The Composition of Comets

This paper is the result of the International Cometary Workshop, held in Toulouse, France in April 2014, where the participants came together to assess our knowledge of comets prior to the ESA Rosetta Mission. In this paper, we look at the composition of the gas and dust from the comae of comets. With the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. We also discuss what has been learned from mass spectrometers during flybys. A few caveats for our interpretation are discussed. With dust, much of our information comes from flybys. They include {\it in situ} analyses as well as samples returned to Earth for laboratory measurements. Remote sensing IR observations and polarimetry are also discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and Philae lander will bring to bear to improve our understanding of comet 67P/Churyumov-Gerasimenko as "ground-truth" for our previous comprehensive studies. Finally, we summarize some of the initial Rosetta Mission findings.Comment: To appear in Space Science Review

Optical properties of dust from Jupiter family comets

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Asteroidal polarimetry, as compared to cometary polarimetry: the case of 21 Lutetia

International audiencePolarimetric measurements provide clues to the classification of solar system objects and to their local properties, through experimental and numerical simulations. Asteroidal polarimetry originates in solar light scattered by an irregular surface, as opposed to cometary polarimetry that comes from scattering by a low-density dust cloud. In both cases, polarimetric phase curves are somehow comparable, with key parameters such as the minimum in polarization, the inversion angle, the maximum in polarization, and their wavelength dependence. Differences between observations obtained at similar phase angles and wavelengths may be attributed to local changes, e.g. variagation of the surface revealed by the rotation for asteroidal polarimetry and existence of halos and jet features with different dust particles for cometary polarimetry. New observations of 21 Lutetia have been recently obtained in three colors and interpreted through laboratory simulations (1); they will be compared with other observations (2) and discussed in relation with the surface images obtained during the Rosetta flyby on 10 July 2010. Also, conclusions about the properties of dust in the coma of 67P/Churyumov-Gerasimenko, the rendezvous target of the Rosetta mission, as inferred from polarimetric observations, will be summarized (3,4)

Imaging Polarimetry of the Dust Coma of the Deep Impact Target Comet Tempel 1

International audienceWe have observed comet 9P/Tempel 1 (the target of the Deep Impact mission) by a polarization imaging technique before and after impact with the 80 cm telescope at Haute-Provence observatory (France). The ejected material is well observed in our images and differences in physical properties are characterized

Properties of dust in inner cometary comae from polarimetric observations

Evidence for changes in the properties of dust particles in cometary comae is provided by observations of the linear polarization of the solar light they scatter, which only depends upon the observational conditions and the dust properties. Local polarimetric changes in a coma under fixed observational conditions point out changes in the properties of the dust (e.g. size distribution, morphology, complex refractive indices and thus albedo). After evidence was obtained from local polarimetric observations of Halley's comet with Giotto spacecraft, techniques of remote polarimetric imaging have been developed, providing polarimetric images of the comae of some comets [e.g. 1]. Up to three different regions may be noticed: a background coma, jet-like features with a higher polarization, and a polarimetric halo extending on less than 2000 km around the photometric center. The possible presence of dust particles with different properties in the innermost coma is of major importance for the Rosetta mission, expected to rendezvous with comet 67P/Churyumov-Gerasimenko in 2014 and to approach its nucleus, while releasing the Philae lander from an altitude smaller than 10 km. Polarimetric observations, as interpreted through both experimental and numerical simulations [e.g., 2, 3], together with recent Stardust and Deep Impact in situ missions, provide clues to the following properties: the dust particles are quite small (0.1 micrometer to a few millimeters); they are very porous and irregular, possibly consisting of fluffy aggregates; they mostly consist of silicates and carbonaceous compounds, although the presence of icy grains cannot be ruled out in the innermost coma. We will discuss the origin of changes between the properties of dust particles present in the innermost coma and those of found further away from the nucleus, as mostly deduced from recent laboratory simulations of dust polarimetric properties. Support from CNES is acknowledged. [1] Hadamcik et al. JQSRT 79, 661, 2003. [2] Hadamcik et al. Icarus 190, 660, 2007. [3] Lasue et al. Icarus 199, 129, 2009

Imaging polarimetry of comet 73P/Schwassmann-Wachmann 3 main fragments during its 2006 apparition

