The composition of cometary ices Bockelée-Morvan, D.; Biver, N.
Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences/Philosophical transactions - Royal Society. Mathematical, physical and engineering sciences,
07/2017, Letnik:
375, Številka:
2097
Journal Article
Recenzirano
Odprti dostop
The chemical composition of cometary ices provides clues for the conditions of formation and evolution of the early Solar System. A large number of molecules have been identified in cometary ...atmospheres, from both ground-based observations and space, including in situ investigations. This includes large organic molecules, which are also observed in star-forming regions. This paper presents a review of molecular abundances measured in cometary atmospheres from remote sensing observations with ground-based and space-based telescopes. The diversity of composition observed in comet populations is presented and discussed.
This article is part of the themed issue ‘Cometary science after Rosetta’.
We present a multi-wavelength study of comet C/2016 R2 (PanSTARRS). This comet was observed on 23 and 24 January 2018 with the IRAM 30 m telescope, and in January to March 2018 with the Nançay radio ...telescope. Visible spectroscopy was performed in December 2017 and February 2018 with small amateur telescopes. We report on measurements of CO, CH3OH, H2CO and HCN production rates, and on the determination of the N2/CO abundance ratio. Several other species, especially OH, were searched for but not detected. The inferred relative abundances, including upper limits for sulfur species, are compared to those measured in other comets at about the same heliocentric distance of ~2.8 AU. The coma composition of comet C/2016 R2 is very different from all other comets observed so far, being rich in N2 and CO and dust poor. This suggests that this comet might belong to a very rare group of comets formed beyond the N2 ice line. Alternatively, comet C/2016 R2 (PanSTARRS) could be the fragment of a large and differentiated transneptunian object, with properties characteristic of volatile-enriched layers.
Context. Since its rendezvous with comet 67P/Churyumov-Gerasimenko (67P), the Rosetta spacecraft has provided invaluable information contributing to our understanding of the cometary environment. On ...board, the VIRTIS and ROSINA instruments can both measure gas parameters in the rarefied cometary atmosphere, the so-called coma, and provide complementary results with remote sensing and in situ measurement techniques, respectively. The data from both ROSINA and VIRTIS instruments suggest that the source regions of H2O and CO2 are not uniformly distributed over the surface of the nucleus even after accounting for the changing solar illumination of the irregularly shaped rotating nucleus. The source regions of H2O and CO2 are also relatively different from one another. Aims. The use of a combination of a formal numerical data inversion method with a fully kinetic coma model is a way to correlate and interpret the information provided by these two instruments to fully understand the volatile environment and activity of comet 67P. Methods. In this work, the nonuniformity of the outgassing activity at the surface of the nucleus is described by spherical harmonics and constrained by ROSINA-DFMS data. This activity distribution is coupled with the local illumination to describe the inner boundary conditions of a 3D direct simulation Monte-Carlo (DSMC) approach using the Adaptive Mesh Particle Simulator (AMPS) code applied to the H2O and CO2 coma of comet 67P. Results. We obtain activity distribution of H2O and CO2 showing a dominant source of H2O in the Hapi region, while more CO2 is produced in the southern hemisphere. The resulting model outputs are analyzed and compared with VIRTIS-M/-H and ROSINA-DFMS measurements, showing much better agreement between model and data than a simpler model assuming a uniform surface activity. The evolution of the H2O and CO2 production rates with heliocentric distance are derived accurately from the coma model showing agreement between the observations from the different instruments and ground-based observations. Conclusions. We derive the activity distributions for H2O and CO2 at the surface of the nucleus described in spherical harmonics, which we couple to the local solar illumination to constitute the boundary conditions of our coma model. The model presented reproduces the coma observations made by the ROSINA and VIRTIS instruments on board the Rosetta spacecraft showing our understanding of the physics of 67P’s coma. This model can be used for further data analyses, such as dust modeling, in a future work.
A spectral survey in the 1 mm wavelength range was undertaken in the long-period comets C/2012 F6 (Lemmon) and C/2013 R1 (Lovejoy) using the 30 m telescope of the Institut de radioastronomie ...millimétrique (IRAM) in April and November−December 2013. We report the detection of ethylene glycol (CH2OH)2 (aGg’ conformer) and formamide (NH2CHO) in the two comets. The abundances relative to water of ethylene glycol and formamide are 0.2–0.3% and 0.02% in the two comets, similar to the values measured in comet C/1995 O1 (Hale-Bopp). We also report the detection of HCOOH and CH3CHO in comet C/2013 R1 (Lovejoy), and a search for other complex species (methyl formate, glycolaldehyde).
