Planetary Radio Q and A: Moonquakes

Posted By Emily Lakdawalla

2009/09/15 05:15 CDT

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This week's Planetary Radio features Robert Zubrin, on his new book How to Live on Mars: A Trusty Guidebook to Surviving and Thriving on the Red Planet On "Questions and Answers" I answered the question:"I remember that the Apollo missions left seismometers on the surface of the Moon. Did they detect any moonquakes?"

I found out about the results of the Apollo Passive Seismic Experiment from this page on the Lunar and Planetary Institute website.

Five of the Apollo missions deployed seismometers on the lunar surface. The Apollo eleven seismometer lasted only three weeks, but others operated until September of 1977. The seismometers successfully detected three different kinds of Moon-shaking events. Two were natural: internally generated moonquakes, and externally caused meteoroid impacts. The seismometers also recorded ground shaking when mission controllers deliberately crashed leftover spacecraft into the lunar surface, including the third stages of Saturn Five rockets and the used ascent stages of the landers.

What's the use of detecting ground motion on the Moon? The shape and timing of seismic waves is by far the best way to study the interior of a solid world. It's because of the Apollo seismic experiment that we know the interior of the moon is cold, and that its crust is about three times thicker than Earth's at 60 kilometers thick. The experiment also told us that the upper 20 kilometers is heavily fractured from past huge impacts. Natural moonquakes were concentrated at about 1000 kilometers depth, and occurred repeatedly at the same sites at monthly intervals, indicating that it's tides from Earth's gravity that cause the Moon's natural quakes.

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