Planetary Astronomy; an Appraisal of Ground-Based Opportunities (1968)

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1 Perspectives INTRODUCTION In classical times, men thought that Jupiter hurled thunderbolts at the unfor- tunates who displeased him. Recent detection of bursts of electrical energy from Jupiter appears to confirm one phase of this hypothesis; but it is obvious that during the intervening centuries the basis of proof required by mankind in its study of natural phenomena has undergone a radical change. Planetary astronomy—the study of objects in the solar system—was for centuries the dominant field in astronomy. The advent of spacecraft and new instruments and techniques have given new impetus to the field. There is reason to believe that despite their diversity the planets, satellites, asteroids, comets, and meteorites have a common origin: that they were created about five billion years ago at the same time the Sun condensed from the interstellar medium. Our present concern is to understand the planetary system—the details of its makeup and the relationships among its members. From studies now being carried out we hope to find records of our early his- tory and clues to our future. Why does the planetary system look the way it does ? Is it accidental, or is it in some sense typical of systems elsewhere in the galaxy? Is life unique to Earth, or does it exist elsewhere? The answers to these questions are some distance in the future, but the information we are gathering is part of the foundation for those answers.

PLANETARY ASTRONOMY CONTENTS OF REPORT To establish an effective space program that is most likely to shed light on the origin of the solar system, a wealth of scientific background information is essential. In the case of the planets, we are still at the stage where more ground- based observations should and can make substantial contributions in the very near future. Requirements in both facilities and manpower must be defined in terms of the present status of the field and its future needs. That is the prime purpose of this report. This report seeks to examine in perspective our present knowledge of the major scientific problems of planetary science and to discuss the specific ob- jects in the solar system as they relate to these problems. Four chapters treat the dynamics of the planetary system, planetary surfaces, atmospheres, and interiors and magnetic fields. They are followed by chapters on new ground-based tech- niques and graduate training in planetary science. The report concludes with the Panel's prime recommendations concerning future developments in the field. Some of the more important advances, described more fully in Chapters 2 through 5, which are likely to be made as a result of further ground-based observations are mentioned in the following paragraphs of this section. The dynamics of the planetary system is a classical field of astronomy which has taxed the skills of mathematical pioneers since early in the seven- teenth century. It is possible that Kepler discovered the reasons for "irregulari- ties" in planetary motions in the same year in which Galileo first used a tele- scope for astronomical observations (1609). Two recent developments have given new impetus to ground-based activity in the field. One is the advent of the digital computer; the other is the tremendous increase in the range, ac- curacy, and resolution of radar, which adds the dimension of distance to mea- surements previously confined to angular position. The distance accuracy already achieved is so great that, when combined with traditional optical angular techniques, an improvement of several orders of magnitude can be achieved in the determination of some important orbital parameters. As a result, the dynamical history of the planets and their satellites can be extended far into the past or projected into the future, and gravitational theories of fundamental importance can be subjected to extremely sensitive tests. Ground-based radar and radio observations can now be used to study plane- tary surfaces. Radar can measure gradual slopes in planetary terrain as well as surface roughness. Radar measures of the Moon, when compared with high- resolution photographs, indicate that the accuracy of the method is very high.

PERSPECTIVES 3 Radio data can be interpreted to yield surface temperatures and, with very high resolution, the distribution of temperature with respect to place and time. Such measurements are of special interest for investigations by means of probes or landers into the existence of life on other bodies in the solar system. The determination of relative abundances of various isotopes in planetary atmospheres as a function of distance from the Sun has a direct bearing on theories of the origin of the solar system. Such isotopic ratios can be com- pared with solar and cosmic abundances to give some idea of the fractionation processes that occurred during the formation and subsequent development of the planets. The close association between the evolution of the terrestrial atmosphere and the development of life on Earth invites comparison with other planetary atmospheres to search for evidence relating to the possible development of extraterrestrial life. Applying this approach in reverse, a general understanding of the origin of atmospheres and the causes of the variety in their composition that we now observe can be expected to lead to new insights into the problems associated with the origin and evolution of life on Earth. Finally, the opportunity to study the general and local circulations as well as condensation and deposition processes in atmospheres having radically dif- ferent compositions, Coriolis forces, and heat budgets than our own will provide rigorous tests for meteorological theories that presently are limited to a single example. A knowledge of the structure and composition of planetary interiors is important not only to a better understanding of the present state of the plane- tary system but also to understanding the manner in which the planetary system was formed and in which it evolved. Observations of planetary magnetic fields cannot now be made from the surface of the Earth except by detecting radiation from charged particles trapped in the field; thus far only Jupiter has displayed such nonthermal emis- sion. The study of this emission permits some understanding of the nature of the Jovian magnetosphere. This is of interest for comparison with the Earth's magnetosphere and may aid in understanding the processes underlying both. Changes in the Jovian magnetic field may well provide a clue to the processes taking place within the planet and on its visible surface. RELATION BETWEEN GROUND-BASED AND SPACE OBSERVATIONS There is little doubt that the most interesting discoveries in the planetary sys- tem will continue to result from space exploration in the form of probes,

