Cometary Grains Research Articles

Refractory comet dust analogues by laser bombardment and arc discharge production: a reference frame for “dusty experiments” on-board ROSETTA

A number of instruments included in the ROSETTA payload are aimed at characterising the cometary dust properties. An important issue will be the nature of these dusts that will include organic (CHON), mixed CHON-silicate, and silicate dust, and ice grains, that are probably arranged in more or less heterogeneous fluffy agglomerates. The dust can be completely amorphous, partially structured and crystalline and these grains may coexist at small scales. Laboratory analysis of the morphological, crystallographic and chemical properties of cometary grain analogues produced in controlled laboratory conditions will play a critical role in the success of the ROSETTA mission. The results obtained by these laboratory analysis can be used to test and calibrate the instruments that will be sent to comet 46P/Wirtanen by this mission. The interpretations of returned scientific data will strongly benefit by the comparison with data recorded by the instruments in ground tests for different classes of analogues previously well characterised with various analytical techniques in the laboratory. In this paper we describe our facilities to produce and characterise elemental carbon, hydrogenated carbon, silicate and mixed carbon-silicate grains by laser bombardment and arc discharge techniques. We use field emission scanning electron microscopy and transmission and analytical electron microscopy to identify the textures, morphologies and grain compositions and structures in analogue samples. Spectroscopy (from far ultraviolet to far infrared) is used to study their optical responses. Some analogue properties, e.g. grain texture and morphology, are similar to those detected in chondritic interplanetary dust particles that may include solid debris from periodic comets.

A systematic study of light scattering by aggregate particles using the microwave analog technique: Angular and wavelength dependence of intensity and polarization

Microwave analog experiments [e.g., Gustafson, 1996a, 1999] allow us to simulate light‐scattering properties of particles of almost any combination of composition, size, and shape. We have taken advantage of this ability and the broadband nature of the University of Florida microwave facility in a systematic study of light scattering by aggregates. We present angular dependencies of intensity and polarization in the scattering angle range 0°–165°, and in the wavelength interval 2.7–4 mm, which simulates 0.430–0.660 μm in the visual. The data are for a variety of sizes, shapes, and compositions of the constituent particles, as well as the number of particles in an aggregate and their packing. Wavelength dependencies are shown to be powerful diagnostics for the interpretation of comet dust observations and we urge observers to report color and polarimetric color. As an example, we estimate the size of constituent particles in cometary grains (0.1–1 μm). We also show that the grains evolve with the distance from the nucleus and that the reason apparently is the loss of some dark matter. Techniques and results reported here are general enough to apply to a broad range of natural and artificial aggregates.

Japanese contribution to in-situ meteoroid and debris measurement in the near Earth space

This paper reviews major results of present studies and recent developments for future missions in the Japanese space program regarding in-situ measurement and collection of micrometeoroids and orbital debris in the near Earth space. Japan’s contribution in this area began with the post flight impact analysis of the Space Flyer Unit (SFU) satellite which was returned to Earth in 1996 after 10-month exposure in space. Despite a decade later than similar efforts first conducted in the USA and Europe, it resulted in a record of over 700 hypervelocity impact signatures, which now forms the nation’s first database of real space impacts being open to public in the Internet. Together with laboratory impact tests, both morphological and elemental analyses of the impact craters yielded new insights of the meteoroid to debris ratio as well as flux variation compared with the previous spacecraft. The next step was a passive aerogel exposure in the STS-85 shuttle mission in 1997. No hypervelocity impact was found there but its experience has been incorporated for designing a microparticle collector to be on-board the Japan Experiment Module-Exposed Facility of the International Space Station. All of such “passive” collection of micro-impact features, however, still leave the significant uncertainty in the quest of their origins. Therefore an aerogel-based “hybrid” dust collector and detector (HD-CAD) is currently under the development. It measures time of impact and deduces impactors’ orbital and physical parameters by detecting impact flash while still capturing them intact. The system is suitable for both (1) sample return missions in LEO as well as to parent bodies of meteoroids, i.e., comets and asteroids, and (2) one-way mission to where the thermal and plasma environment is such that impact induced plasma detectors may suffer from significant noise, e.g., a Mercury orbiter and a solar probe. Together with unambiguous dust samples from a comet by STARDUST and an asteroid by MUSES-C as references, the HD-CAD in the LEO will be able to deduce the accretion rates of the cometary and asteroidal dust grains on the Earth.

Organic substances in cometary grains: comparison of secondary ion mass spectral data and californium-252 plasma desorption data from reference compounds

Secondary ion mass spectra (SIMS) and 252Cf plasma desorption ionization mass spectra (PDMS) of two carboxylic acids and two purine nucleobases have been compared. Results demonstrate that the positive ion mass spectra as well as the negative ion mass spectra are very similar in SIMS and PDMS. The principal characteristics are formation of quasimolecular ions (M + H) + or (M−H) − and loss of small molecules producing nonradical fragment ions; fumaric acid exhibits a remarkable formation of radical cations and anions. A set of simplified rules has been formulated for typical ion production processes involved in these techniques. This work contributes to the development of methods for data interpretation applicable to in situ analyses of cometary grains during the ROSETTA space mission of European Space Agency to the comet 46p/Wirtanen.

