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WHAT WE ARE LEARNING FROM THE PLANETS Natural science editor of The Christian Science Monitor

By Robert C. CowenNatural science editor of The Christian Science Monitor / December 29, 1980

When Neil Armstrong and Edwin E. Aldrin Jr. took that "giant leap for mankind ," they also took a little-noticed hop for planetary science. A laser reflector they left behind on the moon July 21, 1969, opened what was then the new field of precision lunar ranging.

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This has blossomed into an international activity whose first fruits illustrate the fact that exploring other bodies in our solar system gives a better understanding of our own planetary home --

Among other things, lunar laser ranging has brought a new level of precision to measuring Earth's rotation. This reveals tiny wobbles related to winds, seismic activity, and even motions within Earth's liquid core. It promises new insight into processes geophysicists only dimly envision today.

As Derral Mulholland of the University of Texas notes, it now "becomes necessary to observe the skies to learn more about the Earth under our feet." This could be the motto for planetary exploration generally.

Earth cannot be understood in isolation. However intimately scientists study our planet, they still are looking only at one special, albeit humanly crucial, case. Although he is concerned specifically about atmospheres, Harvard University meteorologist Richard Goody emphasizes this general point when he explains: "You keep tuning your theories of the Earth's atmosphere as you gain more [terrestrial] knowlegde. . . . [yet] it is possible to be totally wrong; you have only one set of facts. But if you study the planets, you create a much larger body of knowledge."

Scientists need the solar-system perspective. They need to know how planets have evolved and how they work generally to see where Earth fits in. So as the United States enters a five-year hiatus in its main planetary exploration program, it is worth taking stock of the perspective gained. That's why this three-part series is concerned with what we have learned fromm the planets, not what we have learned aboutm them.

Having made its spectacular survey of the Saturn system in November, the Voyager 1 spacecraft is heading for interplanetary space. Its twin, Voyager 2, is being prepared for its follow-up Saturn survey next August. It will then go on to Uranus, but won't arrive at the planet until January 1986. There are no other projects that can reach planetary objectives between now and the Uranus encounter. Hence the roughly five-year hiatus in new exploration.

The concern scientists feel because of this hiatus, how it developed, and what might be done next is the subject of the final article in this series.

The first two articles are concerned with what has been learned to date. For this purpose, the solar system can be classified naturally into two regions -- the inner, rocky, earthlike planets and the giant gas bags ranging from Jupiter outward.

All of these need to be studied to gain a full understanding of the solar system. But the inner planets -- Mercury, Venus, Earth, Mars (and, you could add , the Moon) -- share a common evolutionary heritage. They are largely solid bodies, some of which probably have liquid cores, and three of which (Venus, Earth, Mars) have atmospheres. Exploration of these bodies has a direct bearing on unraveling Earth's own evolutionary history.

One of the main features that lepas out at anyone who scans a representative set of planetary close-up pictures is the important role that cratering has played in their history. As the planets formed, debris of many sizes from small meteorites to giant asteroids rained in upon them. Scientists had known that this probably happened. But it took firsthand observations to show that the cratered face of the Moon is typical, rather than exceptional, for the inner solar system.