In my youth it was a popular assumption that the “canals” on the planet Mars had been constructed by intelligent beings. Our other near planetary neighbor, Venus, was perpetually covered in clouds and these were assumed to be Earth-like clouds obscuring a surface that could well be exotic in its fauna and flora. The shattering of any such beliefs by the space investigations during the last twenty years marks one of the greatest advances in astronomical knowledge in recent times. Venus, although closely similar to Earth in size and density, has been revealed as a most inhospitable planet, the searing heat on the surface and the poisonous constituents of its clouds making any form of biological evolution out of the question.
The American and Soviet spacecraft had already established this about Venus at the time when the American Viking spacecraft were dispatched to Mars in the summer of 1975. During the long journey until the spacecraft landed in July and September 1976 there was eager anticipation that the biological sampling of the surface would reveal some form of life, however primitive, on the surface of the planet. That hope has not materialized in any positive or decisive manner, but any consequent disappointment has been counteracted by the acquisition of information about the nature of Mars. The two books under review deal with that subject through the analysis of the astonishing high-definition photographs of the planetary surface transmitted to Earth by the series of Mariner and Viking spacecraft after the first successes with Mariner 4 in 1965.
When this spacecraft successfully flew by the planet at a distance of a little under 10,000 kilometers on July 15, 1965, a new and revolutionary era of the exploration of Mars commenced. Until that time our sole knowledge of the planet came from earth-bound observations. Even before the invention of the telescope the observation of the motion of Mars across the sky by Tycho Brahe had presented a problem that seemed insoluble by the Copernican hypothesis that the Earth and the planets moved in circular orbits around the Sun. The motion of Mars against the background of the stars was irregular—for short periods it appeared to reverse its direction of motion with respect to the stellar background. It was this feature that led Kepler to conclude after laboring for ten years that Mars could not be moving in a circular orbit but that the orbit of Mars and the other planets must be elliptical with the Sun at one of the foci of the ellipse. Kepler’s great work, in which he enumerated the first two laws of planetary motion, was published in 1609; almost simultaneously Galileo was viewing Mars for the first time through his small telescope.
It is unlikely that Galileo could see any particular features of the planet, and another half century elapsed before Huyghens made drawings which showed polar caps and some markings of the surface. Another century passed and at the time when William Herschel discovered the planet Uranus in 1781 he was also making extensive observations of Mars as viewed through his excellent telescopes. He measured the inclination of the axis of rotation and from the behavior of the polar cap he concluded that Mars, like Earth, was subject to seasonal variations. Toward the end of the nineteenth century the observations of several astronomers who were able to study the planet under unusually favorable circumstances generated intense excitement about the conditions on the planet.
The orbit of Mars is distinctly elliptical, and since it takes 687 Earth days to travel around the Sun (that is fortythree days short of two Earth years) the distance between Earth and the planet varies from about 400 million kilometers to only 55 million kilometers. Successive close approaches occur every 780 Earth-days, and each century for a few of these approaches the minimum distance of Mars from Earth is less than 60 million kilometers. In 1877 there was such a close approach and it was on this occasion that Giovanni Schiaparelli, the director of the Milan Observatory, made his famous drawings of the planet and introduced a nomenclature of the major features of the planet’s surface that have largely survived to this day.
Schiaparelli’s map portrayed an elaborate network of linear features which he referred to as “canale.” Commonly translated as “canals,” these markings soon became the center of excited discussion about the planet. The observatory that Percival Lowell founded at Flagstaff, Arizona, in 1894 was largely directed to the study of these Martian features. Lowell produced a map of the planet showing five hundred of these canals. At the beginning of this century he maintained that they had been constructed by intelligent beings to distribute water from the polar regions to the equatorial deserts on the planet. It is important to realize that even when the planet is at its closest to Earth the best definition attainable by any telescope on Earth is only about 150 kilometers. Although there was opposition to Lowell’s interpretation of the canals the belief in their existence continued until the advent of the spacecraft. Only three years before the flight of Mariner 4 to Mars responsible papers were published maintaining that the linear markings indicating the presence of the canals existed on the planetary surface.
Twenty years later we are quite certain that these linear “canals” do not exist on the planet and that they were imaginary perceptions of the terrestrial observers of groups of surface markings close to the resolution limit of the telescope. On the other hand entirely new features have been revealed by the spacecraft cameras which, as beautifully revealed by the photographs in these two books, include long sinuous valleys that have all the appearances of dried riverbeds. The real possibility that these actually were rivers in the past history of the planet and that the water is now frozen as permafrost under the planetary surface is one of the most extraordinary and intriguing results of these modern explorations.
Central to these two books is the interpretation of the photographs and other data transmitted to Earth by six American spacecraft that were dispatched to the planet, beginning with Mariner 4 in 1965 and ending when the Viking spacecraft were closed down on August 7, 1980. (During this period six Soviet missions were also sent to the planet.) Ironically the twenty-two pictures of the surface of the planet transmitted by Mariner 4 as it made a fly-by of the planet were both surprising and disappointing. They revealed a hazy view of a cratered surface similar to the highland features of the Moon. The planet seemed to be dull and dead, and even the pessimists had expected to find a more interesting and varied landscape than that of the Moon.
