Coma Of Comet Research Articles

A gas-phase primordial origin of O2 in comet 67P/Churyumov-Gerasimenko

Recent observations made by the Rosetta/ROSINA instrument have detected molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko with abundances at the 1-10% level relative to H2O. Previous studies have indicated that the likely origin of the O2 may be surface chemistry of primordial (dark cloud) origin, requiring somewhat warmer, denser and extreme H-atom poor conditions than are usually assumed. In this study we propose a primordial gas-phase origin for the O2 which is subsequently frozen and effectively hidden until the ice mantles are sublimated in the comet's coma. Our study presents results from a three-phase astrochemical model that simulates the chemical evolution of ices in the primordial dark cloud phase, its gravitational collapse, and evolution in the early protosolar nebula. We find that the O2 abundance can be produced and is fairly robust to the choice of the free parameters. Good matches for the O2:H2O ratio and, to a lesser extent, the N2:CO and CO:H2O ratios are obtained, but the models significantly over-produce N2. We speculate that the low value of N2:O2 that is observed is a consequence of the specific thermal history of the comet.

Rapid variations of dust colour in comet 41P/Tuttle–Giacobini–Kresák

We monitor the inner coma of comet 41P/Tuttle-Giacobini-Kres\'{a}k searching for variations of its colour. Fast changes in colour of the comet 41P/Tuttle-Giacobini-Kres\'{a}k provide important clues for better understanding of the microphysical properties of its dust. Using the 61-cm and 70-cm telescopes we measured the apparent magnitude of the comet with the V and R Johnson-Cousins filters from January 29 until April 25 of 2017. The inner coma ({\sim} 2000 km) reveals fast and significant variations of colour. The most significant change was found between March 3 and 4 of 2017, when it changed from blue with a colour slope S {\approx}(-10.15 {\pm} 3.43){\%} per 0.1 {\mu}m to red with S {\approx} (16.48 {\pm} 4.27){\%} per 0.1 {\mu}m. This finding appears in good accordance with what was previously reported by Ivanova et al. (2017) for long-period comet C/2013 UQ4 (Catalina), suggesting that fast and significant variations of colour of dust could be a common feature of short- and long-period comets. We model observations of comet 41P/Tuttle-Giacobini-Kres\'{a}k using the agglomerated debris particles and conclude that its inner coma consists of a mixture of at least two types of particles made of Mg-rich silicates and organics or Mg-Fe silicates.

A comparison of multiple Rosetta data sets and 3D model calculations of 67P/Churyumov-Gerasimenko coma around equinox (May 2015)

We have used the latest available shape model for gas and dust simulations of the inner coma of comet 67P/Churyumov-Gerasimenko for the period around May 2015 (equinox). We compare results from a purely insolation-driven model with a complementary set of observations made by ROSINA, VIRTIS, MIRO, and OSIRIS within the same period. The observations include - for the first time - inverted MIRO measurements of gas density, temperature and bulk velocity to constrain the model. The comparisons show that, as in November 2014 (Marschall et al., 2016), insolation-driven activity does not provide an adequate fit to the data. Both VIRTIS and MIRO observations indicate that emissions from the Hatmehit and Imhotep regions of the nucleus are strongly depleted in total gas, H2O, and dust emissions in this case. The MIRO inversion provides a challenging constraint to the models as a consequence of the terminator orbit and nucleus pointing of the spacecraft. Nonetheless a consistent picture with a dominance of outgassing from the Hapi region, even at equinox, is clearly evident. An inhomogeneous model consistent with models proposed for the November 2014 time-frame was constructed and provides a better fit to the data. As far as we are aware this is the first time comae data from four Rosetta instruments have been used to constrain within one self-contained model the emission distribution at the nucleus surface and study the dynamics of the gas and dust outflow.

Fine-grained Material Associated with a Large Sulfide returned from Comet 81P/Wild 2.

