Rocky Cometary Particulates: Their Elemental,Isotopic and Mineralogical Ingredients

The determination of the chemical composition of solid cometary dust particles was one of the prime objectives of the three missions to Comet Halley in 1986. The dust analysis was performed by time-of-flight mass-spectrometry. Within the experimental uncertainty the mean abundances of the rock-forming elements in cometary dust particles are comparable to their abundances in CI-chondrites and in the solar photosphere, i.e. they are cosmic. H, C, and N, on the other hand, in cometary dust are significantly more abundant than in CI-chondrites, approach solar abundances, are to some extent related to O, and reside in an omnipresent refractory organic component dubbed CHON. Element variations between individual dust grains are characterized by correlations of Mg, Si, and O, and to a lesser extent of Fe and S. From particle-to-particle variations of the rock forming elements information on the mineralogy of cometary dust can be obtained. Cluster analysis revealed certain groups that partly match the classifications of stratospheric interplanetary dust particles. About half of Halley's analyzed particles are characterized by anhydrous Fe-poor Mg-silicates, Fe-sulfides, and rarely Fe metal. The Fe-poor Mg-silicates link Halley's dust to that of Hale-Bopp as shown by recent IR observations. No significant deviation from normal of the isotopic composition of the elements is unequivocally present with the notable exception carbon: ¹²C-rich grains with ¹²C/¹³C-ratios up to ≈ 5,000 link cometary dust to presolar circumstellar grains identified in certain chondrites. This revised version was published online in June 2006 with corrections to the Cover Date.     

Isotopic analysis of cometary organic matter

Carbon isotope ratios have been measured for CN in the coma of comet Halley and for several CHON particles emitted by Halley. Of these, only the CHON-particle data may be reasonably related to organic matter in the cometary nucleus, but the true range of ¹³C/¹²C values in those particles is quite uncertain. The D/H ratio in H₂O in the Halley coma resembles that in Titan/Uranus. The next decade should substantially improve our understanding of the distribution of C, H, N, and O isotopes in cometary organics. The isotopic composition of meteoritic organic matter is better understood and can serve as a useful analog for the cometary case.     

From interstellar matter to comets: Elemental abundances in interstellar dust and in comet Halley

Analogies between interstellar and cometary matter can be found in their chemical compositions, both in the gaseous and solid phases, but also in the physical processes involved like evidence for ion-molecules reactions at low temperature and for ice irradiation processes. Such analogies can be observed from 3 types of measurements: interstellar spectra, cometary observations, and analyses of interplanetary dust particles, with the help of laboratory simulation experiments. Taking into account all the present available information, a compilation of the elemental abundances in interstellar matter and in comet Halley is derived, without any assumption about the dust to gas ratio. It is found that there is a significant apparent depletion of nitrogen, presently unexplained, in both interstellar and cometary materials.     

Assessing the elemental composition of comet 81P/Wild 2 by analyzing dust collected by Stardust

One of the prime objectives in the analysis of cometary dust collected by the Stardust space mission is to determine the elemental composition of comet 81P/Wild 2. For this analysis, samples captured by two sampling media, silica aerogel and Al foil, were available. While aerogel was qualified to sample the dust almost intact, particles impinging on Al foils produced hypervelocity impact craters with residual cometary matter. Both sample types delivered valuable information on the cometary inventory, even though a slight loss of volatiles was observed for impact residues on Al foils. Altogether an elemental composition close to solar elemental abundances was observed, indicating that the early solar system was chemically rather homogeneous from the innermost regions close to the sun to the outer edge of the solar system, the presumed region of cometary origin.     

