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Great expectations: from canals to craters, picturing the Martian landscape has never been cut and dry.

WHEN PIP, in Charles Dickens's novel Great Expectations, learns that a secret philanthropist has provided money to educate him and turn him into a gentleman, he incorrectly assumes that his mysterious benefactor is Miss Havisham, a strange, manipulative, and mean-spirited woman whose true character he doesn't recognize. He is eventually surprised to discover that his "great expectations" actually were realized by Magwitch, an escaped convict he had once assisted.

Forty years ago Mars inspired great expectations as the Mariner 4 spacecraft reached the planet and on July 14, 1965, transmitted 21 and a fraction video photographs of its surface. Until then, no other planet had been seen at close range.

There was little expectation that Mariner 4 would reveal the exotic planet pictured by Percival Lowell more than a half century before. By 1965 Lowell's vision of Mars as an inhabited world had been contradicted by astronomical facts. Astronomers knew that Mars is too cold and too dry, with an atmosphere too thin, for the planetwide system of canals Lowell imagined that the world's faltering alien civilization had engineered to redistribute water. Most experts instead expected a rocky, Earth-like planet, possibly graced with simple plant life.

Opinion on Martian "canals" was divided. Some argued that the linear features were illusions of our pattern-seeking brains. Others thought that the canals and the seasonal variations in surface color were evidence of what Estonian astronomer Ernst J. Opik described as "strange vegetation of mosses and lichens." Analyzing the surface of Mars in The Oscillating Universe (1960), Opik concluded, "The best explanation still seems to be that the dark areas are covered with vegetation."

Mariner 4, however, did not meet organic expectations. Its first 10 images of Mars disclosed a barren landscape, not at all like Earth. If there were continents, mountain ranges, valleys, or ocean basins on Mars, Mariner 4 didn't see them. The spacecraft's 11th frame, a small piece of real estate in the Atlantis feature on Martian maps, surprised everyone with a crater 151 kilometers (94 miles) in diameter. At least 70 craters larger than 5 km across were seen in the Mariner photographs, and they appeared to be impact scars. Although Mariner 4 examined only 1 percent of the surface, the crater count suggested that Mars might have more than 10,000 similar craters planetwide--far more than Earth, where only a few had been recognized.

Looking for insight into the canals and signs of life, most people didn't expect to see craters on Mars. The Jet Propulsion Laboratory's initial statement on Mariner 4's mission, authored by physicist Robert B. Leighton, geologists Bruce C. Murray and Robert P. Sharp, and spacecraft technicians Richard K. Sloan and J. Denton Allen, acknowledged that our "first close-up look at Mars ... revealed the scientifically startling fact that at least part of its surface is covered with large craters." The preliminary report added, "Frame No. 11 of the Mariner sequence must surely rank as one of the most remarkable scientific photographs of this age."

Hardly anyone had envisioned Mars with craters prior to Mariner 4, but a few had speculated that they might exist. Patrick Moore, one of astronomy's most effective popularizers, has suggested that the late Donald L. Cyr was the first to mention craters on Mars. An engineer and the publisher of Stonehenge Viewpoint, a newspaper from the 1970s dedicated to Earth mysteries and unorthodox archaeology, Cyr discussed craters in a self-published booklet, Life on Mars, in 1944. According to Cyr, the canals are fertile paths on which Martian creatures migrate from one "oasis" to another. The oases, he thought, are craters.

Opik may have been the first professional astronomer to contemplate craters on Mars. Knowing the planet to be "drier than the Sahara and colder than Siberia," Opik didn't consider the Martian craters a seasonal destination for wildlife, but, calling Mars "the stone-battered planet," he conjectured that the oases could be meteoritic craters. The linear features some reported might be surface cracks induced by impact. Opik believed that the crowding of asteroids in the neighborhood of Mars is a thousand times greater than around Earth, and Mars should, then, be carpeted in craters.

At first glance Mariner 4 made Mars look like the Moon, but subsequent missions exposed a richer variety of Martian geology. With the largest volcano in the solar system, a canyon as long as the United States is wide, and signs of ancient seas and water flow, scientist now know that Mars resembles neither Earth nor the Moon. Craters cover nearly its entire surface, and their abundance helped us realize that impacts--seemingly a distinctly lunar experience--were a fundamental process in the solar system's formation. As we dispatched more probes into space, craters showed up everywhere: on the moons of Mars, on Mercury, on asteroids like Gaspra, and on satellites of Jupiter, Saturn, Uranus, and Neptune. Even Venus, beneath its opaque clouds, couldn't conceal impact craters from radar surveys from Earth and by spacecraft in the 1980s and 1990s.

The discovery of craters throughout the solar system rejuvenated an interest in the impact craters closer to home. In 1965 the Mariner 4 science team contrasted Mars with Earth, which was believed to have only a "handful" of impact craters. Because they were so few, little thought was given to the role of bombardment on Earth's development. Over the last 40 years, however, more than 150 terrestrial impact features have been identified. Many lie on very ancient continental shields, Precambrian rock that is geologically stable and less eroded than much of the rest of the planet. This cratering record emphasizes the importance of collisions in the formation of planets and the evolution of their surfaces, and the extensive pounding seen in the lunar landscape records the last episode of heavy cratering, roughly 3.9 billion years ago.

Major impacts have also been linked with mass extinctions and dramatic climate change, and they now modulate our understanding of the evolution of life on Earth.

As Mariner 4 closed in on Mars in early 1965, we expected to see evidence of life beyond Earth, but the cratered surface of Mars cultivated a different sensibility. The more we look at Mars--and at other stone-battered worlds--the more we appreciate the unusual character of Earth. JPL's Mariner 4 press statement signaled the beginning of this change in perspective. The presence of Martian craters, it asserted, "further enhances the uniqueness of Earth within the Solar System." Informed of the spacecraft's findings, President Lyndon B. Johnson reflected, "It may just be that life as we know it, with its humanity, is more unique than many have thought."

The solar system's ubiquitous craters, first billboarded by Mariner 4, have prompted us to understand that Earth, not Mars, is the planet with the great expectations. E. C. Krupp expects the unexpected at Griffith Observatory in Los Angeles.
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Author:Krupp, E.C.
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2005
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