IAR   05382
INSTITUTO ARGENTINO DE RADIOASTRONOMIA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Impact induced compaction of Chondrites and the porosity evolution of chondritic parent bodies.
Autor/es:
E. BEITZ(*); J. BLUM(*); M. G. PARISI; (*) TECHNISCHE UNIVERSITAT BRAUNSCHWEIG, INSTITUT FUR GEOPHYSIK UND ETECHNISCHE UNIVERSITAT
Lugar:
Casablanca
Reunión:
Workshop; In 77th Annual Meeting of the Meteoritical Society; 2014
Institución organizadora:
Meteoritical Society
Resumen:
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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 109
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.