The Collisional Evolution of Undifferentiated Asteroids and the Formation of Chondritic Meteoroids

Abstract

Most meteorites are fragments from recent collisions experienced in the asteroid belt. In such a hyper-velocity collision, the smaller collision partner is destroyed, whereas a crater on the asteroid is formed or it is entirely disrupted, too. The present size distribution of the asteroid belt suggests that an asteroid with 100 km radius is encountered 10 14 times during the lifetime of the Solar System by objects larger than 10 cm in radius; the formed craters cover the surface of the asteroid about 100 times. We present a Monte Carlo code that takes into account the statistical bombardment of individual infinitesimally small surface elements, the subsequent compaction of the underlying material, the formation of a crater and a regolith layer. For the entire asteroid, 10,000 individual surface elements are calculated. We compare the ejected material from the calculated craters with the shock stage of meteorites with low petrologic type and find that these most likely stem from smaller parent bodies that do not possess a significant regolith layer. For larger objects, which accrete a regolith layer, a prediction of the thickness depending on the largest visible crater can be made. Additionally, we compare the crater distribution of an object initially 100 km in radius with the shape model of the asteroid (21) Lutetia, assuming it to be initially formed spherical with a radius that is equal to its longest present ellipsoid length. Here, we find the shapes of both objects to show resemblance to each other.

Evidence for the formation of comet 67P/Churyumov-Gerasimenko through gravitational collapse of a bound clump of pebbles

“…Low-velocity impact experiments of glass spheres of 5 mm radius and 1.37 g mass (and, thus, a density of ρ = 2600 kg m −3 ), dropped from h = 2 mm above the flat surface, consisting of ∼20 layers of dust aggregates with ∼1 mm diameter, yielded intrusions of 1-2 mm (Isensee 2016). As the impact pressure p imp = ρgh ≈ 25 − 50 Pa is constant on a scale length of the impactor radius and then decays inversely proportional to the distance squared (Beitz et al 2016), the resulting density increase was ∼10-30%. As the lithostatic pressure contrast between the two lobes is on the same order as the impact pressure in the impact experiments, we expect a similar density contrast between the two lobes, which matches the findings by Jorda et al (2016).…”

A radiative heating model for chondrule and chondrite formation

“…Alternatively, or in addition, there may be other important fractionation processes operating to enhance the proportion of well-lithified to not-well-lithified primitive asteroidal material impacting the Earth. Primitive material must survive for ∼4.5 Gyr in the asteroid belt, suffering an enormous number of collisions (Beitz et al 2016), which may preferentially deplete the fraction that has lesser tensile strength. Small grains (≤ 100µm) are susceptible to forces from radiation pressure and solar wind that alter their motions and distributions within the Solar System in ways that are different from chondrite-sized objects (Dermott et al 2002).…”

The Physics of Protoplanetesimal Dust Agglomerates. IX. Mechanical Properties of Dust Aggregates Probed by a Solid-projectile Impact

“…To properly simulate the formation and compaction of regolith surfaces, a good knowledge of the impact physics into granular matter is required. Examples of applications of experimental impact studies such as the one described in this paper are high-velocity impacts into asteroidal surfaces with and without regolith (see, e.g., Beitz et al 2016) or dust-aggregate impacts into granular matter (see, e.g., Planes et al 2017).…”