Asteroid Size Research Articles

The asteroids as outcomes of catastrophic collisions

The role of catastrophic collisions in the evolution of the asteroids is discussed in detail, employing extrapolations of experimental results on the outcomrs of high-velocity impacts. We determine the range of the probable largest collision for target asteroids of different sizes during the solar system's lifetime, and we conclude that all the asteroids have undergone collisional events capable of overcoming the material's solid-state cohesion. Such events do not lead inescapably to complete disruption of the targets, because (i) for a previously unfractured target, experiments show that fragments of significant size can survive breakup, depending on the energy and geometry of the collision; (ii) self-gravitation can easily cause a reaccumulation of fragments for targets exceeding a critical size, which seems to be of the order of 100 km. In the intermediate diameter range 100⪅ D ⪅300 km, where formation of gravitationally bound “rubble piles” is frequent, the transfer of angular momentum can be large enough to produce objects with triaxial equilibrium shapes (Jacobi ellipsoids) or to cause fission into binary systems. In the same size range, low-velocity escape of collisional fragments can also occur, leading to the formation of dynamical families. Asteroids smaller than ∼100 km are mostly multigeneration fragments, while for D⪆300 km the collisional process produces nearly spheroidal objects covered by megaregoliths; whether their rotation is “primordial” or collisionally generated depends critically on the past flux of colliders. The complex and size-dependent phenomenology predicted by the theory compares satisfactorily with the observational evidence, as derived both by a classification of asteroids in terms of their size, spin rate, and lightcurve amplitude, and by a comparison between the rotational properties of family and nonfamily asteroids. The fundamental result of this investigation is that almost all asteroids are outcomes of catastrophic collisions, and that these events cause either complete fragmentation of the target bodies or, at least, drastic readjustments of their internal structure, shape, and spin rate.

Rotational properties of asteroids: Correlations and selection effects

Rotational data on 321 asteroids observed as of late 1978 are analyzed. Selection effects within the sample are discussed and used to define a data set consisting of 134 main-belt, nonfamily asteroids having reliably determined periods and amplitudes based on photoelectric observations. In contrast to A. W. Harris and J. A. Burns (1979, Icarus 40, 115–144) we found no significant correlation between rotational properties and compositional type. Smaller asteroids have a greater range of rotational amplitudes than the largest asteroids but are not, on the average, appreciably more elongated. While no definite relationship between asteroid size and rotation rate was found the distribution is not random. The largest asteroids have rotation periods near 7 hr compared with 10 hr for the smaller. A group of large, rapidly rotating, high-amplitude asteroids is recognized. A pronounced change in rotational properties occurs near this size range (diam = 200 ± 50 km) which also corresponds to the size at which a change of slope occurs in the size frequency distribution. We believe this size range represents a transition region between very large, rapidly rotating, low-amplitude (primordial?) objects and smaller ones having a considerably greater range of periods and amplitudes. Asteroids in this transition size range display an increase in rotational amplitude with increasing spin rate; other than this, however, there is no correlation between period and amplitude. The region of low spatial density in the asteroid belt centered near 2.9 AU and isolated from the inner and outer belt by the 2:5 and 3:7 commensurabilities is shown to be a region in which non- C or - S asteroids are overrepresented and which have marginally higher rotational amplitudes than those located in more dense regions. We attribute disagreements between our results and other studies of this type to the inclusion of non-main-belt asteroids and photographic data in the earlier analyses.

Asteroid rotation: I. Tabulation and analysis of rates, pole positions and shapes

Data are presented for the 182 asteroids whose rotational properties are available in the literature. Plots are provided for the asteroid rotational frequency f and lightcurve amplitude Δm versus asteroid size; the latter is determined using standard methods if data are available but otherwise is estimated from asteroid albedos, selected depending on taxonomic type or orbital position. A linear least-squares fit to all the data shows that f increases with decreasing size, confirming McAdoo and Burns' (1973) result; this is demonstrated to be primarily caused by relatively more small non-C than C asteroids in our sample, coupled with a slower mean rotation rate for C asteroids ( P ≈ 11 hr) than non-C asteroids ( P ≈ 9 hr). In terms of the collisional theory of Harris (1979a), this means that the C's are less dense than the other minor planets. Any slight tendency for smaller asteroids to spin faster, even within a taxonomic type, could be due to selection effects; our data are not extensive enough to determine whether the very smallest (⪅ 10-km diameter) spin especially fast. The minor planets of our survey become more irregular at smaller sizes, disputing the conclusions of Bowell (1977b), Degewij (1977), and Degewij et al. (1978), based on other, perhaps more complete, data; selection effects may account for this disagreement. Shapes do not appear to depend on taxonomic type. The dispersion of asteroid rotation rates from the mean is found to be in excellent agreement with a three-dimensional Maxwellian distribution, such as would be developed in a collisionally evolved system. The rotation axes, therefore, appear to be randomly oriented in space. Rotation pole positions are also tabulated and calculated to likely be constant in space over the extent of past observation. Observers are encouraged to measure the rotational properties of faint objects and asteroids of unusual taxonomic types, and to carry out long-time studies of asteroids which over short periods do not seem to vary.

