TY - JOUR
AU1 - Bowler, Sue
AB - A special issue of Monthly Notices of the RAS brings results from Rosetta on the 30th anniversary of Giotto's encounter with comet 1P/Halley, writes Sue Bowler. The Rosetta mission ended on 30 September 2016; after 786 days at comet 67P/Churyumov–Gerasimenko, the European Space Agency spacecraft touched down and shut down. The mission has produced a store of data that will be exploited for years to come, adding to the knowledge about comets and the formation of the solar system. Rosetta's duration and range of instruments, together with those on the lander Philae, have combined to make the mission a major step forward. This article focuses on papers published in a special issue of the journal Monthly Notices of the Royal Astronomical Society based on the ESA's 50th ESLAB Symposium; a second special issue will follow next year, presenting research from the Comets 2016 conference in Toulouse in November. “The publication of these collections of papers, containing the most recent data from the Rosetta mission, represents an important innovation for MNRAS,” says editor-in-chief David Flower. “Given the success of the mission and the significance of its findings, we are delighted that the Rosetta consortium decided that MNRAS was the most appropriate journal for the dissemination of its scientific results.” Open in new tabDownload slide Previous comet missions The ESLAB meeting, celebrating the 30th anniversary of Giotto's encounter with comet 1P/Halley (figure 1), considered data from the Rosetta mission in the context of data from Giotto and previous comet missions including NASA's Deep Impact mission to comet 9P/Tempel 1. Comet 67P/Churyumov–Gerasimenko is a Jupiter-family comet, a group that also includes 103P/Hartley 2, visited by Deep Impact as part of its extended mission; these comets come from the Kuiper belt in the outer solar system. 1P/Halley comes from a different group, the Oort cloud comets, typically longer period comets originating at greater distances from the Sun. 67P is a short-period comet about 4 km across that has a chaotic orbit with a current period of 6.45 years. In 1959 and 1923 it came to within 1 au of Jupiter, changing its orbital path, and making it hard to track the comet more than about 10 orbits back. 1 Open in new tabDownload slide What a difference 30 years makes. (Left) A composite image of the nucleus of comet 1P/Halley, taken by Giotto in 1986 (ESA/MPAE). (Right) Rosetta's OSIRIS camera captured comet 67P from about 15.5 km during its final descent on 30 September 2016. The image measures about 3.2 km across. (Below) The Philae lander on its way to the comet surface (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA). For parts of the Rosetta mission, the comet was visible from Earth; a significant ground-based observing programme was organized to match these more remote observations to data collected at the comet by Rosetta. Snodgrass et al. (2016) summarized the data from robotic telescopes, whose time allocation systems favoured the short observation windows for the comet around perihelion. Photometry data tracked dust production, spectroscopy, the gas output; the comet's brightness seen from Earth peaked two weeks after perihelion and its behaviour largely matched previous perihelia. The first surprises of the mission came as Rosetta approached its target; the irregular, bilobate shape was more akin to a rubber duck than the smooth rounded body of comet simulations (figure 2). Comets 103P/Hartley 2 and 19P/Borrelly also have a bilobate shape, as do some asteroids, suggesting that this configuration could be common. And 67P was dark, with an albedo of just 6%. At close quarters the surface was raw and rugged, with cliffs, dust and boulder fields and circular, pit-like depressions – called pits (figure 3) and first identified as features like sinkholes and the source of outbursts of volatiles and dust from under the surface (Vincent et al. 2015). The craggy surface had very few impact craters because so much of the surface was lost each time the comet approached the Sun; Fulle et al. (2016a) estimate that the nucleus loses an average of 1 m at perihelion, with local losses up to 15 m. 2 Open in new tabDownload slide Pictured on 8 July 2015 from 152 km, activity was building as 67P approached perihelion. Individual streams can be seen rising from the comet's small lobe in particular, while the overall glow of activity accentuates the silhouette of the shadowed portions of the nucleus. (ESA/Rosetta/Navcam) 3 Open in new tabDownload slide Active pit detected in Seth region of 67P, imaged on 28 August 2014 from a distance of 60 km. Image processing reveals fine structures in the shadow of the pit, interpreted as jet-like features rising from the pit. