Cometary organic chemistry: a review from observations,numerical and experimental simulations

This paper is a review dealing with the organic chemistry of comets. It describes how the chemical composition of comets can provide information about the chemistry of the interstellar medium, and the formation of the solar system. We discuss to what extent they could have brought to Earth the ingredients essential to the emergence of life: water and prebiotic compounds. We review all molecules which have been detected or tentatively detected in comets by remote sensing or in-situ observations, inputs of theoretical models, and all other organic species expected to be present from the results of experimental simulations. This compilation yields a list of more than a hundred molecules which can be used as a reference for the preparation of experiments developed for the Rosetta and Deep Space 4 cometary missions. We point out that further experiments are necessary to investigate the connections between the solid and gaseous phases of comets, especially studying the photodegradation of high molecular weight compounds which could be present in the nuclei.     

The neutral atmospheres of comets

In this paper we have endeavored to critically evaluate our present understanding of cometary atmospheres. Following a brief introduction of the significance of the study of cometary atmospheres (Section 1), the relevant photometric and spectroscopic observations are summarized in Section 2.The interaction with the solar radiation, with regard to both the excitation of the observed species as well as the dissociation of stable molecules evaporating from the nucleus, is considered in Sections 3 and 4. The gas phase chemistry likely to take place in the dense inner coma is next considered in Section 5.The exospheric and hydrodynamic models of the expanding cometary atmosphere are considered in detail in Section 6, and both their limitations as well as possible improvements are discussed.The observed chemical composition of the neutral atmosphere and the inferred chemical composition of the volatile component of the nucleus, together with possible variations between different classes of comets is next considered in Section 7, and their possible cosmogonic significance is discussed.In conclusion, some of the important directions in which future research should progress, in order to provide more complete and secure knowledge of cometary atmospheres, are stressed (Section 8).Astrophysics and Space Science Review Paper.     

Hamiltonian Formulation and Perturbations for Dust Motion Around Cometary Nuclei

In this paper we analyze the dynamical behavior of large dust grains in the vicinity of a cometary nucleus. To this end we consider the gravitational field of the irregularly shaped body, as well as its electric and magnetic fields. Without considering the effect of gas friction and solar radiation, we find that there exist grains which are static relative to the cometary nucleus; the positions of these grains are the stable equilibria. There also exist grains in the stable periodic orbits close to the cometary nucleus. The grains in the stable equilibria or the stable periodic orbits won’t escape or impact on the surface of the cometary nucleus. The results are applicable for large charge dusts with small area-mass ratio which are near the cometary nucleus and far from the Solar. It is found that the resonant periodic orbit can be stable, and there exist stable non-resonant periodic orbits, stable resonant periodic orbits and unstable resonant periodic orbits in the potential field of cometary nuclei. The comet gravity force, solar gravity force, electric force, magnetic force, solar radiation pressure, as well as the gas drag force are all considered to analyze the order of magnitude of these forces acting on the grains with different parameters. Let the distance of the dust grain relative to the mass centre of the cometary nucleus, the charge and the mass of the dust grain vary, respectively, fix other parameters, we calculated the strengths of different forces. The motion of the dust grain depends on the area-mass ratio, the charge, and the distance relative to the comet’s mass center. For a large dust grain (> 1 mm) close to the cometary nucleus which has a small value of area-mass ratio, the comet gravity is the largest force acting on the dust grain. For a small dust grain (< 1 mm) close to the cometary nucleus with large value of area-mass ratio, both the solar radiation pressure and the comet gravity are two major forces. If the a small dust grain which is close to the cometary nucleus have the large value of charge, the magnetic force, the solar radiation pressure, and the electric force are all major forces. When the large dust grain is far away from the cometary nucleus, the solar gravity and solar radiation pressure are both major forces.     

