Questions: Planetary Seismology and Interior Structure

Question 1

A seismometer on a distant planetary station records P-waves arriving from a large quake but detects no S-waves. What does this most directly imply about the interior the waves traveled through?

• A. The planet has no seismic activity strong enough to generate S-waves
• B. The waves traveled through a liquid region, which transmits P-waves but blocks S-waves
• C. The planet's crust is too thin to sustain S-wave propagation
• D. The seismometer is too far from the quake source to detect S-waves

Answer: (B)

S-waves are shear waves that require a solid medium; they cannot propagate through liquids. An S-wave shadow zone is therefore direct evidence of a liquid layer in the wave path. This is exactly how Earth's liquid outer core was discovered — and the same logic applies to any planetary body. Distance alone does not prevent S-wave detection; both wave types attenuate with distance, but the key difference is physical state of the medium.

Question 2

Lunar seismograms from Apollo show signals ringing for over an hour after a quake, far longer than on Earth. What explains this?

• A. The Moon experiences much stronger quakes than Earth
• B. The dry, fractured lunar crust scatters seismic energy rather than absorbing it, prolonging the signal
• C. The Moon's large liquid core amplifies and re-emits seismic energy
• D. Apollo seismometers were more sensitive than modern Earth instruments

Answer: (B)

On Earth, water-saturated rocks in the crust absorb seismic energy quickly, damping the signal. The Moon's crust is dry and highly fractured, so energy scatters rather than dissipates — the signal bounces around for a very long time. The Moon's crust having no liquid core to speak of, and seismometer sensitivity being a design choice unrelated to signal duration, make the other options incorrect.

Question 3

The InSight mission determined that Mars has a liquid iron-alloy core with a radius of about 1,830 km without any drill reaching it.

• T. True
• F. False

Answer: True

InSight's single broadband seismometer detected marsquakes whose seismic wave travel times and reflections revealed interior velocity boundaries, including the core-mantle boundary. The liquid state was inferred from seismic behavior (P-waves can pass through, S-waves cannot), just as Earth's liquid outer core was discovered seismically decades before any direct sampling was even conceivable. This is the power of seismology: access to interiors that are otherwise completely unreachable.

Question 4

Having only one seismometer on a planet makes it very difficult to determine anything meaningful about planetary interior structure.

• T. True
• F. False

Answer: False

InSight operated with a single seismometer on Mars and still resolved crustal thickness, mantle seismic velocities, and core radius. Techniques such as surface wave dispersion analysis, receiver functions, and using independently located sources (like meteorite impacts) extract interior information from a single station. Fewer stations mean more uncertainty and creative analysis, but not a complete absence of information.

Question 5

Why can seismology reveal the interior structure of a planet without any direct sampling or drilling?

Model answer: Seismic waves change speed and direction whenever they cross a boundary between materials with different densities or elastic properties. P-waves and S-waves reflect, refract, and convert at these boundaries, and the arrival times and waveforms recorded at the surface encode the entire path traveled. Because S-waves cannot pass through liquids, their absence from certain azimuths identifies liquid regions. By analyzing which phases arrive when, scientists reconstruct the layered velocity structure — and thus the composition and physical state — of the deep interior.

The key principle is that wave behavior at boundaries is diagnostic: reflections indicate boundary depth, refraction angles indicate velocity contrast, and the absence of S-waves indicates liquid. This works across any planet because it depends only on the physics of wave propagation in elastic media, not on any planet-specific assumption.