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.     

Imaging Plasma Density Structures in the Soft X-Rays Generated by Solar Wind Charge Exchange with Neutrals

Both heliophysics and planetary physics seek to understand the complex nature of the solar wind’s interaction with solar system obstacles like Earth’s magnetosphere, the ionospheres of Venus and Mars, and comets. Studies with this objective are frequently conducted with the help of single or multipoint in situ electromagnetic field and particle observations, guided by the predictions of both local and global numerical simulations, and placed in context by observations from far and extreme ultraviolet (FUV, EUV), hard X-ray, and energetic neutral atom imagers (ENA). Each proposed interaction mechanism (e.g., steady or transient magnetic reconnection, local or global magnetic reconnection, ion pick-up, or the Kelvin-Helmholtz instability) generates diagnostic plasma density structures. The significance of each mechanism to the overall interaction (as measured in terms of atmospheric/ionospheric loss at comets, Venus, and Mars or global magnetospheric/ionospheric convection at Earth) remains to be determined but can be evaluated on the basis of how often the density signatures that it generates are observed as a function of solar wind conditions. This paper reviews efforts to image the diagnostic plasma density structures in the soft (low energy, 0.1–2.0 keV) X-rays produced when high charge state solar wind ions exchange electrons with the exospheric neutrals surrounding solar system obstacles.The introduction notes that theory, local, and global simulations predict the characteristics of plasma boundaries such the bow shock and magnetopause (including location, density gradient, and motion) and regions such as the magnetosheath (including density and width) as a function of location, solar wind conditions, and the particular mechanism operating. In situ measurements confirm the existence of time- and spatial-dependent plasma density structures like the bow shock, magnetosheath, and magnetopause/ionopause at Venus, Mars, comets, and the Earth. However, in situ measurements rarely suffice to determine the global extent of these density structures or their global variation as a function of solar wind conditions, except in the form of empirical studies based on observations from many different times and solar wind conditions. Remote sensing observations provide global information about auroral ovals (FUV and hard X-ray), the terrestrial plasmasphere (EUV), and the terrestrial ring current (ENA). ENA instruments with low energy thresholds (\(\sim1~\mbox{keV}\)) have recently been used to obtain important information concerning the magnetosheaths of Venus, Mars, and the Earth. Recent technological developments make these magnetosheaths valuable potential targets for high-cadence wide-field-of-view soft X-ray imagers.Section 2 describes proposed dayside interaction mechanisms, including reconnection, the Kelvin-Helmholtz instability, and other processes in greater detail with an emphasis on the plasma density structures that they generate. It focuses upon the questions that remain as yet unanswered, such as the significance of each proposed interaction mode, which can be determined from its occurrence pattern as a function of location and solar wind conditions. Section 3 outlines the physics underlying the charge exchange generation of soft X-rays. Section 4 lists the background sources (helium focusing cone, planetary, and cosmic) of soft X-rays from which the charge exchange emissions generated by solar wind exchange must be distinguished. With the help of simulations employing state-of-the-art magnetohydrodynamic models for the solar wind-magnetosphere interaction, models for Earth’s exosphere, and knowledge concerning these background emissions, Sect. 5 demonstrates that boundaries and regions such as the bow shock, magnetosheath, magnetopause, and cusps can readily be identified in images of charge exchange emissions. Section 6 reviews observations by (generally narrow) field of view (FOV) astrophysical telescopes that confirm the presence of these emissions at the intensities predicted by the simulations. Section 7 describes the design of a notional wide FOV “lobster-eye” telescope capable of imaging the global interactions and shows how it might be used to extract information concerning the global interaction of the solar wind with solar system obstacles. The conclusion outlines prospects for missions employing such wide FOV imagers.     

Charge Transfer Reactions

Charge transfer, or charge exchange, describes a process in which an ion takes one or more electrons from another atom. Investigations of this fundamental process have accompanied atomic physics from its very beginning, and have been extended to astrophysical scenarios already many decades ago. Yet one important aspect of this process, i.e. its high efficiency in generating X-rays, was only revealed in 1996, when comets were discovered as a new class of X-ray sources. This finding has opened up an entirely new field of X-ray studies, with great impact due to the richness of the underlying atomic physics, as the X-rays are not generated by hot electrons, but by ions picking up electrons from cold gas. While comets still represent the best astrophysical laboratory for investigating the physics of charge transfer, various studies have already spotted a variety of other astrophysical locations, within and beyond our solar system, where X-rays may be generated by this process. They range from planetary atmospheres, the heliosphere, the interstellar medium and stars to galaxies and clusters of galaxies, where charge transfer may even be observationally linked to dark matter. This review attempts to put the various aspects of the study of charge transfer reactions into a broader historical context, with special emphasis on X-ray astrophysics, where the discovery of cometary X-ray emission may have stimulated a novel look at our universe.     

