Collection of cometary dust

Rendezvous Missions to Comets lead to low velocities at the nucleus of the comet. The resulting impact velocity of the cometary dust on a target will range between 10 and 400 m/s. The dust particle which impacts on a target can be collected for a subsequent in-situ analysis.

The collection efficiency of a target depends in addition to obvious geometrical conditions upon the surface of the target. The surface characteristics can be divided into two groups:

• “dirty” surfaces, covered with silicate or hydrocarbon compounds (for example vacuum grease),

• “clean” surfaces, like gold (with additional sputtering).

This paper deals with the experimental and theoretical investigation of the collection efficiency of “clean” targets. Laboratory experiments are described which were conducted at the Technische Universität München, Lehrstuhl für Raumfahrttechnik, and the Max-Planck-Institut für Kernphysik, Heidelberg. In both experiments an electromagnetic accelerator is used to accelerate different types of dust in vacuum to velocities between 10 and 400 m/s.

The target is then examined under the microscope and a secondary ion mass spectrometer (which is a model of the laboratory carried on board of the spacecraft for “in situ” analysis). The adhesion of the dust grains at the target is evaluated experimentally in an ultracentrifuge.     

Thermal emission from cometary dust

The thermal emission from the dust coma of a comet can be analyzed to yield the flux and size distribution of the dust grains and the relative abundance of silicate and absorbing grains.     

Report on the Kiso cometary dust trail survey

Cometary dust trails were first observed by IRAS; they are widely known to be the origins of meteoric showers. A new window has been opened for the study of dust trails, using ground-based observations. We succeeded in obtaining direct images of the 22P/Kopff dust trail with the Kiso 1.05-m Schmidt telescope. Following this initial success, we have continued to perform a dust trail survey at Kiso. As a result of this survey, we have detected dust trails along the orbit of six periodic comets, between February 2002 and March 2004. The optical depth of these dust trails are 10⁻⁹ to 10⁻⁸, which is consistent with IRAS measurements. In this paper, we describe the observations and data reduction procedures, and report the brief result obtained between February 2002 and March 2004.     

Carbonaceous matter in cometary dust and coma.

The analysis of carbonaceous matter in p/Halley's dust and coma via mass spectrometry of positive ions is reviewed. Dust impact generated ions were analyzed by the PUMA instrument aboard VEGA I, and coma plasma ions by the PICCA instrument aboard GIOTTO. For the organic molecules results an overall C:H:O:N ratio of 1.:1.4:0.6:0.1. Most of this polymer material can formally be understood as an aggregation of monomers C2H2, CH2O, and HCN. Special emphasis is given to possible aromatic, especially heterocyclic, and other unsaturated ions, and their importance for abiotic chemical and prebiotic evolution. Aspects of the potential heterogeneous catalysis in liquid water at the inorganic grain backbone structure found by this analysis, too, are also treated.     

Recent infrared observations of comets with UKIRT and IRAS

A preliminary analysis of infrared observations of comets P/Crommelin and P/Tempel 1 is presented. Comet P/Crommelin was observed from UKIRT over the range 1–20 micron, using standard filters. From the shape of the thermal emission spectrum, the temperature of the dust grains is estimated (T = 314 ± ₃₃⁴⁴K) and also the dust production rate (1.3 × 10⁵gs^?1). Comet P/Tempel 1 was observed with the Infrared Astronomical Satellite (IRAS). The emission is found to be considerably extended and there is also evidence for temperature variation of the dust grains as indicated by the 12 to 25 micron flux ratio.     

A capacitor impact sensor (CIS) on board Giotto for detection of cometary dust particles

A low power high reliability impact sensor based on the discharge of a parallel plate capacitor is described. The choice of a surface area of about 1000 cm² and a penetration thickness of 50 micrometers will provide data on the flux density of cometary dust particles in the 5 micrometers diameter range (10⁻¹⁰g). A high noise immunity promotes excellent reliability under conditions of heavy spacecraft bombardment and high plasma densities in the late stages of the 500 km approach distance. Self-limiting of the event rate compression system also provides flux data at arbitrarily high impact rates. The capacitor sensor will be located on the external face of the outer dust shield of Giotto Spacecraft and it will be a part of the DIDSY experiment.     

Heterogeneous grain morphologies and acceleration mechanisms in cometary coma dust dynamics: Mass envelope dispersion

Modelling of the cometary coma with respect to the distribution of dust particles within the coma and tail have been performed by a number of authors /1,2,3/. Applications of the Divine model using a program coded for the Giotto DIDSY sensors have also been made to calculate expected sensor response of the instrument and spacecraft impact rates /4/. For a chosen mass of ～ 10^?10g we use the Divine Reference model /1/ to investigate the effect on the mass envelope of i) a velocity spread in dust particle ejection; and ii) a variation in the particle type. The results show that effects i) and ii) lead to a smoothing-out of the anticipated peak flux at an envelope boundary. A conceptual model to follow the formation and development of dust jets is also presented and effects illustrated for various nucleus rotation periods.     