Context. Studying the coma environment of comet 67P/Churyumov-Gerasimenko (67P) is one of the primary scientific goals of the VIRTIS experiment on the ESA Rosetta mission. Aims. The distribution and ...variability of water vapour and carbon dioxide in the comet’s coma are needed to estimate their production rate, abundances in the nucleus, and the spatial distribution of the active regions. Methods. Infrared emission lines from vibrational bands of water and carbon dioxide at 2.67 and 4.27 μm, respectively, were observed by the VIRTIS-M imaging channel and mapped from close to the nucleus up to ~10 km altitude with a resolution of ~40 m/px. A dataset consisting of 74 observations in the 1−5 μm spectral range acquired from 8 to 14 April 2015 when 67P was at a heliocentric distance of 1.9 AU is analysed in this work. A statistical correlation between the gas distribution and the surface’s active regions was performed. Results. The maximum H2O emission is observed within 3 km from the nucleus and is mainly concentrated above two active regions, Aten-Babi and Seth-Hapi, while the CO2 distribution appears more uniform with significant emissions coming from both the “head” and southern latitude regions. In the equatorial region, the column densities of both species decrease with altitude, although CO2 decreases more rapidly than H2O. The calculated CO2/H2O column density ratios above Aten-Babi and Seth-Hapi are 2.4 ± 0.6% and 3.0 ± 0.7%, respectively. A value equal to 3.9 ± 1.0% is observed at equatorial latitudes in the region encompassing Imothep. Conclusions. VIRTIS-M has mapped the distribution of water vapour and carbon dioxide around the nucleus of 67P with unprecedented spatial resolution. The different water and carbon dioxide outgassing above the surface, seen in the VIRTIS-M data, might be indicative of a different thermal history of the northern and southern hemispheres of 67P.
We present the analysis of ≈100 molecular maps of the coma of comet 67P/Churyumov-Gerasimenko that were obtained with the MIRO submillimeter radiotelescope on board the Rosetta spacecraft. From the ...spectral line mapping of H 216 $_2^{16}$ 216 O, H 218 $_2^{18}$218 O, H 217 $_2^{17}$217 O, CH3OH, NH3, and CO and some fixed nadir pointings, we retrieved the outgassing pattern and total production rates for these species. The analysis covers the period from July 2014, inbound to perihelion, to June 2016, outbound, and heliocentric distances rh = 1.24–3.65 AU. A steep evolution of the outgassing rates with heliocentric distance is observed, typically in rh−16 $r_{\textrm{h}}^{-6}$rh−6 , with significant differences between molecules (e.g. steeper variation for H2O post-perihelion than for methanol). As a consequence, the abundances relative to water in the coma vary. The CH3OH and CO abundances increase after perihelion, while the NH3 abundance peaks around perihelion and then decreases. Outgassing patterns have been modeled as 2D Gaussian jets. The width of these jets is maximum around the equinoxes when the bulk of the outgassing is located near the equator. From July 2014 to February 2015, the outgassing is mostly restricted to a narrower jet (full width at half-maximum ≈80°) originating from high northern latitudes, while around perihelion, most of the gaseous production comes from the southernmost regions ( − 80 ± 5° cometocentric latitude) and forms a 100°–130° (full width at half-maximum) wide fan. We find a peak production of water of 0.8 × 1028 molec. s−1, 2.5 times lower than measured by the ROSINA experiment, and place an upper limit to a 50% additional production that could come from the sublimation of icy grains. We estimate the total loss of ices during this perihelion passage to be 4.18 ± 0.18 × 109 kg. We derive a dust-to-gas ratio in the lost material of 0.7–2.3 (including all sources of errors) based on the nucleus mass loss of 10.5 ± 3.4 × 109 kg estimated by the RSI experiment. We also obtain an estimate of the H 218 $_2^{18}$218 O/H 217 $_2^{17}$217 O ratio of 5.6 ± 0.8.
The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) in March-April 2013 and January 2015, combined with the improved observational capabilities of submillimeter facilities, ...offered an opportunity to carry out sensitive compositional and isotopic studies of the volatiles in their coma. We observed comet Lovejoy with the IRAM 30 m telescope between 13 and 26 January 2015, and with the Odin submillimeter space observatory on 29 January–3 February 2015. We detected 22 molecules and several isotopologues. The H216O and H218O production rates measured with Odin follow a periodic pattern with a period of 0.94 days and an amplitude of ~25%. The inferred isotope ratios in comet Lovejoy are 16O/18O = 499 ± 24 and D/H = 1.4 ± 0.4 × 10-4 in water, 32S/34S = 24.7 ± 3.5 in CS, all compatible with terrestrial values. The ratio 12C/13C = 109 ± 14 in HCN is marginally higher than terrestrial and 14N/15N = 145 ± 12 in HCN is half the Earth ratio. Several upper limits for D/H or 12C/13C in other molecules are reported. From our observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio in an Oort Cloud comet that is not larger than the terrestrial value. On the other hand, the observation of the same HDO line in the other Oort-cloud comet, C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous measurements of D/H in cometary water, this illustrates that a diversity in the D/H ratio and in the chemical composition, is present even within the same dynamical group of comets, suggesting that current dynamical groups contain comets formed at very different places or times in the early solar system.