In a consortium analysis of a large particle captured from the coma of comet 81P/Wild 2 by the Stardust spacecraft, we report the discovery of a field of fine-grained material (FGM) in contact with a large sulfide particle. The FGM was partially located in an embayment in the sulfide. As a consequence, some of the FGM appears to have been protected from damage during hypervelocity capture in aerogel. Some of the FGM particles are indistinguishable in their characteristics from common components of chondritic-porous interplanetary dust particles (CP-IDPs), including glass with embedded metals and sulfides (GEMS) and equilibrated aggregates (EAs). The sulfide exhibits surprising Ni-rich lamellae, which may indicate that this particle experienced a long-duration heating event after its formation but before incorporation into Wild 2.

Decimeter-scale particle characterization in the coma of 73P/Schwassmann-Wachmann 3 using dual-wavelength radar observations

We investigate the centimeter-to-decimeter-scale particle size distribution and dynamics in the coma of Comet 73P/Schwassmann-Wachmann 3 using radar observations obtained shortly after the comet's disintegration in May 2006. We derive the particle size distribution power-law index in the comae of fragments B and C using radar cross sections and circular-polarization ratios observed at S and X bands (2.38 GHz and 8.56 GHz, respectively), and show day-to-day changes in the number of contributing particles. We test three irregular particle morphologies, but find that the particle shape plays a relatively minor role. The power-law indices from 4.0 to 4.8 and major-axis diameters up to about 30 cm provide the best fits to the radar observations, the values depending on the particle shape and the observed radial velocity. A majority of the measured particle velocities exceed the escape velocity of the nuclei.

Formation of cometary O2 ice and related ice species on grain surfaces in the midplane of the pre-solar nebula

Context. Detection of abundant O2 at 1–10% relative to H2O ice in the comae of comets 1P/Halley and 67P/Churyumov-Gerasimenko has motivated attempts to explain the origin of the high O2 ice abundance. Recent chemical modelling of the outer, colder regions of a protoplanetary disk midplane has shown production of O2 ice at the same abundance as that measured in the comet. Aims. We aim to carry out a thorough investigation to constrain the conditions under which O2 ice could have been produced through kinetic chemistry in the pre-solar nebula midplane. Methods. We have utilised an updated chemical kinetics code to evolve chemistry under pre-solar nebula midplane conditions. Four different chemical starting conditions and the effects of various chemical parameters have been tested. Results. Using the fiducial network, and for either reset conditions (atomic initial abundances) or atomic oxygen only conditions, the abundance level of O2 ice measured in the comets can be reproduced at an intermediate time, after 0.1–2 Myr of evolution, depending on ionisation level. When including O3 chemistry, the abundance of O2 ice is much lower than the cometary abundance (by several orders of magnitude). We find that H2O2 and O3 ices are abundantly produced (at around the level of O2 ice) in disagreement with their respective abundances or upper limits from observations of comet 67P. Upon closer investigation of the parameter space, and varying parameters for grain–surface chemistry, it is found that for temperatures 15–25 K, densities of 109−1010 cm−3, and a barrier for quantum tunnelling set to 2 Å, the measured level of O2 ice can be reproduced with the new chemical network, including an updated binding energy for atomic oxygen (1660 K). However, the abundances of H2O2 and O3 ices still disagree with the observations. A larger activation energy for the O + O2 → O3 reaction (Eact > 1000 K) helps to reproduce the non-detection of O3 ice in the comet, as well as reproducing the observed abundances of H2O2 and O2 ices. The only other case in which the O2 ice matches the observed abundance, and O3 and H2O2 ice are lower, is the case when starting with an appreciable amount of oxygen locked in O2. Conclusions. The parameter space investigation revealed a sweet spot for production of O2 ice at an abundance matching those in 67P and 1P, and O3 and H2O2 ice abundances matching those in 67P. This means that there is a radial region in the pre-solar nebula from 120–150 AU, within which O2 could have been produced in situ via ice chemistry on grain surfaces. However, it is apparent that there is a high degree of sensitivity of the chemistry to the assumed chemical parameters (e.g. binding energy, activation barrier width, and quantum tunnelling barrier). Hence, because the more likely scenario starting with a percentage of elemental oxygen locked in O2 also reproduces the O2 ice abundance in 67P at early stages, this supports previous suggestions that the cometary O2 ice could have a primordial origin.

