1000 km

Figure 5.5 A cross-section through the Moon.

so at 1000 km. Further data on internal temperatures have been obtained from Love numbers derived from the 100 mm surface tide at the lunar surface, generated mainly by the Earth. This tide was measured from the round-trip laser travel times to the retroreflectors left by the Apollo missions. This gives the distance at any instant with the required precision. A partially liquid interior is indicated. Such a warm interior is consistent with the stagnant lid provided by the Moon's thick lithosphere.

We also have seismic data. At the Apollo 11, 12, 14-16 sites, seismometers were set up successfully. The Moon is not nearly as seismically active as the Earth, but 'moonquakes' (mostly caused by tidal stresses induced by the Earth, or by surface impacts) have yielded the data in Figure 5.6. These two profiles were seen earlier in Figure 4.9(b) where it was noted that they are each the average of a number of profiles. This has obscured evidence of features visible in most individual profiles. Note especially the large uncertainties, a result of the infrequency of moonquakes and the small number of seismometers, all on one side of the Moon.

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