When Neil Armstrong and Edwin E. Aldrin Jr. took that "giant leap for mankind ," they also took a little-noticed hop for planetary science. A laser reflector they left behind on the moon July 21, 1969, opened what was then the new field of precision lunar ranging.
This has blossomed into an international activity whose first fruits illustrate the fact that exploring other bodies in our solar system gives a better understanding of our own planetary home --
Among other things, lunar laser ranging has brought a new level of precision to measuring Earth's rotation. This reveals tiny wobbles related to winds, seismic activity, and even motions within Earth's liquid core. It promises new insight into processes geophysicists only dimly envision today.
As Derral Mulholland of the University of Texas notes, it now "becomes necessary to observe the skies to learn more about the Earth under our feet." This could be the motto for planetary exploration generally.
Earth cannot be understood in isolation. However intimately scientists study our planet, they still are looking only at one special, albeit humanly crucial, case. Although he is concerned specifically about atmospheres, Harvard University meteorologist Richard Goody emphasizes this general point when he explains: "You keep tuning your theories of the Earth's atmosphere as you gain more [terrestrial] knowlegde. . . . [yet] it is possible to be totally wrong; you have only one set of facts. But if you study the planets, you create a much larger body of knowledge."
Scientists need the solar-system perspective. They need to know how planets have evolved and how they work generally to see where Earth fits in. So as the United States enters a five-year hiatus in its main planetary exploration program, it is worth taking stock of the perspective gained. That's why this three-part series is concerned with what we have learned fromm the planets, not what we have learned aboutm them.
Having made its spectacular survey of the Saturn system in November, the Voyager 1 spacecraft is heading for interplanetary space. Its twin, Voyager 2, is being prepared for its follow-up Saturn survey next August. It will then go on to Uranus, but won't arrive at the planet until January 1986. There are no other projects that can reach planetary objectives between now and the Uranus encounter. Hence the roughly five-year hiatus in new exploration.
The concern scientists feel because of this hiatus, how it developed, and what might be done next is the subject of the final article in this series.
The first two articles are concerned with what has been learned to date. For this purpose, the solar system can be classified naturally into two regions -- the inner, rocky, earthlike planets and the giant gas bags ranging from Jupiter outward.
All of these need to be studied to gain a full understanding of the solar system. But the inner planets -- Mercury, Venus, Earth, Mars (and, you could add , the Moon) -- share a common evolutionary heritage. They are largely solid bodies, some of which probably have liquid cores, and three of which (Venus, Earth, Mars) have atmospheres. Exploration of these bodies has a direct bearing on unraveling Earth's own evolutionary history.
One of the main features that lepas out at anyone who scans a representative set of planetary close-up pictures is the important role that cratering has played in their history. As the planets formed, debris of many sizes from small meteorites to giant asteroids rained in upon them. Scientists had known that this probably happened. But it took firsthand observations to show that the cratered face of the Moon is typical, rather than exceptional, for the inner solar system.
After their initial formation 4.5 billion years ago, and while debris was still raining in, all of the inner planets may have shared a family resemblance in this regard. Erosion, vulcanism, and tectonic (crust-shaping) activity have reshaped the features of the more active planets. But on the Moon -- where there has been little erosion and where vulcanism appears to have ceased some 3 billion years ago -- and on Mercury, craters remain a dominant surface characteristic. These bodies provide basic information (such as the statistics of cratering throughout inner-solar-system history) which can help planetologists reconstruct the more complex histories of Venus, Earth, and Mars.
Earth is the most geologically active of these planets. Erosion by wind and water, continuing extensive vulcanism, and the action of "plate tectonics" have removed virtually all trace of the first billion years from its surface.
Earth's outer layer (the lithosphere) is made up of 10 or so large plates, plus a number of smaller plates. These constantly interact -- bumping together, sliding past one another, or overriding and dipping beneath each other. Most volcanic and earthquake activity is connected with plate interactions and occurs along the edges where plates meet.
Meanwhile, continents move, new oceans open up, and old oceans close as the plates rearrange themselves. Along the sea floor, new plate material is created as magma wells up from midocean ridges and spreads os magma wells up from midocean ridges and spreads out laterally. Old plate material, meanwhile, is being reabsorbed into the underlying mantle where sea-floor plates plunge beneath continent-carrying plates along ocean edges. In this way the sea floor is completely renewed about once every 200 to 250 million years.
