Cometary Origin Research Articles

Meteoroid ablation models

The fate of entering meteoroids in atmosphere is determined by their size, velocity and substance properties. Material from ablation of small-sized meteors (roughly R£0.01–1 cm) is mostly deposited between 120 and 80 km altitudes. Larger bodies (up to meter sizes) penetrate deeper into the atmosphere (down to 20 km altitude). Meteoroids of cometary origin typically have higher termination altitude due to substance properties and higher entry velocity. Fast meteoroids (V>30–40 km/s) may lose a part of their material at higher altitudes due to sputtering. Local flow regime realized around the falling body determines the heat transfer and mass loss processes. Classic approach to meteor interaction with atmosphere allows describing two limiting cases: — large meteoroid at relatively low altitude, where shock wave is formed (hydrodynamical models); — small meteoroid/or high altitudes — free molecule regime of interaction, which assumes no collisions between evaporated meteoroid particles. These evaporated particles form initial train, which then spreads into an ambient air due to diffusion. Ablation models should make it possible to describe physical conditions that occur around meteor body. Several self-consistent hydrodynamical models are developed, but similar models for transition and free molecule regimes are still under study. This paper reviews existing ablation models and discusses model boundaries.

SPECTROSCOPY OF A GEMINID FIREBALL: ITS SIMILARITY TO COMETARY METEOROIDS AND THE NATURE OF ITS PARENT BODY

A detailed analysis of a photographic spectrum of a Geminid fireball obtained in December 14, 1961 at the Ondrejov Observatory is presented. We have computed a synthetic spectrum for the fireball and compared it with the observed spectrum assuming chemical equilibrium in the meteor head. In this way we have determined relative chemical abundances in meteor vapors. Comparing the relative chemical abundances of this Geminid meteoroid with those obtained from meteoroids associated with comets 55P/Tempel-Tuttle and 109P/Swift-Tuttle we found no significant chemical differences in main rock-forming elements. Despite of this similarity, the deepest penetration of the Geminid meteoroids and their ability to reach high rotation rates in space without fragmentation suggest that thermal processing is affecting their physical properties. We suggest that as consequence of space weathering a high-strength envelope is produced around these particles. In this picture, heating processes of the mineral phases could result in the peculiar properties observed during atmospheric entry of the Geminid meteoroids, especially their strength, which is evidenced by its resistance to ablation. Finally, although one meteoroid cannot be obviously considered as representative of the composition of its parent body, we conclude that 3200 Phaethon is able to produce millimetre-size debris nearly chondritic in composition, but the measured slight overabundance of Na would support a cometary origin for this body.

Comet or asteroid shower in the late Eocene?

The passage of a comet shower approximately 35 million years ago is generally advocated to explain the coincidence during Earth's late Eocene of an unusually high flux of interplanetary dust particles and the formation of the two largest craters in the Cenozoic, Popigai and the Chesapeake Bay. However, new platinum-group element analyses indicate that Popigai was formed by the impact of an L-chondrite meteorite. Such an asteroidal projectile is difficult to reconcile with a cometary origin. Perhaps instead the higher delivery rate of extraterrestrial matter, dust, and large objects was caused by a major collision in the asteroid belt.

Chemical abundances determined from meteor spectra - II. Evidence for enlarged sodium abundances in meteoroids

The relative abundances of sodium in meteoroids have been estimated by averaging the composition of the radiating gas along the fireball path produced during their atmospheric entry. We used the method of thermal equilibrium for 13 fireballs produced by meteoroids mainly of cometary origin. The results show greater sodium abundances than those expected for interplanetary dust particles and chondritic meteorites, and interesting differences from the sodium abundance of 1P/Halley comet dust were also found.

Extraction and microanalysis of cosmic dust captured during sample return missions: laboratory simulations

Particles of cometary and asteroidal origin collected at source using dedicated capture cell technologies will be returned to Earth within the next 8 years. Furthermore, coincidental capture of interplanetary dust particles will occur on the exposed surfaces of the Genesis spacecraft. Laboratory simulations using both light-gas-gun and Van de Graaff accelerators have impacted dust analogues at velocities ranging from 5 km s −1 to ca. 72 km s −1 into comparable silicon and aerogel targets. Analysis of the impacts on silicon has shown complete spallation of impact residues for silicate projectiles of 38–53 μm in diameter, however craters formed by 1 μm iron projectiles show that near-intact residues can be preserved. An olivine grain embedded in aerogel has been characterized in situ using Raman micro-spectroscopy. Monte Carlo simulations and laboratory experiments have shown that analytical scanning electron microscopy can also be used to characterize embedded grains. Development of a novel particle extraction methodology using a 266 nm UV laser micro-dissection system has resulted in the recovery of an olivine grain. The extracted particle was then “cleaned up” using focused ion beam (FIB) milling to remove excess aerogel that was fused on the grain surface.

