Moon Structure
LUNAR MAGNETIC FIELD
There is no general lunar magnetic field as large as approximately one-ten-thousandth that of the earth, which seems to indicate that the moon does not now have the molten iron-nickel core comparable to that of the earth believed necessary to produce a magnetic field. But evidence suggests that the moon may have had a stronger magnetic field early in its history. Random magnetic fields up to about 0.6 percent of the earth's field intensity were detected at different sites by the Apollo astronauts, but we do not know the reason for such magnetic anomalies.
SEISMIC ACTIVITY
From seismographs left on its surface we know that seismic events on the moon follow patterns different from those here on the earth. Moonquakes, rare meteorite impacts, and artificially produced vibrations (grenade explosions and crash landings of discarded spacecraft) are transmitted very slowly through the lunar material. They build gradually and then take up to an hour to subside.
Some seismic disturbances have been traced to geologic movements in the rilles; others, to occasional impacts of meteoroid swarms. Moonquakes frequently coincide with tidal stresses triggered by the varying distance between moon and earth. They occur at depths of 600 to 900 kilometers, much deeper than earthquakes. About 80 sou rces for these deep moonquakes have been discovered so far. But compared with the earth's seismic activity, the moon's is fairly subdued; the whole moon releases less than one-tenbillionth of the earth's earthquake energy.
About 35 shallow quakes, presumably tectonic events, have been detected. Thus if the moon is expanding or contracting, it is doing it extremely slowly. In the last 3 billion years thermal and geological activity has been relatively rare. As we have mentioned, most volcanic activity appears to have ceased about 3 billion years ago, but some minor activity may still be going on.
MODEl FOR THE INTERIOR
Seismic data tells us that the crust is about 60 kilometers thick, twice the thickness in kilometers of the earth's crust. The basaltic material of the maria was formed by the impact of large bodies, which provided the energy to melt partially the iron-rich regions below the crustal layer. Heat flow from the deep interior through the lunar crust is no more than about a third of that for the earth. Thus thermally driven processes cannot be so important as for the earth.
The moon's mantle, nearly 800 to 1000 kilometers deep under the lunar crust, is uniformly structured. Most of it may be pyroxene and olivine, minerals containing silicon, oxygen, calcium, magnesium, and iron. The seismic data reinforce the view that the moon's core is unlike earth's metallic core. Probably the lunar core consists of partly molten silicates, with a small metallic center. At present scientists can neither rule out the existence of a small iron core nor prove that one does not exist.
Only a few decades ago the moon was a light in the sky that, even though near, was still part of the remote cosmos. Now it is a place that has been visited by human beings and studied to such a close extent that it seems no longer to be a cosmic body. What is its future role in the affairs of man? There are proposals to establish permanent bases on the moon for astronomical observatories, mining operations, ore refining, or manufacturing; and it is not impossible that moon the moon may become the departure station for manned exploration of the solar system. But from any point of view it is no longer a strange aand distant world.
MODEL FOR THE INTERIOR
Seismic data tells us that the crust is about 60 kilometers thick, twice the thickness in kilometers of the earth's crust. The basaltic material of the maria was formed by the impact of large bodies, which provided the energy to melt partially the i ron-rich regions below the crustal layer. Heat flow from the deep interior through the lunar crust is no more than about a third of that for the earth. Thus thermally driven processes cannot be so important as for the earth.
The moon's mantle, nearly 800 to 1000 kilometers deep under the lunar crust, is uniformly structured. Most of it may be pyroxene and olivine, minerals containing silicon, oxygen, calcium, magnesium, and iron. The seismic data reinforce the view that the moon's core is unlike earth's metallic core. Probably the lunar core consists of partly molten silicates, with a small metallic center. At present scientists can neither rule out the existence of a small iron core nor prove that one does not exist.