Scientists trying to understand our own planet's history are grateful to be able to study other inner-solar-system planets where the surface record is better preserved. As noted, the Moon and Mercury provide good records of the early period when cratering was especially active. Mars and Venus provide intermediate cases.
Mars, with an average radius of 3,390 kilometers (2,107 miles) -- alittle over half that of Earth's 6,371 km (3,963 miles) -- has had considerable volcanic and tectonic activity. But the planet is too small to sustain the vigorous surface-reshaping activity that characterizes Earth. Also its thin air (surface pressure a little less than 1 percent that of Earth) and lack of water keep erosion to a minimum, although there is evidence of some water erosion in the past. The surface preserves a long record of geological activity intermediate between that of Earth and the Moon.
A line tilted about 50 degrees to the equator divides Mars into its two distinctive types of terrain. The southerly hemisphere contains most of the very ancient, heavily cratered lands. North of the line are regions of lightly cratered plains and large volcanoes. Remnants of old craters stick up through these plains to suggest that an original heavily cratered surface has been extensively altered.
Mars has some of the most striking relief and the largest volcanoes known in the solar system. Three of them tower 27 km above the Mars datum level (corresponding to Earth's mean sea level) and 17 km above surrounding plains. An even larger volcano -- Olympus Mons -- rises 25 km above the land. Diameters of these giants range from several hundred kilometers to well over 1,000 km. Earth's largest volcanoes, in Hawaii, are typically only 9 km above the seabed and 120 km across. Vulcanism has played a major role on Mars and is thought to be still active, although there doesn't seem to be much plate tectonic action.
"The result," says Michael H. Carr of the US Geological Survey (USGS), reviewing Martian geology in the journal American Scientist, "is a spectacular planet on which geological features of enormous scale and a wide variety of origins and ages are almost perfectly preserved."
Venus, on the other hand, may be only at the beginning of plate tectonic activity. Comparable to Earth in size (6,260-km radius), the planet had been considered Earth's twin. Too many difference have been found for that image to be sustained. Its atmosphere, composed largely of carbon dioxide (CO2), as is the Martian atmosphere, is many times thicker than our own (surface pressure 95 times that of Earth), with a perpetual cloud that veils the planet. Its surface is hot enough to melt lead (about 470 degrees C.). And the surface features as revealed so far by radar are only vaguely reminiscent of the continents and seabeds of Earth.
Most of the detail has come from data taken by the radar altimeter on the Pioneer Venus spacecraft now orbiting the planet. This covers about 93 percent of the surface. It shows uplands, mountains, and lowlands distributed quite differently from those on Earth. One mountain massif, Maxwell, is slightly higher than Mt. Everest. Some of the plateaus are higher than comparable terrestrial features. About 60 percent of the surface is within 1,000 meters of the mean datum level ("sea level"), however, while only 16 percent is significantly below this level. On Earth, the ocean floors make up 70 percent of the surface.
Although relatively little is yet known about the surface of Venus, the planet appears to be the most active in the inner solar system after Earth. In announcing the radar mapping results this year, USGS geologist Harold Mazursky commented that the surface may be undergoing some kind of plate tectonic action. If true, he observed, it could become "a powerful tool for understanding this powerful process [plate tectonics], which has become the great unifying theme in Earth science."
Between them, Venus and Mars represent alternative geophysical histories sufficiently different from Earth's to enable scientists to see how basic processes common to all three planets work out different contexts. The same can be said for the atmospheres of the planets.
The thick Venusian atmosphere and the thin atmosphere of Mars are certainly different from that of Earth. Yet, as meteorologist Goody has noted, they are "sufficiently like ours so that any model of Earth's atmosphere should work for the others."
Venus is a slowly rotating planet, turning once in 243 Earth days and in a direction opposite from Earth's rotation. Its lower atmosphere seems stagnant. Yet its upper atmosphere layers circle the planet in only four days. The atmoshpere circulates in such a way that there is little temperature contrast around the planet. The equator-to-pole temperature difference is only about 2 percent, contrasting with 40 percent on Mars (which spins at about the same rate as Earth) and 15 percent on Earth.