Isotopic Abundances in Comets

Isotopic ratios in comets provide keys for the understanding of the origin of cometary material, and the physical and chemical conditions in the early Solar Nebula. We review here measurements acquired on the D/H, ¹²C/¹³C, ¹⁶O/¹⁸O, ¹⁴N/¹⁵N, ³²S/³⁴S ratios in dust and gases, and discuss their cosmogonic implications. The prospects for future measurements from cometary space missions and remote sensing observations at millimeter and submillimeter wavelengths are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interstellar Reservoirs of Cometary Matter

We review the evidence for the products of interstellar chemistry in volatile cometary matter. We compare the organic inventory of star-forming cores with that measured in various comets and point out the similarities and differences. The conditions necessary to fractionate interstellar molecules in the heavier isotopes of H, C, O and N are summarised and compared to the measured fractionation ratios in cometary ices. We give a list of future measurements that would shed further light on the putative connection between cometary and interstellar molecules.     

Structural evolution of cometary surfaces

Comets with a high content of organics and light molecules are expected under cosmic radiation to gain a relatively unreactive crust and less volatile material to some ten metres deep. Interstellar dust impacts act to loosen and turn over 1 cm of the surface. We discuss how far this accords with observations of cometary dust halos and new versus old comets. Two key material properties have emerged from recent studies. Firstly, the source of cometary volatiles is not ice in the sense of material with a single sublimation energy. Secondly, the particulates are not simply mineral dust but include much organic material, some of which undergoes chemical processing and exchanges with the gaseous environment. Consistent with these properties, a coherent crust rather than a mantle of loose grains would build up to cover much of the nucleus of periodic comets. It would consolidate by cooking in the solar radiation, especially at peak temperatures around perihelion. There are two disjoint surface phases: one of volatile material, the other the refractory crust, the former deepening into crater-like hollows over successive apparitions. The transition to non-volatile crust is unstable, subject to competing consolidation and disruption processes, and sensitive to seasonal changes. A comet dims and becomes asteroidal as the inert crust extends over the erosion craters, and may only be rejuvenated via collision with a boulder-sized impactor or perturbation of the orbit to smaller perihelion distance.     

Cosac, The Cometary Sampling and Composition Experiment on Philae

Comets are thought to preserve the most pristine material currently present in the solar system, as they are formed by agglomeration of dust particles in the solar nebula, far from the Sun, and their interiors have remained cold. By approaching the Sun, volatile components and dust particles are released forming the cometary coma. During the phase of Heavy Bombardment, 3.8--4 billion years ago, cometary matter was delivered to the Early Earth. Precise knowledge on the physico-chemical composition of comets is crucial to understand the formation of the Solar System, the evolution of Earth and particularly the starting conditions for the origin of life on Earth. Here, we report on the COSAC instrument, part of the ESA cometary mission Rosetta, which is designed to characterize, identify, and quantify volatile cometary compounds, including larger organic molecules, by in situ measurements of surface and subsurface cometary samples. The technical concept of a multi-column enantio-selective gas chromatograph (GC) coupled to a linear reflectron time-of-flight mass-spectrometer instrument is presented together with its realisation under the scientific guidance of the Max-Planck-Institute for Solar System Research in Katlenburg-Lindau, Germany. The instrument's technical data are given; first measurements making use of standard samples are presented. The cometary science community is looking forward to receive fascinating data from COSAC cometary in situ measurements in 2014.     

The Chemical Composition of Interstellar Matter at the Solar Location

The Local Interstellar Cloud (LIC) surrounds the Solar System and sets the boundary conditions for the heliosphere. Using both in situ and absorption line data towards ε CMa we are able to constrain both the ionization and the gas phase abundances of the LIC gas at the Solar Location. We find that the abundances are consistent with all of the carbonaceous dust grains having been destroyed, and in fact with a supersolar abundance of C. The constituents of silicate grains, Si, Mg, and Fe, appear to be sub-solar, indicating that silicate dust is present in the LIC. N, O and S are close to the solar values.     

