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in pictures one could see landscapes and clouds-often strange, to be sure, but landscapes and clouds nevertheless. One could envision spacecraft orbiting a planet or landing on its surface and could identify personally with astronauts stepping onto the bleak and desolate moon. As a consequence NASA had little difficulty in capturing and holding a widespread interest in this aspect of the space science program.

The exploration of the moon and planets began with the Soviet Luna flights in 1959. From that time on, every year at least one mission to the moon or a planet was attempted by the United States or the Soviet Union. The American assault on the moon began with Pioneers, followed by Rangers, then the soft-landing Surveyors. In the summer of 1966 the first of five Lunar Orbiters began the task of mapping almost the entire surface of the moon. Even an Explorer was injected into lunar orbit to study the space environment around the moon. The climax was reached when the Apollo missions began manned exploration of the moon with the orbital flight of Apollo 8 in December 1968 and the first manned landing in July 1969. While Apollo was in progress the Soviet Union conducted a series of sophisticated unmanned lunar missions that included circumlunar flights of Zond spacecraft, which were successfully recovered with pictures they had taken of the moon. More advanced Luna spacecraft soft-landed on the moon, carrying roving vehicles to investigate the lunar surface in situ and radio the information to Soviet stations on Earth, and in some cases to send samples of lunar soil to Earth for investigation in the laboratory. The success of the Soviet unmanned rovers and sampling missions sparked an intense debate between the scientists and NASA, many of the scientists feeling that the unmanned approach to the study of the moon was the wiser, and by far the more economical.

Criticism was blunted, however, by the tremendous success of Apollo. The astronauts brought back hundreds of kilograms of lunar rocks and soil from six different locations, the analysis and study of which quickly engaged the attention of hundreds of scientists throughout the United States and around the world. In addition to collecting lunar samples, the astronauts also set up nuclear-powered geophysical laboratories instrumented with seismometers, magnetometers, plasma and pressure gauges, instruments to measure the flow of heat from the moon's interior, and laser corner reflectors for geodetic measurements from Earth. The geophysical stations operated for many years after the astronauts had left, radioing back volumes of information on the moon's environment and its seismicity. Twice satellites were left behind in lunar orbit for lunar geodesy and to make extended chemical analyses of the lunar surface material from observations of the short-wavelength radiations of the moon.

The United States claimed the first success in planetary exploration when Mariner 2, launched in the summer of 1962, passed by Venus the following December, probing the clouds, estimating planetary tempera

tures, measuring the charged particle environment of the planet, and looking for a magnetic field.35 In 1965 Mariner 4 flew by Mars to take 21 pictures, covering about one percent of the planet's surface. Then Mariner 5 visited Venus, this time getting substantially more data on the atmosphere, including estimates of the ionosphere. It was back to Mars in 1969 with Mariner 6 and 7, which returned some 200 pictures of the surface along with a variety of other measurements. Mariner 9 went into orbit around the Red Planet in November 1971 at a time when the planet was almost completely obscured by a global dust storm. During the next month and a half the spacecraft monitored the clearing of the dust storm, which itself provided much interesting information about the planet. After that the spacecraft's cameras were devoted to the first complete mapping ever achieved of another planet-Mariner 9 returned 7329 pictures of Mars and its two satellites, which permitted drawing up complete topographical maps showing the true nature of the markings that had so long puzzled astronomers. 36

During the 1960s the Soviet Union had also set its sights on the planets. In fact, its attempts appreciably outnumbered those of the United States, since the USSR seized virtually every favorable opportunity to try a launching. But early Soviet planetary endeavors were about as dismal as America's early lunar tries. Not until a Venera spacecraft in 1970 succeeded in penetrating the atmosphere of Venus, returning data on the composition and structure of the atmosphere, did fortune smile on these planetary attempts. In December 1970 Venera 7 landed and returned data from the surface of Venus; Venera 8 followed suit in July 1972, for 50 minutes sending back surface data and analyses of the soil of Venus. Less fortunate, however, were the two Soviet Mars landers which, like Mariner 9, also arrived at the planet during the great dust storm of 1971. The storm that provided the Mariner with an unexpected opportunity to observe the dynamics of the planet's atmosphere may have been the cause of the Soviet spacecraft's failure to land successfully. 37

In November 1973, Mariner 10 left on a journey that would take it first by Venus and then on to Mercury, where the spacecraft arrived in March 1974 taking pictures and making a variety of other measurements. Having completed its first Mercury mission, Mariner 10 was redirected by briefly firing its rockets so that the spacecraft would visit Mercury again in September of 1974. By visiting Mercury several times, Mariner 10 provided the scientists with the equivalent of several planetary missions for little more than the price of one. 38 Also, with the visit to Mercury, scientists at long last had close looks at all of the inner planets of the solar system, including the two satellites of Mars.

