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The origin of cometary matter and the potential contribution of comets to inner-planet atmospheres are long-standing problems. During a series of dedicated low-altitude orbits, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta spacecraft analyzed the isotopes of xenon in the coma of comet 67P/Churyumov-Gerasimenko. The xenon isotopic composition shows deficits in heavy xenon isotopes and matches that of a primordial atmospheric component. The present-day Earth atmosphere contains 22 ± 5% cometary xenon, in addition to chondritic (or solar) xenon.

Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10 −10 to 10 −7 kilograms, and 48 grains of mass 10 −5 to 10 −2 kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails.

Carbon dioxide (CO₂) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO₂ ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO₂ ice-rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO₂ ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination. the CO₂ ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO₂ ice and the depth of the eroded surface layer. We interpret the presence of CO₂ ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

The international Rosetta mission was launched in 2004 and consists of the orbiter spacecraft Rosetta and the lander Philae. The aim of the mission is to map the comet 67P/Churyumov-Gerasimenko by remote sensing, and to examine its environment in situ and its evolution in the inner Solar System. Rosetta was the first spacecraft to rendezvous with and orbit a comet, accompanying it as it passes through the inner Solar System, and to deploy a lander, Philae, and perform in situ science on the comet's surface. The primary goals of the mission were to: characterize the comet's nucleus; examine the chemical, mineralogical and isotopic composition of volatiles and refractories; examine the physical properties and interrelation of volatiles and refractories in a cometary nucleus; study the development of cometary activity and the processes in the surface layer of the nucleus and in the coma; detail the origin of comets, the relationship between cometary and interstellar material and the implications for the origin of the Solar System; and characterize asteroids 2867 Steins and 21 Lutetia. This paper presents a summary of mission operations and science, focusing on the Rosetta orbiter component of the mission during its comet phase, from early 2014 up to September 2016. This article is part of the themed issue ‘Cometary science after Rosetta’.

Main-belt comets are small Solar System bodies located in the asteroid belt that repeatedly exhibit comet-like activity (that is, dust comae or tails) during their perihelion passages, strongly indicating ice sublimation 1 , 2 . Although the existence of main-belt comets implies the presence of extant water ice in the asteroid belt, no gas has been detected around these objects despite intense scrutiny with the world’s largest telescopes 3 . Here we present James Webb Space Telescope observations that clearly show that main-belt comet 238P/Read has a coma of water vapour, but lacks a significant CO 2 gas coma. Our findings demonstrate that the activity of comet Read is driven by water–ice sublimation, and implies that main-belt comets are fundamentally different from the general cometary population. Whether or not comet Read experienced different formation circumstances or evolutionary history, it is unlikely to be a recent asteroid belt interloper from the outer Solar System. On the basis of these results, main-belt comets appear to represent a sample of volatile material that is currently unrepresented in observations of classical comets and the meteoritic record, making them important for understanding the early Solar System’s volatile inventory and its subsequent evolution. Using James Webb Space Telescope observations, spectroscopic identification of a coma of water vapour but no significant CO 2 gas coma is found for the main-belt comet 238P/Read, indicating water–ice sublimation.

The European Rosetta mission has been following comet 67P/Churyumov-Gerasimenko for 2 years, studying the nucleus and coma in great detail. For most of these 2 years the Rosetta Orbiter Sensor for Ion and Neutral Analysis (ROSINA) has analysed the volatile part of the coma. With its high mass resolution and sensitivity it was able to not only detect deuterated water HDO, but also doubly deuterated water, D₂O and deuterated hydrogen sulfide HDS. The ratios for [HDO]/[H₂O], [D₂O]/[HDO] and [HDS]/[H₂S] derived from our measurements are (1.05 ± 0.14) × 10⁻³, (1.80 ± 0.9) × 10⁻² and (1.2 ± 0.3) × 10⁻³, respectively. These results yield a very high ratio of 17 for [D₂O]/[HDO] relative to [HDO]/[H₂O]. Statistically one would expect just 1/4. Such a high value can be explained by cometary water coming unprocessed from the presolar cloud, where water is formed on grains, leading to high deuterium fractionation. The high [HDS]/[H₂S] ratio is compatible with upper limits determined in low-mass star-forming regions and also points to a direct correlation of cometary H₂S with presolar grain surface chemistry. This article is part of the themed issue 'Cometary science after Rosetta'.

