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LUNAR AND PLANETARY PERSPECTIVES ON THE GEOLOGICAL HISTORY OF THE EARTH JAMES W. HEAD III Department of Geological Sciences, Brown University, Providence, RI 02912 USA Abstract. During the latter part of the last century, a profound change took place in our perception of the Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seems to explain disparate observations about the Earth and how it works, and lets us see the Earth as a planet. Secondly, actually seeing the Earth from the Moon, and exploring the other planets provided additional perspectives on our own home planet and hastened the decline of scientific terracentrism. Thirdly, learning that the uniqueness of the Moon in terms of size and aspects of its chemistry may be due to its derivation from the Earth as the result of a giant impact, provided a concrete filial link. Finally, the geological record revealed by exploration of the Moon and planets has provided us with the missing chapters in the dynamic history of the Earth. We now know that gargantuan impact basins formed in Earth’s formative years and that impact events are likely to be the cause of many punctuations in Earth’s biological evolution. Perspectives on ancient tectonic activity are provided by Mercury, Venus, Mars, and the Moon, and show that the Earth has changed considerably since its youth. Widely varying volcanic eruption styles are seen on the planets, providing insight into how puzzling rocks from early Earth history formed. The composition of planetary atmospheres has revealed the unusual nature of Earth’s, and its link to the evolution of life. The atmospheres of the planets have undergone radical changes with time, providing clues to Earth’s history and destiny. Fundamentally different hydrological cycles on Earth, Venus, Europa and Mars, and evidence for significant changes with time, have provided insight into Earth’s history. The probable presence of oceans on Europa and Mars has changed our thinking about the origin and evolution of life on Earth. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a member of a family of planets, each of which provides important missing information and perspective on the other, and together reveal the fabric of the history of the Solar System. Future exploration and perspectives will place our Solar System in the context of all of the others. 1. Initial Perspectives Early observations of the heavens by humans led to awe and superstition, as unexplained and frightening appearances of comets and meteor showers profoundly distracted people from difficult daily lives. Unusual configurations and alignments of celestial bodies were seen in the context of animal forms and deities. Special configurations (e.g., a bright star and crescent Moon), or unusual brightness (e.g., an extremely bright star over a small town in the Middle East), were seen as signs of supreme beings, particularly when linked to unusual earthly events. A common theme was that these signs were warnings or harbingers, and definitely related to humans and our presence here on Earth. Although the gods who controlled these Earth, Moon and Planets 85–86: 153–177, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 154 JAMES W. HEAD III things were clearly superior, nonetheless, they were speaking to us. If we could not understand these things, at least we could put them in a framework that we could understand. A second perspective evolved in parallel to the development of these superstitious and religious frameworks. Empirical observations of the positions of the stars, Sun and Moon, particularly in relation to seasonal changes and cycles of growth, led several early civilizations to attempt to understand the heavens in the context of regular change. Later on, this perspective was sidetracked by attempts to fit the motions of the planets into a cosmos in which Earth (read humans) occupied the central position. Conveniently, everything revolved around the Earth, in that most perfect of ancient Greek geometric figures, the circle. We constantly interpreted our surroundings in terms of our most immediate frames of reference (anything in the sky above us is a direct message to us; flat ground equals flat Earth; the known world is the center of all activity). Acosmic terracentrism is a natural consequence of our lack of perspective on space and time. After all, we are special. 2. The Retreat from Specialness For Western civilization, the retreat from human specialness began with the intellectual and artistic rebirth represented by the Renaissance in the fifteenth and sixteenth centuries. Copernicus, Tycho, Kepler, and Galileo all helped humans to break the bonds of terracentrism and to perceive our surroundings in ever broader frameworks of space and time. Galileo, working in Padua, applied telescopic observations to the nature and motions of the planets and satellites. These observations took us to new dimensions of scale, thus changing our perception of the Solar System and the place of the Earth, and laying the foundations for modern science. Now the Sun was the center of the Solar System. But the road was not smooth. A powerful and vengeful Catholic Church was threatened by these new views; Giordano Bruno was burned at the stake and Galileo was placed under house arrest and forced to recant his views. The scientific mantra of these times might have been “Publish and perish” not “Publish or perish”. In addition, the rich artistic and intellectual treasures produced during the Renaissance temporarily reinforced the concept of human specialness through a triumphant selfcelebration. Later in the millennium, a physical, geological and biological renaissance began to reveal the true age of the Earth, the concept of ‘deep’ time, and the role of longterm biological evolution. Newton introduced quantitative approaches to testing scientific ideas. Geologists began to understand the extent and temporal immensity of the history of the Earth and how events had changed with time. Darwin, a geologist by training, outlined the nature of biological evolution, and explicitly and implicitly, the place and role of humans. By the latter two centuries of the last millennium, the Sun was accepted as the center of the Solar System, the motions LUNAR AND PLANETARY PERSPECTIVES 155 of the planets and satellites were well known, the age of the Earth was known to be over 4.5 billion years, and humans were generally, and begrudgingly, accepted as the product of biological evolution measured over geological time scales. The retreat from human specialness was well underway. But all was not lost; we were still the crowning achievement of this biological evolution, we were still at the top of the tree of life. 3. The Influence of the Exploration of Inner and Outer Space on Perception During the latter part of the final century of the last millennium, profound changes took place in our perception of the Earth as we explored inner and outer space. First, this change was holistic. Prior to this time, the geology of the Earth was seen as regional in nature. Mountain belts and volcanoes were classified, compared and contrasted, to look for common themes in their formation and evolution. But there were no unifying themes in geological sciences for how the planet worked. Concepts like continental drift, put forth to explain the close fit of many continental margins with each other, were seen as eccentric or untestable, primarily because the outer parts of the interior of the Earth were thought to be solid and immobile. Exploration of inner space (the floors of the oceans and the structure of the interior of the Earth) in the years following World War II forever changed our concepts of our own planet. Seafloor exploration revealed that the ocean floors were very young geologically and completely unlike the continents. Probing of the Earth’s interior revealed chemical and mechanical layers in the interior. The outermost of the mechanical layers was a lithosphere, overlying a more mobile substrate called the asthenosphere. The lithosphere was comprised of many adjacent plates, was created at mid-ocean ridges, moved laterally, and was destroyed at subduction zones, where the lithosphere was bent downward and reentered the interior of the planet. This paradigm of “global plate tectonics” showed that the seafloor was spreading apart at amazing geological rates, and that continents were forming and breaking apart as a result of this motion. Earthquakes, mountain belts and volcanoes could all be placed in the context of geological activity at the boundaries of these plates. Plate tectonic theory provided a unifying theme that seemed to explain disparate observations about the Earth and how it works, and for the first time, it let us see the Earth as a planet. A few years of reflection led to the awareness that the dynamism implied by plate tectonics explained the lack of abundant rocks from early Earth history. Two-thirds of the present surface of the Earth formed in the last 5% of the history of our planet! Most of the chapters in the book of Earth history had been destroyed. The second revolution in our perspective came from the exploration of outer space. The launches of Sputnik and Yuri Gagarin made us look upward again, but this time we were prepared to see the cosmos in a broader context of space and time. Early Soviet images of the lunar farside showed a face of a nearby planetary 156 JAMES W. HEAD III Figure 1. Earth from space. Image taken by Apollo astronauts on their way to the Moon. NASA photograph. body previously unseen by life on Earth. As astronauts took their first tentative steps toward the Moon, they looked back in awe at an Earth suspended in the black vastness of the cosmos (Figure 1). Their wistful descriptions of the Earth from the Moon reminded us all of the vastness of space and the specialness of, not us, but our planet. We saw the Earth as a beautiful blue sphere, with no political boundaries and a tenuous and fragile environment. The Apollo photographs of the Earth became an icon for this new awareness. It was rapidly dawning on humans that we were part of a larger planetary environment and that our very activities were destroying it. Words like ecology and environmentalism were in vogue. But human specialness still prevailed; ecology was commonly defined as “the relationship between humans and their environment”. We were still “top dog”. Scientific terracentrism was also still rampant. Several decades of successful application of plate tectonic theory to scientific problems on Earth rapidly brought LUNAR AND PLANETARY PERSPECTIVES 157 our knowledge of recent geologic history to encyclopedic proportions. But again, we were very highly collimated in our perceptions of time. What about the other 80% of Earth history? What happened in the formative years? How did the Earth we observe today get to be the way it is? Where might it be going in the future? How does the Earth compare to the other planets? Could there be information there that might provide a broader perspective on our own home planet? These questions were on the minds of only a very few scientists. 