Figure 63. Crustal evolution in silicate planets. Evidence suggests that earthlike planets all follow similar courses of evolution. Paul D. Lowman, Jr., in Journal of Geology 84 (Jan. 1976): 2, fig. 1; reproduced courtesy of Dr. Lowman.

taken by Earth, but was well behind, only now approaching the tectonic plate stage. The moon and Mercury had long since run the course of their evolution, which terminated well before a tectonic plate stage.

This picture, although consistent with much of the data, could hardly be regarded as more than tentative. It would have to pass the test of further observations and measurement, and stiff debate. But one satisfying feature was the emphasis the theory gave to the kinship of the planets with each other. As theorists had pointed out, if the planets did form from the material of a solar nebula left over after the creation of the sun, then their individual characteristics should depend to a considerable extent on their distances from the sun (fig. 64). Near the sun, where the nebular material would be heated to rather high temperatures by the sun's radiations, one could expect to find planets composed primarily of materials that condense at high temperatures, the silicates and other rock-forming minerals. Moreover, the densities of the planets could be estimated according to distance from the sun by considering what compounds were likely to form at the temperature to be expected at Mercury's distance, which at the distance of Venus, which in the vicinity of Earth, etc. From what was known of the inner planets, they did indeed fit such a picture.

As for the outer planets, one would expect them to consist of large quantities of the lighter substances-hydrogen, helium, ammonia, methane-which could condense out of the solar cloud only at the low temperatures that would exist so far from the sun. Qualitatively, the outer planets also fitted this picture, but quantitatively there were discrepancies. To develop the true state of affairs in proper perspective, an intensive investi

gation of the outer planets was called for, and was on the agenda for the 1970s and 1980s. The investigation got off to an exciting start with the visit. of Pioneer 10 to Jupiter in 1973. It was clear that an exciting period in planetary exploration lay ahead as scientists began to amass data on the atmospheres, ionospheres, and magnetospheres of these strange worlds. While these planets themselves would be quite different from the terrestrial planets, their satellites could be expected to resemble the latter in many ways. Moreover, as many persons pointed out, the satellite systems of Jupiter and Saturn might turn out to be very much like miniature solar systems, particularly the satellites that formed along with the parent planet rather than being captured later. Supporting this view was the early discovery from the Pioneer observations that the four regular satellites of Jupiter decreased in density with increasing distance from the planet, as though they had formed from a cloud of gas and dust that was hotter near the planet than it was farther away.45 The opportunities for important research seemed endless.

Finally there was the question of extraterrestrial life. Space research on fundamental biology was early divided into two areas: (1) the study of terrestrial life forms under the conditions of space and spaceflight, and (2) exobiology. The former was discussed briefly in chapter 16. The latter came to mean the search for extraterrestrial life and its study in comparison

Figure 64. Origin of solar system planets. The higher temperature materials, like silicates, condense nearer the sun; the more volatile substances, farther away.

with Earth life. Most scientists considered the chance of finding life elsewhere in the solar system to be minute, but it was universally agreed that the discovery of such life would be a tremendously important event. Thus, while recognizing the unlikelihood of finding extraterrestrial life, many considered that the potential implications offset the small chance of finding any, and accordingly devoted considerable time to studying in the laboratory the chemical and biological processes that seemed most likely to have been part of the formation of life. They sought out Earth forms that could live under extremely harsh conditions-like arid deserts, the brines. of the Great Salt Lake, or the bitter cold Antarctic-and paid special attention to them. And they devised experiments to probe the Martian soil for 'the kinds of life forms deemed most likely to be there.

But, after a decade and a half, the problem of life on other planets remained open. No life was found on the moon, nor was there any evidence that life had ever existed there. A careful search was made for carbon, since Earth life is carbon-based. In lunar samples a few hundred parts per million were found, but most of this carbon was brought in by the solar wind. Of the few tens of parts per million that were native to the moon, none appeared to derive from life processes.46

Nor was any life found on Mars, even though the two Viking spacecraft with their samplers and automated laboratories were set down in 1976 in areas where once there might have been quite a bit of water.47 Still, the subject could hardly be called closed. If life had been found, that would have settled the question. But that life was not found in two tiny spots on Mars did not prove that there was no life on the planet. So, although the first attempts were disappointing, it could be assumed that future missions to Mars would pursue the question further. Nor would it be likely that exobiological research would be confined to the Red Planet. At the very least, one would expect that, as scientists studied the chemical evolution of the planets and their satellites, they would keep the question of the formation and evolution of life in mind.

While the foregoing has touched upon but a few of the results accruing from the exploration of the solar system, still the reader should be able to derive some insight into the impact that space science was having upon the earth and planetary sciences. For one thing the study of the solar system was revitalized after a long period of relative inactivity. Second, lunar and planetary science became an important aspect of geoscience, attracting large numbers of researchers. Third, the new perspective afforded by space observations gave an immeasurable boost to comparative planetology, a field that made great strides during its first 15 years. Nevertheless, no one doubted that in the mid-1970s comparative planetology still looked forward to its most productive years.

