SP-424 The Voyage of Mariner 10


painting representing early stage of solar system formation?

Chapter 9

A Clearer Perspective


[101] MARINER 10 PROVIDED an important addition to man's view of the inner solar system. Previously, the Apollo program had provided a direct sampling of rocks from another world, thus allowing age-dating of planetary material even further back than was possible on Earth because of major disturbances to the Earth's surface following its formation. An orbiting Mariner spacecraft had surveyed the whole of the planet Mars and revealed unexpected surface features, a planet that was partly primordial and partly molded by volcanism and atmospheric effects.

Venus had been visited by a number of American and Soviet spacecraft, and the general nature of its atmosphere and surface conditions determined, with reasonable, but yet untested inferences regarding its internal structure and evolutionary history. Direct observational data on Mercury were sparse. Its mass and density were known, and evidence existed that its atmosphere was at best tenuous. There were no data on Mercury's surface topography or body characteristics, from which both evolutionary history and internal composition might be inferred.

Mariner 10's investigation of Venus yielded a modest, but important increase in knowledge. Fine-scale markings in the upper atmosphere of Venus that are visible only in ultraviolet light were observed. These "cloud" patterns exhibit a rapid rotation with a 4-day period, much faster than that of the planet itself. While the mechanisms responsible for these remarkable features are not yet understood, their eventual elucidation will be of profound importance to our understanding of Venus's atmosphere. Further, observations of the interaction of the planet's atmosphere with the solar wind were extended significantly. These new data, combined with Mariner's direct, highly precise measurements of hydrogen, helium, carbon and argon abundances, provided better insight into the processes by which planetary atmospheres evolve and are modified by the Sun.

But Mariner 10's principal contribution to the study of the Solar System lay, as expected, in the Mercury observations. A major discovery was Mercury's magnetic field. This result was completely unexpected and very exciting. Mercury was known to be a slowly rotating planet, and the early Mariner 10 pictures had shown it to be one which, like the Moon, had not experienced significant crustal modification by internal activity since its infancy. Our best theories told us that planetary magnetic fields were generated by a dynamo effect which was caused by the presence of a molten electrically conducting core within a rotating planet. Another possible explanation would be the induction of a magnetic field by the interaction of a cold, but conductive planet with the fluctuating magnetic field of the solar wind.

[102] This effect could produce a magnetic field similar to one which was internally generated and very difficult to distinguish from the latter in a single observation.

Mercury III provided a second observation. The third encounter results showed unequivocally that the field is of internal origin, and the explanation of Mercury's magnetic field became a sort of scientific "Catch-22." The field had to be either actively generated at the present or a relic of a previous field. With regard to the latter, the "catch" is that the high temperatures which are required for the generation of a field (i.e., temperatures above the Curie point, when metals lose their induced magnetism ) destroy induced magnetism once the dynamic field-generating process stops. The "catch" in the presently active (dynamo) explanation is the fact that Mercury's surface shows no evidence of internally generated deformation or volcanism for at least 3 billion years. Thus the best explanation for the origin of the field-i.e., a large, hot, thermally convecting core-seems at odds with the clearly primitive state of the planet's crustal development.

The photographic record of Mercury's surface produced by Mariner 10 has allowed planetologists to take a deeper look at the inner Solar System. Some now see a pattern emerging in which there were five major epochs in the building of the terrestrial planets.

The first epoch was one of major accretion, in which the basic mass of each planet came together from Solar System material in a relatively short time of several million years about 4.5 billion years ago. Whether or not this was an accumulation of particles of the same general composition irrespective of distance from the Sun, or a differentiated accumulation of particles depending upon distance from the Sun, is not yet known.

In the former case, all the inner planets would have started out with basically the same materials, ranging from heavy elements such as iron to lighter volatiles such as hydrogen and helium. Then subsequent evolution of the planets would have caused a change to their present states in which the planets differ radically in composition, some having more heavy elements and fewer volatiles than others.

In the latter, generally more favored case, planets closer to the Sun accreted from material richer in the heavier elements, while the material forming those further from the Sun has a greater proportion of the lighter elements. A chemical gradient model allows the condensation from the primordial nebula of different combinations of chemical compounds at various distances from the Sun; some planets would receive water, others would not. Thus the innermost planets, Mercury and Venus, might have accreted in a zone around the Sun where there was little water, whereas the Earth formed in a zone with an abundance of water.

It is further speculated that following the accretion of the planets there was a period of internal heating during which the original accretionary surface was molded. This appears to some scientists to be borne out by observations of the surface of Mercury from Mariner 10. Between the craters there appear to be many areas of an ancient smoothed surface. Some planetologists feel that an early process of melting is evidenced by these "intercrater" plains and that they are the record of the chemical differentiation which appears to be required to explain Mercury's magnetic field, i.e., heavy materials in the core and lighter elements near the surface. However, this differentiation could have taken place during the actual process of accretion; i.e., the heavy materials might have accreted into a protoplanet which later collected lighter materials as these condensed from the primordial nebula. The question of the timing of Hermian differentiation is now being attacked by scientists specializing in planetary thermal evolution models. Their observational limits have been set by Mariner 10 and lunar age dating.

Between 4 and 3.3 billion years ago, according to the evidence of the lunar rocks, the Moon was subjected to bombardment by swarms of large bodies, which created large impact basins and smaller craters accompanied by secondary and tertiary craters. Then, as abruptly as it commenced, this tremendous bombardment ended.