Silicates in Hale–Bopp: hints from laboratory studies

The recent passage of the Hale–Bopp (C/1995 O1) comet has provided the first opportunity to analyse the infrared spectral properties of a bright comet both from the ground and by the ISO space observatory. Previous works have already been dedicated to study the potential candidates to reproduce the cometary feature at 10 μm observed for different comets. We have applied a similar approach to compare the Hale–Bopp (C/1995 O1) spectra with laboratory data. The best fit has been obtained by using a mixture of crystalline Mg-rich olivine (forsterite), amorphous olivine and amorphous carbon grains. Some constraints on the possible cometary grain types derive from our simulation. Aggregates of submicron particles, composed of amorphous and crystalline olivine and amorphous carbon materials seem to be compatible with the cometary emission. Moreover, the possibility of fitting observational data on a wide IR spectra range, offered by ISO, provides interesting hints about the size distribution of grains responsible for the detected features.

Magnetospheric screening of cosmic dust

We investigate the effect of the terrestrial magnetosphere on the dynamics of cosmic dust. We show that the magnetosphere acts as a shield with an efficiency that depends on the size and the velocity of the incoming dust particles. We report on numerical simulations for dust grains of interplanetary, cometary, and lunar origin. We show that magnetospheric effects reduce the flux of interplanetary and lunar dust particles with radii a < 0.1 μm, but the shielding is much less effective for cometary dust grains owing to their high average approach velocity.

Resonant meteoroids from Comet Tempel-Tuttle in 1333: the cause of the unexpected Leonid outburst in 1998

Recent observations of an unexpectedly high incidence of bright Leonid meteors about 16 h before the predicted maximum of the main shower are explained by the ejection of dust grains into the 5/14 mean-motion resonance with Jupiter, principally during the perihelion passage of Comet 55P/Tempel—Tuttle in 1333. The dynamical evolution of resonant grains has the following properties: first, the grains do not spread uniformly around the orbit, but instead librate about a resonance centre within the main stream; secondly, these resonant zones contain a much higher space density of particles than the background stream, with the particle density approaching that of recently ejected cometary grains; thirdly, differential precession between the cometary orbit and the orbits of resonant particles may lead to meteor storms at unexpected times, possibly far removed from that of the normal shower. The presence of resonant dust grains leads to a complex structure within the Leonid meteoroid stream, and is an important general feature of meteoroid streams associated with Halley-type comets, themselves often trapped for long periods in mean-motion resonances.

Hydrocarbon synthesis in cometary grains

Light-weight straight-chain saturated and insaturated hydrocarbons in the C1–C5 range are formed when model material of cometary metal-containing silicate grains, similar to those present in chondritic porous interplanetary dust particles, are allowed to react with gas of variable composition (H2, CO, NH3, H2S) at 5×10−4 atm and 423–523 K. Methyl and methylene species dominate the infrared spectra in the 3.0–4.0 gm region. Gas-gain CO hydrogenation reactions may represent a plausible explanation for the formation of hydrocarbons as well as other organics in comets.

The GIADA experiment for ROSETTA mission to comet 46P/wirtanen: Design and performances

Rosetta is one of the most ambitious missions planned by ESA for the beginning of the next millennium. It will explore from very close a comet nucleus along its trajectory up to perihelion. In the instrument complex forming the scientific payload, the GIADA (Grain Impact Analyser and Dust Accumulator) experiment is devoted to study the cometary dust flux evolution and grain dynamic properties. To achieve the required performances and the expected scientific return, GIADA has been designed as a multi-sensor instrument. It is able to detect grain passage by laser light scattering measurement, particle momentum through piezoelectric transducers and mass flux by means of quartz crystal microbalances. In this paper we describe the technical solutions and performances which have been reached on the development models of GIADA.

Life in the solar system

Life, defined as a chemical system capable of transferring its molecular information via self-replication and also capable of evolving, must develop within a liquid to take advantage of the diffusion of complex molecules. On Earth, life probably originated from the evolution of reduced organic molecules in liquid water. Organic matter might have been formed in the primitive Earth's atmosphere or near hydrothermal vents. A large fraction of prebiotic organic molecules might have been brought by extraterrestrial-meteoritic and cometary dust grains decelerated by the atmosphere. Any celestial body harboring permanent liquid water may therefore accumulate the ingredients that generated life on the primitive Earth. The possibility that life might have evolved on early Mars when water existed on the surface marks it as a prime candidate in a search for bacterial life beyond the Earth. Europa has an icy carapace. However, cryovolcanic flows at the surface point to a possible water subsurface region which might harbor a basic life form. The atmosphere and surface components of Titan are also of interest to exobiology for insight into a hydrocarbon-rich chemically evolving world. One-handed complex molecules and preferential isotopic fractionation of carbon, common to all terrestrial life forms, can be used as basis indicators when searching for life beyond the Earth.

