Lunar near‐surface structure

Seismic refraction data obtained at the Apollo 14, 16, and 17 landing sites permit a compressional wave velocity profile of the lunar near surface to be derived. Although the regolith is locally variable in thickness, it possesses surprisingly similar seismic characteristics. Beneath the regolith at the Apollo 14 Fra Mauro site and the Apollo 16 Descartes site is material with a seismic velocity of ∼300 m/s, believed to be brecciated material or impact‐derived debris. Considerable detail is known about the velocity structure at the Apollo 17 Taurus‐Littrow site. Seismic velocities of 100, 327, 495, 960, and 4700 m/s are observed. The depth to the top of the 4700‐m/s material is 1385 m, compatible with gravity estimates for the thickness of mare basaltic flows, which fill the Taurus‐Littrow valley. The observed magnitude of the velocity change with depth and the implied steep velocity‐depth gradient of >2 km/s/km are much larger than have been observed on compaction experiments on granular materials and preclude simple cold compaction of a fine‐grained rock powder to thicknesses of the order of kilometers. The large velocity change from 960 to 4700 m/s is more indicative of a compositional change than a change of physical properties alone. This high velocity is believed to be representative of the material that forms the lunar highlands.