Showing papers in "Earth Moon and Planets in 1996"

The Delivery of Organic Matter from Asteroids and Comets to the Early Surface of Mars

Abstract: Carbon delivered to the Earth by interplanetary dust particles may have been an important source of pre-biotic organic matter (Anders, 1989). Interplanetary dust is shown to deliver an order-of-magnitude higher surface concentration of carbon onto Mars than onto Earth, suggesting interplanetary dust may be an important source of carbon on Mars as well.

Terrestrial impact craters: Their spatial and temporal distribution and impacting bodies

Abstract: The terrestrial impact record contains currently ~145 structures and includes the morphological crater types observed on the other terrestrial planets. It has, however, been severely modified by terrestrial geologic processes and is biased towards young (≤200 Ma) and large (≥2 0 km) impact structures on relatively well-studied cratonic areas. Nevertheless, the ground-truth data available from terrestrial impact structures have provided important constraints for the current understanding of cratering processes. If the known sample of impact structures is restricted to a subsample in which it is believed that all structures ≥20 km in diameter (D) have been discovered, the estimated terrestrial cratering rate is 5.5±2.7 x 10-15km-2a-1 for D ≥20 km. This rate estimate is equivalent to that based on astronomical observations of Earth-crossing bodies. These rates are a factor of two higher, however, than the estimated post-mare cratering rate on the moon but the large uncertainties preclude definitive conclusions as to the significance of this observation. Statements regarding a periodicity in the terrestrial cratering record based on time-series analyses of crater ages are considered unjustified, based on statistical arguments and the large uncertainties attached to many crater age estimates. Trace element and isotopic analyses of generally siderophile group elements in impact lithologies, particularly impact melt rocks, have provided the basis for the identification of impacting body compositions at a number of structures. These range from meteoritic class, e.g., C-l chondrite, to tentative identifications, e.g., stone?, depending on the quality and quantity of analytical data. The majority of the identifications indicate chondritic impacting bodies, particularly with respect to the larger impact structures. This may indicate an increasing role for cometary impacts at larger diameters; although, the data base is limited and some identifications are equivocal. To realize the full potential of the terrestrial impact record to constrain the character of the impact flux, it will be necessary to undertake additional and systematic isotopic and trace element analyses of impact lithologies at well-characterized terrestrial impact structures.

Rayleigh waves in an orthotropic thermoelastic medium under gravity field and initial stress

Abstract: The aim of the present paper is to investigate the influence both of gravity field and initial stress on the propagation of Rayleigh waves in an orthotropic thermoelastic medium subject to certain boundary conditions. We suppose that the body is under initial stress alonqx 1-direction and incremental thermal stresses. The wave velocity equation has been obtained. Many special cases and comparison with the previous results have been studied.

Light scattering by Gaussian random particles

Abstract: We introduce multivariate lognormal statistics to describe the shapes of small particles, and compute scattering phase matrices in the ray optics approximation. The results help us understand light scattering by solar system dust particles, and thereby constrain the physical properties of, for example, asteroid regoliths and cometary comae. The present stochastic geometry could turn useful in modeling the shapes of asteroids.

The main belt as a source of near-Earth asteroids

Abstract: We investigate the flux of main-belt asteroid fragments into resonant orbits converting them into near-Earth asteroids (NEAs), and the variability of this flux due to chance inter asteroid al collisions. A numerical model is used, based on collision al physics consistent with the results of laboratory impact experiments. The assumed main-belt asteroid size distribution is derived from that of known asteroids extrapolated down to sizes of ≈ 40 cm, modified in such a way to yield a quasi-stationary fragment production rate over times ≈ 100 Myr. The results show that the asteroid belt can supply a few hundred km-sized NEAs per year, well enough to sustain the current population of such bodies. On the other hand, if our collision al physics is correct, the number of existing 10-km objects implies that these objects either have very long-lived orbits, or must come from a different source (i.e., comets). Our model predicts that the fragments supplied from the asteroid belt have initially a power-law size distribution somewhat steeper than the observed one, suggesting preferential removal of small objects. The component of the NEA population with dynamical lifetimes shorter than or of the order of 1 Myr can vary by a factor reaching up to a few tens, due to single large-scale collisions in the main belt; these fluctuations are enhanced for smaller bodies and faster evolutionary time scales. As a consequence, the Earth’s cratering rate can also change by about an order of magnitude over the 0.1 to 1 Myr time scales. Despite these sporadic spikes, when averaged over times of 10 Myr or longer the fluctuations are unlikely to exceed a factor two.

