Chapter 2-5

The Planetary Connections

If our goal in planetary exploration were simply to accumulate a list of impressive discoveries, then we have succeeded beyond our wildest expectations. But there is a larger purpose to our search through the solar system: to discover not just what planets are like, but also how they got that way. What forces formed and shaped young planets in the ancient past? What processes sculptured their surfaces, made or failed to make atmospheres, and brought forth or failed to bring forth life?

We cannot discover the general laws that govern all planets by studying only the Earth. We would never know whether things that are common on Earth (oxygen, water, and life) were abundant or absent elsewhere, and so we have gone to other worlds to find out. A new scientific discipline has arisen, comparative planetology, in which we study as many worlds as possible, looking for common characteristics amid the riot of individuality.

Finding how planets form and grow is motivated by more than just scientific curiosity. The Earth is a planet too. Our world and its life are the results of complex forces operating for billions of years. The more we learn about all planets, the better we can understand our own: its geologic past, the behavior of its atmosphere, and future climatic trends.

By going into space, we have discovered that certain basic planetary characteristics occur throughout the solar system, manifesting themselves in different ways on different planets. These general features include phenomena that affect the Earth itself - volcanism, meteorite bombardment, magnetism, atmospheric evolution, and weather and climate.


Volcanic eruptions seem to be a normal process in the development of terrestrial planets. The Moon is covered with dark lavas that have been identified, analyzed, and age-dated through samples brought to the Earth. Photos of as-yet-unsampled Mars reveal both huge volcanoes and surface rocks that resemble lavas. Mercury shows large surface features that also could be lava flows. Our blurred and indistinct radar views of Venus show some landforms that probably are large volcanoes. The asteroids also seem to have volcanic histories, for some of the meteorites that we find (themselves presumably asteroidal fragments) are pieces of ancient lava flows erupted at the very beginning of the solar system. Even Jupiter's moon lo has its own collection of strange, sulfur-spouting volcanoes.

Meteorite bombardment

Before we went into space, meteorite bombardment seemed an unimportant process. Only about a dozen small meteorite craters were known on the Earth, and many scientists thought that the Moon's craters were all ancient volcanoes. Now we know that intensive bombardment by meteorites in the past was the rule, not the exception, among the planets. The Moon suffered a violent bombardment in its earliest years, more than 4 billion years ago, and the traces are still seen in the heavily cratered lunar highlands.

Other planets show traces of the same ancient cosmic battering. The whole surface of Mercury resembles the lunar highlands, saturated with overlapping craters. The southern half of Mars likewise was battered, although its large craters have been deeply eroded. Venus bears traces of what may have been large impacts. Even Callisto, the icy moon of a gas giant world, displays much the same cratered surface that Mercury does.

Is the Earth unique? It shows no obvious large meteorite craters. Did it somehow escape the bombardment? No. The ancient Earth must have been pounded and shaped by meteorite impacts just like the worlds near it. But the ancient craters, like the Earth's oldest rocks, have been destroyed by the continuous volcanism, erosion, and mountain-building that characterize our planet, Nearly 100 ancient craters, now identified in the geological record, show that the Earth has been, and still presumably is, subject to meteorite impacts.

Meteorite impact has continued through time, though at a much lower rate than in the early bombardment. Smaller craters occur in profusion on the relatively young, dark, lava flows on the Moon, and even tinier microscopic craters, made by bits of cosmic dust, dot the surfaces of exposed lunar rocks. On the Earth, Meteor Crater in Arizona, the best-known impact scar, is less than 50,000 years old. The Tunguska event in Siberia, a violent explosion probably caused by the entrance of a comet into the Earth's atmosphere, occurred within living memory, in 1908.

Planetary magnetism

We have studied and used the Earth's magnetic field for centuries, but it remains a mystery in many ways. It surely is caused by huge electric currents in Earth's iron core. But we still do not know the details of how it is produced, why it varies, and why it completely reverses itself every million years or so.

Space probes have discovered magnetic fields elsewhere in the solar system, extending outward from the Sun and surrounding both some terrestrial and some gas giant planets. Where these fields are present, they shield the planet from the solar wind that pours out from the Sun, and in the region where the solar particles encounter the planetary fields, there arise other remarkable effects: trapped radiation belts, planetary magnetic tails, magnetic storms that cause aurorae, and bursts of radio noise that can be heard across the solar system. In truly great magnetic fields like those of Jupiter, atomic particles may be heated to millions of degrees, and a great electric arc flows between the planet and its moon lo. Indeed, one of the curiosities of the solar system is that two planets as different as Jupiter and the Earth-one terrestrial, one a gas giant-should have similar magnetic properties. Each has a magnetic field, aurorae, radiation belts, and naturally-generated radio noise.