Protosolar nebula and composition of comets

Comets seem to be composed of matter, which is supposed to have the same molecular composition as protosolar nebula. Although there are no unbiased evidence that cometary nuclei retain the molecular composition inherited from the protosolar cloud, the observed properties of comets indicate that there is at least a resemblance between cometary composition and the material properties of dense interstellar clouds. Therefore the origin of comets could be searched in the cold stages of the protosolar nebula and molecular abundances of grain mantles in this nebula may be similar to those in the cometary dust. It is suggested that comets may contain pristine, virtually unaltered protosolar material and their study might be very relevant way to more information about processes in early stages of the solar nebula. Our knowledge about composition of the cometary nucleus is still relatively scarce, but we can partly deduce it from data obtained either by ground-based spectroscopy or by in situ mass spectrometry from space experiments. Most important were the discovery of fluffy CHON particles composed partly or even completely from compounds containing light elements. No consensus concerning the presence of interstellar pristine matter in comet has been reached from various approaches to determine the relationship between comets and interstellar grains. Most of these studies are based on infrared spectroscopy. Another method is the comparison on the chemical models of the protosolar nebula with the volatile compounds of the cometary nuclei. Both gas-phase and grain-surface chemistry are considered and initial gas-phase atomic abundances are assumed to be protosolar. The cometary matter is certainly not identical with the typical material of dense interstellar cool dense clouds, but it is closer to it than any other type of matter in solar system so far accessible to us. The data from comets combined with models of chemical evolution of matter in environment similar as prevailed the early stage of presolar nebula may at least impose constrains on the condition for comet formation. Here presented study is a preliminary contribution to such studies.     

Origin of cometary extended sources from degradation of refractory organics on grains: polyoxymethylene as formaldehyde parent molecule

The radial distribution of some molecules (CO, H₂CO, HNC, …) observed in the coma of some comets cannot be explained only by a direct sublimation from the nucleus, or by the photolysis of a detected parent compound. Such molecules present a so-called extended source in comae. We show in this paper that extended sources can be explained by refractory organic material slowly releasing gas from grains ejected from the cometary nucleus, due to solar UV photons or heat. The degradation products are produced throughout the coma and therefore are presenting an extended distribution. To model this multiphase chemistry we derive new equations, which are applied to Comet 1P/Halley for the case of the production of formaldehyde from polyoxymethylene (POM), the polymer of formaldehyde (-CH₂-O-)_n. We show that the presence of a few percent of POM on cometary grains (a nominal value of ∼4% in mass of grains is derived from our calculations) is in good agreement with the observed distribution, which so far were not interpreted by the presence of any gaseous parent molecule.     

Mass Transfer in the Near-Surface Layer of a Cometary Nucleus: A Gas-Kinetic Approach

A self-consistent model of the kinetically nonequilibrium near-surface layer of a cometary nucleus is developed on the basis of the gas-kinetic approach. The weight method of direct statistical simulation is used to model numerically the two-dimensional gas outflow from an ice sample subjected to radiative heating. The effective coefficient of water ice sublimation is estimated. Mass transfer in a porous ice and mineral (scattering) nonisothermal medium is investigated by the method of test particles, and the effective gas release is evaluated taking into account the proper rotation of the cometary nucleus for various model parameters. In these calculations, allowance is made for the kinetic character of the flow and volume sublimation and condensation of the volatile constituents of the material of the cometary nucleus.     

Models of P/Wirtanen nucleus: active regions versus non-active regions

From the current understanding we know that comet nuclei have heterogeneous compositions and complex structures. It is believed that cometary activity is the result of a combination of physical processes in the nucleus, like sublimation and recondensation of volatile ices, dust grains release, phase transition of water ice, depletion of the most volatile components in the outer layers and interior differentiation.The evolution of the comet depends on the sublimation of ices and the release of different gases and dust grains: the formation of a dust crust, the surface erosion and the development of the coma are related to the gas fluxes escaping from the nucleus. New observations, laboratory experiments and numerical simulations suggest that the gas and dust emissions are locally generated, in the so-called active regions. This localized activity is probably superimposed to the global nucleus activity. The differences between active and inactive regions can be attributed to differences in texture and refractory material content of the different areas.In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices—such as water (the dominant constituent), CO₂, CO- and of dust grains embedded in the ice matrix.By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed.Comet P/Wirtanen is the target of the international ROSETTA mission, the cornerstone ESA mission to a cometary nucleus. The successful design of ROSETTA requires some knowledge of comet status and activity: surface temperatures, amount of active and inactive surface areas, gas production rate and dust flux.     

