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No. 89: Sep-Oct 1993

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Blasting Rocks Off Planets

"Can rocks from the surface of a major planet or satellite be launched into interplanetary space by natural processes? A few years ago the answer to this question would have been a resounding "no" from the experts on both volcanism and impact cratering, the only geological processes known to eject solid material at substantial velocities. Observation, however, has once again confounded expectation."

In the snowy wastes of Antarctica, scientists have picked up meteorites that almost certainly came from the moon and Mars. And near St. Gallen, Switzerland, there was discovered a 22-centimeter block of Malm limestone that was apparently ejected from the Ries impact crater, almost 200 kilometers away, about 15 million years ago. We know all of these rocks are impact debris because they contain shatter cones indicating a violent origin.

Not only did these bits of debris confound expectations, but their shatter cones implied shock-wave pressures far too low to achieve lunar and Martian escape velocities, or even the velocity necessary to propel that chunk of Malm limestone 200 kilometers. Something was wrong somewhere.

It has turned out that shock-wave theory had been misapplied. It is not the pressure that is important in ejecting bits of debris from around the impact site, but rather it is the pressure gradient. Anomaly extirpated!

(Melosh, H.J.; "Blasting Rocks Off Planets," Nature, 363:498, 1993.)

Moral. A.C. Clarke was right again: When a respected scientist says something cannot happen, it probably will!

From Science Frontiers #89, SEP-OCT 1993. 1993-2000 William R. Corliss