Thermal Reaction in Cometary and Pre-cometary Ices: Formation of Thiocarbamate in OCS-CH3NH2 Mixed Ices

Cometary ices are rich sources of organic molecules relevant to prebiotic chemistry and may have played a key role in the delivery of the building blocks of life to Earth. Comets endure thermal processing of their surfaces during their orbits around the Sun. Furthermore, icy grain precursors of cometary materials may have experienced heating in the solar nebula. Despite the important role that this thermal processing could play in the chemistry of ices, little effort has been devoted to understanding the purely thermal reactivity involving electronically stable molecules. Both methylamine (CH3NH2) and carbonyl sulfide (OCS) have been detected in the cometary coma of comets, particularly in 67P-Churyumov–Geramenisko (67P) visited by the Rosetta spacecraft. We present here the first laboratory study addressing the reactivity induced by thermal processing of methylamine and OCS mixed ice. Mimicking a realistic scenario within which the surface of the comet is thermally processed as a result of its path close to the Sun, we demonstrate that the methylthiocarbamate anion in salt with methylammonium, [CH3NHOCS]− [CH3NH3]+, and thiocarbamic acid dimer are formed by nucleophilic addition of OCS on methylamine. This study shows that a thermally driven reaction chemically analogous to the one proposed as a condensing pathway of amino acids in the early Earth may take place in cometary ice. The formed thiocarbamate is a known intermediate in the formation of peptides. Furthermore, the consumption of OCS by this reaction with methylamine may account for the missing sulfur in dense clouds and protostars.

High polarization degree of the continuum of comet 2P/Encke based on spectropolarimetric signals during its 2017 apparition

Context.Spectropolarimetry is a powerful technique for investigating the physical properties of gas and solid materials in cometary comae without mutual contamination, but only a few spectropolarimetric studies have been conducted to extract each component.Aims.We attempt to derive the continuum (i.e., scattered light from dust coma) polarization degree of comet 2P/Encke, free of the influence of molecular emissions. The target is unique in that its orbit is dynamically decoupled from Jupiter, like the main-belt asteroids, but it ejects gas and dust like ordinary comets.Methods.We observed the comet using the Hiroshima Optical and Near-Infrared Camera attached to the Cassegrain focus of the 150 cm Kanata telescope on UT 2017 February 21 when the comet was at the solar phase angle ofα= 75°.7.Results.We find that the continuum polarization degree with respect to the scattering plane isPcont, r= 33.8 ± 2.7% at the effective wavelength of 0.82μm, which is significantly higher than those of cometary dust in a high-Pmaxgroup at similar phase angles. Assuming that an ensemble polarimetric response of the dust of 2P/Encke as a function of phase angle is morphologically similar with those of other comets, its maximum polarization degree is estimated toPmax≳ 40% atαmax≈ 100°. In addition, we obtain the polarization degrees of the C2swan bands (0.51–0.56μm), the NH2αbands (0.62–0.69μm), and the CN-red system (0.78–0.94μm) in a range of 3–19%, which depend on the molecular species and rotational quantum numbers of each branch. The polarization vector is aligned nearly perpendicularly to the scattering plane with an average of 0°.4 over a wavelength range of 0.50–0.97μm.Conclusions.From the observational evidence, we conjecture that the high polarization degree of 2P/Encke might be attributable to a dominance of large dust particles around the nucleus, which have remained after frequent perihelion passages near the Sun.