4. The New and Present Perspective on Earth History Seeing the Earth from the space, walking on the Moon, holding samples from other planetary bodies in our hands, and exploring a host of other planets has indeed provided additional perspectives on our own home planet and hastened the decline of scientific terracentrism. Apollo astronauts completed extensive geological traverses on the Moon (Figure 2). Samples returned from these carefully planned scientific expeditions provided the first documentation of the nature and processes operating in the first one-half of Solar System history (Figure 3). We began to understand that this early history is unlike that seen in later stages of planetary evolution. Geology as a science, in its early development, had to define itself against the “catastrophism” of the great biblical flood. Thus, “uniformitarianism”, the concept that geological processes have operated at about the same rate throughout geological history, was developed. No special circumstances, no “Deus ex machina”, no catastrophic events, were required. A second concept developed at this time added to the underpinnings of geological thought. Geological processes observed to operate today (e.g., volcanism, stream activity, glaciers, etc.) have been operating throughout geological time, and thus “the present is the key to the past”. But the expanded geological record provided by the Moon began to yield important perspectives on these underpinnings. Processes such as impact cratering, which occur so infrequently in recent geological history as to not be part of the geologist’s awareness, were found to dominate earlier planetary history (Figure 4). Individual impact craters were certainly catastrophic locally, and perhaps globally. And clearly the relative proportions of processes operating during different times in planetary history could vary widely. Laboratory analysis of the returned lunar samples provided a further perspective. The elements were the same, and the minerals were familiar, but the proportions were generally different. Rocks that dominated the lunar highlands (anorthosites) were rare and poorly understood on Earth. The maria were made of basalts, a common rock type on Earth, but the proportions of titanium within them were virtually unheard of on Earth. And most importantly, the lunar rocks were extremely dry and had unusual isotopic ratios. Two stunning conclusions were reached from these and other data. First, it appears likely that the Moon formed from the impact of a Mars-sized body into the very early Earth. The melting and ejection of this 158 JAMES W. HEAD III Figure 2a. Astronauts exploring the Moon: (a) Apollo 15 Commander Dave Scott examines the geology of the base of the Apennine Mountains. (b) Apollo 16 Commander John Young jumps a few feet off the lunar surface to get a better view of the Cayley Formation in the Descartes highlands. (c) Apollo 17 Lunar Module Pilot Harrison H. “Jack” Schmitt samples a large boulder at the base of the Taurus Littrow Mountains. NASA Apollo photographs. material into Earth orbit ultimately resulted in the re-collection of the debris to form the Moon. The uniqueness of the Moon in terms of its size and chemistry may thus be due to its derivation from the Earth as the result of a giant impact. In what may have been the ultimate catastrophic event in our local frame of reference, the Moon may indeed have been born from stripping of the outer layers of the Earth. The Earth–Moon system may represent a concrete filial link (Figure 5). And this must have forever changed the course of the evolution of the Earth. Secondly, the anorthositic crust of the Moon formed early in lunar history and appears to be the result of heat associated with intense impact bombardment. The energy associated with the accretion of the Moon may have melted the outer several hundreds of kilometers of the Moon and produced a molten rock (magma) ocean. Low density crystals floated to the top to produce the anorthositic crust. Could the LUNAR AND PLANETARY PERSPECTIVES 159 Figure 2b. Continued. Earth have undergone a similar type of global melting and early crustal evolution? No Earth rocks have been found dating from this period of planetary history. Could the other planets provide clues? The geological record revealed by exploration of the Moon and other planetary bodies has indeed provided us with many of the missing chapters in the dynamic history of the Earth (Figure 3). We now know that even hundreds of millions of years after the accretion of the planets, gargantuan impact basins were forming on planetary surfaces, including Earth’s. The Orientale Basin on the lunar western limb is almost 1000 km in diameter and is among the larger (but not the largest) of the impact structures there. Its rings form a prominent bull’s-eye pattern and its ejecta influences almost an entire lunar hemisphere. Although the depth of excavation is not yet well constrained, it is obvious that some of these impacts must have penetrated to great depths to excavate material from deep within the interior. The influence of the millions of cubic