Planetary Seismology

Buzz Aldrin deploying a seismometer on the Moon during the Apollo 11 mission. (Image courtesy of NASA.)
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Seismology can potentially reveal internal structure and dynamic processes of other rocky bodies—planets, moons, and asteroids— in the solar system, if seismic sensors can be deployed and data retrieved. A very limited amount of seismic data obtained from the Moon during the Apollo program revealed unique and fundamental information about the Moon’s internal structure, including thickness of the surface regolith layer, presence of a low-velocity zone near a depth of 400 km, very low seismic attenuation indicative of very small quantities of fluids in the crust, and the possible existence of a partially molten silicate core. Deployments of seismometers on other planetary bodies can potentially address many significant scientific questions, such as the existence and radius of planetary liquid cores, the extent of water and temperatures within the crust and mantle of Mars, the dimensions of the salt-water ocean on Europa, and the reason for the lack of a magnetic field on Venus. Although every planet presents formidable challenges to seismological approaches, the long reach of seismological methods can provide a bountiful return of important information that cannot be obtained by any other method.
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A return of seismometers to the Moon would provide opportunities to explore outstanding basic questions, including: Does the Moon’s internal structure support the model of lunar formation from ejecta of a large impact on Earth? What is the nature of the mantle-core boundary within the Moon, and what is its connection with deep moonquakes? What is the physical mechanism that controls the correlation between moonquakes and tidal stresses excited by Earth’s gravitational field? Are the mechanisms of failure for deep lunar quakes similar to the mechanisms responsible for deep earthquakes on Earth? Are these events related to solid phase changes in silicate minerals? How large are lateral heterogeneities in composition and structure, as determined using 3D tomography?
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A similar broad range of topics can be addressed by deploying seismometers on Mars, if engineering challenges of designing, building, and deploying rugged seismometers protected from extreme temperatures, winds, and cosmic radiation, can be overcome. Mars is likely to be relatively lacking in tectonic faulting processes, but mapping the crust and lithosphere will be feasible using artificial sources and the new seismic technique of analyzing correlated noise excited by the strong atmospheric winds. Key topics that could be addressed include the radial layering of the crust mantle and core of the planet, the distribution of groundwater/ ice in the near surface, and the internal structural variations associated with the presently enigmatic bimodal surface morphology of the planet. Determining the frequency of impacts and how strongly they vibrate the surface is also of interest.
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Venus and Mercury present formidable environmental challenges, but seismological technologies that can overcome them may be within reach. Smaller planetary bodies like Europa, Ganymede, and Enceladus are good targets for using seismological methods to determine the presence and extent of internal fluids. Asteroids have highly uncertain material properties, and design of seismological probes of their interiors can complement other approaches such as ground-penetrating radar. Given the great payoff from even limited seismic recordings, every mission to a solid body in the solar system should include consideration of the potential for seismological instrumentation and data collection.


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