Lunar seismology: The internal structure of the Moon

Abstract

Tables A1–A4 and Figures A1–A3 are available with entire articleon microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J81‐003;$1.00. Payment must accompany order. The arrival times of direct P and S waves, measured on seismograms recorded from natural lunar seismic events, have been analyzed using linearized inversion and parameter search methods to simultaneously determine event locations, origin times, and structural parameters (seismic velocities). Polarization‐filtered record section plots are correlated with theoretical travel time curves to identify later phases and obtain additional structural and velocity information. Shear wave amplitudes plotted as a function of distance provide data on the existence and magnitude of seismic velocity gradients in the interior. These studies are used to delineate the structure of the moon below the crust to a depth of about 1100 km. This structure has been divided into an upper and a lower mantle. The upper mantle, from 60‐ to 400‐km depth, has average seismic wave velocities of Vp = 7.70 ±0.15 km/s and Vs = 4.45 ± 0.05 km/s with corresponding Q values (taken from the literature) of Qp ∼ 5000 and Qs ∼ 3000. The shear wave velocity decreases with a negative gradient of at most −6 × 10−4 km/s/km, implying a Vs variation of from 4.57 km/s at 60‐km depth to 4.37 km/s at 400‐km depth; this decrease can be accounted for by increasing temperature, and so no major compositional gradient is required in the upper mantle. A small negative P wave velocity gradient may also be present. Between 400‐ and 480‐km depth there is a transition zone with a sharply decreasing shear wave velocity and possibly an accompanying small decrease in Vp. The dominant shear velocity decrease may occur at a 480‐km interface and may represent a compositional change, although the effects of increased temperature cannot be totally ruled out. The lower mantle extends from 480‐km depth to at least 1100‐km depth; few seismic waves recorded by the lunar network penetrate below 1100 km. The average velocities are Vp = 7.6 ± 0.6 km/s and Vs = 4.2 ± 0.1 km/s. Q values (taken from the literature) are Qp ∼ 1500 and Qs ∼ 1000. Below 1100 km there is some indication that the attenuation may increase still further for shear waves, with Qs dropping to a few hundreds or less. This seismic model of the moon is well constrained, with uncertainties on the above values given explicitly by the analysis methods, and so it serves as a strong control on the present‐day lunar compositional and thermal structure.

A new seismic velocity model for the Moon from a Monte Carlo inversion of the Apollo lunar seismic data

“…In pratice this smoothness constraint results in a vertical resolution of approximately 5 kin. In line with a previous method [Goins et al, 1981], the uppermost kilometer of the crust comprising a very low velocity layer, the regolith, was not included in our velocity model, instead a local time correction to the travel times was determined. It should be noted that these corrections are surface consistent, by which we mean that the same time correction beneath a given station is added to the travel time for rays coming from different sources.…”

“…ToksOz et Goins et al, 1981;Nakamura et al, 1974Nakamura et al, , 1976, while the latest study by Nakamura et al (1982) and Nakamura (1983) employed all 81 identified events, which included a greater number of deep moonquakes. Generally, these studies were succesful in determining the gross features of the lunar interior and resulted in the recognition of the Moon as being a differentiated body with a crust and a mantle whose lower parts were thought to be partially molten.…”

The scientific case for renewed human activities on the Moon

“…The Apollo seismometers remained active for up to eight years, and did provide useful information on the structure of the lunar crust and upper mantle (see [16] for a review). However, the deep interior of the Moon was only very loosely constrained by the Apollo seismology -even the existence, never mind the physical state and composition, of a lunar core is uncertain.…”

Sources of Extraterrestrial Rare Earth Elements: To the Moon and Beyond

“…The present day internal architecture of the Moon has been constrained to: an upper mantle from 60 to 400 km; a mantle transition zone from 400 to 800 km; a lower mantle from to (at least) 1100 km; a fluid outer core (350 km radius); and a solid inner core (160 km radius, Figure 3a: [63][64][65]). In the absence of plate tectonics, the lunar crust and mantle have remained physically separate for the past ~4 billion years [64,66].…”