Lunar And Planetary Perspectives On The Geological History Of The Earth · 2013-07-25 · LUNAR AND PLANETARY PERSPECTIVES ON THE GEOLOGICAL HISTORY OF THE EARTH JAMES W. HEAD III

LUNAR AND PLANETARY PERSPECTIVES ON THE GEOLOGICALHISTORY OF THE EARTH

JAMES W. HEAD IIIDepartment 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 ofthe Earth. First, this change was holistic: Plate tectonic theory provided a unifying theme that seemsto explain disparate observations about the Earth and how it works, and lets us see the Earth as aplanet. Secondly, actually seeing the Earth from the Moon, and exploring the other planets providedadditional 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 maybe due to its derivation from the Earth as the result of a giant impact, provided a concrete filiallink. Finally, the geological record revealed by exploration of the Moon and planets has provided uswith the missing chapters in the dynamic history of the Earth. We now know that gargantuan impactbasins formed in Earth’s formative years and that impact events are likely to be the cause of manypunctuations in Earth’s biological evolution. Perspectives on ancient tectonic activity are providedby Mercury, Venus, Mars, and the Moon, and show that the Earth has changed considerably sinceits youth. Widely varying volcanic eruption styles are seen on the planets, providing insight intohow puzzling rocks from early Earth history formed. The composition of planetary atmospheres hasrevealed the unusual nature of Earth’s, and its link to the evolution of life. The atmospheres of theplanets 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 forsignificant changes with time, have provided insight into Earth’s history. The probable presence ofoceans on Europa and Mars has changed our thinking about the origin and evolution of life onEarth. We no longer think of the Earth in isolation. Instead, Earth is now perceived of as a memberof a family of planets, each of which provides important missing information and perspective onthe other, and together reveal the fabric of the history of the Solar System. Future exploration andperspectives 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 un-explained and frightening appearances of comets and meteor showers profoundlydistracted people from difficult daily lives. Unusual configurations and alignmentsof celestial bodies were seen in the context of animal forms and deities. Specialconfigurations (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 signsof supreme beings, particularly when linked to unusual earthly events. A commontheme was that these signs were warnings or harbingers, and definitely related tohumans 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 notunderstand these things, at least we could put them in a framework that we couldunderstand.

A second perspective evolved in parallel to the development of these supersti-tious 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 contextof regular change. Later on, this perspective was sidetracked by attempts to fit themotions of the planets into a cosmos in which Earth (read humans) occupied thecentral position. Conveniently, everything revolved around the Earth, in that mostperfect of ancient Greek geometric figures, the circle. We constantly interpretedour surroundings in terms of our most immediate frames of reference (anything inthe sky above us is a direct message to us; flat ground equals flat Earth; the knownworld is the center of all activity). Acosmic terracentrism is a natural consequenceof 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 in-tellectual and artistic rebirth represented by the Renaissance in the fifteenth andsixteenth centuries. Copernicus, Tycho, Kepler, and Galileo all helped humans tobreak the bonds of terracentrism and to perceive our surroundings in ever broaderframeworks of space and time. Galileo, working in Padua, applied telescopic obser-vations to the nature and motions of the planets and satellites. These observationstook us to new dimensions of scale, thus changing our perception of the SolarSystem 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 wasthreatened by these new views; Giordano Bruno was burned at the stake and Galileowas placed under house arrest and forced to recant his views. The scientific mantraof these times might have been “Publish and perish” not “Publish or perish”. Inaddition, the rich artistic and intellectual treasures produced during the Renaissancetemporarily reinforced the concept of human specialness through a triumphant self-celebration.

Later in the millennium, a physical, geological and biological renaissance beganto reveal the true age of the Earth, the concept of ‘deep’ time, and the role of long-term biological evolution. Newton introduced quantitative approaches to testingscientific ideas. Geologists began to understand the extent and temporal immensityof the history of the Earth and how events had changed with time. Darwin, ageologist by training, outlined the nature of biological evolution, and explicitlyand implicitly, the place and role of humans. By the latter two centuries of the lastmillennium, 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 tobe over 4.5 billion years, and humans were generally, and begrudgingly, acceptedas the product of biological evolution measured over geological time scales. Theretreat from human specialness was well underway. But all was not lost; we werestill the crowning achievement of this biological evolution, we were still at the topof 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 changestook 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 wasseen as regional in nature. Mountain belts and volcanoes were classified, comparedand contrasted, to look for common themes in their formation and evolution. Butthere 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 continentalmargins with each other, were seen as eccentric or untestable, primarily becausethe 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 interiorof the Earth) in the years following World War II forever changed our conceptsof our own planet. Seafloor exploration revealed that the ocean floors were veryyoung geologically and completely unlike the continents. Probing of the Earth’s in-terior revealed chemical and mechanical layers in the interior. The outermost of themechanical layers was a lithosphere, overlying a more mobile substrate called theasthenosphere. The lithosphere was comprised of many adjacent plates, was cre-ated 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 spreadingapart at amazing geological rates, and that continents were forming and breakingapart as a result of this motion. Earthquakes, mountain belts and volcanoes couldall 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 disparateobservations about the Earth and how it works, and for the first time, it let us see theEarth as a planet. A few years of reflection led to the awareness that the dynamismimplied by plate tectonics explained the lack of abundant rocks from early Earthhistory. Two-thirds of the present surface of the Earth formed in the last 5% of thehistory of our planet! Most of the chapters in the book of Earth history had beendestroyed.

