16-03-2015   13:30 hs.

The formation of chondrule dust rims and the collisional evolution of chondritic parent bodies

Dr. E. Beitz – Institut für Geophysik und extraterrestrische Physik, TU Braunschweig

Chondrites are the most abundant group of meteorites. They show a large variety of features like chondrule rims, which need to be in agreement with the formation and evolution of their parent bodies. In the literature the rim formation is predicted as a nebula process in which chondrules acquire their rims either in collisions of chondrules with single dust grains while freely floating in the solar nebula or by dynamic compaction on the parent body in which also the chondrites were compacted. We tested both hypotheses experimentally and found dust rims to be most likely formed in any environment in which dust and chondrules are present simultaneously. Those rims do not show resemblance to the volume filling factors of real chondrule rims and lead to the conclusion that further compaction of them and the entire conglomerate of dust and chondrules called pre-chondrite, is required.
The consolidation of those pre-chondrites can be achieved during the collisional evolution of their parent bodies until they are finally released as meteorites from the asteroid belt. The typical cosmic ray exposer age of a few 10 Myrs determines the time since they became ejecta from collisions and reaches the Earth. Thus, meteorites are fragments form recent collisions in the asteroid belt. The collision speed between two objects of the asteroid belt is given by the eccentricity and inclination of their respective Keplerian orbits. Typical values are on the order of a few km s-1. In such a hyper-velocity collision, the smaller collision partner (projectile) is destroyed, whereas, depending on the mass ratio of the colliding objects, a crater on the larger body (target) is formed or it is entirely destroyed. As we want to predict the porosity distribution of present asteroids that might serve as parent bodies for meteorites, only impactors are considered that do not lead to a catastrophic disruption of the target. The collision frequency of those impactors can be derived from the number-frequency distribution of the present asteroid belt and leads to a number of 1000 millons of encounters on a 100 km radius asteroid during the Solar System’s lifetime. The craters formed by these impacts cover the surface of the asteroid ~20 times.
In order to quantify impact consolidation, we performed high-velocity impact experiments into porous chondrite parent body analogs and measured the degree of compaction. Here, we found a power law relation between the pressure and the volume filling factor as well as the decrease of the pressure in the target to be proportional to the size of the impactor. To apply these results to asteroid sizes, a Monte Carlo code to calculate the compaction of an asteroid due to a statistic bombardment was developed. We compared the ejected material from recent collisions with the shock stage of meteorites of low petrologic type and found a very good agreement, whereas also a clear difference to ejecta from mutual collisions of the first 20 Myrs of the simulation was found. Additionally, we compared the crater distribution of an initially 100 km radius object with a shape model of the asteroid (21) Lutetia, assuming the latter to be initially formed spherical with its radius equal to its longest present semimajor axis. Here, we found the shapes of both objects to show resemblance to each other. Thus, a prediction of the porosity distribution and shock stage of an asteroid can be made.