Magnetic properties vary widely and unpredictably among the terrestrial planets. The Earth has a metal core and a strong magnetic field. The Moon, with no detectable metal core, has no field, but it may have had a strong field in the past because a strange "fossil" magnetism has been detected in many lunar rocks. Mercury has a large metal core, but only a weak magnetic field. Venus probably has a metal core, but it has no field. Mars, which may or may not have a metal core, has no field. The gas giant planets, at least the two visited by spacecraft, seem to be more uniform. Both Jupiter and Saturn have strong magnetic fields, although their "metallic" interior regions probably are made of hydrogen rather than of nickel-iron.

Our studies of planetary magnetism have so far produced more questions than answers. Why do some terrestrial planets with metal cores have magnetic fields (Earth, Mercury) while others (Venus, Mars) do not? If the planet's rotation rate is a factor, why does one slow rotator (Mercury) have a magnetic field while another (Venus) does not? More puzzling is the evidence from Moon rocks that the Moon's magnetic field "turned off" about 3 billion years ago. How could this happen? Might the Earth, or another planet, lose its magnetic field in the future?


Long before the Space Age, we knew that other worlds have atmospheres. But only recently have we been able to make accurate analyses of these atmospheres or to understand something of their histories.

There seem to be two types of atmospheres: original ones, formed from gas present in the primordial dust cloud, and evolved or outgassed ones, whose gases have gradually come out of the interior of the planet, probably as the result of volcanism.

The atmospheres of the gas giant planets are largely original. Their compositions are close to that of the Sun itself (largely hydrogen and helium), and the crushing gravity of these huge worlds would prevent any original gases from escaping. The gases that collected into Jupiter 4.5 billion years ago must be there still.

The terrestrial planets seem to have atmospheres of the outgassed type. Somehow, most of the original gas seems to have been swept away from the terrestrial planets and replaced by other gases from their interiors, such as nitrogen, carbon dioxide, and water.

These outgassed atmospheres differ greatly. Mars and Venus have atmospheres rich in carbon dioxide, but the pressure of Venus'atmosPhere is 10,000 times that of Mars. Although Venus has a thick, heavy atmosphere, its composition suggests that the planet has not outgassed as much as has the-Earth. The atmosphere of Mars has been modified continuously since formation because the low gravity of the planet has allowed much of the nitrogen to escape into space.

Earth's oxygen-rich atmosphere seems unique, but we know that it has been drastically modified by an agent not detected on the other planets: life. Studies of other planets suggest that, just before life developed, Earth's atmosphere may have been much like that of Mars and Venus: probably rich in carbon dioxide and nitrogen. In such an atmosphere, with oxygen lacking, simple organic molecules could combine into more complex ones without being destroyed by oxygen. Eventually, about 4 billion years ago, these complex molecules united to produce simple life forms. At some later time, simple plants began to turn the carbon dioxide into oxygen, and the process has continued to the present, producing the air we now breathe.

It is ironic that the development of life on Earth has finally produced an atmosphere so rich in oxygen that the original chemical reactions that led to life can no longer occur. If life suddenly vanished from the Earth, it might not reappear again. Because living things produced the atmosphere in which we now live, there is a real cause for concern that another kind of life (human beings) could change the atmosphere even further. It happened once before.

Weather and climate

Earth is probably the worst place to learn the laws that govern Earth's weather. Our weather patterns are complicated; they are modified by the planet's rotation, by high mountain ranges, by the huge oceans, and by the water that rises as clouds and falls again as rain. Because of these complications, it is difficult to study the weather and almost impossible to predict it.

We have to examine other worlds with simpler weather patterns in order to learn about our own. Fortunately, the solar system provides a wide variety to study. There are plan- ets that rotate slowly (like Venus) and rapidly (like Jupiter). There is a flat world (Venus), a somewhat moun- tainous world (Mars), and a world that may have no solid surface what- ever (Jupiter). There are thick, dense atmospheres (Venus, Jupiter) and thin ones (Mars). There are atmos- pheres of carbon dioxide (Venus, Mars) and of hydrogen and helium (Jupiter, Saturn). The atmospheres range from superheated (Venus) to freezing (Mars).

We have found similarities to Earth's weather in unlikely places. The circulation of the thin Martian atmosphere is similar to the effects found over Earth's deserts. High-velocity jet streams like our own also run along the belts of Jupiter's atmosphere. Jupiter's Red Spot, three times the size of Earth, bears a strong resemblance to an overgrown terrestrial hurricane.

The atmospheres of other planets give us another important opportunity: to learn how our own atmosphere might change in the future. We can look at other worlds to see how their atmospheres are affected by certain important substances: dust, carbon dioxide, sulfuric acid. In this way, we can understand what will happen to Earth's atmosphere if natural or human activities continue to introduce these materials into it.