Plasma clouds associated with Comet P/Borrelly dust impacts

The NASA DS1 spacecraft encountered Comet P/Borrelly on September 22, 2001 at a distance of ∼2171 km on the sunward side of the comet. The flyby speed was ∼16.5 km s −1. Using high temporal resolution (50 μs) absolute electric field amplitude measurements from a ∼1 m dipole antenna, new features of plasma clouds created by cometary dust impacts have been detected. The pulses have 1/ e exponential decays of ∼650 μs duration, exponentially shaped overshoots with rise times of ∼2 ms, and exponential-shaped overshoot decay times of ∼10 ms. Assuming a plasma temperature of 10 4 K, these pulse features have been explained as plasma cloud space charge effects from the electron, proton and heavy ion portions of the clouds passing the antenna. Complex pulse shapes were also detected. These are believed to be due to either plasma cloud scattering off of the spacecraft, or to secondary impacts. Small electric pulses of duration 10–15 ms of cometary origin were detected but are presently unexplained. The electric component of the plasma wave spectra at closest approach had an f −2.4 power law shape from 10 Hz to 1 kHz. The electron cyclotron frequency was approximately 1 kHz. One possible explanation of the wave spectrum is that whistler mode waves associated with phase steepened cometary plasma waves are dispersed, leading to the broad spectrum. Finally, based on the present results, a new type of low-cost, large-area dust detector is proposed.

Dynamical Models of Kuiper Belt Dust in the Inner and Outer Solar System

We report several results related to the dynamical evolution of dust produced in the Kuiper belt (KB). We show that its particle size frequency distribution in space is greatly changed from the distribution at production, as a result of the combined effects of radiation forces and the perturbations of the planets. We estimate the contribution of KB dust to the zodiacal cloud by calculating the radial profile of its number density near the ecliptic. We also study the contribution of KB dust to the population of interplanetary dust particles (IDPs) collected at Earth, by calculating geocentric encounter velocities and capture rates. Our models show, in contrast with previous studies, that KB dust grains on Earth-crossing orbits have high eccentricities and inclinations and, therefore, that their encounter velocities are similar to those of cometary grains and not asteroidal grains. We estimate that at most 25% in number of captured IDPs have a cometary or KB origin; the KB contribution may be as low as 1%–2%. We present the velocity field of KB dust throughout the solar system; this, together with the number density radial profile, is potentially useful for planning spacecraft missions to the outer solar system.

Samples of stars beyond the solar system: silicate grains in interplanetary dust.

We have identified six circumstellar silicate grains within interplanetary dust particles (IDPs). Their extrasolar origins are demonstrated by their extremely anomalous oxygen isotopic compositions. Three 17O-rich grains appear to originate from red giant or asymptotic giant branch stars. One 16O-rich grain may be from a metal-poor star. Two 16O-poor grains have unknown stellar sources. One of the grains is forsterite, and two are amorphous silicate "GEMS" (glass with embedded metal and sulfides), which is consistent with astronomical identifications of crystalline and amorphous silicates in the outflows of evolved stars. These observations suggest cometary origins of these IDPs and underscore the perplexing absence of silicates among circumstellar dust grains from meteorites.

Gas-phase syntheses for interstellar carboxylic and amino acids

We report experimental results that demonstrate gas-phase, ionic syntheses of glycine and β-alanine, as well as acetic and propanoic acid, from smaller molecules found in space; in doing so, we infer the formation of these acids in the interstellar environment. We show that ionized glycine and β-alanine are produced in the reactions of hydroxylamine ions, NH2OH + , with acetic and propanoic acid respectively. Even more promising in the context of interstellar synthesis are our results that demonstrate the corresponding production of the protonated amino acids from analogous reactions with protonated hydroxylamine. The striking specificity of these syntheses for the β-isomer of alanine suggests that the amino acids of CI (Carbonaceous Ivona) chondrite meteorites are products of interstellar chemistry and supports the hypothesis that these meteorites are of cometary origin.

