A MEETING WITH THE UNIVERSE

Chapter 2-1

A New Solar System

To the ancients, the solar system consisted of a few worlds wandering through a limited region of empty space. To us, it is a huge region containing uncountable bits of solid matter and filled with great magnetic fields and streams of electrically charged atomic particles from the Sun. One of the most visible and exciting activities of the Space Age has been the detailed exploration of this astonishing collection of matter and energy around the Sun. The inventory of solid bodies in the solar family is impressive: nine planets, at least thirty-nine moons, thousands of tiny asteroids, billions of comets, and vast numbers of meteoroids and small particles of cosmic dust.

In recent years, many spacecraft have explored the planets and their moons: over two dozen of these worlds have been studied at close range. This exploration is not a random process.

It proceeds in systematic stages, each stage built on the results of earlier ones. First, there are the reconnaissance or "flyby" missions such as those to Mercury, Jupiter, and Saturn.

Next come exploration missions, carried out by orbiters and landers on the Moon, Mars, and Venus. Finally, there is a stage of intensive study, involving astronaut landings and sample return, which thus far have been accomplished only on the Moon.

As of now, we have made intensive studies of only the closer worlds. Our spacecraft have not been powerful enough to push us very far out from Earth or very far inward toward the Sun. For this reason, most of our missions, especially the earliest ones, have involved the nearest worlds: the Moon, Venus, and Mars. We have only briefly surveyed Mercury, Jupiter, and Saturn. Although one interplanetary probe, Pioneer 10, has passed the orbit of Uranus, the outer planets beyond Saturn are completely untouched.

Even current plans involving definite missions include only a possible Voyager 2 flyby of Uranus in 1986. To return inward to survey Mercury in greater detail than heretofore, or to go outward to study the outermost planets, we will need more powerful spacecraft than exist today.

According to current theory, the Sun and planets formed together in the collapse of a vast cloud of interstellar dust and gas (the protosolar cloud) about 5 billion years ago. The central part of the cloud collapsed to form the Sun, and the planets condensed in orbits around it. One feature of the solar system that all the theories have to take into account is that there are two basically different kinds of planets: the solid terrestrial planets, like the Earth, and gas giants, like Jupiter and Saturn.

The terrestrial planets (Mercury, Venus, Earth, and Mars) lie in the inner solar system, close to the Sun. They take from almost three months to almost two years to orbit around it. They are relatively small, from less than 5000 kilometers to almost 13,000 kilometers (less than 3100 miles to almost 8100 miles) in diameter, and they are solid and rocky, as if they had formed in a hotter part of the original dust cloud from which most of the gases were lost. The samples we have from the Earth, the Moon, and meteorites suggest that all the terrestrial planets are composed largely of fairly heavy elements such as silicon, aluminum, calcium, magnesium, iron, and others, combined with oxygen to make solid minerals and rocks.

Despite their common composition, the terrestrial planets are very different. Consider their atmospheres: Mercury has none, the Earth and Mars have modest atmospheres, and Venus has air so thick, dense, and cloud filled that it forever conceals the planetary surface. The magnetism of terrestrial planets also varies strikingly. The Earth has a fairly strong magnetic field, Mercury has a weaker one, and Venus and Mars apparently have none at all. Even more curious, the rates of rotation vary widely: Earth and Mars each spin on their axes in about one day, but Mercury takes two months to rotate once and Venus takes a full eight months. (Venus also rotates opposite to the direction of rotation of the other planets.)

The critical chemical water (H₂0) varies greatly among the terrestrial worlds. The Earth has vast quantities of liquid water, so much so that it often is referred to as "the water planet." Mars has a much smaller amount, present as ice. Venus may have almost none; only a trace of water has been found so far in its atmosphere. No water has been found on the Moon or in its rocks, although some scientists speculate that frozen water may exist in permanently shadowed regions near the lunar poles. A final mystery is that life, so far as we know, exists and has existed only on the Earth.

Further out in the solar system are the gas-giant planets: Jupiter, Saturn, Uranus, and Neptune. Great globes of dense gas, with little or no rocky material, they formed in cooler parts of the protosolar cloud, so gases and ices were preserved. These planets take from almost 12 years to almost 165 years to circle the Sun, but they spin on their axes remarkably rapidly, in 10 to 16 hours, rather than in days or months.

The giant planets are virtually all atmosphere. On Jupiter and Saturn, we see spectacular banded patterns of swirling, brilliantly colored clouds. At high altitudes, the clouds probably are composed of frozen ammonia crystals; in effect, there is a high layer of ammonia clouds like the high-altitude cirrus clouds of Earth. At lower altitudes Jupiter's clouds are probably made of water and complex molecules. The atmospheres of far-off Uranus and Neptune are hard to study, but show faint traces of clouds as well.

We can only make informed conjectures about the interiors of the gas giants. They may have small rocky cores; if so, the cores are surrounded by layers of solid ice. Around this ice, in Jupiter and Saturn, the enormous pressure of the overlying material has reduced the molecular hydrogen gas (which makes up most of the atmosphere) to a liquid state in which the hydrogen behaves electrically like a metal. In Uranus and Neptune, the hydrogen is dense but may not reach the metallic condition. Currents circulating in the metallic fluids of Jupiter and Saturn generate powerful magnetic fields that surround the two planets in space and trap atomic particles from the "solar wind" that streams outward from the Sun.

The gas giants have solid moons orbiting around them. Not just one or two moons, like Earth and Mars, but whole families; Jupiter and Saturn have more than a dozen moons apiece.

Their moons, recently photographed by the Voyagers 1 and 2 spacecraft, have been revealed as an astonishing collection of distinct individuals: large irregular rocks, worlds of cratered ice, and one moon of incessant glowing volcanic eruptions. The largest of Saturn's moons, Titan, is covered by a dense brownish atmosphere of nitrogen and methane, with minor amounts of other organic molecules.

Within each of the two planetary groups - the terrestrial planets and the gas giants - our space probes have revealed tremendous diversity. Each world is unique, but each also has something in common with the others, so that by studying one, it is possible to discover basic truths that relate to them all.