Apollo Seismometers Measured 3 Components
All of these seismometers measured all three components of ground displacement;
- Up-down
- East-west
- North-south
And the Passive Seismic Experiment was the very first seismometer put on the Moon’s surface. The gadget detected lunar “moonquakes” and gave information about the internal structure of the Moon.
Furthermore, this experiment investigated the distribution of seismic waves through the Moon and presented the first detailed look at the Moon’s internal structure.
So, this device contained four seismometers powered by two panels of solar cells, which turned solar energy into electricity.
Passive Seismic Experiment
The Passive Seismic Experiment used three long-period seismometers and one short-period vertical seismometer for estimating meteorite impacts and moonquakes. The instrument recorded around 100 to 200 hits by meteorites during its entire lifetime.
Data about the duration, strength, and relative direction of the seismic event were relayed to tracking stations back on Earth.
And, because the instrument was powered by solar cells, the tests were only operated during the lunar days.
The First Use of Nuclear Energy In a NASA Crewed Mission
Throughout the 340 hours a Moon night, when temperatures can fall to minus 170ºC, the device was kept to a minimum of minus 54ºC by a radioisotope heater.
It was the first significant use of nuclear energy in a NASA manned mission. And any temperature below this could destroy the instrument.
Furthermore, at the other end of the scale, the scientists tried regulating the daytime heat on the electronic parts by a series of power dumps, cutting off the electrical power systems.
The Apollo Seismic Instrument Package
And then, merely before the lunar night began, the seismometer automatically turned into stand-by mode, halting the transmission of all data.
The seismic instrument package ceased responding to commands at 0400 UT on August 25, 1969, presumably because it was overheating from the blazing and scorching midday sun.
Later Apollo Moon Missions Used an Alternate Design
Next, an alternate design was flown on later Apollo Moon missions.
Furthermore, the Apollo 11 seismometer sent back data in just three weeks but gave a useful first look at the moon’s seismology.
More superior seismometers were deployed at the Apollo 12, 14, 15, and 16 landing sites. And they transmitted data back to Earth until September 1977.
All of these seismometers measured all three components of ground displacement: up-down, north-south, and east-west.
Summary
So, the passive seismic experiment produced several critical scientific results. They were as follows:
Distribution of Lunar Seismic Sources
More numerous than 1700 lunar meteoroid impacts were recorded by the seismometer network, with impactor sizes estimated to be between 0.5 and 5000 kilograms.
And most moonquakes happen at depths of 800-1000 kilometers. Those occur at monthly periods at around 100 different sites. That indicates that the moonquakes are caused by stresses from various changes in lunar tides as the Moon orbits the Earth.
However, the moonquakes are considerably small, often with Richter scale magnitudes less than 2.
Furthermore, the volume of the energy discharged from earthquakes in a typical year is around ten million times larger than that released by moonquakes in a year. Just a few near-surface moonquakes were detected.
Understanding of Lunar Interior Structure
Like Earth, the Moon has a core, mantle, and crust. The moon’s crust is abundant in the mineral plagioclase and has an average crustal depth of 60-70 kilometers, which is about three times the medium crustal thickness on Earth.
The Moon’s mantle extends between the core and the crust and consists mainly of the minerals pyroxene and olivine.
The core is presumably made of mostly sulfur and iron and extends from the center of the Moon out to a range of no more than 450 kilometers, i.e., the core range is less than 25% of the Moon’s radius, and this is quite small.
In contrast, the Earth’s core radius is 54% of the Earth’s radius. Nevertheless, the size of the moon core is not well constrained by existing seismic observations.
So, therefore, better constraints come from the laser-ranging retroreflector and magnetometer experiments.
Attenuation of Seismic Waves
Meteoroid Moon impacts cause massive fracturing in the upper twenty kilometers of the lunar crust.
These fractures, in turn, produce scattering of seismic waves in these regions.
Below twenty kilometers, seismic wave scattering diminishes due to either the closure of these fractures due to rising pressure or a change in the chemical composition of the crust.
Seismic Wave Attenuation is Enhanced at High Temperatures
In the mantle, seismic waves are attenuated considerably less on the Moon than they are on Earth.
Seismic wave attenuation is intensified at high temperatures and in the presence of water. And the low attenuation on the Moon indicates a cool, dry interior.
And because the Moon is much smaller than Earth, it is expected to have cooled more rapidly, creating a cold interior.
The lack of water may be due either to the failure of the Moon to store water when it formed or to a subsequent loss of water to space.
So, below 1000 kilometers depth, seismic wave attenuation increases, perhaps indicating the presence of a small amount of molten Moonrock.
The Apollo Passive Seismic Experiment continued until the command uplink was lost on August 25, 1969. The downlink failed on December 14, 1969. And as of 2018, the Lunar Laser Ranging experiment continues to be operational.
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