Cometary Origin Research Articles

ASTRONOMICAL CATASTROPHES IN EARTH HISTORY

ABSTRACT The boundary between the Cretaceous and Tertiary epochs has now been firmly linked to a large impact structure centered on Chicxulub in Yucatan. This crater has an age of 65.0 ± 0.1 million years. Macro-fossils such as ammonites, and footprints of dinosaurs, are found up to centimeters_ below (but not above!) the Cretaceous-Tertiary (K/T) boundary. It is not yet certain whether the Chicxulub crater was produced by a comet or by an asteroid. However, data on the crater diamter and K/T iridium abundance slightly favor a cometary origin. The causes of the other major extinctions in the geological record, such as those between the Permian and Triassic (P/TR) and between the Triassic and Jurassic (TR/J) remain in doubt. Explosions of nearby supernovae occur too rarely to account for the frequency of major extinctions in the ~600 million year geological record of the Phanerzoic.

A search for shocked quartz grains and impact ejecta in early Silurian sediments on Gotland, Sweden

AbstractAll bentonite and bentonite-resembling layers thicker than a few millimetres from a 120m-thick Early Silurian sequence on Gotland, Sweden, were searched for shocked quartz grains of comet or asteroid impact origin. Although more than 200000 quartz grains from 86 bentonite samples were studied, not one single grain with multiple planar shock features was found. The studied sequence represents sedimentation during a period of about 2 million years. Impact frequencies, estimated from the cratering record and astronomical observations, indicate that during a 2-myr- period on average 20 comet or asteroid bodies larger than 0.5 km in diameter strike the Earth. The number of smaller impacting bodies is many times higher. In the light of this high frequency of impacts, the absence of any shocked-quartz-bearing fallout layer in our sequence indicates that lateral spreading of such ejecta is relatively restricted during small- and medium-scale impact events.The results also show that shocked quartz in general is absent or extremely rare in volcanic ash. This strengthens the case for an impact-related origin of shocked quartz grains in the Cretaceous–Tertiary boundary days.

The dilemma of the new-comet flux

The question of the constancy of the flux rate of “new” comets from the Oort cloud is critical in studies of the origin of comets. The large numbers may be explained by our being in a “comet shower” or else by a loss of intrinsic brightness by new comets during their first passage through the inner solar system—hence the dilemma. Improvements in the quality and quantity of reliable orbits now make possible fairly precise evaluation of the data involved in the dilemma and a realistic comparison of the two alternatives. If we are in a comet shower, the increase with respect to the last several tens of thousands of years is not more than a factor of 1.8 nor a magnitude loss during first passage of more than 0 m .85. This applies if new comets are defined as those with periods greater than 10 6 yr. If the limit is set at 2.8 × 10 6 yr, we may not be really in a comet shower and a good fraction of the comets with periods in the range 10 6 < P < 2.8 × 10 6 yr may be returning comets. The Oort cloud would then be fairly stable to nearly 40,000 AU. If we assume the reality of comet fading in the sense that few new comets are observable on their returns when more distant than 3.0 AU from the Sun, and if we then limit the discussion to comets with perihelion distance q < 3.0 AU, there is no evidence for a shower.

The thermal history of interplanetary dust particles collected in the Earth's stratosphere

Abstract— Fragments of 24 individual interplanetary dust particles (IDPs) collected in the Earth's stratosphere were obtained from NASA's Johnson Space Center collection and subjected to pulse‐heating sequences to extract He and Ne and to learn about the thermal history of the particles. A motivation for the investigation was to see if the procedure would help distinguish between IDPs of asteroidal and cometary origin. The use of a sequence of short‐duration heat pulses to perform the extractions is an improvement over the employment of a step‐heating sequence, as was used in a previous investigation. The particles studied were fragments of larger parent IDPs, other fragments of which, in coordinated experiments, are undergoing studies of elemental and mineralogical composition in other laboratories. While the present investigation will provide useful temperature history data for the particles, the relatively large size of the parent IDPs (∼40 μm in diameter) resulted in high entry deceleration temperatures. This limited the usefulness of the study for distinguishing between particles of asteroidal and cometary origin.