kilometers of ejecta on the early atmosphere and surface is as yet not fully conceivable. Such planetary-scale events were not uncommon in the first third of Solar System history. It is interesting to speculate as to how human culture and religion might have evolved differently if 160 JAMES W. HEAD III Figure 2c. Continued. this gigantic unblinking eye had been directly facing Earth, rather than hidden on the limb (Figure 3). Such spectacular examples of impact events obscure the fact that smaller projectiles were much more abundant and that they dominated the geological record of early planetary history (Figure 4). The lunar geologic record of impact flux, known from the samples returned by Soviet Luna and US Apollo missions, shows a monotonic decrease in the rate of cratering as a function of time (Figure 7). LUNAR AND PLANETARY PERSPECTIVES 161 Figure 3. Comparative geological records of different planets. All planetary bodies formed at essentially the same time, about four and a half billion years ago. Plotted is the percentage of the presently exposed surface that dates from different times in the history of the Solar System. The Earth’s surface is dominated by the young seafloor and continental deposits ringing ancient cratons. The record of the Moon, Mars and Mercury formed in the first half of Solar System history and is still preserved today. Impact cratering and volcanism dominate these one-plate planets. On Venus, the surface has a young Earth-like age, but does not display plate tectonic features. Implicit in the knowledge of this flux is the fact that impact cratering is an ongoing and recurring geological process throughout the history of the planets, including Earth. If we view Earth history backwards from the perspective of recent geological events, most Earth scientists would relegate impact craters to the category of minor curiosity. When viewed from the perspective of the past history of the planets, planetary scientists see impact cratering as an ongoing process operating at many scales, and having substantial geological, environmental, and biological consequences. These two disparate views did not begin to be reconciled until distinctive geochemical anomalies similar to those seen in meteorites were detected in sediments at the Cretaceous–Tertiary (K–T) boundary. The demise of the dinosaurs and the formation of this distinctive world-wide geological boundary is now thought to be due to the impact of a bolide that formed a crater in the Yucatan. Subsequent investigations have shown that impact events are likely to be the cause of many other punctuations in Earth’s biological evolution. The road to the top of the tree of life may not have been direct. What about other geological processes? On Earth, the destruction of the early chapters of history have obscured the origin of plate tectonics. We know it has been operating for at least hundreds of millions of years, but when and how did it start? Perspectives on ancient tectonic activity are provided by Mercury, Venus, 162 JAMES W. HEAD III Figure 4. Lunar craters on the heavily cratered lunar farside. The 75 km diameter King crater, with its lobster-claw-like central peaks, is seen near the center of the picture. NASA Apollo 16 image. Mars, and the Moon, and these records show that the Earth has changed considerably since its youth. The Moon, Mars and Mercury all have heavily cratered surfaces that formed and were modified predominantly in the first half of Solar System history (Figure 4). The stability of these surfaces, and the lack of features associated with plate tectonics on Earth, indicate that these bodies are “one-plate planets”. Their outer mechanical layers, or lithospheres, stabilized early on into one continuous global plate. This stability preserved the important record of early planetary history that we see today. Tectonic movement on these one-plate planets was then largely vertical, with loading by volcanic deposits, subsidence and flexure on the Moon, broad uplift by mantle plume activity on Mars, and minor global shrinkage to produce spectacular scarps on Mercury. Why do these bodies differ so from the Earth? The surface area to volume ratio means that they are good radiators, LUNAR AND PLANETARY PERSPECTIVES 163 Figure 5. A view of the Earth–Moon system from the Galileo spacecraft as it returned from Venus and the asteroid belt. The Moon is closer to the viewer than the Earth and a significant portion of the lunar farside is seen. NASA Galileo photo. losing heat very efficiently. This, together with their small diameters, results in their lithospheres becoming a relatively large percentage of their radii early in history. It is then extremely hard to start the subduction that apparently resulted in plate tectonics on Earth. Breaking a thick rigid layer and pushing it into the interior on a small planet is not easy. But what about Venus, the most Earth-like of the planets in terms of its size, density, and position in the Solar System? Does Venus have plate tectonics? Exploration of Venus was motivated by just such questions and following numerous missions by the Soviet Union and the US, the Magellan mission obtained global high-resolution radar images in the 1990s. These spectacular images (Figure 8) revealed mountain ranges, rift zones, and an extremely young surface geologically (Figure 3), general properties that were very similar to the Earth and its plate tectonic system. But most surprisingly, there was no supporting evidence for plate Figure 6. The Orientale Basin on the Moon (left), which formed by impact about four billion years ago. The basin is almost 1000 km in diameter. NASA Lunar Orbiter photo. A montage of how the Orientale Basin might have appeared if it had formed at the sub-Earth point and was constantly, and unblinkingly, looking down on us (right). 