Impact induced compaction of Chondritic parent bodies and the initial porosity distribution of chondrites

E. BEITZ(*); J. BLUM(*); M. G. PARISI; (*) TECHNISCHE UNIVERSITAT BRAUNSCHWEIG, INSTITUT FUR GEOPHYSIK UND ETECHNISCHE UNIVERSITAT

Jena

Workshop; 92nd Annual Meeting Deutsche Mineralogische Gesellschaft; 2014

Deutsche Mineralogische Gesellschaft

P { margin-bottom: 0.21cm; } 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, too. Especially meteorites that originate from unequilibrated parent bodies (<300 km in size) can provide important insights into the dynamics of chondrite formation and compaction. However, the last fragmentation process in which the meteorites are formed is extremely violent and changes the initial properties of the parent body, as the difference in porosity of carbonaceous chondrites and their parent bodies (C-type asteroids) show [1]. In order to quantify impact consolidation, we performed high-velocity impacts experiments into porous chondrite parent body analogs and measured the degree of compaction. To calculate the pressure, we adopted an impedance-matching algorithm. In our experiments, the volume filling factor (1- porosity) increases with increasing impact pressure with a power of 0.082 and reaches unity for pressures higher than 1 GPa [2]. Thus, we deduced the pressure range required to compact carbonaceous chondrites to their typical volume filling factors of  0.6 - 0.9 [4] to be between 2 and 200 MPa. The experiments showed that the pressure in the target decreases with increasing depth as a function of projectile size. Therefore, the volume beneath the impact shows a variation in volume filling factor. The crater size and, thus, the ejected mass depend on the size of projectile and target as well as on the impact velocity [3]. However, not all of the compacted material is ejected by the impact; some compacted material remains on the parent body and changes its initial properties for further impacts. [1]Consolmagno G. J. et al. 2008. Chemie der Erde 68:1?29. [2]Beitz E. et al. 2013. Icarus. 225, 558-569. [3] Holsapple K. A. 1993. Annual Reviews Earth Planetary Science 21:333-73.