The result of all these space probe missions was the accumulation of volumes of data on the moon and near planets, illuminated with thousands of highly detailed pictures. The photo resolutions exceeded by orders

of magnitude what had been possible through telescopes. When Rangers crashed into the moon the closeup pictures sent back just before the impact were a thousandfold more detailed than the best telescopic pictures previously available (fig. 47). After landing on the surface with its television cameras, Surveyor afforded another thousandfold increase in resolution, revealing the granular structure of the lunar soil and a considerable amount of information on the texture of lunar rocks (fig. 48). In the laboratory Apollo samples put the moon's surface under the microscope, as it were (fig. 49). As for the planets no detail at all had been available before on the surface characteristics of distant Mercury or clouded Venus. Some Mariner 10 pictures afforded better resolution for Mercury than Earthbased telescopes had previously given for the moon (figs. 50-51). Ultraviolet photos of Venus from passing spacecraft showed a great deal of structure in the atmospheric circulation that was hitherto unobservable (fig. 52), while radar measurements from Earth penetrated the clouds to reveal a rough, cratered topography.39 For Mars, the indistinct markings observable from Earth were replaced with sufficient detail to show craters, volcanoes, rifts, flow channels, apparently alluvial deposits, sand dunes, and structure in the ice caps (figs. 53-60). Added to such pictures, data on planetary radiations in the infrared and ultraviolet; surface temperatures; atmospheric temperatures, pressures, and composition (when there was an atmosphere); and charge densities in the ionosphere (when there was an ionosphere)—this wealth of information completely revitalized the field of planetary studies, which had long been quiescent for lack of new data. By the early 1970s comparative planetology was well under way, although one must hasten to add that the task ahead of understanding the origin and evolution of the planets was one of decades, not merely months or years.

Nevertheless, progress was rapid. Much was learned about the mineralogy and petrology of the moon, and by extrapolation probably about the other terrestrial planets. Radioactive dating of lunar specimens led to the conclusion that the moon is probably some 4.6 billion years old, an age consistent with the ages of meteorites and the presumed age of Earth. The moon was found to be highly differentiated; that is, the lunar materials, through total or partial melting, had separated into different collections of minerals and rock types. The maria were mainly basalt, similar to but significantly different from the rocks of the ocean basins on Earth. In contrast the lunar highlands were rich in anorthosite, a rock consisting mainly of the feldspar calcium aluminum silicate. Both maria and highlands were much cratered-as could already be seen from Earth-and one could now see that the crater sizes extended down to the very small, showing that the moon had been bombarded by very small particles as well as by very large objects. The entire surface was covered with fine fragments and soil, broken rocks and rubble-crustal material chopped up by the cratering

process. A considerable amount of glass was found, some of it in coatings splashed onto other rocks, much of it in the form of tiny glass beads of a variety of colors dispersed through the soil.

Reading through the record imprinted on the lunar surface one could bit by bit piece together the course of the moon's evolution. Contrary to expectations of many, before the unmanned and manned space missions showed differently, the present lunar surface was not a virginal record of conditions that existed at the time of the moon's formation. Instead one could discern a continual, sometimes violent process of evolution. Either at the time of formation or very shortly thereafter the moon's crust was molten to a considerable depth. Presumably during this phase the differentiation of lunar materials took place, producing the light-colored, feldsparrich highlands. After solidifying about 4.4 billion years ago, the highlands were bombarded for hundreds of millions of years, probably by material left over from the formation of the moon and planets. This process produced the cratered topography of the highlands still visible today. Between 4.1 and 3.9 billion years ago the bombardment of the moon became cataclysmically violent, with asteroid-sized meteorites gouging out the great basins, hundreds and even thousands of kilometers across (fig. 61). Then, as radioactive dating of lunar materials showed, between 3.8 and 3.1 billion years ago a series of eruptions of basaltic lavas filled the basins to form the maria, or dark regions of the moon clearly visible from Earth. By 3 billion years ago the violent evolution of the moon had come to an end. Cratering and “gardening" of the lunar surface continued, but at about the rates observed today, most of the impacting particles being micrometeor and grain sized, occasionally cobble sized, with the very large impacts exceedingly rare. 40

The moon observed by Apollo instruments was very quiet. The lack of any substantial organized magnetic field suggested that there was no molten core, but the presence of magnetism in lunar rocks indicated that there might have been a core at some time in the past-on the assumption that a liquid iron core was required to generate the lunar field that magnetized the lunar rocks. Seismometer data showed that the moon was at least partly molten below 800 km and suggested a small, possibly iron, core a few hundred km in radius. The energy released by moonquakes detected by the Apollo seismometers was nine orders of magnitude—a billion times-less than that released by earthquakes over a similar period of time, which, however, seemed consistent with the relative sizes of Earth and moon. The seismic data yielded the picture of a lunar crust 50 to 60 km thick, four times the average thickness of Earth's crust, underlain by a mantle solid at the top but partially melted toward the bottom (fig. 62). The very thick crust could account for the lack of any recent volcanic activity.41

Before these investigations it was common to think of the moon as a small body and to suppose that small bodies would remain cold and essen

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