From Comets to Asteroids: When Hairy Stars Go Bald

Abstract: We discuss the essential differences between comets and asteroids. Ironically, with the exception of the rocky asteroids in the inner solar system, most of the objects classified as asteroids at and beyond Jupiter’s orbit are likely to conceal buried volatiles, and thus are more usefully considered as comets.

Changes in meteorological variables in coronel oviedo, paraguay, during the total solar eclipse of 3 november 1994

Abstract: Solar global radiation, air temperature, relative humidity, and wind speed were measured in Coronel Oviedo, Paraguay, within the path of totality of the eclipse of 3 November 1994. Global radiation decreased gradually as the sun was being eclipsed and became negligible during the totality, then increased to their normal values. Surface air temperature decreased significantly, about 3 ‡C, with the lowest value occurring about 7 minutes after totality. Relative humidity, because of its dependence on temperature, increased as a consequence of the temperature decrease. Surface wind speed decreased gradually during the eclipse, as a result of the cooling and stabilization of the atmosphere. Although atmospheric pressure was also measured, it was not possible to record pressure changes clearly associated to the eclipse, since the pressure sensor was only able to detect pressure variations of at least 1 hPa.

The “Shiva Hypothesis”: Impacts, Mass Extinctions, and the Galaxy

Abstract: The “Shiva Hypothesis”, in which recurrent, cyclical mass extinctions of life on Earth result from impacts of comets or asteroids, provides a possible unification of important processes in astrophysics, planetary geology, and the history of life. Collisions with Earth-crossing asteroids and comets ≥ a few km in diameter are calculated to produce widespread environmental disasters (dust clouds, wildfires), and occur with the proper frequency to account for the record of five major mass extinctions (from ≥ 108 Mt TNT impacts) and ~ 20 minor mass extinctions (from 107-108 Mt impacts) recorded in the past 540 million years. Recent studies of a number of extinctions show evidence of severe environmental disturbances and mass mortality consistent with the expected after-effects (dust clouds, wildfires) of catastrophic impacts. At least six cases of features generally considered diagnostic of large impacts (e.g., large impact craters, layers with liigh platinum-group elements, shock-related minerals, and/or microtektites) are known at or close to extinction-event boundaries. Six additional cases of elevated iridium levels at or near extinction boundaries are of the amplitude that might be expected from collision of relatively low-Ir objects such as comets. The records of cratering and mass extinction show a correlation, and might be explained by a combination of periodic and stochastic impactors. The mass extinction record shows evidence for a periodic component of about 26 to 30 Myr, and an ~ 30 Myr periodic component has been detected in impact craters by some workers, with recent pulses of impacts in the last 2-3 million years, and at ~ 35, 65, and 95 million years ago. A cyclical astronomical pacemaker for such pulses of impacts may involve the motions of the Earth through the Milky Way Galaxy. As the Solar System revolves around the galactic center, it also oscillates up and down through the plane of the disk-shaped galaxy with a halfcycle ~ 30±3 Myr. This cycle should lead to quasi-periodic encounters with interstellar clouds, and periodic variations in the galactic tidal force with maxima at times of plane crossing. This “galactic carrousel” effect may provide a viable perturber of the Oort Cloud comets, producing periodic showers of comets in the inner Solar System. These impact pulses, along with stochastic impactors, may represent the major punctuations in earth history.