The inner dust coma of Comet 26P/Grigg-Skjellerup: multiple jets and nucleus fragments?

On 1992 July 10 the European Space Agency's spaceprobe Giotto passed the nucleus of the comparatively inactive comet 26P/Grigg-Skjellerup at a relative velocity of 14 km s(-1). This Giotto Extended Mission (GEM) followed a highly successful encounter in 1986 with Comet 1P/Halley. We present results returned from the Optical Probe Experiment (OPE) and in particular consider data gathered by the channels sensitive to the scattering of solar light by cometary dust grains, in emission-free continuum bands. Owing to the demise of the Halley Multicolour Camera (HMC) during the Halley encounter, and the low number of impacts registered by the Dust Impact Detection System (DIDSY), OPE data offer the best indication of the actual encounter geometry. We find that it is likely that Giotto was on the sunward side of the shadow terminator plane at closest approach, with our modelling results suggesting that Giotto passed similar to 100 km from the nucleus (although distances of up to 300 km cannot be ruled out). We investigate possible causes of the striking 'spike' features, or 'events', in the OPE data. While scattering of sunlight from ejecta particles as a result of dust impacts on the spacecraft body cannot be ruled out, considerations of the hypervelocity impact mechanisms and impact geometry show that this explanation is not without problems, and more investigation is needed before it can be conclusively accepted. As an alternative solution, we find that the complex data profiles can be fitted by jet activity in the innermost coma (which was not resolvable by ground-based observers). One particular event occurring at least 1000 km from the nucleus can be fitted if the OPE line of sight passes close to a nucleus fragment of radius 10-100 m which is situated around 50 km from the spacecraft and which is producing a small dust coma.

Properties of Cometary Dust from Color and Polarization

It is shown that the maximum value of the polarization of light scattered by either spherical or spheroidal polydisperse particles is mainly governed by the composition of grain material (complex refractive index), whereas its color is mainly determined by the grain size distribution. This conclusion is also valid for particles of a more general shape. Relations are obtained which make it possible to evaluate the properties of cometary grains from the color and polarization measured for the cometary continuum at large phase angles. These relations are especially suitable for carrying out a comparative analysis of cometary dust. From our own photometric and polarimetric data of five recent comets we predict their dust properties and compare them with the interpretation of infrared observations available in the literature.

From the formation of bioorganics in interstellar dust to the thermodynamics of self- organization in cometary grains as seeds of life's origins: Are 1025 chances enough?

From the formation of bioorganics in interstellar dust to the thermodynamics of self- organization in cometary grains as seeds of life's origins: Are 1025 chances enough?

Photometry and direct imaging of comet P/Faye 1991 XXI

Broad-band B and V photometry over a period of 74 days including the perihelion passage, and CCD direct-imaging (V and I) is presented for the coma of short-period comet Faye 1991 XXI. The V lightcurve, corrected for the projected diaphragm diameter varying with geocentric distance, and for the cometary dust phase function, reveals a brightness “outburst” at perihelion, corresponding to an increase of the water production rate by about 10–15% during 2.0 days. This “outburst” cannot be explained by an increase of the gas sublimation rate due to a change of temperature with heliocentric distance. Rather, it must be attributed to the activation of a “dormant” active area on a very slowly rotating nucleus, or to a change in the physical structure of the emission region, like an “explosion” of volatile ices, or an exothermal phase transition in the nucleus material. From the total visual brightness of the comet a water production rate of roughly 2 × 10 28 s −1 can be estimated for the time of perihelion at 1.6 AU. The “apparent” post-perihelion brightness decrease is extraordinarily steep with n = 13.2 (or larger), which is difficult to explain by a simple dependence of the sublimation from heliocentric distance. The correction of the lightcurve for the cometary dust phase function of the coma, i.e. reduction to zero phase angle, leads to a “more normal” brightness decrease for Jupiter family comets of n = 6−7. Radial surface-brightness profiles show a ϱ −1 dependence for the inner coma indicating an almost isotropic dust emission of the near-nucleus region. A cut-off of the ϱ −1 profile in the sunward sector can be interpreted as a dust reflection envelope at about 5 × 10 3 km in front of the nucleus. The dust expansion velocities near perihelion are of the order of 200 m s −1 for the smallest grains visible in the coma. Moreover, apart from the sun-tail elongation, a slight deviation from the isotropy of the coma can be recognized perpendicular to the sun-comet radius vector, and may be interpreted in terms of solar radiation pressure acting on cometary dust grains, or anisotropic outgassing of the nucleus.