Solar wind interaction with comets

The planned missions to Comet Halley, which will arrive at the nearest space of the Sun in 1986, have recently revived interest in studying solar wind interaction with comets. Several unsolved problems exist and the most urgent of them are as follows:

1. The character of the solar wind interaction with comets: bow shocks and contact surface formation near comets; similarities and differences of solar- wind interaction with comets and with Venus. The differences are probably associated with a great extension of neutral atmospheres of comets (due to a practical lack of cometary gravitation) and the ‘loading’ of the solar wind flux by cometary ions during the interaction.
2. The anomalous ionization in cometary heads.
3. The problem of the anamalously high accelerations of ions in the plasma tails of comets.
4. The variability of plasma structures observed in cometary tails.

     

The Silicate Material in Comets

Silicates in comets appear to be a mix of high-temperature crystalline enstatite and forsterite plus glassy or amorphous grains that formed at lower temperatures. The mineral identifications from the 10 and 20 μm cometary spectra are consistent with the composition of anhydrous chondritic aggregate IDPs. The origin of the cometary silicates remains puzzling. While the evidence from the IDPs points to a pre-solar origin of both crystalline and glassy components, the signatures of crystalline silicates appear in the spectra of young stellar objects only at a late evolutionary stage, when comets are the likely source of the dust. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

On the Organic Refractory Component of Cometary Dust

Cometary nuclei consist of ices intermixed with dust grains and are thought to be the least modified solar system bodies remaining from the time of planetary formation. Flyby missions to Comet P/Halley in 1986 showed that cometary dust is extremely rich in organics (∼50% by mass). However, this proportion appears to be variable among different comets. In comparison with the CI-chondritic abundances, the volatile elements H, C, and N are enriched in cometary dust indicating that cometary solid material is more primitive than CI-chondrites. Relative to dust in dense molecular clouds, bulk cometary dust preserves the abundances of C and N, but exhibits depletions in O and H. In most cases, the carbonaceous component of cometary particles can be characterized as a multi-component mixture of carbon phases and organic compounds. Cluster analysis identified a few basic types of compounds, such as elemental carbon, hydrocarbons, polymers of carbon suboxide and of cyanopolyynes. In smaller amounts, polymers of formaldehyde, of hydrogen cyanide and various unsaturated nitriles also are present. These compositionally simple types, probably, are essential "building blocks", which in various combinations give rise to the variety of involatile cometary organics. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cometary dynamics

The modern theory of cometary dynamics is based on Oort's hypothesis that the solar system is surrounded by a spherically symmetric cloud of 10¹¹ to 10¹² comets extending out to interstellar distances. Dynamical modeling and analysis of cometary motion have confirmed the ability of the Oort hypothesis to explain the observed distribution of energies for the long-period comet orbits. The motion of comets in the Oort cloud is controlled by perturbations from random passing stars, interstellar clouds, and the galactic gravitational field. Additionally, comets which enter the planetary region are perturbed by the major planets and by nongravitational forces resulting from jetting of volatiles on the surfaces of the cometary nuclei. The current Oort cloud is estimated to have a radius of 6 to 8 × 10⁴ AU, and to contain some 2 × 10¹² comets with a total mass of 7 to 8 Earth masses. Evidence has begun to accumulate for the existence of a massive inner Oort cloud extending from just beyond the orbit of Neptune to 10⁴ AU or more, with a population up to 100 times that of the outer Oort cloud. This inner cloud may serve as a reservoir to replenish the outer cloud as comets are stripped away by the various perturbers, and may also provide a more efficient source for the short-period comets. Recent suggestions of an unseen solar companion star or a tenth planet orbiting in the inner cloud and causing periodic comet showers on the Earth are likely unfounded. The formation site of the comets in the Oort cloud was likely the extended nebula accretion disc reaching from about 15 to 500 AU from the forming protosun. Comets which escape from the Oort cloud contribute to the flux of interstellar comets, though capture of interstellar comets by the solar system is extremely unlikely. The existence of Oort clouds around other main sequence stars has been suggested by the detection by the IRAS spacecraft of cool dust shells around about 10% of nearby stars.     

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.     

X-Rays From Mars

X-rays from Mars were first detected in July 2001 with the satellite Chandra. The main source of this radiation was fluorescent scattering of solar X-rays in its upper atmosphere. In addition, the presence of an extended X-ray halo was indicated, probably resulting from charge exchange interactions between highly charged heavy ions in the solar wind and neutrals in the Martian exosphere. The statistical significance of the X-ray halo, however, was very low. In November 2003, Mars was observed again in X-rays, this time with the satellite XMM-Newton. This observation, characterized by a considerably higher sensitivity, confirmed the presence of the X-ray halo and proved that charge exchange is indeed the origin of the emission. This was the first definite detection of charge exchange induced X-ray emission from the exosphere of another planet. Previously, this kind of emission had been detected from comets (which are largely exospheres) and from the terrestrial exosphere. Because charge exchange interactions between atmospheric constituents and solar wind ions are considered as an important nonthermal escape mechanism, probably responsible for a significant loss of the Martian atmosphere, X-ray observations may lead to a better understanding of the present state of the Martian atmosphere and its evolution. X-ray images of the Martian exosphere in specific emission lines exhibited a highly anisotropic morphology, varying with individual ions and ionization states. With its capability to trace the X-ray emission out to at least 8 Mars radii, XMM-Newton can explore exospheric regions far beyond those that have been observationally explored to date. Thus, X-ray observations provide a novel method for studying processes in the Martian exosphere on a global scale.     