The impact cratering record of Fennoscandia

Abstract: The current database of craterform structures in Fennoscandia contains 22 structures of impact origin and about fifty other structures which lack sufficient evidence for impact. The discovery rate of new structures has been one or two per year during the past ten years. The proven impact structures are located in southern Fennoscandia and the majority have been found in Proterozoic target rocks. The age of the structures varies from prehistoric to ≤ 1 000 Ma and their diameters (D) from 0.04 km to 55 km. Nine of the structures contain impact melt. A characteristic feature of the Fennoscandian impact record is a relatively large number of small (≤ 5 km) but old (≥ 200 Ma) structures: this is a result of success of geophysical methods to discover small but old impact structures in an eroded shield covered with relatively thin overburden. Some of the large circular structures in satellite images and/or in geophysical maps may represent deeply eroded scars of very old impacts, but due to the lack of shock metamorphic features, impact-generated rocks or identified ejecta layers, they cannot yet be classified as impact sites. Two huge structures are proposed here as possible impact sites on the basis of circular satellite images and distinct geophysical anomalies: the Lycksele structure in northern Sweden (D ~ 120 km, see also Witschard, 1984) and the Valga structure in Latvia/Estonia (D ~ 180 km). However, endogeneous explanations, like buried granites, basement domings, or faultbounded blocks are also possible for these structures. Hints, such as distal ejecta layers or impact produced breccia dykes, of an Archaean or Early Proterozoic impact structure have not been found in Fennoscandia so far. New ways of searching for these structures are proposed with particular emphasis on high-resolution integrated geophysical methods. The impact cratering rate in Fennoscandia is ~ 2.0 • 10-14 km-2a-1 (for craters with D ≥ 3 km) corresponding to about two events per every 100 Ma for the last 700 Ma. Due to erosion, this is a minimal estimate but is higher than the global rate probably due to strong research activity for finding impact structures in Fennoscandia.

Star passages through the Oort cloud

Abstract: Stars passing through the Oort cloud eject comets to interstellar space and initiate showers of comets into the planetary region. Monte Carlo simulations of such passages are performed on a representative distribution of cometary orbits. Ejected comets generally lie along a narrow tunnel “drilled” by the star through the cloud. However, shower comets come from the entire cloud, and do not give a strong signature of the star’s passage, except in the inverse semimajor axis distribution for the shower comets. The planetary system is likely not experiencing a cometary shower at this time.

Dynamical Evolution of NEAs: Close Encounters, Secular Perturbations and Resonances

Abstract: We discuss the main mechanisms affecting the dynamical evolution of Near- Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some “fast-track” dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.

Propagational features of higher harmonic tweeks at low latitudes

Abstract: Higher harmonic tweeks observed for the first time at the low latitude station Varanasi (geomag. lat. 14‡ 55′ N) are reported. The analysis of data shows that higher harmonic tweeks are usually not associated with whistlers and occur when the ionization in the lower ionosphere would not increases with height. The Earth-ionosphere waveguide dispersion features play an important role in the propagation of broad band lightning generated signals and their occasional observation as higher harmonic tweeks. It is shown that the conductivity of ground and sea mixed path, forming the lower surface of the waveguide, provide an estimate of the travelled distances of higher harmonic tweeks in the waveguide. The attenuation factors are computed which shows that as the harmonic number increases their probability of observation decreases. The attenuation increases as the frequencies approach the cut-off frequencies and also as the layer height falls. The tweek activity is found to increase during periods of magnetic disturbances.

Analytical extension of lunar libration tables

Abstract: Tables of lunar physical libration defining the analytical dependence upon the parameters of the lunar gravitational field are presented. The tables are obtained on the framework of the “main problem” in lunar libration by integration of the Hamilton equations reduced to the harmonic oscillator equations. The variables of physical libration have been obtained in the form of Poisson series. The distinguishing feature of the tables is that these series are the analytical extension of semianalytical solution computed for a number of dynamical parameters LURE2. A comparison with the Eckhardt's solution is briefly presented. The previously revealed disagreement of the mean inclination of lunar equator to ecliptic with that in Eckhardt's “solution 500” has been maintained.