Processing of cometary grains at the nucleus surface

Processing of cometary grains at the nucleus surface

Scattering Properties of Cometary Grains

AbstractHalley probes (Giotto, Vega) have shown that cometary grains can be extremely dark and might have fluffy structures. These probes have also detected submicron sized particles in the coma of the comet. In view of these observations and to interpret the observed data on several other comets we calculate the angular distribution of the scattered intensity, polarization and albedo for porous and compact particles. For these calculations, we apply discrete dipole approximation (DDA) first to the spheroidal particles, assumed to be made of small scattering elements (dipoles). Then we reduce the number of dipoles systematically to model the porous grains. We study the phase function, linear polarization, and albedo as a function of grain size and packing density.

Sublimation of Interplanetary Dust

AbstractThree topics are briefly reviewed to examine evidence for the nature of dust grains and grain alteration by physical processes related to mainly sublimation. Namely, (1) a change of dust flux detected byin situdust measurements beyond ~ 2 AU from the Sun suggests a disappearance of volatile ices due to sublimation below this distance, (2) an elongation angle dependence of Doppler shifts observed in the zodiacal light can be explained by the introduction of a dependence of the orbital velocity on the radiation pressure forces acting on the grains, taking into account a variation of dust structure with heliocentric distance, and (3) the variation of color temperature observed in cometary coma suggests a loss of carbon content from cometary dust grains due to sublimation. This piece of evidence agrees with the generation of carbon-bearing molecules from extended sources, as infered from observations in cometary comae.

Interplanetary dust interaction with comets: Production of x-rays

Recent results of theoretical investigations related to generation of high-energy (0.1–1 keV) photons in comets due to production of high-temperature (3×10 5 – 10 7 K) plasma clots from collisions of cometary and interplanetary grains at high relative velocities (70–700 km s −1 at heliocentric distances R=0.01–1 AU) are summarized and main features of the process are marked.

Chemical synthesis of lipids and the origin of life

Keeping in mind the importance of amphiphilic lipids for the formation of semipermeable membranes, a review summary of the sources of appropriate precursors, and chemical reactions for the abiotic synthesis of lipids is presented here within the framework of the theory of chemical evolution. It covers the presence in different cosmic environments of precursors for the formation of the biochemical molecules necessary for the emergence of life on Earth. It starts (1) with a short introduction. Then the following matters are briefly reviewed: (2) The circumstellar and interstellar molecules, some of which, could generate straight chain fatty acids through C9. (3) The possible reactions of hydrogenation and hydrolysis of cyanopolyynes which in the presence first of hydrogen and then liquid water could lead to the formation of aliphatic acids. (4) The composition of comets, where the preliminary analysis by mass spectrometry indicate straight chain hydrocarbons through only C5. (5) The organic compounds in carbonaceous chondrites where aliphatic acids through C12 have been identified, although the branched chain isomers are abundant. (6) The synthesis of some biochemical compounds, such as amino acids present in carbonaceous chondrites, which were probably formed by condensation of presolar precursors, aldehydes and ketones, with HCN in the presence of ammonia and liquid water in the meteorite parent body. The isotopic evidence seems to support this interpretation. (7) The formation of the Earth-Moon system by the catastrophic impact of a Mars-size body with the proto-Earth. (8) The subsequent capture of cometary water, organic and inorganic compounds, which must have led to a very reactive primitive Earth's atmospheric environment. The cometary iron-nickel grains could have catalyzed the formation of fatty acids by Fischer-Tropsch reactions. (9) The laboratory synthesis of straight chain fatty acids from C5 through C20 by Fischer-Tropsch processes. The amounts are usually in excess of the yields of aliphatic hydrocarbons. The chemical synthesis of glycerophospholipids including phosphatidylcholine. (10) The formation of liposomes, primarily, from phosphatidylcholine and the encapsulation within them of biopolymers. (11) Speculations on protocellular models of increasing complexity based on liposomes enclosing catalytic biomolecules. (12) Finally, some of the important problems remaining to be solved concerning the experimental approach to the study of the origin of life are briefly considered. It is hoped that in the next century, significant advances will be made in our understanding of the origin of life on Earth.

Ionization mechanisms of cometary comas

A new approach is considered to the problem of ionization of the inner comas of comets connected with two phenomena: meteor-like process due to cometary molecules — interplanetary meteoroids impacts and explosion-type process due to high-velocity collisions between cometary dust grains and interplanetary meteoroids. It is found that the efficiency of explosive ionization exceeds the efficiency of meteor ionization approximately 100 times. The explosive ionization may be possible mechanism for anomaly ionization of the inner comae of dusty comets like Halley 1986 III with the dust to gas production rate ratio more than 0.1.