164 JAMES W. HEAD III LUNAR AND PLANETARY PERSPECTIVES 165 Figure 7. The record of the impact flux on the Moon. Comparison of the number of craters on different geologic units can provide a relative time scale of events. Return of samples from well-known places on regional units is required to provide the basis for the absolute time scale. The absolute time scale derived from Apollo and Luna samples shows that in the first few hundred million years of lunar history, the flux was extremely high, decreasing exponentially between 4 and 3 billion years ago. Although considerably diminished from its early values, the impact flux, and individual events, are still a very important part of the geological processes operating on planetary surfaces. Return of samples from well known units on other planets will provide the exact time scale for those bodies in the future. The number of craters on the vertical axis refers to craters larger than 1 km in diameter per million square kilometer area. 166 JAMES W. HEAD III Figure 8. Tectonic deformation in the mountains of Venus. In this view of a huge dome in the Freya Montes region of Ishtar Terra, numerous tectonic features are testimony to the intense deformation accompanying the creation of this and adjacent tessera terrain. Ringing the dome to the east and west are broad folds caused by shortening and contraction. On top of the dome are seen a set of intersecting extensional structures (graben) indicating that the dome underwent stretching and collapse. Width of the image is about 75 km. NASA Magellan radar image. tectonics! No globe-encircling system of plate boundaries, no evidence for some very young surfaces (where plates were forming), and no evidence for older surfaces (where mature plates were being subducted and destroyed). The distribution of impact craters could not be distinguished from a completely random one, and the global density of craters was so low as to suggest that the surface was only several hundred million years, not billions of years, old. LUNAR AND PLANETARY PERSPECTIVES 167 How could this be? How could this contrast so much with the Earth, where the young average age (Figure 3) is a combination of the older continental surfaces and the very young ocean basins? Geophysicists set about to try to understand this and one of the models they came up with was radically different than our previous thinking. In the absence of plate tectonics, could the vertical buildup of crust on a one-plate planet, if it continues long enough on a large body like Venus, lead to periodic density inversion, vertical foundering of the outer layer, and catastrophic resurfacing of the planet? Could this be how plate tectonics started on the Earth? Among the competing ideas to this is the concept of episodic plate tectonics: in this view, periods in Venus history are alternatively plate-tectonic dominated, and one-plate-planet dominated, and at present we are in a one-plate phase. A third alternative is that plate tectonics previously characterized the surface of Venus, constantly destroying old terrain and producing new, but that due to continuing heat loss over geologic time, the lithosphere thickened, froze up in recent history, and plates stopped moving relative to one another. In this view, Venus is now and forevermore a one-plate planet. Could this be the fate of Earth in its future? These radical ideas are still hotly debated in the scientific community, and no firm consensus has emerged. But the richness of the alternatives has opened our eyes to several new ways of thinking about the history of the Earth. Could there be major changes in the style of global tectonic activity with time? Could global changes and long-term loss of heat from the interior be episodic rather than monotonic? Could the abundance of water on Earth be a critical factor in plate tectonics? Widely varying volcanic eruption styles are seen on the planets, providing insight into how unusual rocks, such as very iron-rich fluid lavas called komatiites, formed early in Earth history. Gigantic lava flows on the Moon are equivalent to 40,000 times the annual output of Kilauea volcano on Hawaii. Venus displays huge flows (Figure 9) that have resurfaced thousands of square kilometers in very short periods of time. These types of eruptions, uncommon on Earth today, may explain the nature, origin and associations of rock types seen in past history. Indeed, the massive outpourings of lava on Venus are now thought to have put so much gas into the atmosphere that surface temperatures increased substantially. As this thermal wave passed into the crust, the style of global tectonic activity is thought to have been influenced. Imagine, atmosphere changes causing changes in the style of deformation of planetary crusts! Massive edifices on Mars rise to over 20 km height (Figure 10) and dwarf the puny Hawaii. The stable one-plate planets can build these large edifices over longlived sources or hot spots, and lay out the complete sequence of deposits with time. This is in