Unusual comets (?) as observed from the Hubble Space Telescope

Abstract: In separate projects, the Hubble Space Telescope has been used to assess the nature of 3 unusual objects: Chiron, Pholus and P/Shoemaker-Levy 9 This paper will compare these objects and discuss how the unique capabilities of the HST may be used to address the issue of cometary activity in each Chiron, which has exhibited obvious cometary characteristics for several years, might have a bound dust coma that is unresolvable from the ground In an attempt to directly observe this bound coma, we have obtained a series of images of Chiron with the HST Planetary Camera Inner coma structure out to 0″2 has been detected From these observations we infer a low bulk nucleus density for Chiron Both HST and ground-based images of 5145 Pholus have been obtained to search for evidence of activity The ground- based data give the most sensitive limits; however, it is shown that the WFPC-2 on HST can give limits 2-3 orders of magnitude more sensitive than conventional ground-based limits Finally, as part of a collaborative effort, we have been obtaining HST observations of SL9 in order to determine the fragment sizes and to assess their nature [ie, cometary vs asteroid) Both ground-based observations from the UH 22m telescope on Mauna Kea and HST observations show that the near-nucleus dust is redder than the sun While FOS spectra did not detect OH emission, the WFPC-2 HST data show that the inner coma remained very circular from July 1993 up until 2 weeks prior to impact, implying continued production of dust

Long-Term Behavior of the Motion of Pluto over 5.5 Billion Years

Abstract: The motion of Pluto is said to be chaotic in the sense that the maximum Lyapunov exponent is positive: the Lyapunov time (the inverse of the Lyapunov exponent) is about 20 million years. So far the longest integration up to now, over 845 million years (42 Lyapunov times), does not show any indication of a gross instability in the motion of Pluto. We carried out the numerical integration of Pluto over the age of the solar system (5.5 billion years ″ 280 Lyapunov times). This integration also did not give any indication of chaotic evolution of Pluto. The divergences of Keplerian elements of a nearby trajectory at first grow linearly with the time and then start to increase exponentially. The exponential divergences stop at about 420 million years. The divergences in the semi-major axis and the mean anomaly (equivalently the longitude and the distance) saturate. The divergences of the other four elements, the eccentricity, the inclination, the argument of perihelion, and the longitude of node still grow slowly after the stop of the exponential increase and finally saturate.

Triaxiality of satellites and small bodies in the Solar System

Abstract: The triaxial figures are very common shape of most of planetary satellites as well as of smaell bodies as asteroids. There are 21 satellites in the Solar System triaxial figures of which were detected in situ evidently (Davies et al., 1995). However, the total number of triaxiaxial satellites in the Solar System should be in fact larger. In this paper the general theory of triaxiality due to tidal forces is discussed in regard to the very recent numerical data. Since they orbit synchronously, as a rule: their orbital periods are equal to the rotational periods, the tidal forces may be responsible for their triaxial figures. On the other hand the origin of triaxiality of asteroids due to another process and the of their figures cannot be axplained by the tidal effects.

Encounters of the Sun with Nearby Stars in the Past and Future

Abstract: The relative space motions of the Sun and nearby stars are considered. The coordinates and velocities of the stars are taken from the Catalogue of Nearby Stars by Gliese and Jahreiss (1991). The minimum space separation between the Sun and every star as well as the corresponding moment of time are calculated by two ways. Firstly, the straight line motions are considered. Secondly, the effect of the Galaxy potential is taken into account. The Galaxy model proposed by Kutuzov and Ossipkov (1989) is used. Twenty five stars approaching the Sun closer than two parsecs are selected. The effects of the uncertainties in the observational data are studied. The influence of the encounters to the Oort cloud is discussed.

