fresh questions.
Combining state-of-the-art telescopic and ground-based laboratory data, Tegler et al.1 have recently reported that the proportions of nitrogen (N2) and methane (CH4), the dominant surface ices on the two largest dwarf planets, Pluto and Eris, are surprisingly similar. More specifically, they found that the N2 and CH4 abundances on Eris are near 90% and 10%, respectively, and that those on Pluto are 97% and 3%. Intriguingly, these abundances are also similar to those on the dwarf planet and Kuiper-belt escapee Triton, which orbits Neptune.
Tegler and colleagues' results, published in the Astrophysical Journal, represent the first quantitative comparison of the abundances of volatile ices on the surface of any bodies beyond Neptune. They have significant implications for understanding Pluto and Eris, as well as the Kuiper belt, the disk-shaped region beyond Neptune's orbit where these two dwarf planets and other bodies reside (Fig. 1). The findings also provide reassurance that the detailed study planned for the Pluto system by NASA's New Horizons mission2, which is now en route for a 2015 fly-by, will be of relevance to a broader suite of small planets common to the outer Solar System.
The discovery of Pluto by Clyde William Tombaugh in 1930 can be considered the technical discovery of the Kuiper belt. But the Kuiper belt's existence was firmly established only in the 1990s, with the discovery of additional bodies there3,4. Interestingly, a wide variety of attributes now known to be common to many large Kuiper-belt objects were first identified in studies of Pluto4. These include Pluto's rocky interior, its icy red surface, the presence of its satellites, its high orbital inclination and its resonant orbit with Neptune (Pluto's orbital period is in the precise ratio of 3/2 of Neptune's orbital period). As such, it is reasonable to refer to Pluto as the harbinger of the Kuiper belt and many of its key attributes.
The discoveries of CH4 and N2 ices on Pluto were reported in 1976 and 1992, respectively5,6. By discovering similar abundances of N2 and CH4 ices on Eris and Pluto, Tegler et al.1 have demonstrated that Pluto's icy surface composition may be common — or at least not uncommon — among large Kuiper-belt worlds, thereby demonstrating another way in which Pluto seems to be a harbinger. Furthermore, because both N2 and CH4 create significant atmospheric vapour pressures at characteristic Kuiper-belt surface temperatures7, an important implication of the authors' discovery1 is that tenuous N2–CH4 atmospheres such as Pluto's (its atmospheric pressure is conceivably a few tens of microbars) may also be a common attribute among planets in the Kuiper belt.
Yet Tegler and colleagues' findings also raise new questions. A pivotal one is why some large Kuiper-belt worlds, such as Eris and Pluto, display N2 and CH4 on their surfaces, whereas others — even those that are similar to Eris and Pluto in both size and location in the Kuiper belt — display only H2O ice on their surface, with no trace8 of either N2 or CH4. A second, related, question concerns comparisons between the surface compositions of Pluto and Eris, and those of comets, which themselves derive from, and are thought to be the building blocks of, dwarf planets. Although the CH4 fractions on Pluto and Eris are not unlike those seen in some comets9, it is puzzling that they display so much N2 on their surfaces when comets are apparently uniformly N2 poor9.
The ongoing rapid advance of ground-based astronomical facilities offers hope that, within this decade, such questions will be answered. The obvious route would be to apply Tegler and colleagues' methods — which involve both ground-based infrared spectroscopy and laboratory-based spectral studies of ice mixtures — to many more Kuiper-belt planets and smaller bodies.
Adding to the likelihood that such questions will be resolved in this decade are two important space missions now en route to their targets. One is the European Space Agency's flagship Rosetta comet orbiter, which will make the most detailed and comprehensive exploration ever imagined10 of a comet (and Kuiper-belt escapee). Rosetta will arrive at its target, comet 67P/Churyumov–Gerasimenko, in mid-2014. Then, just one year later, NASA's New Horizons mission2 will reconnoitre Pluto and all three of its known moons in exquisite detail.
Of particular relevance for surface-composition studies is the fact that both missions carry sensitive infrared mapping spectrometers. These spectrometers will, for the first time, reveal the distribution of N2, CH4 and many other compounds across the surface of a dwarf planet and search for them across a comet. What's more, they will, by dint of the close proximity of their spacecraft to the respective targets, also be able to look into the near-surface interiors of these representatives of comets and dwarf planets. This will be accomplished by examining the surface compositions of subsurface windows afforded by craters, fissures and exposed, vertically bedded layering where it is present on these bodies.
Tegler et al.1 have revealed both compositional insight into, and commonalities among, the two largest planets of the Kuiper belt. It is up to future research teams, working with even more advanced facilities than those used by the authors, to address the questions that this discovery has raised, and to determine how much more diversity or commonality there is in surface composition among the planets of the Kuiper belt.