International audienceWe have observed the dust ejected by parts of the nucleus (so-called fragments or components) of comet 73 P/Schwassmann-Wachmann 3 during seven consecutive nights from 2006, April 27 to May 3 by imaging polarimetry using the 0.8 m telescope at OHP (Observatoire de Haute-Provence, France). Three fragments were observed, B and C main fragments on all nights and G fragment on two nights at 24 h interval. Fragment C, which almost behaves as a normal comet, presents some night-to-night evolution on polarization maps together with some sunward-jets morphology. Fragment B, as noticed by numerous observers, continues to fragment, with clues to the presence of large secondary fragments, tailward on the intensity images; an increase of activity is noticed on May 2. Jets and fans are observed sunward, with a larger extension in fragment C than in B. Fragment G is fainter and, as fragment B, it continues to fragment. A short sunward jet is detected on the rotational gradient image together with an important tailward structure. The integrated polarization for the two main fragments is typical of polarization of high-Pmax comets. An important evolution is observed from night-to-night on the polarization maps. Fragment C presents, in two nights at 48 h interval, a lower polarization in the inner coma, neither observed in the intermediate night nor later. A high polarization is also observed on the two sides of the lower polarization regions. In fragment B, the regions around the secondary fragments have a higher polarization than the surrounding coma, They are easily detected in the treated intensity images. As usually, the polarization increases when the phase angle increases. Numerous observers have found similar chemical compositions for the two main fragments together with differences in their optical properties, suggesting heterogeneities in the physical properties during the aggregation of the original nucleus and/or changes after the ejection of dust particles

Analogues of interplanetary dust particles to interpret the zodiacal light polarization

International audienceSome local properties of the dust particles that build the interplanetary dust cloud may be deduced from the light they scatter (with emphasize on their linear polarization). The ratio of organics (absorbing particles) to silicates (more transparent particles) was suggested through numerical simulations to reproduce the shape of the polarimetric phase curve and the decrease in polarization with decreasing solar distance.Reproducing these properties, through measurements on real dust particles lifted in microgravity conditions, in the PROGRA2 light scattering experiment. Using dust analogues with parameters similar to those derived from numerical simulations.Analogue particles, previously tested for the two main sources of dust in the inner interplanetary cloud (comets and asteroids) were used in the experiment. The ratio between fluffy aggregates and compact particles was kept constant. Five samples were studied, corresponding to mixtures with the organics / silicates ratio as defined in the numerical simulation.We show that we can reproduce by experimental simulations the polarimetric properties of particles present in the inner interplanetary dust cloud, i.e. their polarimetric phase curve in the symmetry plane at 1.5 au from the Sun and variations of polarization at a 90° phase angle as a function of the solar distance between 0.5 au and 1.5 au. The effect of the different parameters suggests a size distribution of the particles following a power law with coefficients of (−3 ± 0.5) for a size-range of 10–100 μm and (−4.4 ± 0.6) for a size range of 100–200 μm, with a steep cutoff at about 10 μm, a constant ratio of (35 ± 10) % in mass of fluffy aggregates versus compact particles and a decreasing ratio of organics with decreasing solar distance. Such results are discussed in the context of recent evidence on cometary dust particles from the Rosetta mission to a Jupiter family comet

Dust coma of Comet C/1999 S4 (LINEAR): imaging polarimetry during nucleus disruption

International audienceImaging and polarimetry of Comet C/1999 S4 (LINEAR) during its disruption provide information about the physical properties of the scattering dust particles, and some insight into the structure of the nucleus. A significant decrease in the brightness was noticed, together with a drastic change in the shape of the dust coma. The whole-coma polarization increased, which was typical of a comet with a near 27 percent maximum in polarization, the increase being comparable to previous observations for comets suffering a limited fragmentation. An important gradient in the intensity on the solar side corresponds to the ejected material. The degree of polarization in this region is higher than generally observed in jets and it increases with time as the nucleus (or its fragments) breaks up and ejects relatively large and compact particles. In the surrounding coma, these large particles are fragmented on short time-scales, indicated by the decrease of polarization. These results suggest that the fragile nucleus was not, as far as the physical properties of the dust are concerned, differentiated, and that it was possibly built of primordial cometesimals originating from the same formation region

Polarization imaging of comets at geocentric distances smaller than 0.5 au: Comet 73P/Schwassmann-Wachmann 3

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