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA) Active pits? Rosetta's extended stay at the comet allowed researchers a closer look at the pits, also seen on comets 9P/Tempel 1 and 81P/Wild 2. These features on 67P are typically a few hundred metres across and around 200 m deep, with flat floors. Detailed modelling presented in Monthly Notices by Guilbert-Lepoutre et al. (2016) suggests that thermal effects causing sublimation would not bring enough energy to mobilize water and other volatiles at such depths in just one orbit of the comet. Large-scale experiments using radio data (RSI and CONSERT) suggest a bulk density of 533 kg m−3 for the nucleus (Pätzold et al. 2016) and that the nucleus is fairly homogeneous; there are no large cavities. Deep Impact measured the bulk density of 9P/Tempel 1 at 400–450 kg m−3 (Davidsson et al. 2016); other comets measured fall above and below these values, although errors are large for these calculations. For 67P, porosities of around 70–80% were measured by the CONSERT experiment (Kofman et al. 2015), matching that estimated for 9P/Tempel 1 from the Deep Impact collision (Ernst & Schultz 2007). Fulle et al. (2016a) note that the dust proportions, in particular, affect the usefulness of comet models that assume an ice matrix; a mineral matrix containing ice is a closer match. Although 67P has a rough surface, its composition is not especially variable; water ice makes up about 1% of the surface on average, with a few water-rich bright patches (VIRTIS data; Fulle et al. 2016a). In smooth terrain, such as that in the Hapi region (in the neck of the comet), VIRTIS detected 15% water ice at sunrise, probably rising from the comet's interior and freezing in near-surface layers in the night (de Sanctis et al. 2015). There was a surface dust-to-water mass ratio of 6 when the comet was at 3.5 au, to between 6 and 100 around perihelion (these values are minima; Fulle et al. 2016b). The dust is thought to comprise crystalline and glassy silicates, organic molecules, including glycine, and iron and nickel compounds, similar to carbonaceous chondrite meteorites; most of it in the coma is in the form of denser particles (Rotundi et al. 2015). In contrast to this low overall density, the data from the Philae lander indicate a stronger surface layer. Philae touched down initially in a region called Agilkia on the “head” of the comet, but did not stay on the surface; it bounced before settling in a crevice in Abydos. Its final location was identified only a few weeks before the end of the mission, on its side by a cliff-face. Agilkia had a strong layer overlaid by a granular layer about 20 cm deep; Abydos did not have loose material; the penetrator on Philae (MUPUS) could not reach more than 27 mm depth, suggesting lower porosity at the surface (Brouet et al. 2016). They suggest that the stronger surface layer may arise from the effects of sunlight, possibly compacting dust and ice by sublimation and redeposition cycles, or sintering. Rosetta's detailed observation of the comet provided ample evidence that the comet has a layered structure in its outer parts, and that the two lobes each have an outer onion-like structure that is absent from the neck region (Massironi et al. 2015). Guilbert-Lepoutre et al. (2016) raise the possibility that dusty outer layers of the comet could protect the subsurface for long enough for volatile movement to produce layering, perhaps extending to several hundred metres depth. Dust movement from one part of the comet to another also produces some layering, but the origin of these layers hundreds of metres into the comet remains unclear. Massironi et al. (2015) conclude that the layering is primordial, part of the accretion process that formed the comet. There is other evidence that primordial features of the comet have survived. Poulet et al. (2016) identifies consolidated granular material across much of the landscape in the panorama image sent back by the Philae lander. This image includes part of the Philae probe which helps estimate the size of these irregularly shaped grains, called pebbles; the authors cite a range between 3.7 and 16.25 mm, with very few at the larger end of the distribution. Poulet et al. (2016) reject recent cometary activity and processes such as solar wind erosion or the effects of sunlight to produce these pebbles and their size distribution. Fulle et al. (2016b) combine the average density of the nucleus with GIADA data on the composition and size of dust particles in the coma to conclude that these pebbles have compositions consistent with chemical abundances in the early solar system, from carbonaceous chondrites and solar abundances. They conclude that 67P is indeed preserving pebbles from the formation of the solar system, as also suggested by Davidsson et al. (2016). The comet and pebble densities suggest that the comet formed by aggregation, possibly as two bodies, early in the evolution of the solar system and has not experienced any significant impacts since then. Fulle et al. (2016b) conclude that the balance of evidence favours an origin in the gentle gravitational collapse of a cloud of pebble-sized bodies. The timing of the neccessarily gentle collision that produced the bilobate form of 67P remains unclear. Massironi et al. (2015) favour an early collision, but recent modelling suggests that the rubber duck shape would be unlikely to survive for several billion years. Jutzi et al. (2016a,b) argue that the relatively gentle collision to produce the bilobate comet we now see would preserve primordal material, but would also leave the neck area as a weak point. Their models predict enough collisions over the 4 billion years of solar system history to break the comet apart. They argue that the current shape formed in the past billion years, but agree with Massironi et al. (2015) that