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From: dipper@utastro.UUCP (Debbie Byrd)
Newsgroups: net.astro
Subject: StarDate:
Message-ID: <405@utastro.UUCP>
Date: Tue, 23-Jul-85 02:00:52 EDT
Article-I.D.: utastro.405
Posted: Tue Jul 23 02:00:52 1985
Date-Received: Wed, 24-Jul-85 08:21:01 EDT
Organization: U. Texas, Astronomy, Austin, TX
Lines: 33

The Earth has earthquakes -- and the moon has moonquakes.  More about
them -- right after this.

May 23:  Moonquakes

When NASA's Apollo astronauts went to the moon, they came back minus
some instruments.  These instruments were left behind to study lunar
phenomena from Earth -- long after the astronauts had returned.

One type of instrument was a seismograph, the same device use on Earth
to measure the intensity of eathquakes.  During the years it was
operational, the seismograph let us listen to vibrations in the body of
the moon.  The vibrations had a variety of sources.  Some came from our
own spacecraft landing and taking off.  Some resulted when meteorites
struck the moon.  And still other came from moonquakes.

Moonquakes are similar to earthquakes -- but they're generally much
milder, and they occur less often.  Earth spawns more than a million
earthquakes each year, while the moon has only a few hundred
moonquakes.  The moon is far less geologically active than Earth.  Its
most recent active period probably occurred billions of years ago.

The seismograph also revealed a surprising fact about the crust of the
moon, which is the layer of rocks that extends down about 40 miles.
This layer was found to vibrate extensively -- to ring like a bell for
several hours, after such small blows as those from spacecraft
deliberately impacted onto the moon.  The Earth can also vibrate in
this way, but it does so to a much lesser degree.  It's thought that
the moon rings because its surface crust is thicker and more rigid than
Earth's.

Script by Deborah Byrd.
(c) Copyright 1984, 1985 McDonald Observatory, University of Texas at Austin