Cometary Deuterium

Deuterium fractionations in cometary ices provide important clues to the origin and evolution of comets. Mass spectrometers aboard spaceprobe Giotto revealed the first accurate D/H ratios in the water of Comet 1P/Halley. Ground-based observations of HDO in Comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), the detection of DCN in Comet Hale-Bopp, and upper limits for several other D-bearing molecules complement our limited sample of D/H measurements. On the basis of this data set all Oort cloud comets seem to exhibit a similar ratio in H₂O, enriched by about a factor of two relative to terrestrial water and approximately one order of magnitude relative to the protosolar value. Oort cloud comets, and by inference also classical short-period comets derived from the Kuiper Belt cannot be the only source for the Earth's oceans. The cometary O/C ratio and dynamical reasons make it difficult to defend an early influx of icy planetesimals from the Jupiter zone to the early Earth. D/H measurements of OH groups in phyllosilicate rich meteorites suggest a mixture of cometary water and water adsorbed from the nebula by the rocky grains that formed the bulk of the Earth may be responsible for the terrestrial D/H. The D/H ratio in cometary HCN is 7 times higher than the value in cometary H₂O. Species-dependent D-fractionations occur at low temperatures and low gas densities via ion-molecule or grain-surface reactions and cannot be explained by a pure solar nebula chemistry. It is plausible that cometary volatiles preserved the interstellar D fractionation. The observed D abundances set a lower limit to the formation temperature of (30 ± 10) K. Similar numbers can be derived from the ortho-to-para ratio in cometary water, from the absence of neon in cometary ices and the presence of S₂. Noble gases on Earth and Mars, and the relative abundance of cometary hydrocarbons place the comet formation temperature near 50 K. So far all cometary D/H measurements refer to bulk compositions, and it is conceivable that significant departures from the mean value could occur at the grain-size level. Strong isotope effects as a result of coma chemistry can be excluded for molecules H₂O and HCN. A comparison of the cometary ratio with values found in the atmospheres of the outer planets is consistent with the long-held idea that the gas planets formed around icy cores with a high cometary D/H ratio and subsequently accumulated significant amounts of H₂ from the solar nebula with a low protosolar D/H. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cometary Refractory Grains: Interstellar and Nebular Sources

Comets are heterogeneous mixtures of interstellar and nebular materials. The degree of mixing of interstellar sources and nebular sources at different nuclear size scales holds the promise of revealing how cometary particles, cometesimals, and cometary nuclei accreted. We can ascribe cometary materials to interstellar and nebular sources and see how comets probe planet-forming process in our protoplanetary disk. Comets and cometary IDPs contain carbonaceous matter that appears to be either similar to poorly-graphitized (amorphous) carbon, a likely ISM source, or highly labile complex organics, with possible ISM or outer disk heritage. The oxygen fugacity of the solar nebula depends on the dynamical interplay between the inward migration of carbon-rich grains and of icy (water-rich) grains. Inside the water dissociation line, OH^? reacts with carbon to form CO or CO₂, consuming available oxygen and contributing to the canonical low oxygen fugacity. Alternatively, the influx of water vapor and/or oxygen rich dust grains from outer (cooler) disk regions can raise the oxygen fugacity. Low oxygen fugacity of the canonical solar nebula favors the condensation of Mg-rich crystalline silicates and Fe-metal, or the annealing of Fe-Mg amorphous silicates into Mg-rich crystals and Fe-metal via Fe-reduction. High oxygen fugacity nebular conditions favors the condensation of Fe-bearing to Fe-rich crystalline silicates. In the ISM, Fe-Mg amorphous silicates are prevalent, in stark contrast to Mg-rich crystalline silicates that are rare. Hence, cometary Mg-rich crystalline silicates formed in the hot, inner regions of the canonical solar nebula and they are the touchstone for models of the outward radial transport of nebular grains to the comet-forming zone. Stardust samples are dominated by Mg-rich crystalline silicates but also contain abundant Fe-bearing and Fe-rich crystalline silicates that are too large (?0.1 μm) to be annealed Fe-Mg amorphous silicates. By comparison with asteroids, the Stardust Fe-bearing and Fe-rich crystalline silicates suggests partial aqueous alteration in comet nuclei. However, aqueous alteration transforms Fe-rich olivine to phyllosilicates before Mg-rich olivine, and Stardust has Mg-rich and Fe-rich olivine and no phyllosilicates. Hence, we look to a nebular source for the moderately Fe-rich to nearly pure-Fe crystalline silicates. Primitive matrices have Mg-Fe silicates but no phyllosilicates, supporting the idea that Mg-Fe silicates but not phyllosilicates are products of water-rich shocks. Chondrule-formation is a late stage process in our protoplanetary disk. Stardust samples show comet 81P/Wild 2 formed at least as late to incorporate a few chondrules, requiring radial transport of chondrules out to perhaps >20 AU. By similar radial transport mechanisms, collisional fragments of aqueously altered asteroids, in particular achondrites that formed earlier than chondrules, might reach the comet-forming zones. However, Stardust samples do not have phyllosilicates and chondrules are rare. Hence, the nebular refractory grains in comet 81P/Wild 2, as well as other comets, appear to be pre-accretionary with respect to asteroid parent bodies. By discussing nebular pathways for the formation of Fe-rich crystalline silicates, and also phyllosilicates and carbonates, we put forth the view that comets contain both the interstellar ingredients for and the products of nebular transmutation.     