Helium and neon in comet 81P/Wild 2 samples from the NASA Stardust mission

AbstractHelium and neon distributions are reported for a variety of Stardust comet 81P/Wild 2 samples, including particle tracks and terminal particles, cell surface and subsurface slices from the comet coma and interstellar particle collection trays, and numerous small aerogel blocks extracted from comet cells C2044 and C2086. Discussions and conclusions in several abstracts published during the course of the investigation are included, along with the relevant data. Measured isotope ratios span a broad range, implying a similar range for noble gas carriers in the Wild 2 coma. The meteoritic phase Q‐20Ne/22Ne ratio was observed in several samples. Some of these, and others, exhibit 21Ne excesses too large for attribution to spallation by galactic cosmic ray irradiation, suggesting exposure to a solar proton flux greatly enhanced above current levels in an early near‐Sun environment. Still others display evidence for a solar wind component, particularly one C2086 block with large abundances of isotopically solar‐like helium and neon. Eighty‐nine small aerogel samples were cut from depths up to several millimeters below the cell C2044 surface and several millimeters away from the axis of major track T41. A fraction of these yielded measurable and variable helium and neon abundances and isotope ratios, although none contained visible tracks or carrier particle fragments and their locations were beyond estimated penetration ranges for small particles or ions incident on the cell surface, or for lateral ejecta from T41. Finding plausible emplacement mechanisms and sources for these gases is a significant challenge raised by this study.

On the ion-neutral coupling in cometary comae

In a cometary coma, the ion-neutral decoupling distance, sometimes referred to as the ion exobase or collisionopause, can be defined as the cometocentric distance, r(in), where ions, initially movi ...

Origin of Molecular Oxygen in Comets: Current Knowledge and Perspectives.

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard the Rosetta spacecraft has measured molecular oxygen (O2) in the coma of comet 67P/Churyumov-Gerasimenko (67P/C-G) in surprisingly high abundances. These measurements mark the first unequivocal detection of O2 in a cometary environment. The large relative abundance of O2 in 67P/C-G despite its high reactivity and low interstellar abundance poses a puzzle for its origin in comet 67P/C-G, and potentially other comets. Since its detection, there have been a number of hypotheses put forward to explain the production and origin of O2 in the comet. These hypotheses cover a wide range of possibilities from various in situ production mechanisms to protosolar nebula and primordial origins. Here, we review the O2 formation mechanisms from the literature, and provide a comprehensive summary of the current state of knowledge of the sources and origin of cometary O2.

Spectrophotometric differences of gas-dust environment of selected Jupiter's family comets and long-period comets with retrograde motion

The physical parameters of selected comets of the Jupiter family (22P/Kopff, 81P/Wild 2, 103P/Hartley 2) and the long-period comets with retrograde motion C/2006 W3 (Christensen), C/2009 K5 (McNaught), C/2009 P1 (Garradd) were obtained on the basis of optical spectra with resolution ((λ/Δλ) = 1500 and (λ/Δλ) = 14000). The spectra of comets were obtained in 2009–2012 at the mountain observatory « Peak Теrskоl» (Russian Federation, North Caucasus) with the help of the reflecting telescope Zeiss-2000 (D = 2.0 m; F2 = 16 m).Based on the Haser model some physical parameters of gas-and-dust coma of comets (energy fluxes, molecule number, gas productivity, relative dust productivity) were estimated. The physical parameters of the neutral gas and dusty atmospheres of comets indicate that, at equal heliocentric distances, the gas and dust productivity of the considered long-period comets is higher than the productivity of Jupiter-family comets. The average values for gas productivity for the considered LPC are QCN = 6.99 × 1025 mol/s, QC3 = 5.35 × 1024 mol/s, QC2 = 3.17 × 1026 mol/s, QNH2 = 4.64 × 1026 mol/s. The average values for gas productivity for the considered Jupiter-family comets are QCN = 1.52 × 1025 mol/s, QC3 = 1.54 × 1024 mol/s, QC2 = 2.12 × 1025 mol/s, QNH2 = 1.75 × 1025 mol/s. The average values for dust productivity for the considered long-period comets is lg Afρ = 4.241; for the considered Jupiter-family comets is lg Afρ = 2.922.This can be explained by age-old disintegration or differences in the origin of different types of comets. Relative dust productivity (Afρ) of long-period comets with the retrograde motion before the perihelion is higher (taking into account the heliocentric distance).The doublet Na (D1 = 588.9 nm, D2 = 589.6 nm) at the considerable heliocentric distance in the spectra of comets 22Р/Kopff (r = 1.8 AU), 103Р/Hartley 2 (r = 1.06 AU), C/2009 K5 (McNaught) (r = 1.5 AU) was identified. The detailed identification of emission spectral lines for comets 103P/Hartley 2 and C/2009 K5 (McNaught) was carried out. The spectra of Hartley 2 have a much smaller number of molecular emission lines.

