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Science 4 August 2006:
Vol. 313. no. 5787, p. 588
DOI: 10.1126/science.313.5787.588b
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This Week in Science
The peculiar shape of the Moon has puzzled scientists since Laplace drew attention to it in 1799. The Moon's axes have been swelled by spinning and tidal stretching, but the deviations are too large for a body in the current lunar orbit. One explanation is that the Moon's orbit was different early in its history when its shape froze as the lunar magma ocean solidified. However, models have so far failed to fit the precise lunar dimensions. Garrick-Bethell et al. (p. 652; see the Perspective by Innanen) show that the Moon's shape can be explained if it had been in an eccentric orbit 100 million years after its formation. If the lunar magma ocean solidified during a period of substantial lunar eccentricity, when the Moon's semimajor orbital axis was about 22 to 28 Earth radii, the "fossil bulge" seen today in the lunar gravity field can be matched. The authors constrain the possible orbits; permitted ones that include a high-eccentricity synchronous orbit, and one with a 3:2 resonance of spin frequency to orbit frequency, as is presently the case for Mercury.
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Science. ISSN 0036-8075 (print), 1095-9203 (online)