Neither the twenty-five pictures from Mariner 6 in July 1969 nor the thirty-three from Mariner 7 in August 1969 changed this opinion in any substantial manner. The next spacecraft—two Mariner 9s designed to be launched in the spring of 1971 and to be placed in orbit around the planet—were initially dogged by ill-fortune. The first of these failed after launching. The second was successfully placed in orbit around Mars on November 14, 1971, but once more bad luck seemed to attend the mission. At that time the surface was completely obscured from view by a great dust storm. Not until January 1972 was the surface sufficiently clear for the Mariner’s cameras to take useful photographs.
From that moment until the mission ended in October 1972, Mariner 9 revealed the extraordinary diversity of the planet—enormous volcanoes, great canyons, apparent flood features, fracture systems, and pitted terrains. More than seven thousand pictures were transmitted to Earth; almost the entire surface was photographed with a resolution of 1 kilometer and many regions with a resolution of 100 meters. Canyons four times deeper than the Grand Canyon of Arizona were revealed and volcanic features rising 27 kilometers above the average surface level—or three times the height of Mount Everest. Evidently by sheer chance the earlier Mariners had revealed an untypical region of the planet’s surface. Perhaps the most exciting features of the Mariner 9 photographs were the sinuous valleys, hundreds of miles long and resembling dry terrestrial river valleys.
With this foretaste of the nature of the surface of Mars, the advent of the Viking spacecraft, designed to land on the surface and equipped with highly sophisticated equipment, was awaited with deep interest. Launched in the summer of 1975 the two Viking craft made successful and historic landings on July 20 and September 3, 1976. For the first time close-up views from the surface were transmitted to Earth while the carrier ships remained in orbit around the planet. The Viking lander photographs in these books transport one to fantastic regions of desolate rock-strewn plains, while the 60,000 pictures of the surface transmitted from the two orbiting spacecraft have mapped the planet over two Martian years. The entire surface is now mapped with a resolution of 200 meters and large areas have been photographed with a resolution of 10 meters. This is a resolution 15,000 times better than we had available from the Earth-based telescopes twenty years ago. The books by Victor Baker and Michael Carr are an intriguing and valuable beginning to the interpretation of this vast amount of detail.
The authoritative nature of both books is beyond question. Victor Baker is Professor of Geosciences at the University of Arizona, and Michael Carr of the US Geological Survey was the leader of the Viking Orbiter Imaging team. Both authors write about the same topic—the nature and history of the planetary surface—and both base their work on the vast and impressive array of photographs from the Mariner and Viking missions to the planet. The conclusions of the two authors do not differ in broad outline although in detail variations in emphasis or certainty are revealed.
A count of craters according to their size provides an index of the impact rates of meteorites and other debris on Mars throughout its 4.5-billion-year history. Probably in the early stages there was both extensive melting of the planet’s crust and “outgassing,” i.e., the release of gases from beneath the surface into the atmosphere. About 3.9 billion years ago the crust solidified and there was also a rapid decline in the rate of impact of interplanetary objects on Mars. At this stage the landscape of the planet stabilized and from that point attempts can be made to reconstruct the subsequent geologic history.
Today the atmosphere of Mars is thin compared with the Earth’s terrestrial atmosphere, but in those early stages Mars had a dense atmosphere. Neither Earth nor Mars retained its early dense primordial atmosphere, and we do not know what catastrophic event in the solar system caused the loss. However, the evidence is that the channels on Mars were formed by water in those early stages. Subsequently volcanic activity continued to produce water. Today there is no evidence of surface water on the planet. Both Carr and Baker agree that the water is now trapped in a vast artesian well system below the thick permafrost that developed as the atmosphere thinned. The amount trapped in the polar regions is not known.
The development of the major surface features as traced by the authors, and the comparison of this development with those of the Earth and the Moon, make a fascinating story. Both authors agree that in some ways Mars resembles Earth—for example it has an atmosphere, although a much thinner one than Earth, and a surface modified over billions of years by wind to produce a variety of landforms. Both on Mars and on Earth the surface features provide the evidence for deformation and volcanism. However, there is a major difference. The geology of Earth is dominated by large-scale movements of the planet’s crusts—plate tectonics—leading to efficient outgassing and the formation of the continents and mountain chains. This has not happened on Mars. The crust of the planet is fixed and the major features we observe today are the result of impact and volcanism. The outgassing is incomplete. Many of the detailed features of the planet and the differences from Earth are explicable by these factors.
The story may be unfinished but one is left with the impression that the major features in the history of the planet have been substantially clarified, to the extent that, as Baker points out, the study of the Martian landscape is stimulating planetologists to ask new questions about Earth. For those interested in possible life-forms on Mars, there is in Carr’s book a special chapter by Harold P. Klein on the Viking biological investigations. The results so far have been negative, showing the “absence of detectable quantities of organic carbon compounds” in the Viking lander material, although as Mr. Klein points out the samples analyzed came from the “essentially featureless areas of the planet.”* He concludes that “the available information does not warrant optimism that new approaches will result in different conclusions about biology on Mars.”
The splendidly reproduced photographs of the surface of Mars are the predominant interest in both books. Again and again as the pages turn the reader must surely gasp with astonishment at the extraordinary feat of American scientists and engineers in procuring this detailed evidence of a distant and inhospitable land. At the end one is left in astonishment at the special conditions that allowed life to develop on Earth and not on our neighboring planet.
June 10, 1982
Klein writes: “If future experiments on Mars, or with returned samples, confirm the absence of organic materials in other more diversified regions of Mars (e.g., in the polar regions or in deeper layers of the surface), this information could significantly affect current ideas about the conditions under which organic chemical evolution may proceed during planetary evolution, and it could well affect probability estimates of the distribution of life in the universe.” ↩