X-Ray and extreme ultraviolet emissions from comets

There is significant progress in the observations, theory, and understanding of the x-ray and EUV emissions from comets since their discovery in 1996. That discovery was so puzzling because comets appear to be more efficient emitters of x-rays than the Moon by a factor of 80000. The detected emissions are general properties of comets and have been currently detected and analyzed in thirteen comets from five orbiting observatories. The observational studies before 2000 were based on x-ray cameras and low resolution (E/E1.5–3) instruments and focused on the morphology of x-rays, their correlations with gas and dust productions in comets and with the solar x-rays and the solar wind. Even those observations made it possible to choose uniquely charge exchange between the solar wind heavy ions and cometary neutrals as the main excitation process. The recently published spectra are of much better quality and result in the identification of the emissions of the multiply charged ions of O, C, Ne, Mg, and Si which are brought to comets by the solar wind. The observed spectra have been used to study the solar wind composition and its variations. Theoretical analyses of x-ray and EUV photon excitation in comets by charge exchange, scattering of the solar photons by attogram dust particles, energetic electron impact and bremsstrahlung, collisions between cometary and interplanetary dust, and solar x-ray scattering and fluorescence in comets have been made. These analyses confirm charge exchange as the main excitation mechanism, which is responsible for more than 90% of the observed emission, while each of the other processes is limited to a few percent or less. The theory of charge exchange and different methods of calculation for charge exchange are considered. Laboratory studies of charge exchange relevant to the conditions in comets are reviewed. Total and state-selective cross sections of charge exchange measured in the laboratory are tabulated. Simulations of synthetic spectra of charge exchange in comets are discussed. X-ray and EUV emissions from comets are related to different disciplines and fields such as cometary physics, fundamental physics, x-rays spectroscopy, and space physics.     

Cometary Dust

This review presents our understanding of cometary dust at the end of 2017. For decades, insight about the dust ejected by nuclei of comets had stemmed from remote observations from Earth or Earth’s orbit, and from flybys, including the samples of dust returned to Earth for laboratory studies by the Stardust return capsule. The long-duration Rosetta mission has recently provided a huge and unique amount of data, obtained using numerous instruments, including innovative dust instruments, over a wide range of distances from the Sun and from the nucleus. The diverse approaches available to study dust in comets, together with the related theoretical and experimental studies, provide evidence of the composition and physical properties of dust particles, e.g., the presence of a large fraction of carbon in macromolecules, and of aggregates on a wide range of scales. The results have opened vivid discussions on the variety of dust-release processes and on the diversity of dust properties in comets, as well as on the formation of cometary dust, and on its presence in the near-Earth interplanetary medium. These discussions stress the significance of future explorations as a way to decipher the formation and evolution of our Solar System.     