Video observations, atmospheric path, orbit and fragmentation record of the fall of the Peekskill meteorite

Abstract: Large Near-Earth-Asteroids have played a role in modifying the character of the surface geology of the Earth over long time scales through impacts. Recent modeling of the disruption of large meteoroids during atmospheric flight has emphasized the dramatic effects that smaller objects may also have on the Earth’s surface. However, comparison of these models with observations has not been possible until now. Peekskill is only the fourth meteorite to have been recovered for which detailed and precise data exist on the meteoroid atmospheric trajectory and orbit. Consequently, there are few constraints on the position of meteorites in the solar system before impact on Earth. In this paper, the preliminary analysis based on 4 from all 15 video recordings of the fireball of October 9, 1992 which resulted in the fall of a 12.4 kg ordinary chondrite (H6 monomict breccia) in Peekskill, New York, will be given. Preliminary computations revealed that the Peekskill fireball was an Earth-grazing event, the third such case with precise data available. The body with an initial mass of the order of 104 kg was in a pre-collision orbit with a = 1.5 AU, an aphelion of slightly over 2 AU and an inclination of 5°. The no-atmosphere geocentric trajectory would have lead to a perigee of 22 km above the Earth’s surface, but the body never reached this point due to tremendous fragmentation and other forms of ablation. The dark flight of the recovered meteorite started from a height of 30 km, when the velocity dropped below 3 km/s, and the body continued 50 km more without ablation, until it hit a parked car in Peekskill, New York with a velocity of about 80 m/s. Our observations are the first video records of a bright fireball and the first motion pictures of a fireball with an associated meteorite fall.

Are Cratering and Probably Related Geological Records Periodic

Abstract: The controversial topic of periodicity in geological records in relation to astronomical modeling is reviewed. Although impact cratering, frequency distribution of geomagnetic reversals, and mass extinction of fauna yield periods when certain tests are applied, none of them can be regarded significant in the sense of mathematical statistics. The first two records yield periods of 30 Myr, while the faunal-extinction record yields a period of ~ 26 — 27 Myr. It seems that although catastrophes in the form of large impacts trigger mass extinctions, certain geophysical or geological conditions need be satisfied for mass extinctions to be realized. One should not expect to find an indisputable periodicity in cratering record because random impacts by asteroids are dominant. Thus, the earthcrossing cometary flux modulated by the galactic tidal force appears consistent with the weak detected periodicity.

On the importance of dust in cometary nuclei

Abstract: The icy conglomerate model introduced by Whipple more than 40 years ago has been widely accepted in cometary science because it is able to describe numerous cometary phenomena. In this model comets are described as a conglomerate of ices and dust where the ices represent the major component. However, some recent observations seem to favour dust rich comets. The purpose of this paper is to summarize the observational facts supporting the dominance of refractories in comets and to discuss the consequences of a dust dominated nucleus for cometary physics.

What can meteoroid streams tell us about the ejection velocities of dust from comets

Abstract: The parent bodies of a number of major meteoroid streams are not in doubt and the orbits of these parents are also well determined. For these major streams individual orbits for a significant number of member meteoroids have also been determined. There is a significant spread in the determined values of the semi-major axis of individual meteoroids in a particular stream and this paper assumes that this spread is caused primarily by a variation in the ejection process and draws conclusions regarding the value of the ejection velocities from this.

Determination of some physical parameters of the Moon with Lunar Laser Ranging data

Abstract: Information about the structure of lunar interior and evolution could be obtained from measurements of lunar free librations, gravitational field, dissipation etc.. In this paper the precision of determining free librations with Lunar Laser Ranging (LLR) data are estimated. Using the observing data from four telescopes for eighteen years the amplitudes and phases of free librations, the moments of inertia ratio of The Moon were determined.