Energetic Particle Showers Over Mars from Comet C/2013 A1 Siding Spring

AbstractThis paper is a phenomenological description of multispacecraft observations of energetic particles caused by the close flyby of comet C/2013 A1 Siding Spring with Mars on 19 October 2014. This is the first time that cometary energetic particles have been observed at Mars. The Mars Atmosphere and Volatile EvolutioN (MAVEN)‐solar energetic particle (SEP) and the Mars Odyssey‐High Energy Neutron Detector (HEND) instruments recorded evidence of precipitating particles, which are likely O+ pickup ions, during the ~10 hr that Mars was within the region of the comet's coma. O+ pickup ions were also detected several hours after, although whether their origin is the comet or space weather is not conclusive. We discuss the possible origin of those particles and also the cause of an additional shower of energetic particles that HEND observed between 22 and 35 hr after the comet's closest approach, which may be related to dust impacts from the comet's dust tail. An O+ pickup ion energy flux simulation is performed with representative solar wind and cometary conditions, together with a simulation of their energy deposition profile in the atmosphere of Mars. Results indicate that the O+ pickup ion fluxes observed by SEP were deposited in the ionosphere around 105 to 120 km altitude, and they are compared with precomet flyby estimations of cometary pickup ions. The comet's flyby deposited a significant fluence of energetic particles into Mars' upper atmosphere, at a similar level to a large space weather event.

The near-nucleus gas coma of comet 67P/Churyumov-Gerasimenko prior to the descent of the surface lander PHILAE

Context.The European Space Agency (ESA) Rosetta mission was the most comprehensive study of a comet ever performed. In particular, the Rosetta orbiter, which carried many instruments for monitoring the evolution of the dusty gas emitted by the cometary nucleus, returned an enormous volume of observational data collected from the close vicinity of the nucleus of comet 67P/Churyumov-Gerasimenko.Aims.Such data are expected to yield unique information on the physical processes of gas and dust emission, using current physical model fits to the data. We present such a model (the RZC model) and our procedure of adjustment of this model to the data.Methods.The RZC model consists of two components: (1) a numerical three-dimensional time-dependent code solving the Eulerian/Navier-Stokes equations governing the gas outflow, and a direct simulation Monte Carlo (DSMC) gaskinetic code with the same objective; and (2) an iterative procedure to adjust the assumed model parameters to best-fit the observational data at all times.Results.We demonstrate that our model is able to reproduce the overall features of the local neutral number density and composition measurements of Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) Comet Pressure Sensor (COPS) and Double Focusing Mass Spectrometer (DFMS) instruments in the period August 1–November 30, 2014. The results of numerical simulations show that illumination conditions on the nucleus are the main driver for the gas activity of the comet. We present the distribution of surface inhomogeneity best-fitted to the ROSINA COPS and DFMS in situ measurements.