•The surface of comet 67P/CG has been imaged by the VIRTIS instrument aboard ROSETTA.•Refractory polyaromatic organics mixed with opaque minerals account for the low albedo.•Semi-volatiles organics ...(solid at 220K) induce a broad band centered at 3.2µm.•Laboratory photolytic/thermal residues formed from interstellar ice analogs are fair analogs.•No hydrated minerals are detected, suggesting no link with carbonaceous chondrites.
The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument aboard the Rosetta spacecraft has performed extensive spectral mapping of the surface of comet 67P/Churyumov-Gerasimenko in the range 0.3–5µm. The reflectance spectra collected across the surface display a low reflectance factor over the whole spectral range, two spectral slopes in the visible and near-infrared ranges and a broad absorption band centered at 3.2µm. The first two of these characteristics are typical of dark small bodies of the Solar System and are difficult to interpret in terms of composition. Moreover, solar wind irradiation may modify the structure and composition of surface materials and there is no unequivocal interpretation of these spectra devoid of vibrational bands. To circumvent these problems, we consider the composition of cometary grains analyzed in the laboratory to constrain the nature of the cometary materials and consider results on surface rejuvenation and solar wind processing provided by the OSIRIS and ROSINA instruments, respectively. Our results lead to five main conclusions: (i) The low albedo of comet 67P/CG is accounted for by a dark refractory polyaromatic carbonaceous component mixed with opaque minerals. VIRTIS data do not provide direct insights into the nature of these opaque minerals. However, according to the composition of cometary grains analyzed in the laboratory, we infer that they consist of Fe-Ni alloys and FeS sulfides. (ii) A semi-volatile component, consisting of a complex mix of low weight molecular species not volatilized at T∼220K, is likely a major carrier of the 3.2µm band. Water ice contributes significantly to this feature in the neck region but not in other regions of the comet. COOH in carboxylic acids is the only chemical group that encompasses the broad width of this feature. It appears as a highly plausible candidate along with the NH4+ ion. (iii) Photolytic/thermal residues, produced in the laboratory from interstellar ice analogs, are potentially good spectral analogs. (iv) No hydrated minerals were identified and our data support the lack of genetic links with the CI, CR and CM primitive chondrites. This concerns in particular the Orgueil chondrite, previously suspected to have been of cometary origin. (v) The comparison between fresh and aged terrains revealed no effect of solar wind irradiation on the 3.2µm band. This is consistent with the presence of efficient resurfacing processes such as dust transport from the interior to the surface, as revealed by the OSIRIS camera.
Aims. We present the spatial and diurnal variation of water outgassing on comet 67P/Churyumov-Gerasimenko using the H216O rotational transition line at 556.936 GHz observed from Rosetta/MIRO in ...August 2014. Methods. The water line was analyzed with a non-LTE radiative transfer model and an optimal estimation method to retrieve the H216O outgassing intensity, expansion velocity, and gas kinetic temperature. On August 7−9, 2014 and August 18−19, 2014, MIRO performed long steady nadir-pointing observations of the nucleus while it was rotating around its spin axis. The ground track of the MIRO beam during the observation was mostly on the northern hemisphere of comet 67P, covering its three distinct parts: the so-called head, body, and neck areas. Results. The MIRO spectral observation data show that the water-outgassing intensity varies by a factor of 30, from 0.1 × 1025 molecules s-1 sr-1 to 3.0 × 1025 molecules s-1 sr-1, the terminal gas expansion velocity varies by 0.17 km s-1 from 0.61 km s-1 to 0.78 km s-1, and the terminal gas temperature varies by 27 K from 47 K to 74 K. The retrieved coma parameters are co-registered with local environment variables such as the subsurface temperatures, measured in the MIRO continuum bands, the local solar time, illumination condition, and beam location on nucleus. The spatial variation of the outgassing activity is very noticeable, and the largest outgassing activity in August 2014 occurs near the neck region of the nucleus. The outgassing activity in the neck region is also found to be correlated with the local solar hour, which is related to the local illumination condition.
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