The second revolution in our perspective came from the exploration of outerspace. The launches of Sputnik and Yuri Gagarin made us look upward again, butthis time we were prepared to see the cosmos in a broader context of space andtime. Early Soviet images of the lunar farside showed a face of a nearby planetary


Figure 1. Earth from space. Image taken by Apollo astronauts on their way to the Moon. NASAphotograph.

body previously unseen by life on Earth. As astronauts took their first tentativesteps toward the Moon, they looked back in awe at an Earth suspended in theblack vastness of the cosmos (Figure 1). Their wistful descriptions of the Earthfrom the Moon reminded us all of the vastness of space and the specialness of, notus, but our planet. We saw the Earth as a beautiful blue sphere, with no politicalboundaries and a tenuous and fragile environment. The Apollo photographs of theEarth became an icon for this new awareness. It was rapidly dawning on humansthat we were part of a larger planetary environment and that our very activities weredestroying it. Words like ecology and environmentalism were in vogue. But humanspecialness still prevailed; ecology was commonly defined as “the relationshipbetween humans and their environment”. We were still “top dog”.

Scientific terracentrism was also still rampant. Several decades of successfulapplication of plate tectonic theory to scientific problems on Earth rapidly brought


our knowledge of recent geologic history to encyclopedic proportions. But again,we were very highly collimated in our perceptions of time. What about the other80% of Earth history? What happened in the formative years? How did the Earthwe 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 therethat might provide a broader perspective on our own home planet? These questionswere 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 fromother planetary bodies in our hands, and exploring a host of other planets hasindeed provided additional perspectives on our own home planet and hastenedthe decline of scientific terracentrism. Apollo astronauts completed extensive geo-logical traverses on the Moon (Figure 2). Samples returned from these carefullyplanned scientific expeditions provided the first documentation of the nature andprocesses operating in the first one-half of Solar System history (Figure 3). Webegan to understand that this early history is unlike that seen in later stages ofplanetary evolution. Geology as a science, in its early development, had to defineitself against the “catastrophism” of the great biblical flood. Thus, “uniformitari-anism”, the concept that geological processes have operated at about the same ratethroughout geological history, was developed. No special circumstances, no “Deusex machina”, no catastrophic events, were required. A second concept developed atthis time added to the underpinnings of geological thought. Geological processesobserved to operate today (e.g., volcanism, stream activity, glaciers, etc.) have beenoperating 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 im-portant perspectives on these underpinnings. Processes such as impact cratering,which occur so infrequently in recent geological history as to not be part of thegeologist’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 timesin planetary history could vary widely.

Laboratory analysis of the returned lunar samples provided a further perspect-ive. The elements were the same, and the minerals were familiar, but the propor-tions were generally different. Rocks that dominated the lunar highlands (anorthos-ites) were rare and poorly understood on Earth. The maria were made of basalts, acommon rock type on Earth, but the proportions of titanium within them were vir-tually unheard of on Earth. And most importantly, the lunar rocks were extremelydry and had unusual isotopic ratios. Two stunning conclusions were reached fromthese and other data. First, it appears likely that the Moon formed from the impactof a Mars-sized body into the very early Earth. The melting and ejection of this


Figure 2a. Astronauts exploring the Moon: (a) Apollo 15 Commander Dave Scott examines thegeology of the base of the Apennine Mountains. (b) Apollo 16 Commander John Young jumps a fewfeet 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 theTaurus Littrow Mountains. NASA Apollo photographs.

material into Earth orbit ultimately resulted in the re-collection of the debris toform the Moon. The uniqueness of the Moon in terms of its size and chemistry maythus be due to its derivation from the Earth as the result of a giant impact. In whatmay have been the ultimate catastrophic event in our local frame of reference, theMoon 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 thismust have forever changed the course of the evolution of the Earth.

