by Paul Weissman from Nature 523, 42–43 (02 July 2015) doi:10.1038/523042a
Analyses of images taken by the Rosetta spacecraft reveal the complex landscape of a comet in rich detail. Close-up views of the surface indicate that some dust jets are being emitted from active pits undergoing sublimation.
When do 18 holes not make for a pleasant afternoon playing golf? When the 18 holes are located on the surface of a comet speeding through the Solar System. Vincent et al.(1) describe the holes, also called pits, that comprise one of the many discoveries of the European Space Agency’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P). The Rosetta spacecraft went into orbit around 67P in August 2014, and the surprises have been coming fast since then. Vincent et al. propose a mechanism for the formation of the pits and identify them as one of the sources of active dust jets.
Comets are the most primitive bodies in the Solar System; they are the remnants of its formation process. Comets therefore retain a physical and chemical record of the conditions and materials in the solar nebula — the gas and dust cloud out of which the Sun and planets formed 4.56 billion years ago. Conveniently, comets have spent most of that time in two very cold storage locations: the Kuiper belt beyond the orbit of Neptune and the spherical Oort cloud outside the planetary region, stretching halfway to the nearest stars. The distant Oort cloud is the source of the long-period comets that have orbital periods ranging up to millions of years. The Kuiper belt is the source of the Jupiter-family comets, such as 67P, which typically have periods of less than 20 years and orbital dynamics that are strongly affected by Jupiter.
As a comet approaches the Sun and warms up, the central solid part, known as the cometary nucleus (comprised of volatile ices and primitive meteoritic material), begins to sublimate and becomes enveloped by a freely outflowing atmosphere called the coma. One of the first surprises for Rosetta, the first ever comet-rendezvous mission, was the odd shape of the target comet’s nucleus (Fig. 1a)(2). Although some nuclei comprised of two large pieces and looking like a bowling pin had been observed before by fly-by missions to other comets, the two lobes of 67P sit on top of each other, with a narrow ‘neck’ in between. There is intense speculation as to how this odd configuration may have formed. Did two cometary nuclei gently collide randomly in the solar nebula, or is the nucleus a single piece that has been oddly sculpted by sublimation processes? Although the former is the more likely scenario, some scientists on the mission suspect the latter.
Rosetta’s camera system, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS), is comprised of narrow-angle and wide-angle digital cameras. As the OSIRIS team of scientists2 began to map the surface of the nucleus using the cameras, they discovered 18 pits on the surface, which Vincent et al. now describe more thoroughly. The cometary nucleus has a diameter of approximately 4 kilometres. The pits are typically about 200 metres in diameter and about 180 metres deep. Pit-like features have been observed on other cometary nuclei, but the morphology of the pits on 67P has not been seen before. They typically have cylindrical shapes with circular openings and near-vertical walls (although at least one pit seems to be lying at a steep angle). And some of the pits are clearly active: images of pits that are illuminated by sunlight show dust jets emanating from their walls and/or floors (Fig. 1b).
How did the pits form? Vincent et al. suggest that they are ‘sink holes’, which formed when material near the surface of the nucleus collapsed into the low-density interior. Rosetta’s Radio Science Investigation team has found(2) that the nucleus has an average bulk density of only 470 ± 45 kilograms per cubic metre, about half the density of solid water ice. But the Grain Impact Analyser and Dust Accumulator instrument has measured(3) a dust-to-ice mass ratio of 4 ± 2, suggesting that silicates and organics, rather than ices, make up about 80% of the mass of the nucleus. This in turn implies that 75–85% of the nucleus interior is empty space, a parameter known as porosity. A high porosity is predicted by the leading scenarios for the internal structure of cometary nuclei, which suggest that they are aggregates(4) of smaller, icy bodies that gently came together in the solar nebula. These aggregates are also referred to as rubble piles(5). This concept has provided insights into the behaviour of comets, such as random and other splitting events.
The morphology of 67P’s surface is dominated in some areas by large, flat-floored basins, similar to features seen on the nucleus of comet(6) Wild 2. It has been suggested that these are sublimation basins that slowly widen as the walls sublimate, leaving large, non-volatile particles that cover the basin floor. The basins cannot be impact craters because they have the wrong size distribution (there are too many large ones), and because not many impact craters are expected on a small cometary nucleus such as 67P.
Could the pits described by Vincent et al. be the precursors of the basins, slowly widening as their walls sublimate? Many of the pits found by OSIRIS are located in the same region on the nucleus where many of the large sublimation basins are found. Both comet 67P and comet Wild 2 are relatively young — that is, they have only recently (within the past 60 years) been perturbed by the gravitational field of Jupiter to perihelion distances (the point in their orbit closest to the Sun) at which it is warm enough for water ice in the nucleus to sublimate, and at which the activity that manifests itself as the bright cometary coma and tails begins. If this is so, why are sublimation basins not observed on other, perhaps older, Jupiter-family comets such as Tempel 1 and Hartley 2? Older nuclei may have accumulated thicker layers of non-volatile materials that have buried the sublimation basins and substantially lowered the activity levels of those comets.
Rosetta has already indicated that it has more surprises for us. On 13 June 2015, the orbiter began receiving signals from the Philae lander, which is on the surface of the comet nucleus and was last heard from in November 2014. With its batteries recharging, Philae probably has much more information to transmit about its final landing location. Also, the activity of the nucleus is expected to reach a maximum soon after the comet passes through perihelion at 1.25 astronomical units from the Sun (a point about 25% farther from the Sun than Earth’s orbit) in mid-August 2015. Rosetta will then follow 67P away from the Sun as cometary activity begins to wane. What changes will we see on the nucleus surface? And how will this alien golf course look from Rosetta’s vantage point then?
(1) Vincent, J.-B. et al. Nature 523, 63–66 (2015).
(2) Sierks, H. et al. Science 347, aaa1044 (2015).
(3) Rotundi, A. et al. Science 347, aaa3905 (2015).
(4) Donn, B. & Hughes, D. in 20th ESLAB Symp. Exploration of Halley’s Comet (eds Battrick, B. et al.) 523–524 (ESA, 1986).
(5) Weissman, P. R. Nature 320, 242–244 (1986).
(6) Kirk, R. et al. in 46th Lunar and Planetary Science Conf. Abstr. 2244 (2015).