Large-scale seismic data were obtained from seismometers located on the Moon by the NASA Apollo missions from 1969 to 1977. According to previous analysis of the lunar seismic data, we found that deep moonquakes occur periodically from identical sources at a depth of about 700 to 1200km.
The Passive Seismic Experiment was the first seismometer placed on the Moon’s surface. It detected lunar "moonquakes" and provided information about the internal structure of the Moon.
This experiment studied the propagation of seismic waves through the Moon and provided the first detailed look at the Moon's internal structure.
This deviceused three long-period seismometers and one short- period vertical seismometer for measuring meteorite impacts and moonquakes, recording about 100 to 200 hits by meteorites during its lifetime.
Data regarding the strength, duration, and approximate direction of the seismic event were relayed to tracking stations on Earth.
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, and 16 landing sites and transmitted data to Earth until September 1977.
he ALSEP program began on March 31 1963 with a series of meetings between NASA Headquarters, the Jet Propulsion Laboratory (JPL), and the Goddard Space Flight Center (GSFC). The first choice of experiments was proposed in December 1963, and consideration was given to those experiments that promised maximum return for least weight and complexity. Suggested experiments were active and passive seismic devices, instruments to measure the surface bearing strength, magnetic field, radiation spectrum, soil density, and gravitational field.
During July 1965, the National Academy of Sciences Space Science Board met at Woods Hole, Massachusetts, to consider the most desirable areas of space study. The board proposed 15 major items for lunar exploration. Some of them were:
What is the internal structure of the Moon?
What is the actual geometric shape of the Moon?
What is the present internal energy regime of the Moon?
What is the composition of the lunar surface?
What principal processes were responsible for the present structure of the Moon?
What is the present tectonic pattern and distribution of tectonic activity on the Moon?
What are the dominant processes of erosion, transport, and deposition of material on the lunar surface?
What volatile substances are present on or near the lunar surface?
Are there organic and/or proto-organic molecules on the Moon?
What is the age of the Moon?
What is the history of dynamic interaction between the Earth and the Moon?
From the list of questions from the Woods Hole meeting, it was decided it was possible to design a complete lunar geophysical station, which would give this information. It could be housed in a storage bay of the Lunar Module and be deployed on the surface of the Moon by the astronauts.
This instrument contained four seismometers powered by two panels of solar cells, which converted solar energy into electricity. It used three long-period seismometers and one short- period vertical seismometer for measuring meteorite impacts and moonquakes, recording about 100 to 200 hits by meteorites during its lifetime. Data regarding the strength, duration, and approximate direction of the seismic event were relayed to tracking stations on Earth. Because it was only powered by solar cells, the experiment only operated during the lunar days. During the 340 hour lunar night, when temperatures can plummet to minus 170ºC the instrument was kept to a minimum of minus 54ºC by a radioisotope heater, the first major use of nuclear energy in a NASA manned mission. Any temperature below this could damage the instrument.
At the other end of the scale the scientists tried controlling the daytime heat on the electronic components by a series of power 'dumps', cutting off the systems electrical power. Then, just before the lunar night began, the seismometer automatically shifted into stand-by mode, stopping transmission of all data. The seismic instrument package stopped responding to commands at 0400 UT August 25 1969, probably from overheating from the hot midday sun. An alternate design was flown on later missions.
To determine sub-surface properties and measure extremely small vibrations of the lunar surface caused by distant moonquakes, man-made explosions, and spacecraft impacts. When the instrument vibrated in response to movement of the ground surface, the inertia of the central lever and the mass on the end caused it to vibrate in sympathy, which was detected electronically by the capacitor effect of the mass on the end of the lever. An internal set of motors kept the seismometers constantly level within a few seconds of arc. Seismic motions were recorded on Earth with a magnification factor of 10 million. The network of four instruments deployed during Apollo enabled the seismologists to locate moonquakes in three dimensions, and to study the seismic velocities and propagation characteristics of the lunar subsurface materials. The PSEs measured daily meteorite impacts and an average of two moonquakes per month, up to depths of 800 kilometers. Earthquakes on the Earth exceed one million per year. On the Moon, there may be up to 300. And they are much smaller than the ones on Earth. It was noted that there was increased activity when the Moon was farthest from, and nearest to, the Earth. Signals generated by heating at sunrise on the Moon's surface was recorded by ALSEP each lunar day.
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, and 16 landing sites and transmitted 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 separated by distance, 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 onto the Moon after they were discarded. These man-made crashes produced seismic events of known times and locations and helped to calibrate the network of seismometers.
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 50 kilometers. 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 sulphur 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 came from the laser ranging retro-reflector and magnetometer experiments.
Distribution of Lunar Seismic Sources. More than 1,700 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 total absence of water on the Moon is due to its formation from dry volatile-depleted material, a consequence of the impact of a Mars-sized body with the Earth. Below 1000 kilometers depth, seismic wave attenuation increases, possibly indicating the presence of a small amount of molten rock.
Scientists hoped that moonquakes and meteorite impacts would answer two fundamental questions: (1) does the Moon have a molten core; and (2) what is the deep interior of the Moon like? To answer these questions they needed at least one impact event of at least 1019 ergs on the far side of the Moon. On May 13 1972 a near-side event with an energy of 1,100 kilograms was recorded. On September 19 1973 a large back-side event with an energy of 1018 ergs took place. On July 17 1972, only three months after it was set up, Apollo 16’s seismometer registered the largest impact ever recorded on the moon when a meteorite hit the far side of the moon near Mare Moscoviense, located at 26ºN 147ºE.
Over the 8 years of the ALSEP’s lives, around 10,000 moonquakes and 2,000 meteorite impacts were registered by the seismometers.
