GEOLOGY: Lunar History

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Science  22 Sep 2000:
Vol. 289, Issue 5487, pp. 2005c

Earth's tides are produced by the orbital interactions of the moon around the rotating Earth and those of the Earth around the sun. Tidal friction has slowed Earth's rotation (lengthening the day) and altered the moon's orbit. This history, extending back to about 1 billion years ago, has been inferred and measured by analyzing sedimentary rocks that fortuitously preserve features—such as rhythmic layering or crossbeds—produced by tides.

Eriksson and Simpson now analyze sedimentary rocks from South Africa dating to 3.2 billion years ago that apparently have a tidal origin. This age is much closer to the time of the moon's origin and would allow, if their interpretation is correct, inference of the moon's orbit in the Archean. Their analysis implies that tides were not unusually strong then and that the Archean lunar orbit was not markedly elliptical, making it similar to that seen today. This conclusion is consistent with formation of the moon by coalescence of material that was generated by an early giant impact into the proto-Earth (which would produce a near-circular orbit), as opposed to the capture of an already formed moon (which would have resulted in a highly elliptical orbit). — BH

Acknowledgments

Geology 28, 831 (2000).

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