MOONQUAKES, THE MOON'S
INTERIOR, AND THE MYSTERIOUS
MAGNETIC FIELD

We know about moonquakes from four seismometers set up by the Apollo missions. Besides telling us how many and how strong moonquakes are, the data acquired by the Apollo seismic network help us figure out something about the nature of the Moon's interior. On Earth, seismology has allowed us to know that the planet has a thin crust (20-60 km over continents, 8-10 km over ocean basins), a thick silicate mantle (down to 2900 km), and a large metallic iron core (2900 km to the center at 6370 km). The Moon is quite different. The crust is thicker than Earth's continental crust, ranging from 70 km on the Earth-facing side to perhaps 150 km on the farside. The mare basalts represent a thin veneer on this mostly plagioclase-rich crust, averaging only about 1 km in thickness (inferred mostly from photogeological studies). Evidence from samples collected on the rims of the large basins Imbrium and Serentatis and from remote sensing instruments carried onboard two Apollo missions, the Clementine Mission, and the forthcoming Lunar Prospector Mission suggest that the lower crust may not contain as much plagioclase as does the upper half of the crust. Beneath the crust is the lunar mantle, which is the largest part of the Moon. There might be a difference in rock types above and below a depth of 500 km, perhaps representing the depth of the lunar magma ocean. Beneath the mantle lies a small lunar core made of metallic iron. The size of the core is highly uncertain, with estimates ranging from about 100 km to 400 km.

That little core is important, though. The Moon does not have much of a magnetic field, so the lunar core is not generating magnetism the way Earth's core is. Nevertheless, it did in the past. Lunar rocks are magnetized, and the strength of the magnetic field has been measured by special techniques. Also, older rocks have stronger magnetism, suggesting that the Moon's magnetic field was stronger in the distant past, and then decreased to its weak present state. Why this happened is unknown. What is known is this: you cannot navigate around the Moon using a compass!

There are other mysteries about the Moon's magnetism. Although the field was always weak and is extremely weak now, there are small areas on the Moon that have magnetic fields much stronger than the surrounding regions. These magnetic anomalies have not been figured out. Some scientists have associated them with the effects of large, basin-forming impacts. Others have suggested that the ionized gases produced when comets impact the Moon might give rise to strong magnetic anomalies in the crater ejecta. The jury is still out. The Lunar Prospector Mission will thoroughly map the distribution of magnetic anomalies, perhaps helping to solve this mystery.

THE MOON'S ORIGIN: A BIG WHACK
ON THE GROWING EARTH

For a long time, the most elusive mystery about the Moon was how it formed. The problem baffled philosophers and scientists for hundreds of years. All of the hypotheses advanced for lunar origin had fatal flaws, even though partisans tried tenaciously to explain away the defects. The capture hypothesis, which depicts capture of a fully formed Moon by Earth, suffered from improbability. Close encounter with Earth would either result in a collision or fling the Moon into a different orbit around the Sun, probably never to meet up with Earth again. The fission hypothesis, in which the primitive Earth spins so fast that a blob flies off, could not explain how Earth got to be spinning so fast (once every 2.5 hours) and why Earth and the Moon no longer spin that fast. The double-planet hypothesis pictures Earth and the Moon forming together, a two-body system from the start. This idea has trouble explaining Earth's rotation rate and how the moon-forming material got into orbit around Earth and stayed there, rather than falling to Earth. (These problems with total amount of spinning involve both Earth's rotation and the Moon's motion around Earth. The amount of rotation and revolving is quantified by a physical property called angular momentum.) The problem was so frustrating that some scientists suggested that maybe science had proved that the Moon does not exist!

The annoying problems with the classical hypotheses of lunar origin led scientists to consider alternatives. This search led to the seemingly outlandish idea that the Moon formed when a projectile the size of the planet Mars (half Earth's radius and one-tenth its mass) smashed into Earth when it had grown to about 90% of its present size. The resulting explosion sent vast quantities of heated material into orbit around Earth, and the Moon formed from this debris. This new hypothesis, which blossomed in 1984 from seeds planted in the mid-1970s, is called the giant impact theory. It explains the way Earth spins and why Earth has a larger metallic core than does the Moon. Furthermore, modern theories for how the planets are assembled from smaller bodies, which were assembled from still smaller ones, predict that when Earth was almost done forming, there would have been a body nearby with a mass about one-tenth that of Earth. Thus, the giant impact hypothesized to have formed the Moon is not an implausible event. The chances are so high, in fact, that it might have been unavoidable.

One would think that an impact between an almost Earth-sized planet and a Mars-sized planet would be catastrophic. The energy involved is incomprehensible. Much more than a trillion trillion tons of material vaporized and melted. In some places in the cloud around the Earth, temperatures exceeded 10,000°C. A fledgling planet the size of Mars was incorporated into Earth, its metallic core and all, never to be seen again. Yes, this sounds catastrophic. But out of it all, the Moon was created and Earth grew to almost its final size. Without this violent event early in the Solar System's history, there would be no Moon in Earth's sky, and Earth would not be rotating as fast as it is because the big impact spun it up. Days might even last a year. But then, maybe we would not be here to notice.

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