Physico-Chemistry of Comets: Models and Laboratory Experiments

Until cometary matter can be studied in-situ or cometary samples are brought back to Earth for analysis, theoretical models and laboratory studies remain a crucial tool for revealing the nature of cometary matter. Constraints on the nature of the primordial material available for incorporation into comets and other solar system material comes from analysis of data from space-based and ground-based observatories. The structure of the nuclear ice component, which may have coexisting amorphous/crystalline phases and include clathrates and other trapped guest molecules, strongly influences the cometary outgassing properties. This paper reviews laboratory work on ice and carbon aceous compounds and discusses their significance for cometary chemistry. Special emphasis will be given to studies on the thermal processing of ices and their implications for the structure changes and subsequent release of volatiles. We also describe the preliminary results of a model of nuclear outgassing, and discuss how such model scan be used to infer the chemical structure of the nuclearices. Furthermore, we confront cometary data with the analysis of carbonaceous meteorites. Recent laboratory results on volatile compounds and the macro molecular structure of carbonaceous meteorites allow us to investigate the link of small bodies in the Solar System. Until ROSETTA will land on comet Wirtanen and study directly the nuclear composition, laboratory measurements of ice and refractory analogs will — together with the analysis of meteorites —significantly improve our knowledge on the origin and structure ofcomets.     

Insolation of a cometary crater at the stage of dust-jet formation

An important cause of the activation and development of active processes on the surface of a cometary nucleus is direct solar radiation illuminating a part of the surface that is not shielded by dust. The intensity of solar radiation near the surface of a cometary nucleus depends on the thickness of the dust cloud above the active area. If the size of the dust cloud noticeably changes, the intensity considerably depends on time. In the present paper, we consider the nonlinear equation of radiative transfer in a dust cloud growing towards the incident wave front with a constant velocity. The change in the intensity of direct solar radiation along the dust jet originating from the active surface area of a cometary nucleus has been found. For the sake of comparison, the linear equation of radiative transfer was solved in the framework of this task. It turns out that the linear approach to the solution of the considered problem suggests a noticeable loss in the amount of direct radiation participating in the dust-jet formation. This loss is comparable with the intensity of solar radiation incident to the active area of a cometary nucleus after scattering in the cometary atmosphere.     

The volatile fraction of the cometary nucleus

In order to prepare a flyby mission to Comet Encke, six different sources of information on the possible chemical composition of the cometary nucleus are compared. These are: the neutral and charged radicals and molecules observed in cometary spectra; the chemical composition of type I carbonaceous chondrites; the meteor spectra; the metallic ions collected in the upper atmosphere and correlated with the meteor shower associated with Comet Encke; and finally the volatile molecules observed in a volatile-rich sample of lunar soil, that were interpreted as a possible cometary impact. Possible molecular abundances for the volatile fraction of Comet Encke are tentatively proposed.     

Present and Future Cometary Science with the IRAM Plateau de Bure Interferometer

Interferometric observations are essential to probe the molecular emission in the inner cometary atmospheres and study the outgassing from the nucleus. Mapping the continuum emission can provide information about the dust and/or nucleus properties. We present here a summary of the observations of the dust and gas coma of comet 17P/Holmes and nuclear observations of 8P/Tuttle, both carried out with the IRAM interferometer at Plateau de Bure (PdBI) in 2007–2008. The observations of these two comets demonstrate the ability of the PdBI in terms of cometary science. In the near future, several improvements will be made (new receivers at 0.8 mm, a new wide-band correlator) allowing more frequent and more detailed studies of comets. On the long term, NOEMA, an expansion project, may add up to six antennas to the Plateau de Bure Interferometer, and increase the baseline lengths. Such an instrument would offer a complement to ALMA to track comets of the northern hemisphere with about half the sensitivity of ALMA for continuum studies.     