contrast to the Earth, where such volcanoes are smaller, formed in production-line-like manner, moved laterally away from the source, and then are subducted and destroyed. Determination of the composition of planetary atmospheres has revealed the unusual nature of Earth’s, and its link to the evolution of life. The atmosphere of Venus and Mars is predominantly carbon dioxide in contrast to the nitrogen and 168 JAMES W. HEAD III Figure 9. Huge bright and dark lava flows in the Lada Terra region of Venus converge on a low point in a north–south ridge, and pour through this saddle and out into the surrounding plain. These flows have traveled almost 700 km eastward from their source region. The total area of the flow field exceeds 500,000 km2 , similar to some ancient flood basalt provinces on Earth. NASA Magellan radar image. LUNAR AND PLANETARY PERSPECTIVES 169 Figure 10. Perspective view of Olympus Mons, a gigantic volcano on Mars about 600 km in diameter and over 20 km high. Viking Orbiter photomosaic overlain on Mars Orbiter Laser Altimeter altimetric data. Vertical exaggeration is about twenty times. oxygen-rich Earth atmosphere. Analysis of ancient Earth rocks and inventory of carbon dioxide stored in carbonate rocks (such as limestones) on the Earth shows that the Earth originally had as much carbon dioxide as other planets, but the evolution of life has changed the atmosphere considerably. In addition, analysis of the planetary atmospheres and the geological record of the planets shows that they have undergone radical changes with time, providing clues to Earth’s history and destiny. The polar caps of Mars have been recognized for over a hundred years, but recent observations of the Moon and Mercury reveal evidence for volatile-rich polar ice deposits there. Planetary degassing products and cometary impact debris apparently migrate to the polar cold traps and produce deposits even in the extreme thermal environment of Mercury. The prospect of sampling the geologic record of volatiles contained in these caps is extremely exciting. The larger and more accessible polar caps of Mars contain a stratigraphic record of many hundreds of layers (Figure 11) which could provide the keys to understanding recent climatic change there. Earth has always been thought of as the water planet. Water dominates the surface, is abundant (occurring in glaciers, rivers, lakes and oceans covering almost two-thirds of the planet), is extremely significant in weathering, and is thought to be essential in the formation and nurturing of life. But recent exploration has shown that water may have played a very important role on other planets too, and that oceans may not be the exclusive purview of Earth. Although liquid water is not now stable under present conditions on Mars, we see evidence for ancient glacial deposits, streams, rivers, and lakes. Indeed recent evidence is consistent with the presence of a huge ocean filling the northern lowlands of Mars earlier in its history (Figure 12). On Europa, the second of the Galilean satellites (Figure 13), we now have evidence that a global ocean covers the surface and that it is frozen over, but likely still liquid today below the surface. These new perspectives on environments have changed our frame of reference in thinking about the formation and evolu- 170 JAMES W. HEAD III Figure 11. Layers in the north polar cap of Mars seen in a Mars Orbiter Camera image. These layers, as small as a few meters thick, are thought to be related to changing conditions on the surface and greater and lesser amounts of deposition of ice (bright) and dust (dark). These layers may be related to the same kind of obliquity variation in the orbital axis that are responsible for the ice ages on Earth. LUNAR AND PLANETARY PERSPECTIVES 171 Figure 12. Hypothesized position of an ocean in the northern lowlands of Mars in its earlier history. In this topographic map of the northern hemisphere of Mars derived from Mars Orbiter Laser Altimeter (MOLA) data, the black area is the low topography proposed to have been occupied by a huge standing body of water. The Tharsis region is seen as a high on the right. Much of the water may have entered the basin from outflow channels entering at the lower left. NASA MOLA data. tion of life on Earth. Radically different hydrological cycles on Earth, Mars, and Europa, and evidence for significant changes with time, have also provided insight into Earth’s history. 5. The Lessons for the History of Earth, Our Home Planet How have these new views changed our perception about the history of the Earth? We now know that the following themes must be considered in the reconstruction of those missing chapters of Earth history. − Formation of the Earth from accretion of planetesimals. − Derivation of the Moon from the Earth as the result of a gigantic (Mars-sized) impact event. • Late addition of a large amount of material from elsewhere in the Solar System. • Stripping of the early atmosphere of the Earth. • Loss from the early Earth of a considerable amount of the solid upper layers. 