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Planetary Impact Cratering and Man
| Article
# : |
14003 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
2 / 1988 |
2,132 Words |
| Author
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Thomas J. Ahrens
Thomas J. Ahrens is professor of geophysics at the California
Institute of Technology. |
Recent astronomical observations of several nearby stars, all similar to the Sun, have shown clouds of dust and gas in orbit about them. Present theory on the origin of the planets suggests that these dust clouds may represent the early stages of solar systems. These clouds, composed of rock, carbon, and ice, coalesce to form kilometer-sized objects called planetesimals that in turn, via mutual gravitational interaction, collide with each other and eventually form planets. The young planets continue to grow via impact with smaller objects. Since the impactors grow at the same time the planets grow, larger planetesimals impact in more violent events, until most of the intervening material has been taken up by the planets.
Other events of our solar system's history are explained most convincingly by interplanetary collisions. The Moon was probably formed when an object about the size of Mars collided with Earth. Moon rocks had arrived on Earth long before man landed there--apparently ejected to escape velocity by a meteor's impact on the lunar landscape. The same process has sent samples of Martian rock earthward.
The conclusion seems to be that impacts played an important part in the development of the solar system and, as will be seen, the beginnings of life on Earth. Impacts may have had a less frequent but nevertheless earth-shaking effect on Earth's later history, producing drastic climatic changes and sudden extinction of thousands of life forms. In fact, these dramatic events may bear serious implications for the evolution of life and the eventual ascent of man.
Craters Everywhere
In the early 1960s the major controversy in planetary science--a debate that raged for nearly a decade--centered on whether the craters on the Moon were volcanic or impact craters. In the 10 years that followed, more photographs from Ranger and Surveyor spacecraft and Moon samples from the Apollo landings in 1969 and from the Soviet sample-return mission in 1970 left little doubt that most craters were due to impact. In fact, a very large fraction of lunar rocks are fused breccias (fig. 1) resulting from numerous impacts on the porous, rocky surface of the Moon.
Surprisingly, photo images of the ancient terrain of both Mercury (fig. 2) and Mars (fig. 3) revealed heavily cratered surfaces, as are a majority of the icy surfaces of the satellites of Jupiter, Saturn, and Uranus (fig. 4). Even Phobos and Deimos, the tiny
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