Quantitative Analysis of H2OComa Images Using a Multiscale MHD Model with Detailed Ion Chemistry

The observation of ions created by ionization of cometary gas, either by ground-based observations or byin situmeasurements can give us useful information about the gas production and composition of comets. However, due to the interaction of ions with the magnetized solar wind and their high chemical reactivity, it is not possible to relate measured ion densities (or column densities) directly to the parent gas densities. In order to quantitatively analyze measured ion abundances in cometary comae it is necessary to understand their dynamics and chemistry. We have developed a detailed ion–chemical network of cometary atmospheres. We include production of ions by photo- and electron impact-ionization of a background neutral atmosphere, charge exchange of solar wind ions with cometary atoms/molecules, reactions between ions and molecules, and dissociative recombination of molecular ions with thermal electrons. By combining the ion–chemical network with the three-dimensional plasma flow as computed by a new fully three-dimensional MHD model of cometary plasma environments (Gombosiet al.1996) we are able to compute the density of the major cometary ions everywhere in the coma. The input parameters for our model are the solar wind conditions (density, speed, temperature, magnetic field) and the composition and production rate of the gas. We applied our model to Comet P/Halley in early March 1986, for which the input parameters are reasonably well known. We compare the resulting column density of H₂O⁺with ground-based observations of H₂O⁺from DiSantiet al.(1990). The results of our model are in good agreement with both the spatial distribution and the absolute abundance of H₂O⁺and with their variations with the changing overall water production rate between two days. The results are encouraging that it will be possible to obtain production rates of neutral cometary constituents from observations of their ion products.     

Triple F—a comet nucleus sample return mission

The Triple F (Fresh From the Fridge) mission, a Comet Nucleus Sample Return, has been proposed to ESA’s Cosmic Vision program. A sample return from a comet enables us to reach the ultimate goal of cometary research. Since comets are the least processed bodies in the solar system, the proposal goes far beyond cometary science topics (like the explanation of cometary activity) and delivers invaluable information about the formation of the solar system and the interstellar molecular cloud from which it formed. The proposed mission would extract three sample cores of the upper 50 cm from three locations on a cometary nucleus and return them cooled to Earth for analysis in the laboratory. The simple mission concept with a touch-and-go sampling by a single spacecraft was proposed as an M-class mission in collaboration with the Russian space agency ROSCOSMOS.     

The interaction of the solar wind with a comet

The flow of plasma on the sunward side of a comet is investigated by means of an axialsymmetric model based on hydrodynamics modified by source terms. The model assumes a given curvature of the isobaric surfaces, which corresponds to paraboloids around the nucleus of the comet. The flow on the axis can be represented by a solution of a system of seven ordinary differential equations (respectively five in case of pure photo-ionization). The flow pattern always contains a widely detached bow shock and a contact discontinuity separating a cavity with purely cometary plasma from the transition region containing also solar wind ions. The model is applied to the special case where the cometary gas is ionized by the solar UV radiation only. Numerical solutions are integrated for five levels of production of neutral gas by the comet and for seven typical situations in the undisturbed solar wind. The results imply standoff distances of the stagnation point from the nucleus of the order of 10 000 km or more, and distances of the bow shock of the order of 10⁶–10⁷ km.     

