Lunar Near‐Surface Structure

Abstract

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. Seismic refraction experiments were executed on the lunar surface by Apollo astronauts during missions 14, 16, and 17 as part of an integrated set of geophysical experiments called Alsep (Apollo lunar surface experiments package). For the study of the earth's crust, seismic refraction techniques are often used. The basic technique involves the recording of seismic waves produced by the detonations of explosive charges placed at different distances from an array of seismometers (geephones). The travel times of the resulting seismic waves are analyzed in a straightforward fashion to determine the variation of seismic velocity with depth and as a result provide a direct method of probing the lunar interior.The results of these experiments when they are combined with the seismic signals from other man-made impacts from spent rocket stages have allowed a model of the near-surface lunar structure to be constructed. In this paper we describe and analyze the seismic data pertinent to the near surface of the moon. SEISMIC INSTRUMENTATIONAt the Apollo 14 and 16 sites the astronauts deployed a 91-m linear array of three geephones, and several seismic energy sources were used: an astronaut-activated thumper device with

GRAIL gravity constraints on the vertical and lateral density structure of the lunar crust

“…The thin dashed line depicts the result obtained using a theoretical model for spherical pores [ Smith , ; Christensen , ], which resembles numerically derived results [ Mogilevskaya et al , ]. Six different lunar seismic models are displayed (see section S7 in the supporting information): in green, an early, shallow model [ Cooper et al , ] depicted as a linear trend excluding the biased [ Nakamura , ] Apollo 17 Lunar Module impact data; in cyan, a pre‐Apollo 17 era crustal model [ Kovach and Watkins , ]; in red and blue, post‐Apollo era models [ Toksöz et al , ; Nakamura , ]. Two more recent models are also included, in pink [ Lognonné et al , ] and in pale gray scale [ Khan and Mosegaard , ].…”

Thickness of Lunar Mare Basalts: New Results Based on Modeling the Degradation of Partially Buried Craters

“…Mare basalt thicknesses have been investigated by many studies, which can be divided into four general classes: direct measurements using elevation differences of lava flow fronts and layering features in crater walls (e.g., Robinson et al, ; Schaber, ; Stickle et al, ), subsurface sounding radar using a spaceborne or ground penetrating radar (e.g., Oshigami et al, ; Phillips et al, ; Xiao et al, ), geophysical techniques based on seismology and gravity (e.g., Cooper et al, ; Gong et al, ; Talwani et al, ), and investigations of impact craters, including partially buried craters, modification of the crater size‐frequency distribution, and the composition of crater ejecta (e.g., De Hon, ; Hiesinger et al, ; Thomson et al, ). Each of these methods measures a different “thickness” (total thickness, thickness of the last flow, or thickness since a crater formed), and has different spatial and temporal resolutions.…”

“…These methods depend on the density contrast between mare and highland materials, and therefore can provide the total basalt thickness. Using the seismic refraction data collected at the Apollo 17 landing site, the total basalt thickness near the Taurus‐Littrow valley was estimated to be 1.4 km (Cooper et al, ). With newly acquired gravity data from the Gravity Recovery and Interior Laboratory (GRAIL) mission, the total basalt thickness on the western nearside hemisphere (19°S–45°N, 68°W–8°W) was estimated to be 740 m on average (Gong et al, ).…”

The Subsurface Coherent Rock Content of the Moon as Revealed by Cold‐Spot Craters

“…The Apollo program included both passive (Apollo 12, 14, and 15) and active (Apollo 14, 16, and 17) seismic experiments which provided much of our knowledge of the lunar subsurface from the classic regolith layer down to the core. The Apollo 14 and 16 sites were estimated to have regolith depths of 8.5 m and 12.2 m, respectively (Watkins & Kovach, ), and the Apollo 17 seismic experiment revealed several possible layers (Cooper et al, ). However, seismic data are degenerate and there are multiple vertical structures that are consistent with the data (see review in Wilcox et al, ).…”