5. The Moon: stepping stone to the planets
The lunar surface
Rocks from the Apollo missions
During the Apollo landings from 1969 to 1972 a dozen people roamed the Moon taking hundreds of rock samples, placing them in labeled bags, and returning 382 kilograms of Moon rocks to Earth in sealed containers. These specimens from another world have permitted scientists to decipher the composition of the lunar crust, and to reconstruct our satellite's history.
None of the rocks brought back from the Moon contain any moisture or hydrated minerals, and they show no signs of having been exposed to water. The oxygen that is on the Earth in rocks and water forms only rocks on the Moon. So, there is no water in the Moon rocks, and there hasn't been any for billions of years. The Moon is lifeless, as we might expect from the lack of water. Extensive testing revealed no evidence for life, past or present, among the lunar samples. They contain no living organisms, fossils or native organic compounds. Thus, the Moon is a desolate place, barren of life.
Scanning the surface
The Apollo rock and soil samples came from only six sites on the near side, chosen mainly to be safe and easy to get to. A global view of the Moon's surface composition therefore had to wait until the Clementine spacecraft surveyed the unexplored regions on both the near and far sides.
The Clementine global data was used to map the abundance and distribution of iron on the Moon, showing that the dark, near-side maria consist of iron-rich lava, containing up to 14 percent iron by weight. In contrast, iron is practically absent in the near-side highland crust and across vast tracts of the far side, at about 3 percent iron by weight. These regions of very low iron content are dominated by aluminum-rich anorthosite.
Until Clementine, we also had no global map of the topography of the Moon. The laser altimeter on the spacecraft fired pulses of light at the Moon once every second and timed how long it took for the light beam to travel down to the lunar surface and back again. This enabled scientists to determine the distance to the surface, over and over again, with an accuracy of 50 meters. When these distances were combined with knowledge of the spacecraft orbit, maps of the elevation, or topography, of the entire lunar surface were obtained.
Possible water ice at the lunar poles
Bright radar echoes, returned to Clementine from the south pole of the Moon, suggested that this region may contain radar-reflective water ice. Lunar Prospector then strengthened the possibility of water ice at the south pole, and also discovered what appears to be additional ice near the north pole. During its passes over the poles, an instrument on Lunar Prospector detected substantial quantities of hydrogen, which mission scientists attributed to water ice found in permanently shaded areas near both poles. They estimate that there could be as much as 6 billion tons, or 6 x 1012 kilograms, of water ice located in the polar regions.
Since the Moon's rotation axis is orientated nearly perpendicular to the ecliptic plane, the lunar poles are never tilted toward the Sun by more than a very small amount. This means that the bottoms of craters at the poles are in constant shadow and in a perpetual deep freeze of 50 to 70 degrees kelvin. Any ice deposited in these frozen reservoirs would be preserved indefinitely in the eternal dark and cold.
The magnetized Moon
The Moon has no overall dipolar magnetic field, at least none that is strong enough to be detected. Its magnetic moment is at least 10 million times weaker than the Earth's. Yet, some of the lunar rocks returned to Earth are magnetized. They have survived since the time that molten rocks covered the Moon and solidified 3 to 4 billion years ago, preserving fossilized remnants of ancient magnetic fields.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University