Ground-based observations of the dust emission from comet Halley

A preliminary analysis of the dust emission from comet Halley is presented based on large scale observations of its dust tail. Selected images obtained between February 22 and May 10, 1986 are compared to synchrone-syndyne graphs to infer the history of the dust production and the properties of the dust, at least qualitatively. Quantitative modeling of the dust tall has also been initiated and preliminary results are shown for the cases of isotropic and anisotropic (jet) dust production.     

Cometary dust observations by optical in-situ methods

Remote optical observations of comets provide information only along the whole line of sight and require some assumptions to be interpreted. Due to the advent of cometary space missions, a two-step strategy has been defined to derive without any assumption spatial distribution and physical properties of dust by in-situ optical observations. First, an $\text{Optical Probe Experiment}$, suitable for a fast fly-by, should provide passive in-situ measurements in the direction of the approaching (or receding) comet near encounter; by suitably differencing such observations, the brightness and polarization per $\text{unit volume}$ can be recovered along the trajectory of the spacecraft. Secondly, a $\text{Light Scattering Dust Analyzer}$, suitable for a rendez-vous mission, should permit the determination of the scattering properties of $\text{individual particles}$. Both experiments also provide a connecting link between non-optical in-situ measurements (from mass spectrometers or impact detectors) and remote optical observations.     

Charging effects in the cometary environment of Halley

Electrostatic charging of the GIOTTO spacecraft in different impact induced charged particle environments was studied with 3D numerical particle-in-cell models. The simulation results are assessed according to first experimental results of GIOTTO and VEGA instruments. Decreasing and even negative spacecraft potentials measured on VEGA near closest approach to the comet suggest the influence of the cometary plasma, which is also confirmed by corresponding numerical simulations.     

The cometary contribution to prebiotic chemistry.

Different estimates based on dynamical considerations, lunar cratering rates, Solar System chemical abundances, and the single-impact theory on the origin of the Earth-Moon system suggest that comets and other related small, volatile-rich primitive minor bodies captured by the Earth during the early Archean must have been a major source of volatiles on our planet. It is likely that a substantial fraction of the organic molecules present in the colliding cometary nuclei, which may have included nitrogen bases and the precursors of amino acids, were destroyed due to the high temperatures and shock wave energy associated with the collision. However, the presence of H2O, CN, CH, CO, CO2 and other carbon-bearing molecules and radicals in the atmosphere of the Sun and in circumstellar shells around carbon-rich stars suggests that at least simple carbon species could have survived the cometary collisions. Under the anoxic conditions thought to prevail in the prebiotic terrestrial paleoatmosphere, the post-collisional formation of a large number of excited molecules and radicals, and the rapid quenching of the expanding gaseous ball may have led, upon rapid cooling, to the formation of molecules of biogenic elements and to their eventual deposition in localized environments where complex organic compounds of biochemical significance may have been produced and accumulated.     

Radiation-induced syntheses in cometary simulated models.

The behavior of an aqueous-dominant multicomponent cometary model is examined at high doses of ionizing radiation. The system is composed of a water mixture of HCN (0.2 mol dm-3), CH3CN (0.04 mol dm-3), C2H5CN (0.02 mol dm-3), CH3OH (0.12 mol dm-3) and HCO2H (0.01 mol dm-3. It was exposed to gamma rays at doses up to 18.5 MGy. The chemical kinetic database used in the computer treatment of experimental data consists of 79 reactions. A complex mixture of products has been synthesized: gases, amino acids, carboxylic acids and polymeric material. The results suggest that the pristine material in cometary nuclei may have been chemically altered by the action of cosmic rays and embedded radionuclides.     

Ultraviolet spectroscopy of cometary comae: An update

Since its launch in 1978 the $\text{International Ultraviolet Explorer}$ ($\text{IUE}$) satellite observatory has been used to record ultraviolet spectra of nearly two dozen comets. These observations have been applied principally to studies of the composition, chemistry and evolution of the gaseous coma and more recently, with the substantially increased data base, to comparative analyses. The observations of Comets Bowell (1982 I) and Cernis (1983?) at a heliocentric distance of ≈ 3.4 AU show these two comets to be virtually identical and pose problems for water ice vaporization models. The most significant recent result from $\text{IUE}$ was the discovery of S₂ in the Earth-approaching comet IRAS-Araki-Alcock (1983d) and the use of the S₂ emission as a monitor of short-term variations in cometary activity. In early 1984, periodic comet Encke was observed for the second time by $\text{IUE}$, this time post-perihelion.     