Cosima – High Resolution Time-of-Flight Secondary Ion Mass Spectrometer for the Analysis of Cometary Dust Particles onboard Rosetta

The ESA mission Rosetta, launched on March 2nd, 2004, carries an instrument suite to the comet 67P/Churyumov-Gerasimenko. The COmetary Secondary Ion Mass Anaylzer – COSIMA – is one of three cometary dust analyzing instruments onboard Rosetta. COSIMA is based on the analytic measurement method of secondary ion mass spectrometry (SIMS). The experiment’s goal is in-situ analysis of the elemental composition (and isotopic composition of key elements) of cometary grains. The chemical characterization will include the main organic components, present homologous and functional groups, as well as the mineralogical and petrographical classification of the inorganic phases. All this analysis is closely related to the chemistry and history of the early solar system. COSIMA covers a mass range from 1 to 3500 amu with a mass resolution m/Δm @ 50% of 2000 at mass 100 amu. Cometary dust is collected on special, metal covered, targets, which are handled by a target manipulation unit. Once exposed to the cometary dust environment, the collected dust grains are located on the target by a microscopic camera. A pulsed primary indium ion beam (among other entities) releases secondary ions from the dust grains. These ions, either positive or negative, are selected and accelerated by electrical fields and travel a well-defined distance through a drift tube and an ion reflector. A microsphere plate with dedicated amplifier is used to detect the ions. The arrival times of the ions are digitized, and the mass spectra of the secondary ions are calculated from these time-of-flight spectra. Through the instrument commissioning, COSIMA took the very first SIMS spectra of the targets in space. COSIMA will be the first instrument applying the SIMS technique in-situ to cometary grain analysis as Rosetta approaches the comet 67P/Churyumov-Gerasimenko, after a long journey of 10 years, in 2014.     

Cometary and interstellar dust grains: Analysis by ion microprobe mass spectrometry and other techniques

A survey of microanalytical measurements on interplanetary dust particles (IDPs) and interstellar dust grains from primitive meteorites is presented. Ion microprobe mass spectrometry with its capability to determine isotopic compositions of many elements on a m spatial scale has played a special role. Examples are measurements of H, N, and O isotopes and refrectory trace elements in IDPs, and C, N, Mg, and Si isotopes in interstellar silicon carbide grains, and C and N isotopes as well as H, N, Al and Si concentrations in interstellar graphite grams. Possible future instrumental developments are also outlined.     

Dust Near The Sun

We review the current knowledge and understanding of dust in the inner solar system. The major sources of the dust population in the inner solar system are comets and asteroids, but the relative contributions of these sources are not quantified. The production processes inward from 1 AU are: Poynting-Robertson deceleration of particles outside of 1 AU, fragmentation into dust due to particle-particle collisions, and direct dust production from comets. The loss processes are: dust collisional fragmentation, sublimation, radiation pressure acceleration, sputtering, and rotational bursting. These loss processes as well as dust surface processes release dust compounds in the ambient interplanetary medium. Between 1 and 0.1 AU the dust number densities and fluxes can be described by inward extrapolation of 1 AU measurements, assuming radial dependences that describe particles in close to circular orbits. Observations have confirmed the general accuracy of these assumptions for regions within 30° latitude of the ecliptic plane. The dust densities are considerably lower above the solar poles but Lorentz forces can lift particles of sizes < 5 μm to high latitudes and produce a random distribution of small grains that varies with the solar magnetic field. Also long-period comets are a source of out-of-ecliptic particles. Under present conditions no prominent dust ring exists near the Sun. We discuss the recent observations of sungrazing comets. Future in-situ experiments should measure the complex dynamics of small dust particles, identify the contribution of cometary dust to the inner-solar-system dust cloud, and determine dust interactions in the ambient interplanetary medium. The combination of in-situ dust measurements with particle and field measurements is recommended.     

