The Hollow Moon hypothesis proposes that Earth's Moon is either wholly hollow or otherwise contains a substantial interior space. No scientific evidence exists to support the idea; seismic observations and other data collected since spacecraft began to orbit or land on the Moon indicate that the planet has a thin crust, extensive mantle and small, dense core, although overall it is much less dense than Earth.
Hollow Moon adherents can be broadly grouped in two major camps, both of which are considered by the scientific community to be unsupported by evidence:
- The Moon's formation process produced a hollow sphere through natural means. Details vary widely; the proposed mechanism is usually an undefined or pseudoscientific factor in accretion-disk dynamics.
- The Moon is an artifact (the "Spaceship Moon Theory"). This often corresponds with beliefs in UFOs or ancient astronauts.
- 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).
- 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.
- 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.
NASA astronaut Neil Armstrong described the Moon as "ringing like a bell," leading to arguments that it must be hollow like a bell. Lunar seismology experiments since then have shown that the lunar body has shallow "moonquakes" that act differently than quakes on Earth, due to differences in texture, type and density of the planetary strata, but no evidence of any large empty space inside the body.
The Moon's density is 3.34 g/cm3 (3.34 times an equal volume of water) whereas the Earth's is 5.5. Proponents argue that this indicates the Moon must have a large cavity inside it.
Some proponents argue that lunar craters are too shallow to be easily explained. A recent study indicates that larger craters on the near side may be a reflection of the thickness of the crust.
- In H.G. Wells's book The First Men in The Moon (1901), Wells describes the insect-like "Selenites" who live inside a hollow Moon.
- Edgar Rice Burroughs wrote an adventure novel, The Moon Maid (1922), set in the interior of a postulated hollow Moon which had an atmosphere and was inhabited. In the sequel, The Moon Men, natives of the hollow Moon invade and conquer Earth. Burroughs also wrote the Pellucidar series, set in the interior of a hollow Earth.
- That Hideous Strength (1945) by C. S. Lewis takes place on Earth, but a hollow Moon is an important part of the novel's background, and is known by its inhabitants as "Sulva."
- The Lomokome Papers, Herman Wouk's little-known venture into science fiction (1947), also uses the concept.
- Neznaika on the Moon, (1965), by Russian author Nikolay Nosov.
- In Shelton Ranasinghe's book Heaven at SETI's Doorstep (2011), scientists working for the SETI project detect signals coming from inside the Moon.
- In David Icke's book Human Race Get off Your Knees – The Lion Sleeps No More (David Icke Books, 2010. ISBN 0955997313.), Icke discusses the idea of a hollow moon.
- Measurements of the lunar induced magnetic moment in the geomagnetic tail: Evidence for a lunar core?
- BASA Research: Moon has Earth-like core
- Khan, A. (2002). "An inquiry into the lunar interior: A nonlinear inversion of the Apollo lunar seismic data". Journal of Geophysical Research 107 (E6): 5036. Bibcode:2002JGRE..107.5036K. doi:10.1029/2001JE001658. ISSN 0148-0227.
- Khan, A.; J. A. D. Connolly, J. Maclennan, K. Mosegaard (2007). "Joint inversion of seismic and gravity data for lunar composition and thermal state". Geophysical Journal International 168 (1): 243–258. Bibcode:2007GeoJI.168..243K. doi:10.1111/j.1365-246X.2006.03200.x. ISSN 0956-540X.
- Nakamura, Y.; D. Lammlein, G. Latham, M. Ewing, J. Dorman, F. Press, N. Toksoz (1973). "New Seismic Data on the State of the Deep Lunar Interior". Science 181 (4094): 49–51. Bibcode:1973Sci...181...49N. doi:10.1126/science.181.4094.49. ISSN 0036-8075. PMID 17769823.
- Konopliv, A. S.; Binder, AB; Hood, LL; Kucinskas, AB; Sjogren, WL; Williams, JG (1998). "Improved Gravity Field of the Moon from Lunar Prospector". Science 281 (5382): 1476–1480. Bibcode:1998Sci...281.1476K. doi:10.1126/science.281.5382.1476. PMID 9727968.
- NASA Moon Fact Sheet
- Garrick-Bethell, I.; Wisdom, J; Zuber, MT (2006). "Evidence for a Past High-Eccentricity Lunar Orbit". Science 313 (5787): 652–655. Bibcode:2006Sci...313..652G. doi:10.1126/science.1128237. ISSN 0036-8075. PMID 16888135.
- Griffiths, David B. (1989). Introduction to electrodynamics. Englewood Cliffs, N.J: Prentice Hall. ISBN 0-13-481367-7.
- NASA Science News: "Moonquakes"
- MIT: "Researchers find that huge craters on the near side of the moon may overstate the intensity of asteroid impacts about 4.1 billion years ago."
- see Wouk - Lomokome Papers at epinions.com
- http://www.agoracosmopolitan.com/home/Frontpage/2008/07/18/02474.html — "Earth's Moon and Human Evolution", by Noel Huntley, Ph.D.