Impactor Population Research Articles

Doublet craters and the tidal disruption of binary asteroids

Out of approximately 28 known terrestrial impact craters with diameters greater than 20 km, at least 3 are doublets created by the nearly simultaneous impact of objects of comparable size. The large size and separation of these doublets rule out atmospheric breakup as the agent of dispersion, while previous investigations of fragmentation within the Roche Limit are similarly unable to account for the dispersion of the doublets. Both the frequency of occurrence and the morphology of the craters and their ejecta blankets make very low angles of impact highly improbable. In this paper we investigate the possibility that tidal disruption of a population of compact binary asteroids might account for doublet craters. After detailed orbital integrations we find that although such asteroids are often disrupted by tidal forces well outside the Roche limit, the magnitude of the separations, averaged over a random population of impactors, is too small to account for the observed doublet craters. The only rational explanation we can find for the occurrence of doublet craters is an initial population of kilometer-size Earth-crossing objects containing between 10 and 20% of binaries separated by four times or more of their mean diameter. We offer this result as a prediction that may be tested by future radar observations of Earth-approaching asteroids.

On the properties of comets, asteroids, and terrestrial planet impactors

This review discusses problems related to understanding the compositional spectrum of the material interacting with Earth's atmosphere. The most important issue is therefore the properties of Earth-crossing objects, but these must be related to their place of origin and mode of transport and evolution. In terms of chemistry of minor bodies, the role of comets as an outward extension of the trends seen among the asteroids is briefly discussed. However, attention is focussed on bulk properties bearing on the question whether the Earth impactor population is dominated by asteroidal fragments originating in the main belt or if it has an important contribution from defunct comets. Cometary evolution is argued to be governed by the formation and blowoff of thin dust mantles, and it appears likely that a large fraction of the short-period comets reach a stage of complete coverage. If so, they form a rich source population for the supply of near-Earth asteroids but a fragile one in the sense that rejuvenation of cometary activity is easily induced by meteoroid impacts.

Crater densities and crater ages of different terrain types on Ganymede

Crater densities and size-frequency distributions on Ganymede were measured for separate areas of dark furrowed and dark smooth terrains, reticulate terrain, complex grooved terrain, and groups of groved polygons and groove lanes having a variety of relative ages. These measurements were used first, together with results of previous studies, to test alternate models of heliocentric and planetocentric crater-forming bombardments of Ganymede and Callisto. The spatial distribution of crater production functions on Ganymede is consistent with a single impactor population having dominated cratering of that satellite's observed surface. In addition, relative crater ages of different Ganymedean material deposits are better reconciled with the deposits' stratigraphic relations if a heliocentric impactor population is assumed. These results are consistent with results of previous studies, interpreted here to indicate that the spatial distributions of most classes of craters on Ganymede and younger craters on Callisto are more consistent with heliocentric than with planetocentric bombardment. The measurements were then used to infer the sequence and style of surface modification of Ganymede; bounds on absolute ages depend on the assumption of heliocentric bombardment. Dark terrain was emplaced during a prolonged period of volcanic resurfacing of older, heavily cratered terrain, probably at least 10 8 yrs in duration and at least 3.8–4.0 Gyr ago. Furrows generally formed on blanket-like deposits of dark material, typically hundreds of meters or more in thickness, soon after their emplacement; furrow formation continued globally throughout the period of dark-material deposition. Next, dark terrain was deformed locally by reticulate terrain formation. Light terrain emplacement subsequently began, at least regionally within “complex grooved terrain.” It continued for an extended period (perhaps several times 10 8 yrs) in grooved lanes, grooved polygons, andsmooth terrain, where a light-material layer averaging several hundred meters or more in thickness was emplaced.

The solar system cratering record: Voyager 2 results at Uranus and implications for the origin of impacting objects

The cratering record at Uranus shows two different crater populations of different ages. The old crater population occurs on the heavily cratered surfaces of Oberon, Umbriel, and Miranda, while the younger one is found on Titania, Ariel and the resurfaced areas of Miranda. Since only the young population occurs on Titania, this satellite must have experienced a global resurfacing event which obliterated the older population prior to the impact of objects causing the younger one. The old crater population is characterized by an abundance of large craters and a relative paucity of small ones. The young crater population, however, has an abundance of small craters and a paucity of large ones relative to the old population. Furthermore, the abundance of small craters and the paucity of large craters increases with decreasing density. This change in the size distribution is consistent with a population of impactors that evolved with time by mutual collision, and therefore was probably in planetocentric orbits. In fact, both crater populations may be the result of accretional remnants in planetocentric orbits that evolved with time by mutual collisions. If so, then the higher crater density on Miranda compared to Oberon and Umbriel suggests that both Oberon and Umbriel were also resurfaced early in their histories. A comparison of the Solar System cratering record from Mercury to Uranus (19 AU) shows different crater populations at different locations in the Solar System. Computer simulations using a modified Holsapple-Schmidt crater scaling and short-period comet impact velocities to recover the projectile diameters from the cratering record produce different projectile populations in different parts of the Solar System. Furthermore, adjusting the Jovian crater curve to match that in the inner Solar System requires differences in the impact velocities that are unrealistic for objects in heliocentric orbits. These results suggest that the Solar System cratering record cannot be explained by a single family of objects in heliocentric orbits, e.g., comets. One possible explanation is that the cratering record is the result of different families of objects (possibly accretional remnants) indigenous to that region of the Solar System in which the different crater populations are found. Thus, in the inner Solar System, the impactors responsible for heavy bombardment were in heliocentric orbits with semimajor axes less than 3 AU. In the outer Solar System, they may have been in planetocentric orbits around each of the Jovian planets.