Polarization of Light Scattered by Cometary Dust Particles: Observations and Tentative Interpretations

Analysis of the polarization of light scattered by cometary particles reveals similarities amongst the phase curves, together with some clear differences: i) comets with a strong silicate emission feature present a high maximum in polarization, ii) the polarization is always slightly lower than the average in inner comae and stronger in jet-like structures. These results are in excellent agreement with the Greenberg model of dust particles built up of fluffy aggregates of much smaller grains. Also, they suggest the existence of different regions of formation, and of different stages of evolution for the scattering particles inside a given cometary coma. This revised version was published online in June 2006 with corrections to the Cover Date.     

The ionospheres and plasma tails of comets

The present understanding of cometary ionospheres and plasma tails is critically evaluated. Following a brief introduction of the significance of the study of cometary ionospheres and tails (Section 1), the observational statistics and spectroscopic observations are summarized in Sections 2 and 3.The complicated and time varying morphology of the plasma tail and the ionosphere as revealed both by photographs as well as visual drawings is discussed in Section 4.The evidence for a strong comet-solar wind interaction, the possible nature of this interaction and also the use of comets as probes of the solar wind are considered in the next 3 sections (5, 6, 7). This is followed by a discussion of the various processes so far proposed for the ionization of cometary gases and their limitations (Section 8).Hydrodynamic models of the solar wind-comet interaction, which refers essentially to the region outside the tangential discontinuity, are presented and evaluated in Section 9. A discussion of the ion chemistry and structure of the region inside the tangential discontinuity (which is here referred to as the cometary ionosphere) follows in Section 10.The largely indirect evidence for the existence of substantial magnetic fields in cometary ionospheres and type 1 tails is evaluated and their likely origin is considered in Section 11. The associated electric currents; their size and closure as well as their importance as sources of ionization in the inner coma are also discussed.Finally in Section 12, some of the directions in which future research should progress, in order to provide a more complete and secure knowledge of cometary ionospheres and plasma tails, are stressed.     

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.     

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.     

Reservoir for Comet Material: Circumstellar Grains

Primitive meteorites and interplanetary dust particles contain small quantities of dust grains with highly anomalous isotopic compositions. These grains formed in the winds of evolved stars and in the ejecta of stellar explosions, i.e., they represent a sample of circumstellar grains that can be analyzed with high precision in the laboratory. Such studies have provided a wealth of information on stellar evolution and nucleosynthesis, Galactic chemical evolution, grain growth in stellar environments, interstellar chemistry, and the inventory of stars that contributed dust to the Solar System. Among the identified circumstellar grains in primitive solar system matter are diamond, graphite, silicon carbide, silicon nitride, oxides, and silicates. Circumstellar grains have also been found in cometary matter. To date the available information on circumstellar grains in comets is limited, but extended studies of matter returned by the Stardust mission may help to overcome the existing gaps.     

Temporal and Spatial Variations of Heliospheric x-ray Emissions Associated with Charge Transfer of the Solar Wind with Interstellar Neutrals

A simple model has been developed that demonstrates that heliospheric X-ray emission can account for about 25%–50% of observed soft X-ray background intensities. Similar to cometary soft X-ray emission, these X-rays are thought to be produced in the heliosphere due to charge transfer collisions between heavy solar wind ions and interstellar neutrals. A more complex model has now been developed to take into account temporal and spatial variations of the solar wind and interstellar neutrals. Measured time histories of the solar wind proton flux are used in the model and the results are compared with the ‘long-term enhancements’ in the soft X-ray background measured by ROSAT for the same time period. This revised version was published online in August 2006 with corrections to the Cover Date.     

Infrared astronomy on the midcourse space experiment

The Midcourse Space Experiment (MSX) is a multiple objective experiment scheduled to fly by the end of 1994. Infrared photometry and interferometry will be obtained by a solid hydrogen cooled, off-axis telescope of 35 cm unobscured primary aperture. The sensitivities of the line scanned arrays are comparable to IRAS bands 1 and 2 but the spatial resolution is some 30 times better. Nine broadly defined astronomy experiments are planned for the 18 month cryogen phase of the mission. Four of these experiments survey regions not adequately covered by previous infrared missions: the zodiacal cloud near the sun and the anti-solar direction, the Galactic Plane where IRAS sensitivities were limited by confusion and the gaps left by the IRAS survey. The higher sensitivity obtained from raster scans will probe Galactic structure and create intermediate spatial resolution maps of extended sources such as HII regions, the Magellanic Clouds and nearby galaxies. Measurements are also planned on a number of solar system objects such as planets, asteroids, the dust bands, comets and cometary debris trails. Moderate resolution spectra of a number of bright, discrete, extended sources will be obtained as well as low resolution spectral mapping along the Galactic Plane and Zodiacal dust cloud.     

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.