it took place in a way that did not destroy primordial structures in the comet. Further evidence that this comet preserves primordial features comes from the presence of molecular oxygen, discussed by Taquet et al. (2016). Atomic oxygen is the third most abundant element in the universe, but molecular oxygen does not produce a strong infrared spectroscopic signal because it is not dipolar. Bieler et al. (2015) found that O2 was the fourth most abundant molecule at 67P, at about 4% of the abundance of water (using the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, ROSINA). Molecular oxygen was also found at comet 1P/Halley by the Giotto mission strongly correlated with water; reanalysis of Giotto Neutral Mass Spectrometer data by Rubin et al. (2015) puts it at levels comparable with the 67P data. Modelling by Taquet et al. (2016) suggests that the molecular oxygen in the coma of 67P comes from sublimation of water ice from the comet surface, rather than from gas-phase chemistry within the coma. The oxygen could have been primordial, trapped in water ice around dust grains before the comet formed, either outside or inside the protoplanetary disc; Taquet et al. (2016)'s models suggest that molecular oxygen could form and survive through the chemical networks thought to prevail in dark interstellar clouds and protoplanetary discs. Indeed, O2 has been detected in ρ Oph A, a dense core containing relatively warm gas (Liseau et al. 2012). Furthermore, the models indicate that the O2 formed before the protoplanetary disc did; ice formed within the protoplanetary disc, possibly during luminosity outbursts from the young star, would also trap other volatile species such as carbon monoxide and nitrogen, which are not found at 67P. A further link to molecular clouds comes from the deuterium/hydrogen ratio in water. The D/H ratio has been widely used to test if Earth's water could have been delivered by comets after accretion. Both C1 chondrite meteorites – from asteroids – and Earth's oceans have D/H = 0.016%, as does Jupiter-family comet 103P/Hartley 2, raising the suggestion that comets and asteroids could have brought water to our planet. But Oort cloud comets have significantly higher values, D/H = 0.03% on average, and 67P has D/H = 0.053% (Altwegg et al. 2014; figure 4). The latter value is more than three times greater than the value for Earth's oceans, approaching that found in dense interstellar clouds (0.1–1%; Teixeira et al. 1999), providing a link back to primordial conditions (Fulle et al. 2016a). But the data from comet 67P indicate that the origin of Earth's water is complicated and the isotope composition of comet water more variable than had been thought, even within the Jupiter and Oort cloud comet families. 4 Open in new tabDownload slide The deuterium/hydrogen ratio in water observed in various bodies, including in Earth's oceans, shown by the horizontal line at 1.56 × 10−4. Points show planets and moons (blue), chondritic meteorites from the asteroid belt (grey), comets from the Oort cloud (purple) and Jupiter-family comets (pink). Diamonds represent data obtained in situ. The lower part of the graph shows D/H measured in H2 in the atmosphere of the giant planets and an estimate of the typical value in H2 for the protosolar nebula, from which all solar system objects formed. Rosetta's ROSINA instrument measured water vapour from 67P and found the D/H ratio to be 5.3 × 10−4, more than three times greater than for Earth's oceans. (Data from Altwegg et al. 2014 and references therein) Magnetism Rosetta also examined the magnetic properties of the comet. Philae's unexpectedly complex path across the near-surface parts of the comet allowed a closer examination of its intrinsic magnetism – and found that it was negligible, posing a problem for models of the presolar nebula that rely on magnetic alignment (Auster et al. 2015). Rosetta's journey with the comet allowed researchers to track the start of the interaction of the comet with the solar wind and the emergence of a magnetosphere (Richter et al. 2015). Close to perihelion, a diamagnetic cavity developed, similar but not identical to the one observed by Giotto at comet 1P/Halley (Koenders et al. 2015). Further information about the plasma environment of 67P came when a coronal mass ejection reached the comet and spacecraft on 5–6 October 2015, at 1.4 au from the Sun. The CME impact took place at a time when Rosetta was within the coma and had not been detecting solar wind ions; they had been deflected by the magnetosphere. At the time of the CME impact, Rosetta detected solar wind ions again, suggesting a compression of the plasma environment; Edberg et al. (2016) report increased suprathermal electron flux (at 10–200 eV increased by a factor of between 5 and 10) and cold plasma density (by a factor of 10, up to around 600 cm−3). At the same time, the background magnetic field increased by about 2.5 times, from 40 to 100 nT, with spikes reaching more than 200 nT. The magnetic field data is interpreted by Edberg et al. (2016) as showing the formation of magnetic flux ropes, either by shearing within the plasma or by magnetic reconnection. The surface of 67P proved to be a source of interest throughout the mission, with data from Rosetta's remote-sensing instruments combining with