Comparative studies of the cratered terrains of Mars, and now Mercury, show that these planets, too, were subjected to similar bombardment. Crater densities observed on these three widely separated planets suggest that the source of the bombarding objects could be remote from the inner Solar System, and not necessarily in the asteroid belt, as was thought before the beginning of spacecraft planetary exploration. In addition, the use of a remote source area for this uniform [103] bombardment flux implies simultaneity of the bombardment for all bodies in the inner Solar System. This episodic, nonuniformitarian theory, although philosophically objectionable in terms of the orderly operation of a natural system which can be described statistically, has considerable appeal to those concerned with the practical problems involved in describing the early history of the Solar System. Final heavy bombardment, if episodic, simultaneous and ubiquitous in the inner Solar System, provides a " marker horizon " for dating epochs in planetary evolution. One suggestion for the origin of the bombarding bodies is the penetration of a large ( 1023 grams) body into the inner Solar System on a Venus satellite escape trajectory, followed by its fragmentation as it passed Venus inside the Roche limit (a distance within which the gravitational energy of the larger body causes the smaller one to fragment). Another is a sudden disruption of a very large body in the vicinity of the asteroid belt. Note that the latter model, while not requiring a remote source, still implies simultaneity.

On Mars and Mercury the bombardment did not destroy all the old surface. On the Moon it did. We cannot be sure what happened on Venus, although radar observations do show evidence of large craters on that planet. On the Earth there is steadily emerging evidence of a period of extensive bombardment of the most ancient continental cores like the Canadian shield. In the main, of course, Earth's bombardment history has been erased by subsequent events. However, it is important to note that all the planets of the inner Solar System were most likely subjected to bombardment by asteroid-sized bodies at some time subsequent to accretion and chemical differentiation.

As the impacts subsided, the Moon entered another phase of evolution-volcanism. Lava flowed into the big impact basins and filled the floors of large craters. On the Moon these flows are very evident. Not so on the Earth and Mars. The smooth post-bombardment plains of Mercury strongly resemble the lunar maria, and many planetologists argue that they also represent an epoch of extensive volcanism. Others, however, point out that no primary volcanic features (domes, vents, pit-craters, etc.) have been recognized on Mercury, and caution that the marelike plains may have been formed by impact-melt processes. It has been determined by direct measurement that on the Moon the latest lava flows occurred about 3.3 billion years ago. It appears, based on crater counts, that the smooth plains on Mercury occurred about the same time, assuming a simultaneity in the heavy bombardment of both planets.

The final phase of planetary evolution is represented on the Earth by a tectonic phase in which convection within the mantle gave rise to shield volcanoes, subduction zones, sea-floor spreading centers, and the motion of crustal plates. The major volcanoes on Mars and the great plateau on which they are found represent the manifestation of tectonic processes. On Venus, too, some of the radar data suggest that although Mariner 10 shows an almost spherical planet, much closer to a sphere than is the Earth, the surface itself has great irregularities. This may indicate that convective forces have been actively molding the surface of Venus too.

There have been postulations that Mars may be entering an active phase of tectonism, leading to an Earthlike planet in the future. There has been speculation that the great Coprates chasm represents the beginning of rifts in the continental mass and shows the start of a breakup into continental plates. However, the results of the Mercury encounter, taken in conjunction with comparative crater counts, seem to strengthen the view that the Martian volcanic activity took place hundreds of millions of years ago, not so recently as was first supposed. This leads to a picture of a planet whose evolution toward a tectonically active Earthlike body has run its course, arrested in its infancy by an insufficient supply of internal heat. The latter presumably is traceable to the initial chemical constitution of the planet.

It is clear from the evidence on hand that the Moon and Mercury have been quiescent since the formation of marelike smooth plains shortly after the cessation of heavy bombardment. Mercury, unlike the Moon, appears to have gone through a tectonic phase of sorts, i.e., crustal shortening. This occurred quite early, during the bombardment epoch, and probably represents a simple crustal adjustment to a slight shrinking of the large Hermian core. As far as can be gathered today, the planet-building process stopped about 3.5 billion years ago as the episodic period of bombardment ended and the inner Solar System was cleared of much of its debris. After that, the [104] individual planets were on their own; they evolved from the materials then within them.

The past decade of planetary exploration has provided mankind with a completely new view of the inner planets which has reflected into our view of the Earth. Planetology has developed into a comparative science in which the broader viewpoint of accurate information concerning all the terrestrial planets-even though still fragmentary-is allowing scientists to take a much harder look at many aspects of Earth sciences-geology, geophysics, climatology and even meteorology. While currently we by no means know all there is to know about how the planets were formed, speculations are based on direct knowledge, knowledge that could never be gained by remote observations from Earth.

Our shiny, fragile spacecraft have given us a perspective totally unavailable to the greatest scientists of the past. Planetary exploration is essentially a cultured activity-a creature and indicator of the level and nature of our civilization. Kenneth Clark, standing on the Pont des Arts in Paris, regarding the Institute of France, the Louvre and Notre Dame, speculated on the definition of civilization, saying "what is civilization? I don't know...But I think I can recognize it when I see it." In Mariner 10 and its views of little known worlds, we can recognize an element of our own.

David Morrison, reporting in the scientific journal Icarus in the First International Colloquium on Mercury held at Caltech in 1975, stated " This Colloquium demonstrated the degree to which Mariner 10 observations have plucked Mercury from obscurity, so that now data on this planet are providing important input to discussions of the origin and early chemical and dynamical evolution of the solar system as well as to theories of planetary surfaces and interiors...it seems certain that the Mariner data will continue to be analyzed for many years to come, and that this planet is now firmly fixed in both public and scientific consciousness as a real world, as interesting and unique as is each of the other planets."