Hollow Moon - Scientific Perspective

Scientific Perspective

Mainstream scientific opinion on the internal structure of the Moon overwhelmingly supports a solid internal structure with a thin crust, an extensive mantle and a small denser core. This is based on:

  1. Seismic observations. Besides Earth, the Moon is the only planetary body with a seismic observation network in place. Analysis of lunar seismic data have helped constrain the thickness of the crust (~45 km) and mantle, as well as the core radius (~350 km).
  2. Moment of inertia parameters. For the Moon, moment of inertia parameters have demonstrated that the core is ~1.4% of the total mass. One such parameter, the normalized polar moment of inertia, is 0.393+/-0.001. This is very close to the value for a solid object with radially constant density, which would be 0.4 (for comparison, Earth's value is 0.33). The normalized polar moment of inertia for a hollow Moon would have a higher value, closer to 0.67. In other words, the moment of inertia parameters indicate that the core of the Moon is both dense and small, with the rest of the Moon consisting of material with nearly-constant density.
  3. Fine-scale variation (e.g., variation along the orbit of the Lunar Prospector orbiter) of the lunar gravitational field, which is consistent with geologic processes involving a crust, mantle, and core.

The large-scale gravitational field of the Moon, however, is unaffected by the internal distribution of mass if the internal density is assumed to vary only radially. For example, had the Moon been replaced with a point object of identical mass, the current gravitational field would continue to exist at distances greater than the ~1700 km lunar radius. This can be derived directly for a spherically symmetric Moon by applying the integral form of Gauss's law. Therefore, the large-scale gravitational field of the Moon does not convey any information about the internal radial distribution of mass. Hollow Moon proponents would, however, have to account for the incredible density of the Moon's crust if it were in fact hollow. As gravitational pull is determined by mass, a hollow moon would require an inordinately dense crust to achieve observed gravitational values.

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