Origin of comets

Origin of comets

Deuterium enrichments in interplanetary dust

Interplanetary dust particles (IDPs) collected in the Earth's stratosphere comprise a diverse assemblage of materials with both cometary and asteroidal origins. Laboratory studies of IDPs show that they are chemically, mineralogically and isotopically heterogeneous on submicrometer scales. Many IDPs appear to have escaped the significant hydrous and thermal alteration processes which have strongly modified even the most primitive meteorites from their original compositions. Enrichments in deuterium are common among IDPs, with the largest anomalies occurring among the most fragile (cluster) particles. The D/H ratios of cluster IDPs vary by two orders of magnitude, reaching a maximum of .008, roughly 50 times the terrestrial value. Significant, but much smaller effects have been previously observed in primitive meteorites, and are attributed to the survival of molecular cloud material. The larger, and more highly variable deuterium excesses observed among IDPs suggest that they may contain well preserved presolar molecular cloud materials. The chemical characterization of the deuterium-rich phases is still at an early stage.

Extraterrestrial samples from low Earth orbits: techniques for their collection and analysis

In the context of dust samples collections in space, the COMET experiment (Collecte en Orbite de Matière ExtraTerrestre) was proposed for the first time in 1982. The idea of such an experiment was to collect grains with identified cometary parent body, instead of mixing all extraterrestrial contributions present in low Earth orbit. It was thus proposed to install collectors inside hermetic boxes, to have these boxes mounted outside a space station, orbiting the Earth and to have the capability of choosing the time and duration of the collection. Since 1985, the COMET experiment has been exposed three times to space (COMET-1, in October 1985 during the encounter of the Earth with the Draconid meteor stream; the EUROMIR-95 instrument, exposing collectors, during the crossing by the Earth of the Orionid meteor stream associated to comet P/Halley and, in November 1998, during the crossing by the Earth of the Leonid meteor stream associated to comet Temple-Tuttle, COMET-99). Specific collection techniques, and corresponding analytical procedures have been developed. The collected particles are the only ones accessible in the laboratory with a known cometary origin, before the return to Earth (2006) of the Stardust mission, which will collect cometary grains in the tails of comet Wild 2. Such a challenge justifies the tremendous efforts brought into play, and that are summarized here.

Clementine Observations of the Zodiacal Light and the Dust Content of the Inner Solar System

Using the Moon to occult the Sun, the Clementine spacecraft used its navigation cameras to map the inner zodiacal light at optical wavelengths over elongations of 3≲ϵ≲30° from the Sun. This surface brightness map is then used to infer the spatial distribution of interplanetary dust over heliocentric distances of about 10 solar radii to the orbit of Venus. The averaged ecliptic surface brightness of the zodiacal light falls off as Z(ϵ)∝ϵ −2.45±0.05, which suggests that the dust cross-sectional density nominally falls off as σ( r)∝ r −1.45±0.05. The interplanetary dust also has an albedo of a≃0.1 that is uncertain by a factor of ∼2. Asymmetries of ∼10% are seen in directions east–west and north–south of the Sun, and these may be due the giant planets' secular gravitational perturbations. We apply a simple model that attributes the zodiacal light as due to three dust populations having distinct inclination distributions, namely, dust from asteroids and Jupiter-family comets (JFCs) having characteristic inclinations of i∼7°, dust from Halley-type comets having i∼33°, and an isotropic cloud of dust from Oort Cloud comets. The best-fitting scenario indicates that asteroids + JFCs are the source of about 45% of the optical dust cross section seen in the ecliptic at 1 AU but that at least 89% of the dust cross section enclosed by a 1-AU-radius sphere is of a cometary origin. Each population's radial density variations can also deviate somewhat from the nominal σ( r)∝ r −1.45. When these results are extrapolated out to the asteroid belt, we find an upper limit on the mass of the light-reflecting asteroidal dust that is equivalent to a 12-km asteroid, and a similar extrapolation of the isotropic dust cloud out to Oort Cloud distances yields a mass equivalent to a 30-km comet, although the latter mass is uncertain by orders of magnitude.

Long-term evolution of Oort Cloud comets: capture of comets

ABSTRA C T We test different possibilities for the origin of short-period comets captured from the Oort Cloud. We use an efficient Monte Carlo simulation method that takes into account nongravitational forces, Galactic perturbations, observational selection effects, physical evolution and tidal splittings of comets. We confirm previous results and conclude that the Jupiter family comets cannot originate in the spherically distributed Oort Cloud, since there is no physically possible model of how these comets can be captured from the Oort Cloud flux and produce the observed inclination and Tisserand constant distributions. The extended model of the Oort Cloud predicted by the planetesimal theory consisting of a non-randomly distributed inner core and a classical Oort Cloud also cannot explain the observed distributions of Jupiter family comets. The number of comets captured from the outer region of the Solar system are too high compared with the observations if the inclination distribution of Jupiter family comets is matched with the observed distribution. It is very likely that the Halley-type comets are captured mainly from the classical Oort Cloud, since the distributions in inclination and Tisserand value can be fitted to the observed distributions with very high confidence. Also the expected number of comets is in agreement with the observations when physical evolution of the comets is included. However, the solution is not unique, and other more complicated models can also explain the observed properties of Halley-type comets. The existence of Jupiter family comets can be explained only if they are captured from the extended disc of comets with semimajor axes of the comets a , 5000 au. The original flattened distribution of comets is conserved as the cometary orbits evolve from the outer Solar system era to the observed region.