Modelling of cometary nuclei: Planetary missions preparation

The planning of planetary missions requires the knowledge as much as possible accurate of the target. This knowledge, which cannot be obtained only by ground based observations, can be supported by theoretical modeling. This is particularly true in the case of Rosetta, a cornerstone ESA mission that will be launched in January 2003, and of its target, the comet 46P/Wirtanen. In this paper we show how, using a nucleus thermal evolution model, it is possible to foresee the temperature of the nucleus surface and the activity level along the orbit and define the range of possible values for these properties. Activity level close to the aphelion depends on the presence of CO. Surface temperatures depend on the physical properties attributed to the dust: temperatures reached by a crust mainly composed by silicatic grains differ from the temperatures reached by a crust mainly composed by organic grains.     

Radiolysis of aqueous formaldehyde relevant to cometary environments.

The radiation chemistry of aqueous solutions of formaldehyde was studied in order to obtain an insight into the possible role of ionizing radiation on cometary environments. Aqueous solutions of 1.0 mol dm-3 formaldehyde were exposed to gamma-radiation in the dose range from 0.01 to 1200 kGy at 298 K. The radiation chemical yield of decomposition of formaldehyde was determined to be: G(-CH2(OH)2)-26.3 +/- 1.2. The high radiation chemical yield of decomposition was explained by a chain reaction initiated by the radical CH(OH)2 with formaldehyde. Computer fitting of the experimental data gives k(CH(OH)2 + CH2(OH)2)- 8.0xl0(1) dm3 mol-1 s-1. In the computer treatment of experimental findings we used 54 equations to consider the radiolysis of water and 11 reactions for the radiolysis of aqueous formaldehyde. Based on previous estimates of the total dose of ionizing radiation that comets have accumulated over 4.6 billion years, we predict a radiation damage-depth curve of formaldehyde in comet nuclei.     

Thermal model and thermo-mechanical stresses in cometary nuclei

Spherically symmetric radial temperature profiles of cometary nuclei have been determined numerically (and for simplified models analytically) in dependence on the orbital position of the periodic comet Halley. These temperature fields in the nucleus are connected with thermal stress fields which have been calculated with the assumption of elastic properties of cometary matter. The remarkable result is the possible existence of stresses, strong enough to cause internal cracking of the nucleus and break-ups of the cometary surface. This may be essential understanding normal cometary activity as well as outbursts and splits.     

Extreme starburst or central activity in X-ray luminous IRAS galaxies

ROSAT All Sky Survey observations of IRAS galaxies have revealed up to now a number of 10 optically non-Seyfert galaxies with X-ray (0.1–2.4) luminosities up to a few 10⁴³erg · s⁻¹ (Boller et al. 1992). The sources are brighter than previous detection limits of a few 10⁴¹erg · s⁻¹ as found by Stocke et al. (1991) or Green, Anderson and Ward (1992) for Einstein sources. The optical classification is based on follow-up observations which indicate clearly the non-Seyfert (LINER and HII region-like galaxies) nature. Our investigations reveal that galaxies classified as non-Seyferts on the basis of optical spectroscopy may reach exceptionally high X-ray luminosities which are similar to that of Seyfert galaxies. On the basis of the present observational material we suppose a hidden low luminosity AGN in the centre of these objects as the source of energy production. The objects are of interest when evaluating starburst versus central activity.     

Simulation of cometary jets in interaction with the solar wind

The influence of cometary jets on the solar wind interaction is studied with a 3D hybrid simulation. Anisotropic outgassing patterns were until recently not considered in cometary simulations, despite strong anisotropies found at observations. Comet 67P Churyumov–Gerasimenko, the target of the ROSETTA mission, was chosen as a case study for a simulation series. The cometary outgassing at 2.7 AU is modeled to originate from a single sun-facing jet with different levels of collimation, from isotropy to extremely thin jets. As no bow shock is present at this distance, solar wind patterns resulting from the anisotropic outgassing become more apparent. We find narrower jets to increase the standoff distance of the plasma interaction structures. Also, the Mach cone is wider and stronger for certain jet profiles. The magnetic field remains unable to propagate through the coma, resulting in strong draping patterns for narrow jets due to the increased standoff distance.     

Intensity profiles of the multiple scattering light near the cometary nucleus

The multiple scattering of solar radiation in the cometary atmosphere is treated with the method of successive scattering. Referring to in situ measurements of comet Halley about the size and spatial distributions of dust, the optical thickness τ₁ of dust has been estimated, i.e. τ₁=0.03 at wavelength λ=0.62μm in a quiet time, but τ₁=0.3 when the outbursts/jets occur. In the derivation of τ₁, optical properties of dust including a mixing ratio of absorbing to silicate grains, are determined based on the polarimetry of P/Halley at λ=0.62μm observed during the phase angles over Nov. 1985 to May 1986 at the Dodaira Station of Tokyo Astronomical Observatory.

It is found that a temporary enhancement of τ₁ leads an increase of the upward reflected intensity when the surface albedo A of the nucleus is less than 0.04, but the reverse is true when A>0.04. On the other hand, the intensity of the downward radiation at the surface of the nucleus always decreases as an increase of τ₁.