4 PLANETARY ASTRONOMY orbiters, or landers. However, the ground-based astronomer is in a position to assist his space colleagues materially. Since space measurements demand such long preparation, any new information their designers can get is of great im- portance. For example, improved ground-based measurements of the surface pressure of the Martian atmosphere supplementing data from the radio occulta- tion experiment aboard Mariner IV have provided important data for the design of a Martian lander. Ground-based observations are often more effective and are almost always considerably less expensive than measurements made in space. Ground-based instruments are capable of data rates several orders of magnitude greater than those of space instruments. They have a much longer useful lifetime than space equipment: lifetimes of several decades are not unusual for astronomical tele- scopes. Ground-based observations permit a degree of flexibility not ap- proached by space techniques in that effective use can be made of recently acquired information and new technology. Spacecraft instrumentation, on the other hand, is designed several years before launch to make one type of obser- vation. The data may thus be obtained by outmoded equipment or under condi- tions that had not been foreseen several years before. When the ground-based observer realizes that one technique is not obtaining the desired information, he is free to modify it or even abandon it in favor of another in a relatively short time. <0 18 35 55 45 *! uiQ ss zl 55 65 1900 J9201940 1960 1980 2000 YEAR FIGURE 1 Oppositions of Mars during the twentieth century. The August 10, 1971, opposition is one of the most favorable.

PERSPECTIVES 1969 1970 1971 1972 1973 1 T 1 1 i i 1 l l 1 l l l l i i MARS JUPITER VENUS .. — — - - •» CAT1IDM I 1 1 ' ' i — i ' i i i i 1 1 I i FIGURE 2 Favorable times for planetary observation, 1969-1973. However, if ground-based astronomy is to make an effective contribution, the time element is an important factor: observations can be used best if the data are obtained and analyzed as far in advance of flights as possible. An addi- tional time constraint is that the planets can be observed to advantage only at certain times: the most favorable times for observing Mars occur for only a few months every two years (Figure 1); there are similar but far less restricted opportunities for observing Jupiter, Venus, and Saturn (Figure 2). Since a space measurement exchanges resolution for comprehensiveness (i.e., the higher the resolution, the more limited the area studied), it is ex- tremely important to know how representative the spaceborne measurements are and to direct such devices as planetary landers to the most interesting loca- tions. Such information can be provided by ground-based techniques. Ground-based observations have limitations, however, which are largely caused by the Earth's atmosphere. One limitation is the result of the atmo- sphere's opacity to radiation of all but a few wavelength bands. Its opacity to gamma, x-ray, and ultraviolet radiation cannot be circumvented from the ground. Balloon techniques open up the ultraviolet to some extent, but observations outside the Earth's atmosphere are still necessary to obtain many important measurements in this region of the spectrum. The opacity in the infrared is somewhat more tractable; observatories at high altitudes in very dry climates open up several windows in the infrared, and airplanes flying above the tropopause permit further significant gains. A second limitation on ground-based techniques is set by the turbulence of the terrestrial atmosphere. This turbulence places an effective limit on the resolution of which any Earth-based instrument is capable. For a number of

6 PLANETARY ASTRONOMY years this factor restricted severely the information that could be obtained from several types of measurements. It is no longer clear, however, that we have reached the limits of resolution. Better observing sites have been found, and new techniques of image scanning and image reconstitution, discussed in Chapter 6, are now just beginning to be used in planetary astronomy. Con- siderable improvements in resolution have already been made, and the limits to which these techniques can be extended have not yet been evaluated. New techniques in radio and radar planetary astronomy are just beginning to be exploited. Here limits are set not by the atmosphere but by instrumental characteristics; great increases in resolution are now technically possible. We are fortunate that modern equipment and new observing techniques are becoming available at this time; the techniques of ground-based astronomy not only provide an opportunity to enrich our knowledge of the solar system but should also make it possible to ensure greater effectiveness in planning the space program.