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Asteroids as rubble piles - Mathilde

from MetSoc 98 suggests as does other reports
that asteroids are piles of loose rubble:

"The bulk density of Mathilde is very low, about 1.3
gm/cm 3 . Even assuming that Mathilde is made of the
least dense known meteoritical materials (carbonaceous
chondritic, which seems likely from its C-type reflectance
spectrum), there must be additional porosity. One
possibility is that Mathilde is a rubble pile, with large
internal voids. Another is that Mathilde is composed of
material even more porous (on a fine scale) and less
dense than carbonaceous chondrites. We must remember
that the Earth’s atmosphere biases meteorite collections
toward unusually strong examples of carbonaceous
meteorites. Whatever the nature of Mathilde’s porosity,
it must damp the propagation of shock waves through the
interior of the body and may also dramatically reduce
ejecta velocities, thereby confining and limiting the
damage done exterior to a crater’s periphery. This style
of cratering has never been recorded on any other solar
system body, but it could be typical for primitive
asteroids and comets, of which Mathilde is the first to be
closely studied."

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