172 JAMES W. HEAD III Figure 13. Galileo image of the surface of Europa in the anti- Jovian region. Note the general lack of impact craters and the cracked nature of its frozen water-ice surface layer. The dark wedge-shaped area in the middle of the image is about 15 km wide, and represents the cracking and opening of the European crust, much in the way sea-floor spreading operates on Earth. The icy layer seen in this image likely overlies a global ocean at depth. The width of the image is about 170 km. NASA Galileo image. LUNAR AND PLANETARY PERSPECTIVES 173 • Massive changes in the internal constitution and thermal structure of early Earth. • Does this event explain the differences between Earth and Venus? • What are other implications of this concrete filial link? − Subsequent continuing high impact flux. − Formation of large impact basins excavating deep into the planet and spreading ejecta widely, influencing the atmosphere and any biota. − Delivery of rocks (and any available microbes) from the surfaces of other planetary bodies to Earth as meteorites. − Launch of rocks (and any available microbes) from the Earth’s surface to other planetary bodies as meteorites. − Effusion of large volcanic outpourings over short periods of time, influencing the atmosphere, hydrosphere, and biosphere. − Possible episodic (not just monotonic) heat loss from the interior, regional and perhaps global in scale; this would influence the atmosphere and surface temperatures, and perhaps the tectonic style. − Internal density instabilities causing global changes in volcanism and tectonism, and possible resurfacing of the entire planet. Could this initiate plate tectonics? − Planetary contraction due to thermal evolution (cooling or phase changes). Could this initiate plate tectonics? − Continuing role of impact events throughout the history of the Earth; potential large-scale modification of the atmosphere and the biota at numerous but random times. In the coming decades, these concepts derived from the last forty years of Solar System exploration will be folded into our ongoing reconstruction of the missing chapters in Earth history to produce a radically different picture of the formative years of our own home planet. In the words of T. S. Elliot, “We will not cease from exploration, and the end of all of our exploring will be to arrive where we started and know the place for the first time”. 6. New Perspectives in Time and Space Terracentrism and human specialness are on the run, being replaced by new scales of insight in time and space. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a member of a family of planets, each member of which provides important missing information and perspective on the other, and together reveal the fabric of the history of the Solar System. Our perception of life has changed radically from the photosynthesis-based, mammalian-dominated tree, leading inexorably to humans, the ultimate evolutionary achievement. Life is now seen everywhere on Earth, even in the most extreme environments, from hightemperature deep-sea volcanic vents, to the deepest, hottest mines, deriving energy 174 JAMES W. HEAD III form a host of different reactions and eating toxic waste and radioactive material. Indeed life is now thought by many to have originated in such high-temperature environments. Assessment of the terrestrial biota underlines again that bacteria are phenomenally important in terms of sheer numbers, biomass, consistency and survivability. Mars rocks may contain microfossils. Rocks can be readily transported from planet to planet throughout geological history, and the interiors of rocks are survivable environments for microorganisms. Rather than the crowning achievement of organic evolution, humans are now seen as just another marginal species. Bacteria rule. Evolution is not directed, and certainly not directed toward us. Evolution is a random process involving multiple chance and chaotic events. Life may have arrived on Earth from Mars inside meteorites almost 4 billion years ago. Humans may owe their day in the Sun to a chance impact that terminated a successful global population of reptiles and paved the way for opportunistic mammals. Humans could easily suffer the same fate as the dinosaurs. Bacteria would still rule. We have had just a few years to enjoy this recently-acquired Solar Systemcentered perspective. Ongoing and future exploration and perspectives will place our Solar System in the context of all of the others that are now known to exist around other stars, and the many more soon to be discovered. The continuing retreat from terracentrism and human specialness is underlined by the incredible diversity of solar system arrangements that have been encountered in the last few years. Of course, our quest is driven by the desire to find a planet “like Earth”. 7. Into the Future: Beyond Human Specialness We have analyzed the historical self-perception of the role of humans and examined our past perceptions in understanding Earth and the cosmos. These assessments show some progress, but abundant mistakes, wrong turns, and prejudices. The record demonstrates that human thinking is almost by definition, limited in space and time (Figure 14). Indeed, limited thinking in space and time is a fundamental characteristic of almost all animal species, and is almost certainly an inheritance from our genetic forebearers. Our daily lives are so dominated by short-term local events that we must struggle mightily to break these bounds. In the short-term, local environment, we are the dominant species. But as we have expanded our ability to probe over greater distances and longer time scales, and understand what we see in these dimensions, human specialness disappears. In this millennium, humans will continue