Direct statistical simulation of the near-surface layers of a cometary atmosphere. II: A nonspherical nucleus

The study presents the results of numerical simulations of mass-transfer processes in the near-surface layer of the cometary nucleus and in the inner part of the cometary atmosphere, which is formed under the action of solar radiation. The gas-kinetic model of the inner part of the cometary atmosphere surrounding a spherical nucleus (Skorov et al., 2004) is extended to the case of a nonspherical nucleus with axial symmetry. After high-resolution images of comets 19P/Borrelly and Wild 2 have been obtained by Deep Space 1 and Stardust spacecraft, such an extension seems to be vital and important. The nucleus and the inner part of the coma are closely related to each other because of the permanent exchange of energy and mass; therefore, they are modeled consistently. As in the first part of our study, the boundary conditions at the inner boundary of the simulation domain, which are necessary for gas-kinetic simulations, were determined from the self-consistent model of heat and mass transfer in a porous cometary nucleus that was developed earlier by the authors. The model took into account the volumetric character of the radiation absorption in a porous sublimating medium, the kinetic regime of the transport of sublimation products in the pores, and the backward gas fluxes from the coma due to intermolecular collisions. We considered different models of the nucleus structure that determined the effective gas production. Using the direct simulation Monte Carlo method, we computed the two-dimensional gas flow from a heterogeneous nonspherical cometary nucleus. The simulations were performed using the SMILE software. The parallel computer implementation of the software made it possible to calculate the spatial structure of the gas flow for the entire circumnucleus zone.     

The Electrostatic Blow-Off of Cometary Grains and Variations and Outbursts of Brightness of Comets at Large Heliocentric Distances

Interaction between the strong solar wind and the dark side of a cometary nucleus was considered. It was calculated that the potential of the dark side of the cometary nucleus could be numerically large and negative. Assuming that the nucleus of the comet has surface mantle which consists of loose, fine dust-ice particles, it was shown that cometary particles could electrostatically levitate over the nucleus. It was examined how this phenomenon affected the changes in the cometary brightness. Calculations were carried out for realistically assumed values of a large range of cometary parameters. It was shown that the considered mechanism could lead to the variations of cometary brightness, sometimes even to the outbursts of brightness.     

Lunar Beagle and Lunar Astrobiology

The study of the elements and molecules of astrobiological interest on the Moon can be made with the Gas Analysis Package (GAP) and associated instruments developed for the Beagle 2 Mars Express Payload. The permanently shadowed polar regions of the Moon may offer a unique location for the “cold-trapping” of the light elements (i.e. H, C, N, O, etc.) and their simple compounds. Studies of the returned lunar samples have shown that lunar materials have undergone irradiation with the solar wind and adsorb volatiles from possible cometary and micrometeoroid impacts. The Beagle 2’s analytical instrument package including the sample processing facility and the GAP mass spectrometer can provide vital isotopic information that can distinguish whether the lunar volatiles are indigenous to the moon, solar wind derived, cometary in origin or from meteoroids impacting on the Moon. As future Lunar Landers are being considered, the suite of instruments developed for the Mars Beagle 2 lander can be consider as the baseline for any lunar volatile or resource instrument package.     

The influence of the nucleus shape on the maximum size of grains ejected from a comet by gentle sublimation and jet‐like features

This paper deals with obtaining the maximum size of cometary grains ejected from nuclei of different shapes. Two mechanisms in terms of grain ejection from comets are taken into consideration. The first one is dragging of particles by outflowing gas molecules released by gentle sublimation from the comets. The second one is related with gas jets from the cavities in a nucleus by cometary jet‐like phenomena. We focused on ellipsoidal shapes of cometary nuclei but with different flattening. Calculations have been carried out for a large range of cometary parameters. It has been shown that for fixed mass of the nucleus the maximum size of grains is an increasing function of the nucleus flattening. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Golf: A resonance spectrometer for the observation of solar oscillations

GOLF (Global Oscillations at Low Frequencies) is an instrument to study the line-of-sight velocity of the solar photosphere, to be flown on the SOHO satellite in 1995. It uses a sodium vapour cell in resonance scattering mode, in order to measure the absolute Doppler shift of the solar sodium absorption lines. We detail laboratory tests to determine the performances of the cell built for the experiment. The results are in good agreement with numerical simulations of the resonance processes. As a final result, we can conclude that the level of performances required for the flight instrument will be obtained.