The Population of Near-Earth Objects Discovered by Spacewatch

Abstract: In the past three years the Spacewatch program at the University of Arizona’s Lunar and Planetary Laboratory has discovered ~45% of the new Earth Approaching asteroids and found evidence for an unheralded population of small (~10m) objects in the inner solar system. This success is due to the automated Moving Object Detection Program (MODP) which searches successive scans over the same region for objects showing consistent motion. Highlights of recent discoveries, an update on research, and the development and potential of the new 1.8m Spacewatch facility will be discussed.

Impacts into oceans and seas

Abstract: Impacts of cosmic bodies into oceans and seas lead to the formation of very high waves. Numerical simulations of 3-km and 1-km comets impacting into a 4 km depth ocean with a velocity of 20 km/sec have been conducted. For a 1-km body, depth of the interim crater in the sea bed is about 8 km below ocean level, and the height of the water wave is 10 m at a distance of 2000 km from the impact point. As the water wave runs into shallows, a huge tsunami hits the coast. The height of the wave strongly depends on the coastal and sea bed topography. If the impact occurred near the shore, the huge mass of water strikes the cliffs and the near shore mountain ridges and can cause displacement of the rocks, initiate landslides, and change the relief. Thus, impact into oceans and seas is an important geological factor. Cosmic bodies of small sizes are disrupted by aerodynamic forces. Fragments of a 100- m radius comet striking the water surface create an unstable cavity in the water of about 1 km radius. Its collapse also creates tsunami. A simple estimate has been made using the light curves from recent atmosphere explosions detected by satellites. The results of our assessment of the characteristics of meteoroids which caused these intense light flashes suggests that fragments of a 25-m stony body with initial impact velocity 15 to 20 km/sec will hit the surface. For a 75-m iron body striking the sea with a depth of 600 m, the height of the wave is 10 m at 200-300 km distance from the impact.

Origin and bulk chemical composition of the Galilean satellites and the primitive atmosphere of Jupiter: A pre-Galileo analysis

Abstract: A theory for the origin and bulk chemical composition of the Galilean satellites is presented — to coincide with the start of the 2-year orbital tour of this satellite system by the Galileo Orbiter. The theory is based on the author's modern Laplacian theory of solar system origin (Prentice 1978a). The nub of the work reported here is that the Jupiter system is indeed a miniature planetary system that formed by much the same physical and chemical processes that were responsible for the condensation of the sun's own family of planets. In particular, a phenomenon of supersonic turbulent convection which I claim caused the proto-solar cloud to rid excess spin angular momentum, by shedding a concentric family of orbiting gas rings at the present planetary orbits, may also have operated with similar effect within the proto-Jovian cloud. Several predictions are made for the bulk chemical composition and physical structure of the icy Galilean satellites which, it is hoped, can be tested by the Galileo Orbiter. The mean density of Callisto is consistent with that of a chemically homogeneous body consisting of about 50% rock, 45% water ice, and 5% ammonia ice, incorporated as the hydrate NH3·H2O. Such a higher-than-solar mass abundance ratio of rock to ice arises naturally within the proto-Jovian cloud since (i) only 34% of the available H2O vapor within the gas ring shed by the proto-solar cloud at Jupiter's orbit was condensed in solid form, and (ii) gravitational sedimentation of solids onto the mean orbit of the proto-solar gas ring leads to an enhancement in the heavy element fraction of the captured primitive Jovian atmosphere. All in all, I predict Jupiter's primitive atmosphere to be enhanced by a factor ζen ≈ 2 in its rock mass fraction (including S) and by a factor ≈ 1.3 in its water content, relative to solar abundances. NH3 and CH44 are present in almost solar proportions. Initially, Ganymede consisted of a chemically uniform mixture of rock and water ice in the proportions 0.524 : 0.476. The observed mean density of this satellite, however, lies midway between the mean densities expected for homogeneous and fully differentiated rock/ice bodies. The calculations presented here suggest that this body is about half-differentiated. I predict that the Galileo Orbiter will find the mean axial moment-of-inertia factor of Ganymede to be 0.35 ± 0.01. The circum-Jovian gas ring from which Europa condensed had a temperature of 302 K and a mean orbit gas pressure of 2.8 bar. Initially, this satellite consisted of a uniform mix of hydrated rocks, of which brucite Mg(OH)2 was the principal constituent. The observed mean density of Europa coincides with that expected for this mix, provided that its 9.4% native H2O content is now fractionated from the rock and resides at the satellite surface, forming a frozen mantle some 155 km thick. Regretfully, the mean density of Io cannot be matched by the solid composition reported here. Perhaps this satellite has a molten interior.