the results of Philae's 60-hour prime mission on the comet surface. Topography affects the pattern of insolation and must play a part in the subsurface movement of volatiles, sublimation and development of active areas on the surface (Fulle et al. 2016a). It is possible that the combination of topography and the rotation and orbit of the nucleus can lead to long-lasting cold traps and resulting persistent lateral variations in the levels of sublimation, for example (Guilbert-Lepoutre et al. 2016). One surprise in the Rosetta results is that water appears to be emitted from everywhere on the surface of this comet, whether sites are smooth or rugged. Philae's PTOLEMY instrument detected water at its first landing site (Wright et al. 2015) and the jets observed at relatively close quarters include water ice and water. OSIRIS (Optical, Spectroscopic and Infrared Remote Imaging System) data analysed by Gicquel et al. (2016) indicated that models relying on the sublimation of icy aggregates comprising dirty grains of ice between 5 and 50 µm could match the brightness of the jets seen. Comet 67P showed strong seasonal and day/night variations in its overall activity. The southern lobe of the comet has high levels of carbon monoxide ice, and the northern one more water ice; the orientation of these lobes with respect to the Sun affects the composition of material released to the coma. Snapshots of coma compositions for distant comets may therefore not capture the overall composition of the comet unless such a body can be observed for complete rotation cycles (A'Hearn 2004). There was also a seasonal effect in the movement of dust from the south lobe to the northern one; more dust moved at perihelion and at night. This dust makes the smooth plains in Hapi, and the movement leaves the southern polar regions low in dust. 5 Open in new tabDownload slide Rosetta's OSIRIS narrow-angle camera captured this image of 67P during the spacecraft's final descent on 30 September 2016. The image measures about 225 m across. (ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA) A notable surprise from Rosetta so far is the difficulty experienced in identifying active areas at the comet's surface. It had been thought that comet jets were emitted from active areas of the comet, considered to be small, discrete patches of the nucleus surface, rich in water ice. Models of the distribution of dust and ice on the coma found that models with active areas and those that assumed an even distribution of water ice on the surface produced a good fit to the night and day variations of the gas flux into the coma (Fulle et al. 2016a). One problem is that the resolution of the models used so far is 50 m; it is possible that active areas smaller than this may exist. Mechanisms of dust emission from the surface of the comet remain poorly understood (Fulle et al. 2016b), although it was found that the dust emitted from the nucleus was surprisingly dry, including proportionately less water than the 15% in the nucleus as a whole. The dust in the coma produces very little water; these dust grains also do not get smaller with time or distance from the nucleus, as they would if they were losing water. This suggests that this dust comprises relatively dry compact particles (Fulle et al. 2016b). The behaviour of larger aggregates of dust and ice in a fountain event was investigated by Agarwal et al. (2016), who found that decimetre and larger fragments were found in the coma, as anticipated from observations of the debris trail of the comet and the polarization of light scattered in the coma. More larger particles were found in the coma than had been assumed in comet modelling, which have assumed that coma dust was dominated by micron-sized particles. This study tracked individual particles in the coma and found that half of them were moving towards the nucleus. Gas drag forces entrain the particles as they move away from the surface; the rocket force coming from ice sublimation appears to be the main driver for their movement back to the comet surface; their accelerations are an order of magnitude larger than would be expected from gravity alone. These aggregates did decrease in brightness as they moved away from the comet, suggesting that the ice within them was sublimating. Conclusion Overall, 67P sprang some surprises: its dust content, its activity, the nature of its water. This comet retains information from the earliest times of the solar system, modified to varying degrees by its repeated passages into the inner solar system. It has shown that the shape of a comet strongly influences how its surface evolves at perihelion; many existing comet models assume a spherical body. The traditional summary of a comet as a dirty snowball simply does not fit 67P, with six times as much dust as ice. It is to be hoped that the wealth of data coming out of the Rosetta mission now and in the future will give us a much more realistic picture of the workings of these surprisingly complex and varied bodies. 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TI - From Giotto to Rosetta
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atw224
DA - 2016-12-01
UR - https://www.deepdyve.com/lp/oxford-university-press/from-giotto-to-rosetta-EIBlUiXIgI
SP - 6.37
EP - 6.40
VL - 57
IS - 6
DP - DeepDyve
ER -