Planetary Seismology and Interior Structure

Core Idea

Seismic waves from quakes and impacts propagate through planetary interiors, reflecting and refracting at boundaries with different acoustic properties. Analysis of waveforms (P-waves, S-waves, surface waves) reveals interior velocity structure and compositional boundaries without requiring direct sampling. Seismology has revolutionized understanding of lunar and Martian interiors.

Explainer

You already know that seismic waves travel through rock at speeds determined by the material's density and elastic properties, and that P-waves (compressional) and S-waves (shear) behave differently — P-waves travel through both solids and liquids, while S-waves propagate only through solids. On Earth, this difference is what revealed the liquid outer core: S-waves vanish in a "shadow zone" because they cannot pass through molten iron. Planetary seismology extends exactly the same logic to other worlds, using quakes, impacts, or artificial sources to illuminate interiors that are otherwise completely inaccessible.

The Apollo missions placed seismometers on the Moon between 1969 and 1972, providing the first extraterrestrial seismic dataset. Lunar seismology revealed a crust about 30–45 km thick, a mantle with distinct upper and lower regions, and a small, partially molten core. The Moon turned out to be a surprisingly noisy body — deep moonquakes occur in clusters at specific depths around 700–1,100 km, triggered by tidal stresses from Earth's gravity. These repeating sources acted as natural controlled experiments, since waves from the same location but recorded at different stations illuminated different interior paths. One striking feature of lunar seismograms is their extreme duration: seismic signals ring for over an hour because the dry, fractured lunar crust scatters energy rather than absorbing it, unlike Earth's water-saturated rocks that damp vibrations quickly.

NASA's InSight mission, which landed on Mars in 2018, brought planetary seismology into the modern era. Its single broadband seismometer, SEIS, detected over a thousand marsquakes during its operational lifetime. The data revealed that Mars has a thick crust (24–72 km depending on location), a mantle with seismic velocities suggesting an olivine-rich composition similar to Earth's upper mantle, and a liquid iron-alloy core with a radius of roughly 1,830 km — larger and less dense than expected, implying a significant fraction of light elements like sulfur dissolved in the core. This finding reshaped models of Martian formation and thermal history. InSight also recorded seismic signals from meteorite impacts, providing both seismic data and independently located sources, which tightened constraints on crustal structure.

The fundamental challenge of planetary seismology is working with far fewer stations than Earth-based networks. Earth has thousands of seismometers; the Moon had four simultaneously active stations at best; Mars had one. With fewer stations, locating quake sources and resolving interior structure requires creative techniques — using surface wave dispersion, reflected phases, and receiver functions to extract maximum information from limited data. Despite these constraints, seismology remains the most powerful tool for determining what lies beneath a planetary surface, and future missions to Europa, Titan, and Venus all include seismic instrumentation in their concept studies.