to try to turn the tide of the retreat from human specialness, and will do their utmost to avoid surrender. But the odds are not with us. Not too many years ago we were the center of the cosmos and the star atop the tree of life. Now we are not. But this should not be a threat. The past tells us that we have a very exciting ride ahead as we probe to ever greater dimensions of space and time, explore outer and inner space, and perhaps other dimensions as well. LUNAR AND PLANETARY PERSPECTIVES 175 Figure 14. The density of humans occupying different parts of the space and time scales. Most humans are concerned with their immediate surroundings (space) and with their short term activities (time). Few individuals are considering broader dimensions of the space and time scales, and even those occupy it only for a short period of time. The decisions that are made and the actions that are taken in the lower left part of the diagram may have immense consequences for the upper right hand portion. Similarly, inaction in this area, and inattention to the broader scales of space and time, can have equally important effects. Modified from D. H. Meadows et al., The Limits to Growth, Universe Books, New York, 1974. In fact, early in this millennium it will likely be conclusively shown that the very exploration ethic that we think of as uniquely human, is instead a manifestation of the survival of species. Of course, this is not so obvious to us as yet in species other than ourselves. The importance of maintaining an open mind to new and seemingly heretical ideas cannot be understated. The very things that bind us perceptually to a small area of space and time (Figure 14) also make us threatened by and resistant to new and changing ideas. The socialization and acculturation processes work heavily against individual and creative thinking. We must recognize this and strive to overcome these effects. But progress is clearly being made and the times are very, very exciting. And the future is as promising as it is incomprehensible. Several years 176 JAMES W. HEAD III ago, a student entered my office in utter frustration, having been unable to find a thesis topic on which no previous work had been done. In answer to her query, my colleague simply said, “Oh, don’t worry, almost everything is not yet known”. Acknowledgements I would like to acknowledge the influence of the following individuals and their writings: E. O. Wilson, Carl Sagan, Ray Bradbury, Rodney Brooks, Isaac Asimov, Thomas S. Kuhn, T. S. Elliot, Arthur C. Clarke, Stephen Jay Gould, David R. Scott, John Young, Harrison Schmitt, John McPhee, Lionel Wilson, and Richard S. Williams, Jr. Discussions with many students at Brown University have challenged, developed and sharpened these thoughts. Selected Readings Allegre, C.: 1992, From Stone to Star, A View of Modern Geology, Harvard University Press, CT. Alvarez, W.: 1997, T. Rex and the Crater of Doom, Princeton University Press, Princeton. Beatty, J. K., Petersen, C. C., and Chaikin, A. (eds): 1999, The New Solar System, Sky Publishing Corporation, Cambridge, MA. Boorstein, D. J.: 1983, The Discoverers, Random House, New York. Chaikin, A.: 1994, A Man on the Moon, Viking Press, New York. Ciba Foundation: 1996: Evolution of Hydrothermal Ecosystems on Earth (and Mars?), John Wiley & Sons Ltd, West Sussex. Goldsmith, D.: 1997, Worlds Unnumbered: The Search for Extrasolar Planets, University Science Books, Sausalito, CA. Goldsmith, D.: 1997, The Hunt for Life on Mars, Penguin, New York. Greeley, R.: 1993, Planetary Landscapes, Chapman & Hall, New York. Grinspoon, D. H.: 1997, Venus Revealed: A New Look below the Clouds of Our Mysterious Twin Planet, Addison-Wesley, Reading, MA. Harland, D. M.: 1999, Exploring the Moon: The Apollo Expeditions, Springer Praxis, London. Hartmann, W. K.: 1993, Moons and Planets, Wadsworth, Belmont, CA. Krupp, E. C.: 1983, Echoes of the Ancient Skies, Harper and Row, New York. Light, M.: 1999, Full Moon, Alfred A. Knopf, New York. MacKenzie, F. T.: 1998, Our Changing Planet: An Introduction to Earth System Science and Global Environmental Change, Prentice Hall, NJ. McSween, H. Y.: 1995, Stardust to Planets: A Geological Tour of the Universe, St. Martin’s Griffin, New York. Morrison, D. and Owen, T.: 1988, The Planetary System, Addison-Wesley Publishing Company, Reading MA. Norton, O. R.: 1998, Rocks from Space, Mountain Press Publishing, Missoula, MT. Oreskes, N.: 1999, The Rejection of Continental Drift: Theory and Method in American Earth Science, Oxford University Press, New York. Sagan, C.: 1982, Cosmos, Random House, New York. Shirley, J. H. and Fairbridge, R. W.: 1997, Encyclopedia of Planetary Sciences, Chapman & Hall, London. Spudis, P. D.: 1996, The Once and Future Moon, Smithsonian Institution Press, Washington, DC. LUNAR AND PLANETARY PERSPECTIVES 177 Strom, R. G.: 1987, Mercury: The Elusive Planet, Smithsonian Institution Press, Washington, DC. Ward, P. D. and Brownlee, D.: 2000, Rare Earth: Why Complex Life Is Uncommon in the Universe, Copernicus, New York. Weissman, P. R., McFadden, L., and Johnson, T. V. (eds.): 1999, Encyclopedia of the Solar System, Academic Press, San Diego. Wilhelms, D. E.: 1993, To a Rocky Moon: A Geologist’s History of Lunar Exploration, University of Arizona Press, Arizona. Zebrowski, E.: 1997, Perils of a Restless Planet: Scientific Perspectives on Natural Disasters, Cambridge University Press, Cambridge.