The COMET experiment

Abstract— The COMET program is a program for the collection of micron to submicron interplanetary dust particles in low Earth orbits. Since collection takes place as the Earth crosses a given meteor stream, the particles are mainly of cometary origin. The grain remnants, located at their impact positions on high purity metallic collectors, are analysed in the laboratory for chemical and isotopical identification. The COMET‐1 experiment took place in 1985 October, during encounter with the Draconid meteor stream, related to the Giacobini‐Zinner comet. The fluence of extraterrestrial grains that had impacted our detectors was ∼10x higher than the value of the mean meteroid fluence at ∼1AU, which suggests that most of the grains originated from the Giacobini‐Zinner comet. One of the most important results of their chemical analysis was that ∼90% of them are enriched in low Z elements (C and O have undoubedly been identified). They could contain a CHON phase similar to that observed in the close environment of Halley's nucleus. The first imagery of the grain remnants by field emission scanning electron microscopy suggests that they are very low density aggregates still present at the impacting positions which, in most cases, are very different from the impact craters observed for the same mean relative velocity for full grains of the same size. These results show that the COMET program has constituted an important step towards the analysis of cometary material and the understanding of the evolution of the early Solar System.

The gravitational sculpting of the Kuiper belt

Results are presented of numerical integrations over billion year time scales of the orbital evolution of more than one thousand test particles on initially low-inclination, low-eccentricity orbits within the proposed Kuiper belt beyond Neptune. Particles which eventually crossed Neptune's orbit often showed long periods (up to several billion years) of relatively low-eccentricity oscillations punctuated by a very rapid jump to Neptune-crossing eccentricity. This flux may be the ultimate source of present-day short-period comets. It is found here that there exists a correlation between Liapunov and crossing times in the Kuiper belt. None of the particles in the study with Liapunov time scales greater than about 1 Myr actually became a Neptune-crosser in 4 Gyr. An intricate structure to the region between 35 and 45 AU is found at the end of the billion year simulation. Implications for the origins of short-period comets and the detectability of objects currently in the Kuiper belt are discussed.

Properties of the Oort cloud and the origin of comets

We report the results of an N-body simulation with 100 stars in the solar neighbourhood and 400 Oort cloud bodies to determine the internal structure and stability of the Oort cloud and to show various physical processes revealing the formation of comets from the Oort cloud. Effects due to Jupiter and Saturn as well as local vertical oscillation of the Solar system within the galactic disc are also included. We follow the evolution of the Oort cloud and the stars for 1.6 × 108 yr, both with and without the planets. We reproduce the shape of the observed 1/a distribution for the comets. We determine the evolution of the number density of the cloud. The perturbation due to the planets causes fluctuation in the number density at both the inner and the outer edges of the Oort cloud. The stellar encounter also causes some fluctuation, mostly at the outer edge. In general, the Oort cloud appears to be very stable under stellar perturbations, with a half-life well beyond 3 million years. We obtain the energy transport rate to the clouds. This result agrees well with existing analytical results.

Book Review: Comets and Their Malevolence: The Origin of Comets

Book Review: Comets and Their Malevolence: The Origin of Comets

Possible cometary origin of heavy noble gases in the atmospheres of Venus, Earth and Mars