Primordial metamorphism of asteroids via electrical induction in A T Tauri-like solar wind

Simple evolutionary models of asteroids of various sizes and solar distances have been constructed assuming unipolar electrical induction heating due to passage of the Sun through a T Tauri phase with an increased magnetic field. Typical T Tauri conditions and an elementary solar wind model were used to calculate induced currents in modlels assuming electrical conductivities appropriate for carbonaceous material. Two restrictions with opposite dependence upon radius dominate the results. The electrical insulating tendency of a cold surface favors heating of larger bodies. The current-limiting backpressure of the induced magnetic field favors heating of smaller bodies. Thus it is found that maximum heating, in some cases sufficient for melting, occurs for model asteroids at the inner edge of the belt and with (model-dependent) radii from 25 to 250 km. This effect, if operant, would have produced a primordial distribution of metamorphosed asteroids primarily occurring at small solar distance and intermediate size. The observational evidence for such a distribution is unclear because the primordial distribution has likely been considerably modified by collisions, particularly at smaller sizes. There does seem to be some consistency with the model in the distribution of the largest asteroids, though data are sparse. In particular, this model seems relevant to the well-known dichotomy between Ceres and Vesta.

4. On the Early Thermal History of Chondritic Asteroids Derived by 244-Plutonium Fission Track Thermometry

Cooling curves were determined for ordinary chondrites within the time- and temperature intervals from ~4.6 to ~4.1 × 109 years and ~1400 K to 300 K respectively. This was done by analyzing the 244-plutonium fission track record in whitlockite and adjacent mineral track detectors. The resulting cooling rates constrain the sizes of the parent asteroids to ~120 - 200 km in radius. These dimensions together with the fact that chondrites had never been molten, suggest lower 26Al/27Al initial ratios for ordinary chondritic than for Allende refractory materials.

7. Relationship Between Meteorites, Asteroids and Comets

There seem to be no objection any more to regard the asteroid belt as the past and present source of initial and intermediate parent-bodies of meteorites. Initial parent bodies were of the size of the present-day largest asteroids; the irregular size of medium sized asteroids suggests intermediate parent bodies fragmented by collisions. Finally, most of the terminal parent bodies can be identified with Earth-crossing or grazing objects. In contrast, icy planetesimals of the outer zone of the primeval nebula underwent almost no thermal evolution, but only a complicated orbital evolution leading to the comets observed now. This does not exclude the possibility that some extinct comets end up on asteroid-type orbits, although their nuclei might still contain ices covered by an insulating crust that can preserve them for a very long time. These could be the parent bodies of bolides, as opposed to meteorites.

Asteroid size distribution: Implications for the origin of stony‐iron and iron meteorites

The diameter‐frequency distribution of large asteroids is analyzed, based on polarimetric and radiometric albedo data. The population of carbonaceous objects shows a high degree of fragmentation. But a class of apparently stony‐iron asteroids shows a relative excess frequency between diameters of 100 to 200 km. It is proposed that the latter objects are predominantly metallic cores of a group of chemically differentiated planetesimals all of which (except Vesta) have suffered catastrophic collisions. Collision frequencies in the asteroid belt may have been much greater than previously supposed.