As noted in a review of planetary atmospheres in the National Science Foundation's magazine Mosaic, Goody says this called for a new look at the way meteorologists account for planetary temperature distributions. Earth's temperature distribution has been explained in terms of global winds and sea currents fueled by heating in the tropics and strongly influenced by the planet's rotation. Such a picture could fit Mars. But Venus scarcely rotates at all.Yet its atmosphere appears to be even more well mixed than that of Mars or Earth.
Harvard's Goody found the average surface air pressure to be a critical factor whose importance had escaped notice. The pole-to-equator temperature difference seems directly correlated with surface pressure. The greater the pressure (and hence the denser the air), the more thorough the mixing.
Conway Leovy of the University of Washington draws another fundamental inference from the seeming paradox of a strong east-west air circulation on a slowly rotating planet. Theories developed primarily for Earth predict that such a zonal circulation, as it is called, with winds blowing more or less parallel to latitude circles, will develop as the atmosphere "tries" to transport excess heat from tropics to poles. The influence of Earth's rotation is given a key role in establishing such winds.
Although his suggestion is only tentative at this stage, Professor Leovy says that it may be that the balance of forces that govern atmospheric motion produces zonal winds, whether or not a planet rotates, and that these winds may arise in several ways. Fundamentally, "there may be no such thing as a nonrotating atmosphere," given a difference in solar heating from equator to pole, he says.
If true, this would be a new principle to add to atmospheric textbooks -- an important scientific dividend from studying unearthly weather.
One of the more disturbing upshots of this study is the connection between the sizzling-hot Venusian surface and the thick heat-absorbing CO2 atmosphere. Some scientists, such as Thomas M. Donahue of the University of Michigan, Andrew Ingersoll of the California Institute of Technology, and Carl Sagan of Cornell University have suggested that Venus may have started out more like Earth, with oceans on its surface. But being closer to the sun, relatively more of the water would have evaporated and reside in the air than on Earth. Its atmosphere would have evolved differently.
Both CO2 and water vapor absorb heat radiated from a planet's surface and raise the surface temperature (the so-called greenhouse effect). As Earth evolved, most of the CO2 was removed by formation of carbonate rock in the sea. The residual greenhouse effect merely helps maintain livable temperatures. On Venus, with more water in the atmosphere and less in the oceans, things may have gone differently. A "runaway greenhouse" effect may have developed, leading to a sizzling planet.
One problem with this theory is that its creators can't yet explain what happened to the water. Venus has little of it today. However, it does make the point that greenhouse effects can have serious consequences. It raises a warning about the buildup of CO2 in Earth's atmosphere from the burning of fossil fuesl.
As Donahue notes in the Mosaic review,". . . our studies of Venus opened up the possibilities of a runaway on Earth. The conditions arem present. We have the water. We have the carbon dioxide. We are putting more and more carbon dioxide into the atmosphere. . . . And we don't know where the runaway point is. . . .'
These few examples reinforce Derral Mulholland's point that looking out to other planets enriches our understanding of Earth. And the lunar laser ranging (LLR) work, in which his program at the McDonald Observatory is the world leader , shows that this is not just confined to Earth's atmosphere and surface.
Earth's rotation includes tiny, unpredictable wobbles which now can be measured with high precision by following the relative motion of Earth and Moon. The friction between large-scale wind systems and the surface may cause some of them. Seismic activity within the outer layer and the interaction of the planet's liquid core with overlying solid material may cause other wobbles. The ability to track the wobbles themselves is an important first step to a better understanding of what is involved.
More intriguing, perhaps, is the vindication LLR has given Einstein's general theory of relativity -- his theory of gravity. A key point is that there should be no difference in the kind of mass that gives rise to gravity and the kind of mass that determines how a body reacts to a force (so-called inertial mass). some critics have challenged this equivalence of gravitational and inertial mass -- the so-called equivalence principle.
If the critics were right and Einstein wrong, this would show up as an anomalous deviation of about one meter in the motion of the moon. LLR could find no such deviation. IT has confirmed the equivalence principle so solidly that this particular challenge has lost its interest.
Einstein always did like the larger perspective.