All of which leads to the usual question. If space science was having such a profound impact on Earth and planetary sciences, was space

science producing a scientific revolution in the field? In the broad sense, no. But, viewed strictly from within the discipline there were indeed numerous revolutionary changes. Much new information was accumulated, permitting the theorist to deal in a realistic way with topics about which one could only speculate before. Many had to relinquish pet ideas about the nature of the lunar surface or the markings on Mars. Proponents of a cold moon were faced with incontrovertible evidence of extensive lunar melting. No pristine lunar surface was to be found; instead a substantial evolution had marked the moon's first one and a half billion years. Far from being an inert planet, Mars turned out to be highly active. Of course, in different aspects of the subject many investigators had been on the right track. Noted astronomers R. B. Baldwin and E. J. Öpik had correctly anticipated that many of the features of Mars were due to craters. 48 Gerard Kuiper had been sure that volcanism was important on the moon, as he explained many times to the author. Thomas Gold had been certain that the lunar surface would contain a great deal of fine dust. Yet, no one had succeeded in putting the separate pieces together in satisfactory fashion. Thus, the most revolutionary aspect of space science contributions to the earth and planetary sciences was probably in helping to develop an integrated picture of the moon and near planets. This was an enormous expansion of horizons, an expansion that could be expected to continue with each new planetary mission.

INVESTIGATION OF THE UNIVERSE

As space scientists were busily altering the complexion of solar system research, space methods were also profoundly affecting the investigation of the universe. Here space science could contribute in a number of ways to solar physics, galactic and metagalactic astronomy, and cosmology, including a search for gravitational waves, observations to determine whether the strength of gravity was changing with time, and studies of the nature of relativity. But the contribution of space techniques to these areas was qualitatively different from those in the planetary sciences. Whereas rockets and spacecraft could carry instruments and sometimes the observers themselves to the moon and planets to observe the phenomena of interest close at hand, this was not possible in astronomy. The stars and galaxies would remain as remote as before, and even the sun would continue to be a distant object extremely difficult to approach even with automated spacecraft because of the tremendous heat and destructive radiation.

The connection between the scientist and the objects of study would continue to be the various radiations coming from the observed to the observer. But rockets and satellites would increase the variety of radiations that the scientist could study by lifting telescopes and other instruments above Earth's atmosphere, which was transparent only in the visible and

some of the radio wavelengths. This extension of the observable spectrum proved to be as fruitful to the prober of the universe as were the lunar and planetary probes to the student of the solar system.

As stated in chapter 6, rocket astronomy began in 1946 when sounding rockets were outfitted with spectrographs to record the spectrum of the sun in hitherto hidden ultraviolet wavelengths. In 1948 x-ray fluxes were detected in the upper atmosphere, after which rocket investigations of the sun ranged over both ultraviolet and x-ray wavelengths. Inevitably experimenters turned their instruments on the skies, and when they did various ultraviolet sources were found. In 1956 the Naval Research Laboratory group found some celestial fluxes that might have been x-rays, but the real significance of x-rays for astronomy had to await more sensitive instruments that did not become available until the early 1960s.

In the meantime sounding rocket research on the sun's radiations moved on apace. Investigators from a number of institutions continued to amass detail on the sun's spectrum in the near and far ultraviolet, which was important in understanding the quiet sun and normal sun-earth relationships. But the real excitement proved to be with the x-rays. It was these, rather than the ultraviolet wavelengths, that came into prominence with high solar activity. When satellites came into being, they were put to use in making long-term, detailed measurements of the sun's spectrum in all wavelengths. On 7 March 1962 the first of NASA's Orbiting Solar Observatories went into orbit, to be followed by a series with steadily improving instrumentation. The Naval Research Laboratory built and launched a series of Solrad satellites, intentionally less complex than the OSOS, to provide a continuous monitoring of the sun in key wavelengths. But, while satellites came into prominence in the 1960s, sounding rockets, some of them launched at times of solar eclipse, continued to yield important results. In fact, some scientists felt that the most significant work on the sun came from sounding rockets rather than from the far more expensive satellites.

NASA's Orbiting Solar Observatories continued into the 1970s, the first one of the decade being OSO 7, launched on 29 September 1971. An important event for solar research was the launching of Skylab in 1973. In this space laboratory astronauts studied the sun intensively using a special telescope mount built for the purpose. Although the high cost of Skylab's solar mission in dollars and time to prepare and conduct the experiments was distressing to many of the scientists, nevertheless the results were extremely important for solar physics, some of them providing solutions to long unsolved problems.

Sounding rocket experiments were also fruitful in stellar astronomy. Perhaps the most significant event in rocket and satellite astronomy occurred when American Science and Engineering experimenters, with an Aerobee rocket flown on 12 June 1962, discovered the first x-ray sources