A new interpretation of the mechanical properties of the Gibeon Meteorite

A set of results on the temperature and strain rate dependence of the tensile properties of the Gibeon meteorite reported by Gordon in 1970 is reinterpreted. It is shown that these results indicate that the meteorite material undergoes a ductile-brittle transition in low strain rate tensile tests at about 50°K. It is estimated that under asteroidal impact conditions the transition temperature would be about 200°K. Thus provided that the internal hydrostatic pressure did not depress the transition temperature significantly, the material would fragment in a brittle manner yielding meteoritic fragments free of plastic deformation. An approximate calculation of the effect of pressure on the transition temperature shows that brittle behavior would not occur in asteroids having radii greater than about 140 km. Thus since meteoritic fragments rarely show signs of gross plastic deformation, it is concluded that this is an approximate upper limit to the size of the parent asteroids. Work on the nickel composition profile in octahedrites by J. A. Wood (1964) led to an estimate of the radii of the parent asteroids of about the same magnitude. Our value of 140 km is probably subject to an error of about 50% in either direction. The larger asteriods now present in the solar system are of about this size.

A Lunar Chronology

The Moon was formed either at the same time as the Earth or only a short time later, about 4.6 billion years ago. After formation, there was a period of intense bombardment of the lunar surface by objects of asteroidal size. They formed the large basins now filled with mare basalts. It is not at present clear whether these impacts occurred in a series in time or in a group at about the same time. It does seem clear that there was a significant change in the bombardment rate 3.9 to 4 billion years ago. The author is professor of geochemistry in the Department of Earth and Space Sciences, State University of New York, Stony Brook, N. Y.

An alternative model for the formation of iron meteorites

The formation of iron meteorites is, at present, considered to have been a high temperature process. It seems generally accepted that these bodies are derived from molten cores of small planets with some 10 z km in diameter. They are thought to have been broken up later. This hypothesis implies a heating up of the rather small bodies and a subsequent separation of liquid metal from a silicate phase. For this hypothesis it is necessary that in the beginning short-living radioactive nuclides acted as a heat source. The separation mechanism of metals and silicates, however, is difficult to imagine because the gravitational forces in such small bodies appear to be insufficient. Even the moon has been found to have no or only a small metal core. Moreover, the disruption of a body of asteroidal size up to its core presents severe difficulties. Problems also arise regarding the structure of certain iron meteorites. Many features are incompatible with a fused metallic phase at the time of enveloping materials such as crystals of Laurencite (FeC12) and broken silicates with sharp corners. Neither of them could have survived temperatures near the melting point of the iron phase in which they have been found (P. Ramdohr [1] ). All this suggests that the existing models of iron meteorite formation are not quite satisfactory. As an alternative method for iron meteorite formation, a mechanism working primarily at low temperatures could have great advantages. We propose as a mechanism the thermal decomposit ion of metal carbonyls (Mex(CO)y). It is well known that comets contain carbon monoxide, iron and nickel, among other constituents and therefore a reaction leading to carbonyls is not at all unlikely. The formation of Fe(CO)s and Ni(CO)4 is now performed industrially (Mond [2] ). This type of reaction is exothermic (50 kcal/mole). It can be performed at atmospheric pressure and below 100°C, although the BASF uses several hundred atmospheres in order to gain time (Zimmermann [3] ), and iron sulfide as a catalyst. If ironand nickel-carbonyls are heated to about IO0°C, decomposit ion takes place and the free metals are formed together with carbon monoxide. 100°C Me (CO)x ~ Me + xCO

Cohenite: its occurrence and a proposed origin

Cohenite is found almost exclusively in meteorites containing from 6 to 8 wt.% Ni. On the basis of phase diagrams and kinetic data it is proposed that cohenite cannot form in meteorites having more than 8 wt.% Ni and that any cohenite which formed in meteorites having Ni content lower than 6 wt.% decomposed during cooling. A series of isothermal sections for the system Fe Ni C has been constructed between 750 and 600°C from published information on the three constitutent binary systems. The diagrams indicate that the presence of a few tenths of a per cent carbon in a Ni Fe alloy may reduce the temperature at which kamacite separates from taenite by more than 50°C. Hence C in iron meteorites may be partly responsible for the postulated supercooled nucleation of kamacite in meteorites proposed by recent authors. Cohenite found in meteorites probably formed over the temperature range 650-610°C. For compositions approximating those of metallic meteorites, the greater the C or Ni content of the alloy, the lower the temperature of formation of cohenite. The presence of cohenite in meteorites indicates neither high nor low pressures of formation. However, the absence of cohenite in meteorites containing the assemblage metal + graphite requires low pressures during cooling. Such meteorites therefore cooled in parent bodies of asteroidal size, or near the surface of large bodies.

Mars: Age of Its Craters

If the craters on Mars were formed by asteroidal particles having spatial distributions independent of asteroid size, and if both spatial distributions and total numbers are independent of time, the average age of the craters may be less than 300 million years.