Mercury Unveiled

As the innermost planet of the solar system, Mercury is almost always obscured by the sun's harsh glare. Under the best viewing conditions, it never appears as more than a hazy disk in earth-bound telescopes. Last week, as the Mariner 10 passed only 400 miles from the planet, some of the mystery about Mercury was finally dispelled. Radioing back the first close-up pictures of the Mercurian surface, the robot ship unveiled a bleak, cratered and totally forbidding world.

"It's like the moon and it isn't," said Donald E. Gault, one of the scientists monitoring the Mariner data at NASA'S Jet Propulsion Lab in Pasadena, Calif. The pictures showed that Mercury's craters are much flatter and thinner-rimmed than the moon's and resemble giant pie pans--an indication that they may have been worn down by some yet-to-be-identified erosional process. Like most of their lunar counterparts, Mercury's craters were apparently created by impacts of asteroid-size chunks of material rather than by volcanic eruptions. Indeed, one crater, about 25 miles across, was blasted out of the rim of a 60-mile-wide crater by an impact that threw off raylike splashes of debris.

The photographs show no mountains similar to the moon's, but there are indications of several great escarpments or cliffs, some of them hundreds of miles long. More puzzling still, there are distinctly nonlunar bumps, hillocks and rills. Some of the rills are remarkably straight, while others twist and turn, almost as if they had been carved out by flowing water. One photograph reveals two overlapping craters with a flow of material--possibly lava--on top of them. That would suggest a period of lava flows that may have followed a major bombardment from space.

The biggest surprise was the discovery of a weak magnetic field (only 1% as strong as the earth's) and an extremely thin atmosphere of helium, argon and perhaps other gases (less than 0.1% as dense as the earth's). Although the earth's magnetic field is generally attributed to the churning of molten iron in the spinning planet's core, Mercury seems to rotate much too slowly to produce such a dynamo effect. But scientists offer alternative explanations. Mercury's magnetic field may be created externally by bombardment of charged particles from the sun hitting the atmosphere--or it may be left over from an epoch when the spin was faster. The presence of an atmosphere is equally difficult to explain because the planet's gravity is too weak to prevent a gaseous envelope from escaping into space. But, says Project Scientist James A. Dunne, some gases could be continually trapped from the stream of solar particles or released from within the planet by the slow decay of radioactive elements.

As they continued their study of the flood of data from Mariner 10 at week's end, the champagne-sipping scientists were elated by the spacecraft's performance. They now think that Mariner may have enough fuel left when it again crosses Mercury's orbit in September to guide the ship over one of the planet's poles, which were hidden from view during last week's flyby.

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