Models that trace the origin of noble gases in the atmospheres of the terrestrial planets (Venus, Earth and Mars) to the 'planetary component' in chondritic meteorites confront several problems. The 'missing' xenon in the atmospheres of Mars and Earth is one of the most obvious; this gas is not hidden or trapped in surface materials. On Venus, the absolute abundances of neon and argon per gram of rock are higher even than those in carbonaceous chondrites, whereas the relative abundances of argon and krypton are closer to solar than to chondritic values (there is only an upper limit on xenon). Pepin has developed a model that emphasizes hydrodynamic escape of early, massive hydrogen atmospheres to explain the abundances and isotope ratios of noble gases on all three planets. We have previously suggested that the unusual abundances of heavy noble gases on Venus might be explained by the impact of a low-temperature comet. Further consideration of the probable history of the martian atmosphere, the noble-gas data from the (Mars-derived) SNC meteorites and laboratory experiments on the trapping of noble gases in ice lead us to propose here that the noble gases in the atmospheres of all of the terrestrial planets are dominated by a mixture of an internal component and contribution from impacting icy planetesimals (comets). If true, this hypothesis illustrates the importance of impacts in determining the volatile inventories of these planets.

Extraction of helium from individual interplanetary dust particles by step‐heating

Abstract Fragments from 20 individual particles, collected in the Earth's stratosphere and believed to be interplanetary dust particles (IDPs), were obtained from NASA's Johnson Space Center collection and subjected to step‐heating to see if differences in the release pattern for 4He could be observed which might provide clues to the origin of the particles. Comparisons were made to the release pattern for 18 individual lunar surface grains heated in the same manner. Twelve of the IDP fragments contained an appreciable amount of 4He, 50 percent of which was released by the time the particles were heated to approximately 630 °C. For the 18 individual lunar grains the corresponding average temperature was 660 °C. The 3He/4He ratios found for these fragments agreed well with those found for deep Pacific magnetic fines believed to be of extraterrestrial origin, and were comparable to those which have been observed for the solar wind and lunar surface soil grains. Four of the IDP fragments contained appreciably less 4He, and this was released at a higher temperature. The remaining four fragments had too little 4He to permit a determination. From Flynn's analyses of the problem of the heating of IDPs in their descent in the atmosphere, the present results suggest that the parent IDPs of the 12 particles which contained an appreciable amount of 4He suffered very little heating in their descent and are likely of asteroidal origin, although one cannot rule out the possibility that at least some of them had a cometary origin and entered the earth's atmosphere at a grazing angle. Mineralogical and morphological studies on fragments companion to those used in the present investigation are under way. When these are completed, a more definite picture should emerge.

Paradoxes of the problem of the Tunguska meteorite

This paper presents basic data on the physics of the Tunguska explosion and the results of searches for matter from the Tunguska meteorite. Concepts that this phenomenon was complex in nature and had a high degree of complexity are substantiated. The author points out the difficulty of interpreting the Tunguska phenomenon within the framework of the existing scientific paradigm, including the currently most widely held hypothesis that the Tunguska meteorite had a cometary origin. A number of paradoxical circumstances pertaining to the trajectory of the Tunguska cosmic body, the geophysical consequences of the Tunguska disaster, and its attendant biological consequences in the blast region are pointed out. The need for development of alternative interpretations of the Tunguska phenomenon is addressed.

The Origins of Comets

Do we live in astronomically disappointing times? Are the skies duller than they've been in the past‐comets less spectacular, meteors less frequent, and the zodiacal lights dimmer? Certainly as an amateur astronomer I have been haunted by that impression. When the only comets in recent times have been West, Kohoutek, and Halley—hardly worth writing home about—one could certainly wonder at the interest that is apparent in treatments of these subjects in historical texts. Why were the ancients so fascinated, fearful, and focused on these faint celestial fuzzballs that can only be seen when carefully pointed out?

Origin of short period comets

Reasons for interest in the origin of short-period comets and the difficulties of computing their long-term dynamcal evolution are reviewed. The relative advantages of a source region in an extended ‘inner core’ of the Oort cloud or a compact ‘comet belt’ just beyond the planetary system are finely balanced, and it is premature to consider the problem solved. A complication is that some comets belonging to the Jupiter family may be part of a time-dependent system, possibly the remains of a giant comet such as Chiron which could have been part of the system ≈ 104 yr ago. The origin of short-period comets plays a pivotal role in many areas of solar system science: planet formation, the source of water (possibly life) on the terrestrial planets, the cratering record on the terrestrial planets and satellites of the outer planets, and the environmental impact posed by massive bodies and their decay products in the Earth's near-space environment.

