[meteorite-list] Meteorites On The Moon

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:23 2004
Message-ID: <3D6C245E.51119672_at_bhil.com>

Hi, List,

    "Will any meteorite survive impact on the Moon?"

    In general, a meteorite will remain intact on impact (or only
crack into large fragments) if it hits a hard surface at a speed
slower than the velocity of sound inside the body of the
meteorite.
    Huh? Well, the shock wave of the impact traveling "back"
through the impactor has the same velocity as the impactor's
speed relative to the surface it hits. If the velocity of the
shock wave is greater than the "local" velocity of sound, the
result will be shredded impactor, the spalled material coming off
the back of the impactor until there's no impactor left.
    Since meteorites form in weak gravity, microgravity, or
almost no gravity, they are structurally weak compared to earth
rocks, not very well consolidated. Hence, the speed of sound is
low in meteorites: in chondrites, for shear waves 600 to 1200
meters per second and for transverse waves 2000 to 4200 meters
per second.
    Even if a meteoroid started out "standing still" with respect
to the Moon, by the time it reached the lunar surface and became
a meteorite it would be traveling at the lunar escape (or more
properly, terminal) velocity of 2380 meters per second. Of
course, they don't start out standing still but encounter the
Moon with velocities identical to those of meteoroids that
encounter the Earth.
    Because of the Moon's lesser gravity, they will reach the
lunar surface moving about 8800 meters per second slower than if
they had encountered the Earth, but there are probably no impacts
of less than 5000 meters per second (and the average impact speed
is more like 10,000 to 20,000 meters per second).
    These impact speeds are more than sufficient to completely
disintegrate 99.99% (and probably a lot more nines than just two)
of all meteoritic objects that hit the Moon.
    The best chance for a survivor would be a good solid iron
approaching the Moon on a prograde orbit of high eccentricity
from the inner solar system that manages a "graze" landing on a
track nearly tangent to the lunar surface at a point where that
surface is almost perfectly flat and is one lucky rock to boot.
    Despite having a plenitude of dry deserts, the Moon is NOT a
good place to go looking for meteorites.


Sterling K. Webb
Received on Tue 27 Aug 2002 09:16:15 PM PDT