We will briefly recapitulate the beginning of modern cometary physic. Then we will assess the results of the cometary flyby missions previous to ESA’s Rosetta rendezvous with comet 67P/Churyumov–Gerasimenko. Emphasis is given to the physical properties of cometary nuclei. We will relate the results of the Rosetta mission to those of the flybys. A major conclusion is that the visited cometary nuclei seem to be alike but represent different stages of evolution. Coma composition and appearance are not only controlled by the composition of the nucleus but also strongly influenced by the shape and rotation axis orientation of the nucleus and resulting seasons that generate varying surface coverage by back fall material. Rosetta showed that the coma composition is not only varying spatially but also strongly with time during the perihelion passage. Hence past interpretations of cometary coma observations have to be re-considered. Finally, we will try to assess the impact of the cornerstone mission leading to a critical evaluation of the mission results. Lessons learned from Rosetta are discussed; major progress and open points in cometary research are reviewed.

Solar heating of a cometary surface provides the energy necessary to sustain gaseous activity, through which dust is removed 1 , 2 . In this dynamical environment, both the coma 3 , 4 and the nucleus 5 , 6 evolve during the orbit, changing their physical and compositional properties. The environment around an active nucleus is populated by dust grains with complex and variegated shapes 7 , lifted and diffused by gases freed from the sublimation of surface ices 8 , 9 . The visible colour of dust particles is highly variable: carbonaceous organic material-rich grains 10 appear red while magnesium silicate-rich 11 , 12 and water-ice-rich 13 , 14 grains appear blue, with some dependence on grain size distribution, viewing geometry, activity level and comet family type. We know that local colour changes are associated with grain size variations, such as in the bluer jets made of submicrometre grains on comet Hale–Bopp 15 or in the fragmented grains in the coma 16 of C/1999 S4 (LINEAR). Apart from grain size, composition also influences the coma’s colour response, because transparent volatiles can introduce a substantial blueing in scattered light, as observed in the dust particles ejected after the collision of the Deep Impact probe with comet 9P/Tempel 1 17 . Here we report observations of two opposite seasonal colour cycles in the coma and on the surface of comet 67P/Churyumov–Gerasimenko through its perihelion passage 18 . Spectral analysis indicates an enrichment of submicrometre grains made of organic material and amorphous carbon in the coma, causing reddening during the passage. At the same time, the progressive removal of dust from the nucleus causes the exposure of more pristine and bluish icy layers on the surface. Far from the Sun, we find that the abundance of water ice on the nucleus is reduced owing to redeposition of dust and dehydration of the surface layer while the coma becomes less red. Spectral analysis of the VIRTIS dataset shows two opposite seasonal colour cycles in the coma and on the surface of comet 67P/Churyumov–Gerasimenko, indicating an orbital water-ice cycle.

The surface and subsurface of comets preserve material from the formation of the solar system. The properties of cometary material thus provide insight into the physical and chemical conditions during their formation. We present mass spectra taken by the Ptolemy instrument 20 minutes after the initial touchdown of the Philae lander on the surface of comet 67P/Churyumov-Gerasimenko. Regular mass distributions indicate the presence of a sequence of compounds with additional -CH 2 - and -O- groups (mass/charge ratios 14 and 16, respectively). Similarities with the detected coma species of comet Halley suggest the presence of a radiation-induced polymer at the surface. Ptolemy measurements also indicate an apparent absence of aromatic compounds such as benzene, a lack of sulfur-bearing species, and very low concentrations of nitrogenous material.

So far, only two interstellar objects have been observed within our Solar System. While the first one, 1I/‘Oumuamua, had asteroidal characteristics, the second one, 2I/Borisov, showed clear evidence of cometary activity. We performed polarimetric observations of comet 2I/Borisov using the European Southern Observatory Very Large Telescope to derive the physical characteristics of its coma dust particles. Here we show that the polarization of 2I/Borisov is higher than what is typically measured for Solar System comets. This feature distinguishes 2I/Borisov from dynamically evolved objects such as Jupiter-family and all short- and long-period comets in our Solar System. The only object with similar polarimetric properties as 2I/Borisov is comet C/1995 O1 (Hale-Bopp), an object that is believed to have approached the Sun only once before its apparition in 1997. Unlike Hale-Bopp and many other comets, though, comet 2I/Borisov shows a polarimetrically homogeneous coma, suggesting that it is an even more pristine object. Polarimetry provides information about physical characteristics of cometary dust. Here, the authors show that the polarization of interstellar comet 2I/Borisov exceeds the typical values for comets, and this together with its polarimetrically homogenous coma suggests a more pristine nature of the object.