Noble gases in micrometeorites from the Transantarctic Mountains

The bulk of extraterrestrial matter currently accreted by the Earth is in the form of micrometeorites (MMs) and interplanetary dust particles (IDPs), thus they may have collectively made a substantial contribution to the volatile inventory of the Earth and the other terrestrial planets. We have performed a complete noble gas study, accompanied by a complete petrographic characterization, of MMs from the Transantarctic Mountain (TAM) collection in the size range ∼300 to ∼1000 µm that fell over an extended time period during the last ∼1 Ma. Our noble gas study includes krypton and xenon, which have been largely missing in previous work. Helium and neon are dominated by a solar component, with generally lower abundance in scoriaceous MMs than in unmelted ones, and also generally lower in abundance than in previously studied MMs, which may be explained by the larger particle size (surface/volume ratio) of the MMs we studied. Considering an enhanced MM flux in the early Solar System, such MMs may have supplied a significant fraction of Earth’s neon. A number of MMs have kept what was probably their pre-terrestrial He/Ne ratio, from which we infer that the observed solar component is retained in a tiny surface region not affected by atmospheric entry. The abundances of (volume-correlated) heavier gases are similar to what was found in previous studies of smaller MMs. While Ar contains both solar and “planetary” contributions, the heavy noble gases (Kr, Xe) generally show “planetary” patterns but are often also compromised by terrestrial contamination as evidenced by an enhanced Kr/Xe ratio. Kr and Xe in a subset of scoriaceous MMs are dominated by isotopically fractionated air, possibly acquired during the passage through Earth’s ionosphere. Those not obviously affected by air show isotopic ratios similar to primitive meteorites (the Q component), thus primordial heavy gases supplied to the Earth by MMs are likely as those found in macroscopic meteorites. There is no evidence for the presence of a “cometary” Xe component as identified in the coma of comet 67P/Churyumov-Gerasimenko, hence a cometary source for a significant fraction of MMs in the studied size range is unlikely. Cosmogenic helium, neon and argon were detected in several cases. Cosmic ray exposure ages were calculated based on cosmogenic 21Ne in combination with the Poynting-Robertson effect, but depend on assumptions about atmospheric entry loss. Still, several cases are consistent with an origin from the asteroid belt (even assuming no loss) and one scoriaceous MM (#45b.17) would have to originate from beyond Jupiter. In at least two cases, including #45b.17, the isotopic composition of cosmogenic Ne appears to be inconsistent with predominant production in small particles free-floating in space, however; much of the irradiation of these MMs may have occurred when they were part of larger parent bodies.

Summer outbursts in the coma of comet 67P/Churyumov–Gerasimenko as observed by Rosetta–VIRTIS

We present an analysis of transient events observed by the Visible InfraRed Thermal Imaging Spectrometer, instrument aboard Rosetta, for the dates of 2015 August 10, September 13 and 14, during the two months surrounding the comet perihelion passage of the Rosetta spacecraft. We detected and characterized events with life-times ranging from 26 min down to 6 min. The temporal evolution of the outburst shows a sudden increase of radiance from quiescent coma to the maximum in a few minutes. This rapid onset is correlated with a change of the visible dust colour from red, 15-18± 3 per cent/100 nm, to bluer with values of 7-10± 0.3 per cent/100 nm. The dust morphology of these outbursts can be classified into two main types: narrow and collimated plumes (August 10, September 13) and broad blobs (September 14). The observations suggest that there are localized regions on the surface that are more prone to outbursts than the rest of the nucleus. The projected dust velocity during the outburst events ranges between 22.2 ± 2.2 m s and 64.9 ± 10.6 m s . The total ejected mass during an outburst event is estimated to be between 10 and 500 tons for a duration of 6-26 min assuming size distribution indices between -2.5 and -3. -1 -1