Magnetic properties of asteroids from meteorite data — Implications for magnetic anomaly detections

Abstract: Magnetic measurements of meteorites suggest that small bodies (e.g. asteroids) in the Solar System have small but distinct magnetic fields produced by the bulk remanent magnetisation (NRM) of the body. Here we report calculations of magnetic fields of small bodies, assuming that they can be approximated as homogeneously magnetised spheres with dipole moments derived from NRM data on known meteorites. The magnetic fields are compared with the field of the asteroid 951 Gaspra measured by spacecraft Galileo in 1991 (Kivelson et al. , 1993). The result of this comparison suggests that the field of Gaspra could be caused by an L-, H- or E-chondritic or a pallasite body. The spectral reflectance data on Gaspra suggest, however, that it is a basaltic achondrite. The problem can be resolved if Gaspra is a differentiated body, its surface material being closer to that of basaltic achondrites, and the bulk closer to ordinary chondrites or pallasites. We also present magnetic anomaly profiles along the surface of Mars such as would be measured with a magnetometer installed on a Rover-type vehicle by assuming that the main sources of the surface anomalies are the NRMs of the boulders on the Martian surface. The NRM values are taken from the data measured on SNC meteorites. The results suggest large oscillations in magnetic field intensity at the Martian surface.

Comets, meteorites and atmospheres.

Abstract: The relatively low value of Xe/Kr in the atmospheres of Earth and Mars seems to rule out meteorites as the major carriers of noble gases to the inner planets. Laboratory experiments on the trapping of gases in ice forming at low temperatures suggest that comets may be a better choice. It is then possible to develop a model for the origin of inner planet atmospheres based on volatiles delivered by comets added to volatiles originally trapped in planetary rocks. The model will be tested by results from the Galileo Entry Probe.

Orbits of Short Period Comets Captured from the Oort Cloud

Abstract: Oort cloud comets occasionally obtain orbits which take them through the planetary region. The perturbations by the planets are likely to change the orbit of the comet. We model this process by using a Monte Carlo method and cross sections for orbital changes, i.e. changes in energy, inclination and perihelion distance, in a single planet-comet encounter. The influence of all major planets is considered. We study the distributions of orbital parameters of observable comets, i.e. those which have perihelion distance smaller than a given value. We find that enough comets are captured from the Oort cloud in order to explain the present populations of short period comets. The median value of cos i for the Jupiter family is 0.985 while it is 0.27 for the Halley types. The results may explain the orbital features of short period comets, assuming that the active lifetime of a comet is not much greater than 400 orbital revolutions.

Interpretation of HMC images by a combined thermal and gasdynamic model

Abstract: Images of comet Halley's nucleus taken by the HMC camera during the GIOT-TO encounter in 1986 show that a major part of the total dust production is localized in a few active areas which are the sources of gas-dust jets. The global dust distribution in the inner coma is dominated by two main jets roughly directed to the sun. A combination of a 1D thermal nucleus model with an axisymmetric continuum model of the jet outflow was used to investigate the properties of the inner coma. Detailed investigations show that the characteristics of the observed jets can be reproduced by outgassing from free sublimating active areas of a few km in diameter, a dust to gas ratio of 1–2.5 and a size distribution dominated by the larger grains. It is further shown that most of the observational constraints provided by the HMC data can be met simultaneously by a model of three jets superimposed on a weak background.

On the Existence of Small Comets and Their Interactions with Planets

Abstract: Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size–distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.