We have already found several examples worthy of study and concern. Dusty Martian sandstorms may imitate the heating or cooling effects of dust, produced by volcanoes or by human beings, in our own air. The fine particles (aerosols) of sulfuric acid that form the corrosive clouds of Venus may help us to understand acid rain and other kinds of sulfur pollution here on Earth. The atmospheres of Mars and Venus, rich in carbon dioxide, may teach us to predict what will happen to our own atmosphere as the burning of fossil fuels continues to pour more carbon dioxide into the air. Will Earth warm considerably or not at all?

Climate is simply the weather of a planet over long periods of time. We know that the Earth's climate has not been stable. It has been both hotter and colder in the geological past, and the recent Ice Ages are only the latest events in these variations. We do not know what caused these climate changes: gradual changes in the Earth's orbit around the Sun, changes in the Earth's oceans and atmosphere, or perhaps even changes in the Sun itself? We know even less about what climatic changes may occur in the future. Yet civilization is dangerously vulnerable to these changes. Will the climate grow warmer, melting the icecaps and flooding our seacoasts? Or will it grow colder, freezing the seas and wiping out agriculture?

Again, other planets can help us find the answers. We now know that other planets have experienced climate changes, so that the Earth is not unique in this respect. Mars shows the traces of a warmer, wetter time, a period when its atmosphere was thick and great floods scoured channels across its surface. Mars may also have been colder in the past than it is now. The layers of sediment around the polar caps suggest that those caps may have been larger in the past. If we could show that Mars'hot and cold climates had occurred at times responding to the Earth's, then the underlying cause might be ascribed to variations in the light from the Sun itself.

Venus, our other neighbor world, is not too helpful yet. We do not know enough about its surface to understand if there are traces of previous climates. In a cooler time, Venus might really have been "Earth's Twin", with rivers cutting winding channels across it; those channels may remain. We need to explore in great detail both Mars and Venus, those worlds that bracket the Earth, before we will fully understand the development of the Earth's climate. The flood of data and discovery in the Space Age has given us our first glimpse of the mechanisms that con- trol the birth and development of planets. We have found that worlds which seem totally different at first glance are actually linked by common bonds: volcanoes, meteorite craters, magnetism, and atmospheres. But further investigation and exploration are required to clearly define the forces that make planets what they are.

From what we have already done and learned, we can suggest and plan the next steps: The more we understand about other worlds, the more we will learn about the past and present nature of the Earth. Did huge meteorite impacts begin the process by which the Earth produced continents and ocean basins? Why is the Earth a highly evolved planet, with active volcanoes and mountain-building, while the other terrestrial planets seem quiet and dead? Why has the Earth developed a temperate environment where life has flourished, while nearby Venus and Mars seem lifeless and inhospitable?

There are other questions about the Earth'sjuture that we must also answer. How are we changing our atmosphere, and what will these changes do to us? Will the Earth's climate change soon, and how? Is the Earth's magnetic field decreasing? What will happen to us if it vanishes, even for a short time? When will an asteroid hit the Earth, as many have done in the past? These are questions of more thanjust scientific interest. Their answers may determine the survival of people and civilization.

The Space Age has given us a new view of many worlds, but most especially of our own Earth. On our first trips to the Moon, the Earth suddenly appeared to human eyes as a tiny blue world of life, isolated in a vast, uncaring blackness. Now that we have explored further, we see that the Earth is not alone. It is one of a family of worlds, all different, each an individual, but all formed at the same time, shaped by the same forces, and developing in related ways. We can no longer hope to understand the Earth, its past, and its future, without studying and understanding its companions in space around the Sun.

Selected Readings

Beatty, J.K., B.O'Leary, and A.Chaikin 1981 (Eds.),
The New Solar System
(Cambridge, Mass.: Sky Publishing Co.,
and New York: Cambridge University Press).

Chapman, C.R. 1977,
The Inner Planets
(New York: Charles Scribner's Sons).

Cooper, H.S.F. 1980,
The Search for Life on Mars
(New York: Holt, Rinehart, and Winston).

French, B.M. 1977,
The Moon Book
(New York: Penguin Books).

Guest, J., P. Butterworth, J. Murray,
and W. O'Donnell 1979,
Planetary Geology
(New York: John Wiley and Sons).

Kaufmann, W.J., III 1979,
Planets and Moons
(San Francisco: W.H. Freeman and Co.).

Nininger, H.H. 1952,
Out of the Sky:
An Introduction to Meteorites
(New York: Dover Books).

The Solar System, 1975
(San Francisco: W.H. Freeman and Co.).

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