On the Relationship Between Comet P/Machholz and the Quadrantid Meteor Stream

The Quadrantid meteor shower has been recognized for more than 150 years. The dynamics of the corresponding stream is peculiar due to the high orbital inclination and, for some particles, the closeness of the 2/1 mean motion resonance with Jupiter. It has been the subject of many investigations relating to the structure of the stream and its nodal retrogression as well as its long-term history and its likely cometary origin. Thus Hamid and Youssef (1963) found that the jovian secular perturbations lead to very large changes in the inclination and perihelion distance of typical stream particles with a period around 4000 yrs. From a more extensive study by Williamset al.(1979) it was obvious that this period is not unique but may vary considerably between different particles. Related to this behaviour is also the investigation by Froeschlé and Scholl (1982) who performed an extensive study of three-dimensional orbits at the 2/1 resonance. The orbits remain confined in the resonance zone and are stable in Hill's sense. Close encounters with Jupiter are avoided through the action of three main protection mechanisms:σlibration around 0,ωlibration aroud 90°, ande–ωcoupling, although most orbits exhibit large variations in inclination and eccentricity. Investigating in more detail a Quadrantid-like meteor stream, Froeschlé and Scholl (1986) also found a nonuniform nodal retrogression and an unusual progression. This behaviour causes a formation of arcs which was not found for other meteor streams in resonance with Jupiter – however, almost never having large inclinations.

Cometary origin of carbon and water on the terrestrial planets

An early high-temperature phase of the protosolar accretion disk is implied by at least three different telltales in chondrites and confirmed by peculirities in the dust grains of comet Halley. The existence of this high-temperature phase implies a large accretion rate hence a massive early disk. This clarifies the origin of the Kuiper Belt and of the Oort cloud, those two cometary populations of different symmetry that subsist today. Later, when the dust sedimented and was removed from the thermal equilibrium with the gas phase, a somewhat lower temperature of the disk explains the future planets' densities as well as the location beyond 2.6 AU of the carbonaceous chondrite chemistry. This lower temperature remains however large enough to require an exogenous origin for all carbon and all water now present in the Earth. The later orbital diffusion of planetesimals, which is required by protoplanetary growth, is needed to explain the origin of the terrestrial biosphere (atmosphere, oceans, carbonates and organic compounds) by a veneer mostly made of comets.

Cometary Origin of Carbon and Water on the Terrestrial Planets

The origin of carbon and water on the terrestrial planets is not trivial, because their presence seems to be excluded by the high temperatures requested by the accretion disk models (Morfill 1988) as well as by Lewis' (1973) adiabat that can explain the different densities of the planets. These high temperatures imply that all dust was outgassed and dehydrated and most carbon stored in CO, that is in the gas phase, before the separation of dust from gas, that occurred by dust sedimentation to the mid-plane of the accretion disk. Thermochemical equilibrium was easily reached, since chemical kinetics (Lewis &amp; Prinn 1980) had time constants shorter than the time needed to agglomerate the first planetesimals (Weidenschilling 1988).

