Lunar Seismology
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Lunar seismology is the study of ground motions of the
Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...
and the events, typically impacts or
moonquakes A quake is the result when the surface of a planet, moon or star begins to shake, usually as the consequence of a sudden release of energy transmitted as seismic waves, and potentially with great violence. The types of quakes include earthquake, ...
, that excite them.


History

Several seismographic measuring systems have already been installed on the Moon and their data made available to scientists (such as those from the
Apollo Lunar Surface Experiments Package The Apollo Lunar Surface Experiments Package (ALSEP) comprised a set of scientific instruments placed by the astronauts at the landing site of each of the five Apollo program, Apollo missions to land on the Moon following Apollo 11 (Apollos Apol ...
). The existence of moonquakes was an unexpected discovery from
seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...
s placed on the Moon by Apollo astronauts from 1969 through 1972. The Apollo 11 instrument functioned through August of the landing year. The instruments placed by the
Apollo 12 Apollo 12 (November 14–24, 1969) was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, by NASA from the Kennedy Space Center in Florida. Commander Charles ...
, 14, 15, and 16 missions were functional until they were switched off in 1977. Moonquakes are not believed to be caused by
tectonic plate Plate tectonics (, ) is the scientific theory that the Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since 3–4 billion years ago. The model builds on the concept of , an idea developed durin ...
movement (as earthquakes are), but by
tidal force The tidal force or tide-generating force is the difference in gravitational attraction between different points in a gravitational field, causing bodies to be pulled unevenly and as a result are being stretched towards the attraction. It is the ...
s between Earth and the Moon. Further data is needed to clarify the origins and effects of the forces causing moonquakes. The
Chandrayaan-3 Chandrayaan-3 ( ) is the third mission in the Chandrayaan programme, a series of Exploration of the Moon, lunar-exploration missions developed by the ISRO, Indian Space Research Organisation (ISRO). The mission consists of a Chandrayaan-2#Vikra ...
mission by
Indian Space Research Organisation The Indian Space Research Organisation (ISRO ) is India's national List of government space agencies, space agency, headquartered in Bengaluru, Karnataka. It serves as the principal research and development arm of the Department of Space (DoS), ...
had a payload ILSA (Instrument for Lunar Seismic Activity), to acquire data about moonquakes. It detected a presumed natural event on 26 August. It is suspected to be a moonquake.


Major findings


Moonquakes

Several categories of
moonquakes A quake is the result when the surface of a planet, moon or star begins to shake, usually as the consequence of a sudden release of energy transmitted as seismic waves, and potentially with great violence. The types of quakes include earthquake, ...
were recorded. Hundreds of deep moonquakes were recorded along with 28 shallow events. The deeper quakes are caused by tidal forces with the Earth and tended to occur in clusters. The shallow events have tectonic origins. Although more rare than deep events, the shallow events were larger, with body wave magnitudes > 5.5 and stress drops exceeding 100 MPa. Other sources of seismic activity included meteorite impacts and artificial signals from lunar modules.


Structure of the Lunar Interior

One key finding was an improved understanding of the structure of the deep lunar interior, including the existence of a solid inner-core and sharp core-mantle boundary and a partial-melt layer at the base of the lunar mantle. The solid core has a radius of about 240 km and is surrounded by a much thinner liquid outer core with a thickness of about 90 km. The partial melt layer sits above the liquid outer core and has a thickness of about 150 km. The mantle extends to within 45 ± 5 km of the lunar surface.


Limitations of the current dataset

*Scattering from the megaregolith Strong variations in material properties near the surface of the Moon, likely caused by a long history of impact cratering, are thought to be responsible for the complex seismic waveforms that lack clear reflected arrivals that would provide clear seismological evidence of a lunar core. *Selenographical distribution All of the Apollo seismometers were placed on the near side of the Moon. The relative dearth of moonquakes observed on the far side of the Moon has been interpreted as either (1) evidence for an attenuating core or (2) observational bias given that it is easier to detect events of a fixed magnitude that are closer to the sensors.


Future plans

Due to the success of the Apollo seismometers, several space agencies including
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
have expressed interest in funding future seismic missions to the Moon. NASA's Planetary Science Decadal Survey for 2012-2022 lists a lunar geophysical network as a recommended New Frontiers mission. The mission would be tasked with enhancing the knowledge of the lunar interior using several identical landers distributed over the lunar surface. A network of arrays would be able to better constrain lunar seismicity, especially on the far side of the Moon. In early 2018,
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
established the Development and Advancement of Lunar Instrumentation (DALI) program. DALI funds research to support the Science Mission Directorate's Planetary Science Division, including the desired lunar geophysical network. NASA awarded five DALI grants in 2024, including research on ground-penetrating radar and a magnometer system for determining properties of the lunar core.


References

{{The Moon Geology of the Moon Seismology