Cold electrons at comet 67P/Churyumov-Gerasimenko

Context. The electron temperature of the plasma is one important aspect of the environment. Electrons created by photoionization or impact ionization of atmospheric gas have energies ~10 eV. In an active comet coma, the gas density is high enough for rapid cooling of the electron gas to the neutral gas temperature (a few hundred kelvin). How cooling evolves in less active comets has not been studied before. Aims. We aim to investigate how electron cooling varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders of magnitude during the Rosetta mission. Methods. We used in situ data from the Rosetta plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps and mutual impedance probe measurements, we determined at which time cold electrons formed at least 25% of the total electron density. We compared the results to what is expected from simple models of electron cooling, using the observed neutral gas density as input. Results. We demonstrate that the slope of the Langmuir probe sweep can be used as a proxy for the presence of cold electrons. We show statistics of cold electron observations over the two-year mission period. We find cold electrons at lower activity than expected by a simple model based on free radial expansion and continuous loss of electron energy. Cold electrons are seen mainly when the gas density indicates that an exobase may have formed. Conclusions. Collisional cooling of electrons following a radial outward path is not sufficient to explain the observations. We suggest that the ambipolar electric field keeps electrons in the inner coma for a much longer time, giving them time to dissipate energy by collisions with the neutrals. We conclude that better models are required to describe the plasma environment of comets. They need to include at least two populations of electrons and the ambipolar field.

Reactive collision of electrons with CO+ in cometary coma

Context. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. Aims. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO+ molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. Methods. The cross-section calculations were based on multichannel quantum defect theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. Results. The cross-sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO+ – relevant for the electronic temperatures observed in comets – are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Conclusions. Among all reactive collisions taking place between low-energy electrons and CO+, the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O(3P), O(1D), O(1S), C(3P) and C(1D) produced in the cometary coma at small cometocentric distances.

Spectrum of the Short-Period Comet 2P/Encke in the Apparition of 2003

Spectra of comet 2P/Encke obtained on November 15 and 22, 2003, in wavelength ranges of λ = 410–580 and 560–720 nm with the Universal Astronomical Grating Spectrograph (UAGS) mounted at the 1-m Zeiss-1000 telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences is analyzed. The spectral resolution is 0.5 nm. In the cometary coma, the emission bands of C2, CN, CH, and NH2 molecules were detected. The normalized spectral gradient of the reflectivity is (11.95 ± 3.45) and (7.90 ± 2.81)% per 100 nm in wavelength bands of 450–567 and 565–676 nm, respectively. The values of the parameter Afρ are (77 ± 9), (81 ± 9), and (79 ± 9) cm for the BC, GC, and RC continuum filters, respectively, which is indicative of a very low dust content in comet Encke. With the Haser model, we determined the production rate of gas molecules at (1.12 × 1025) and (3.82 × 1024) mol/s for C2 (Δν = 0) and NH2 (0,10,0) molecules, respectively.

Coma morphology of comet 67P controlled by insolation over irregular nucleus

While the structural complexity of cometary comae is already recognizable from telescopic observations1, the innermost region, within a few radii of the nucleus, was not resolved until spacecraft exploration became a reality2,3. The dust coma displays jet-like features of enhanced brightness superposed on a diffuse background1,4,5. Some features can be traced to specific areas on the nucleus, and result conceivably from locally enhanced outgassing and/or dust emission6–8. However, diffuse or even uniform activity over topographic concavity can converge to produce jet-like features9,10. Therefore, linking observed coma morphology to the distribution of activity on the nucleus is difficult11,12. Here, we study the emergence of dust activity at sunrise on comet 67P/Churyumov–Gerasimenko using high-resolution, stereo images from the OSIRIS camera onboard the Rosetta spacecraft, where the sources and formation of the jet-like features are resolved. We perform numerical simulations to show that the ambient dust coma is driven by pervasive but non-uniform water outgassing from the homogeneous surface layer. Physical collimations of gas and dust flows occur at local maxima of insolation and also via topographic focusing. Coma structures are projected to exhibit jet-like features that vary with the perspective of the observer. For an irregular comet such as 67P/Churyumov–Gerasimenko, near-nucleus coma structures can be concealed in the shadow of the nucleus, which further complicates the picture. Images of 67P's nucleus from the Rosetta spacecraft, together with numerical simulations, show that the jet-like features of cometary comae can be produced by diffuse activity focused by the nucleus topography as well as non-uniform insolation over the surface.