Observations of solar wind ion charge exchange in the Comet Halley coma

Giotto Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) observations of solar wind ions show charge exchange effects and solar wind compositional changes in the coma of comet Halley. As the comet was approached, the He(++) to proton density ratio increased until about 1 hour before closest approach after which time it decreased. Abrupt increases in this ratio were also observed in the beginning and near the end of the so-called Mystery Region (8.6 - 5.5(10)(exp 5) km from the comet along the spacecraft trajectory). These abrupt increases in the density ratio were well correlated with enhanced fluxes of keV electrons as measured by the Giotto plasma electron spectrometer. The general increase and then decrease of the He(++) to proton density ratio is quantitatively consistent with a combination of the addition of protons of cometary origin to the plasma and loss of plasma through charge exchange of protons and He(++). In general agreement with the solar wind proton and He(++) observations, solar wind oxygen and carbon ions were observed to charge exchange from higher to lower charge states with decreasing distance to the comet. The more abrupt increases in the He(++) to proton and the He(++) to O(6+) density ratios in the mystery region require a change in the solar wind ion composition in this region while the correlation with energetic electrons indicates processes associated with the comet.

Cometary origin of carbon, nitrogen and water on the Earth

Two independent assumptions are substantiated; firstly, that the Earth accreted from dust particles that were hot enough not to contain any volatiles; secondly, that after the accretion was finished, all the volatiles of the biosphere, including the atmosphere and the oceans, were brought by a cometary bombardment.

A recent cometary origin for Saturn's rings?

Gravitational torques and erosion by micrometeoroids may cause rapid evolution of Saturn's rings, possibly suggesting formation of the rings within the last 10 8 years. Optically thin and/or narrow rings at Jupiter, Uranus, and Neptune may form repeatedly by disruption of inner satellites. However, present-day creation of Saturn's more massive rings in this way is unlikely. An alternative scenario for ring origin is tidal disruption of a large comet, such as Chiron, passing within Saturn's Roche radius. Subsequent collisions among the fragments lead to a flat, equatorial ring, while collisions with inner satellites produce a Population II-like set of craters on the moons. Assuming a nominal flux of Saturn-crossing bodies, 10–100 close passages occur in 4.5 Gyr. Most close passages leave no bound debris, so that approximately one ring capture event occurs in the age of the solar system. This rate is comparable to the expected frequency of disruption of ring parent satellites by cometary bombardment. These results imply that the rings result from a recent unlikely tidal or collisional event, or that the arguments for rapid evolution of Saturn's rings are erroneous and the rings are primordial. However, the flux of Saturn-crossers is highly uncertain; if the flux is much higher, the rings are probably the recent result of tidal disruption. Ongoing searches for outer Solar System planetesimals and future data from Cassini will help determine the rings' age and origin.

Problems of origin of asteroids and comets

Abstract— Various hypotheses of the origin of asteroids and comets are briefly discussed. Interaction of planetesimals in the asteroid zone (AZ) with the gas, their perturbations by proto‐Jupiter, and sweeping them out by more massive Jupiter zone bodies when they penetrated the AZ are considered. If the gas was turbulent, it could prevent a settling of dust particles to the equatorial plane of the disk and formation of dust condensations due to gravitational instability. Then particles grew by sticking upon collision. Gas moved radially due to turbulent viscosity and its dissipation. Small particles moved more‐or‐less together with the gas. As a result of gas drag, larger particles and bodies moved relative to the gas in the direction of increasing gas pressure. Gas would remove much of the solid material from the AZ if most bodies larger than a few km disintegrated by collisions into fragments smaller than a few tens of meters. Most of these fragments would then move into the Martian zone, and the small mass of Mars would have no explanation.Resonant perturbations of asteroids by Jupiter are discussed. In the model of a small mass disk they could scan through the asteroid belt due to changes in Jupiter's distance from the Sun that occurred when this planet accreted the gas and ejected the bodies from the solar system. Such a scanning considerably accelerated the removal of asteroids from the AZ. Massive Jupiter zone bodies with large orbital eccentricities that crossed the AZ were probably efficient at sweeping out bodies. Larger bodies increased the random velocities of the remaining asteroids at close encounters to the present values ∼ 5 km/s.Restrictions on the runaway growth of giant planets, on the relative velocities of bodies and the disk surface density that follow from the consideration of the origin of the asteroid belt and the cometary cloud are considered.