Are organic molecules produced by nebular Fischer-Tropsch processes preserved in comets?

Chondritic porous interplanetary dust particles, thought to be of cometary origin, contain solid phases that were likely produced by gas-grain reactions in nebular environments. In the course of laboratory simulations addressed to reproduce under nebular conditions such distinctive mineralogies, simple organic molecules are formed through Fischer-Tropsch-type processes, suggesting that part of the organic volatiles in comets may have had a nebular origin. On the basis of these experiments, the relative abundances of homologous volatile hydrocarbons and the implications for the possible existence of thiols and sulfonic acids in comets are discussed.

Cometary Origins

Cometary Origins

Search for meteor showers associated with Near-Earth Asteroids

Observed meteor showers associated with some Near-Earth asteroids (NEAs) is one of the few criteria that such asteroids may be considered to be candidate extinct cometary nuclei. In order to reveal new NEA- meteor shower associations, we calculated the secular variations of the orbital elements of 17 Taurid Complex asteroids with allowance for perturbations from six planets (Mercury-Saturn) over one cycle of variation of perihelia arguments. The Earth-crossing class of these NEAs and theoretical geocentric radiants and velocities of their meteor showers were determined and compared with available observational data. It turns out that each Taurid Complex asteroid is associated with four meteor showers. This is evidence for the cometary origin of these asteroids.

The Origin of Halley-Type Comets: Probing the Inner Oort Cloud

We have integrated the orbits of 27,700 test particles initially entering the planetary system from the Oort cloud in order to study the origin of Halley-type comets (HTCs). We included the gravitational influence of the Sun, giant planets, passing stars, and Galactic tides. We find that an isotropically distributed Oort cloud does not reproduce the observed orbital element distribution of the HTCs. In order to match the observations, the initial inclination distribution of the progenitors of the HTCs must be similar to the observed HTC inclination distribution. We can match the observations with an Oort cloud that consists of an isotropic outer cloud and a disklike massive inner cloud. These idealized two-component models have inner disks with median inclinations that range from 10° to 50°. This analysis represents the first link between observations and the structure of the inner Oort cloud.

1999 KUIPER PRIZE LECTURE: Cometary Origin of the Biosphere

Most of the biosphere was brought on the primitive Earth by an intense bombardment of comets. This included the atmosphere, the seawater and those volatile carbon compounds needed for the emergence of life. Comets were thrown into the inner Solar System by the strong perturbation induced by the growth of the giant planets' cores. The bulk of the Earth's bombardment came from those comets that accreted in Jupiter's zone, where the original deuterium enrichment had been diminished by steam coming from the hot, inner parts of the Solar System. This steam had condensed into icy chunks before their accretion into larger cometary nuclei. In contrast, comets that accreted in the zones of the outer giant planets kept their interstellar isotopic enrichments. Those comets contributed to the Earth's bombardment for a small amount only; they were mostly ejected into the Oort cloud and are the major source of the long-period comets observed today. The short-period comets, which come from the Kuiper Belt, should also have the same interstellar enrichment. The deuterium enrichment of seawater, accurately predicted by the previous scenario, has become one of the best telltales for the cometary origin of our biosphere. This cometary origin may have far-reaching cosmological consequences, in particular for the origin of life in other planetary systems.

Lightcurve of asteroid Nereus

An Apollo asteroid (4660) Nereus is one of the most accessible asteroids by spacecraft. Photometric observations of light variation of Nereus were performed in order to support a mission scenario planning and to elucidate the origin of the asteroid. The results revealed that Nereus has a long rotational period, 15.1 ± 1.2 hours, and that the amplitude of light variation is fairly large. Its slow rotation is preferable for a landing or touch down scenario of spacecraft. On the other hand, the comparison of lightcurve data between small main-belt asteroids and comet nuclei suggests that Nereus may be an extinct cometary nucleus, rather than a migrated asteroid from the main-belt. The comparison of the observed data with laboratory impact experiments also implies that the asteroid may not be a fragment from a catastrophic impact event in the main-belt. All the other data already obtained, such as by colorimetry, are not inconsistent with the cometary origin, either.