Apollo Expeditions to the Moon|
IMPLANTING SENSORS ON THE MOONSerious space probe studies of the Moon began with the Soviet Zond and Luna, and with the U.S. Ranger, Surveyor, and Lunar Orbiter. Armed with the information obtained by unmanned probes, the United States carried out the Apollo manned missions, implanting nuclear-powered geophysical laboratories at several landing sites, including seismometers, magnetometers, plasma and pressure gauges, heat flow instruments, and laser corner reflectors. These laboratories continue to operate long after the astronauts have returned to Earth. On two missions, the lunar surface laboratories were supplemented by satellites left in orbit to support geodetic studies. Detailed studies of lunar surface composition by X-ray fluorescence and radioactivity measurements were made from the Apollo spacecraft itself. Most importantly. the astronauts brought back from their six landing missions hundreds of pounds of lunar rocks and soil, which, together with the small amount returned by Soviet unmanned sample missions, have since been the subject of analysis and study by scientists around the world.
How old is the Moon? This was the first major question. Although a few thought the Moon would prove to be considerably younger than Earth. most felt it would turn out to be of the same vintage as our own planet. From radioactive dating of the lunar rocks and soil it is now established that the Moon formed about 4.6 billion years ago. The Moon is, indeed, very old, as old as Earth, and is telling us much about the earliest years of our planet and of the solar system. But to our great surprise the lunar rocks show that the Moon's surface is no longer in its original condition, having undergone considerable change in the first 1.5 billion years. A few of the highland rocks in the Apollo samples are older than 4 billion years - at least one is 4.6 billion years old, the probable time of formation and initial melting of the Moon - but most of them have been shocked and remelted by meteoroid impacts 4 billion years ago. Most of the mare lava flows are from 3 to slightly less than 4 billion years old. indicating that substantial melting and flow occurred on the Moon long after its formation. Gravitational energy released during the aggregation of the Moon, heat generated by radioactive elements in the lunar material, and thermal effects of the huge impacts that sculptured the lunar surface all undoubtedly contributed to the early melting of the outer layers.
What is the physical condition of the lunar surface? The question generated countless heated debates before the lunar landings. It was clear from the visible meteorite cratering that the surface must be thoroughly chopped up. How fine would the material be? Would it be loosely or densely packed? Could it hold a spacecraft landed upon it? How thick would the layer of rubble be? Some spoke of deep deposits of finely powdered dust into which a spacecraft would sink out of sight. Now we know that the Moon's surface is everywhere covered with many feet of fragmented material, or regolith. A sizable fraction of the soil is very fine dust, but the material is sufficiently cohesive and well packed that it easily supported the Apollo spacecraft, the astronauts, and their lunar rover.
Of what is the Moon made? This was a question about which one could only speculate from afar. It could be essentially meteoritic material. It might be like the Earth's crust. Or it might be something entirely different. Any proposition put forth, however, had to satisfy the constraint that the Moon's density is only 3.36 , considerably lighter than Earth's average density of 5.5. One of the major questions was: Is there any water there? Water would be the most important constituent that one might hope to find on the Moon. It would be essential for the support of any microbial life there. It would be an invaluable resource for lunar bases that might be established in the remote future. No one expected to find water exposed on the surface - it would quickly evaporate in the airless, oven-hot environment of the lunar day - but many supposed that there might be substantial subsurface water in the form of ice as a permafrost. Even if free water were not found, it was fully expected that there would be water of crystallization in the lunar minerals.
Learning at firsthand about the stuff of which the Moon is made from the actual lunar material carried back by the Apollo astronauts has been one of the most exciting scientific undertakings of our day. More than 700 scientists, including several hundred from twenty other countries, have spent uncounted hours analyzing samples, conducting laboratory tests, and theorizing over the significance of what was being revealed for the first time. Annually at the Johnson Space Center they have assembled to share their findings and to try to explain them. No water was found. Lunar material is very dry. with practically no water of crystallization. The hydrous minerals so common on Earth arc exceedingly rare in the lunar rocks and soil. Moreover, lunar material is depleted in most volatile elements. suggesting a very hot processing at some time in their history. The rocks in the lunar maria arc similar to, though significantly different from, lavas on Earth. Highland samples show a considerable separation of lunar material into different minerals, showing a differentiation like that responsible for the wide variety of rocks and minerals on Earth. Moon material is neither exactly like the meteorites nor exactly like the Earth's crust, but all three could have had a common source with a different history.