Apollo 16 Mission
Science Experiments - Passive Seismic
The Passive Seismic Experiment detected lunar "moonquakes" and provided information about the internal structure of the Moon.
The Passive Seismic Experiment studied the propagation of seismic waves through the Moon and provided our most detailed look at the Moon's internal structure. The Apollo 11 seismometer returned data for just three weeks but provided a useful first look at lunar seismology. More advanced seismometers were deployed at the Apollo 12, 14, 15, and16 landing sites and radioed data to Earth until September 1977. Each of these seismometers measured all three components of ground displacement (up-down, north-south, and east-west).
If a seismic event is observed by three or more seismometers, the time and location of the event can be determined. Because seismic waves from distant events travel deeper into the Moon than waves from nearby events, by measuring events at various distances from the seismometer, one can determine how seismic velocities vary with depth in the Moon. In turn, this information can be used to study the Moon's internal structure. Most of the events observed by the seismometers were due either to moonquakes or to meteoroid impacts. However, the third stages of several Saturn 5 rockets and the ascent stages of several lunar modules were deliberately crashed into the Moon after these spacecraft were no longer needed. These man-made crashes produced seismic events of known times and locations and helped to calibrate the network of seismometers.
The Passive Seismic Experiment produced several important scientific results:
Knowledge of Lunar Interior Structure. Like the Earth, the Moon has a crust, mantle, and core. The lunar crust is rich in the mineral plagioclase and has an average crustal thickness of 60-70 kilometers, which is about 3 times the average crustal thickness on Earth. The lunar mantle lies between the crust and the core and consists mostly of the minerals olivine and pyroxene. The core is probably composed mostly of iron and sulfur and extends from the center of the Moon out to a radius of no more than 450 kilometers, i.e., the core radius is less than 25% of the Moon's radius, which is quite small. In comparison, the Earth's core radius is 54% of the Earth's radius. However, the size of the lunar core is not well constrained by existing seismic observations. Better constraints come from the laser ranging retroreflector and magnetometer experiments.
Distribution of Lunar Seismic Sources. More than 1700 meteoroid impacts were recorded by the seismometer network, with impactor masses estimated to be between 0.5 and 5000 kilograms. Most moonquakes occur at depths of 800 to 1000 kilometers. These occur at monthly intervals at about 100 distinct sites, indicating that these moonquakes are caused by stresses from changes in lunar tides as the Moon orbits the Earth. These moonquakes are quite small, mostly with Richter scale magnitudes less than 2. The amount of energy released by earthquakes in a typical year is about 10 million times larger than that released by moonquakes in a year. Only a few near-surface moonquakes were detected.
- Attenuation of Seismic Waves. Meteoroid impacts cause heavy fracturing in the upper 20 kilometers of the lunar crust. These fractures in turn cause scattering of seismic waves in these regions. Below 20 kilometers, seismic wave scattering decreases as a result of either closure of these fractures due to increasing pressure or of a change in chemical composition of the crust. In the mantle, seismic waves are attenuated much less on the Moon than they are on Earth. Seismic wave attenuation is enhanced at high temperatures and in the presence of water, and the low attenuation on the Moon indicates a cold, dry interior. Because the Moon is smaller than Earth, it is expected to have cooled more rapidly, producing a cold interior. The absence of water may be due either to the failure of the Moon to accumulate water when it formed or to subsequent loss of water to space. Below 1000 kilometers depth, seismic wave attenuation increases, possibly indicating the presence of a small amount of molten rock.