MIDAS – The Micro-Imaging Dust Analysis System for the Rosetta Mission

The International Rosetta Mission is set for a rendezvous with Comet 67 P/Churyumov-Gerasimenko in 2014. On its 10 year journey to the comet, the spacecraft will also perform a fly-by of the two asteroids Stein and Lutetia in 2008 and 2010, respectively. The mission goal is to study the origin of comets, the relationship between cometary and interstellar material and its implications with regard to the origin of the Solar System. Measurements will be performed that shed light into the development of cometary activity and the processes in the surface layer of the nucleus and the inner coma. The Micro-Imaging Dust Analysis System (MIDAS) instrument is an essential element of Rosetta’s scientific payload. It will provide 3D images and statistical parameters of pristine cometary particles in the nm-μm range from Comet 67P/Churyumov-Gerasimenko. According to cometary dust models and experience gained from the Giotto and Vega missions to 1P/Halley, there appears to be an abundance of particles in this size range, which also covers the building blocks of pristine interplanetary dust particles. The dust collector of MIDAS will point at the comet and collect particles drifting outwards from the nucleus surface. MIDAS is based on an Atomic Force Microscope (AFM), a type of scanning microprobe able to image small structures in 3D. AFM images provide morphological and statistical information on the dust population, including texture, shape, size and flux. Although the AFM uses proven laboratory technology, MIDAS is its first such application in space. This paper describes the scientific objectives and background, the technical implementation and the capabilities of MIDAS as they stand after the commissioning of the flight instrument, and the implications for cometary measurements.     

Remote Observations of the Composition of Cometary Volatiles

The volatile species released in the coma are an important clue to the composition of the cometary nucleus ices. Their identification and the measurement of their abundances is possible by remote sensing. Considerable progress has been made recently using radio and infrared spectroscopy, especially with the observations of the two exceptional comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp).) 24 molecules likely to be parent molecules outgassed from the nucleus have now been identified. Significant upper limits exist for many other species, and the presence of unidentified lines suggests that further species are to be identified. In addition, isotopic varieties have been observed for hydrogen, carbon, nitrogen and sulphur. We will review these results with a special emphasis on the reliability of the identifications and of the molecular production rate determinations. A critical point is to assess whether a given species is a genuine parent molecule outgassed from nuclear ices, or is a secondary product coming from grains or from gas-phase photochemistry. Ground-based spectral imaging, such as radio interferometry, may help resolving this problem. This revised version was published online in June 2006 with corrections to the Cover Date.     

Dust Environment Modelling of Comet 67P/Churyumov-Gerasimenko

Dust is an important constituent of cometary emission; its analysis is one of the major objectives of ESA’s Rosetta mission to comet 67P/Churyumov-Gerasimenko (C–G). Several instruments aboard Rosetta are dedicated to studying various aspects of dust in the cometary coma, all of which require a certain level of exposure to dust to achieve their goals. At the same time, impacts of dust particles can constitute a hazard to the spacecraft. To conciliate the demands of dust collection instruments and spacecraft safety, it is desirable to assess the dust environment in the coma even before the arrival of Rosetta. We describe the present status of modelling the dust coma of 67P/C–G and predict the speed and flux of dust in the coma, the dust fluence on a spacecraft along sample trajectories, and the radiation environment in the coma. The model will need to be refined when more details of the coma are revealed by observations. An overview of astronomical observations of 67P/C–G is given, because model parameters are derived from this data if possible. For quantities not yet measured for 67P/C–G, we use values obtained for other comets, e.g. concerning the optical and compositional properties of the dust grains. One of the most important and most controversial parameters is the dust mass distribution. We summarise the mass distribution functions derived from the in-situ measurements at comet 1P/Halley in 1986. For 67P/C–G, constraining the mass distribution is currently only possible by the analysis of astronomical images. We find that both the dust mass distribution and the time dependence of the dust production rate of 67P/C–G are those of a fairly typical comet.