Secondly, the anorthositic crust of the Moon formed early in lunar history andappears to be the result of heat associated with intense impact bombardment. Theenergy associated with the accretion of the Moon may have melted the outer severalhundreds 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


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. Couldthe other planets provide clues?

The geological record revealed by exploration of the Moon and other planetarybodies has indeed provided us with many of the missing chapters in the dynamichistory of the Earth (Figure 3). We now know that even hundreds of millions ofyears after the accretion of the planets, gargantuan impact basins were formingon planetary surfaces, including Earth’s. The Orientale Basin on the lunar westernlimb 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 andits ejecta influences almost an entire lunar hemisphere. Although the depth ofexcavation is not yet well constrained, it is obvious that some of these impactsmust have penetrated to great depths to excavate material from deep within theinterior. The influence of the millions of cubic kilometers of ejecta on the earlyatmosphere and surface is as yet not fully conceivable. Such planetary-scale eventswere not uncommon in the first third of Solar System history. It is interesting tospeculate as to how human culture and religion might have evolved differently if


Figure 2c. Continued.

this gigantic unblinking eye had been directly facing Earth, rather than hidden onthe limb (Figure 3).

Such spectacular examples of impact events obscure the fact that smaller pro-jectiles were much more abundant and that they dominated the geological recordof early planetary history (Figure 4). The lunar geologic record of impact flux,known from the samples returned by Soviet Luna and US Apollo missions, showsa monotonic decrease in the rate of cratering as a function of time (Figure 7).


Figure 3. Comparative geological records of different planets. All planetary bodies formed at essen-tially the same time, about four and a half billion years ago. Plotted is the percentage of the presentlyexposed surface that dates from different times in the history of the Solar System. The Earth’s surfaceis dominated by the young seafloor and continental deposits ringing ancient cratons. The record ofthe Moon, Mars and Mercury formed in the first half of Solar System history and is still preservedtoday. Impact cratering and volcanism dominate these one-plate planets. On Venus, the surface has ayoung 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 ongoingand recurring geological process throughout the history of the planets, includingEarth. If we view Earth history backwards from the perspective of recent geolo-gical events, most Earth scientists would relegate impact craters to the categoryof minor curiosity. When viewed from the perspective of the past history of theplanets, planetary scientists see impact cratering as an ongoing process operatingat many scales, and having substantial geological, environmental, and biologicalconsequences. These two disparate views did not begin to be reconciled until dis-tinctive geochemical anomalies similar to those seen in meteorites were detectedin sediments at the Cretaceous–Tertiary (K–T) boundary. The demise of the dino-saurs and the formation of this distinctive world-wide geological boundary is nowthought 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 causeof many other punctuations in Earth’s biological evolution. The road to the top ofthe tree of life may not have been direct.

What about other geological processes? On Earth, the destruction of the earlychapters of history have obscured the origin of plate tectonics. We know it hasbeen operating for at least hundreds of millions of years, but when and how didit start? Perspectives on ancient tectonic activity are provided by Mercury, Venus,


Figure 4. Lunar craters on the heavily cratered lunar farside. The 75 km diameter King crater, withits 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 consid-erably since its youth. The Moon, Mars and Mercury all have heavily crateredsurfaces that formed and were modified predominantly in the first half of SolarSystem history (Figure 4). The stability of these surfaces, and the lack of featuresassociated with plate tectonics on Earth, indicate that these bodies are “one-plateplanets”. Their outer mechanical layers, or lithospheres, stabilized early on intoone continuous global plate. This stability preserved the important record of earlyplanetary history that we see today. Tectonic movement on these one-plate planetswas then largely vertical, with loading by volcanic deposits, subsidence and flexureon the Moon, broad uplift by mantle plume activity on Mars, and minor globalshrinkage to produce spectacular scarps on Mercury. Why do these bodies differ sofrom the Earth? The surface area to volume ratio means that they are good radiators,


Figure 5. A view of the Earth–Moon system from the Galileo spacecraft as it returned from Venusand the asteroid belt. The Moon is closer to the viewer than the Earth and a significant portion of thelunar farside is seen. NASA Galileo photo.

losing heat very efficiently. This, together with their small diameters, results in theirlithospheres becoming a relatively large percentage of their radii early in history.It is then extremely hard to start the subduction that apparently resulted in platetectonics on Earth. Breaking a thick rigid layer and pushing it into the interior on asmall 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? Ex-ploration of Venus was motivated by just such questions and following numerousmissions by the Soviet Union and the US, the Magellan mission obtained globalhigh-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 platetectonic system. But most surprisingly, there was no supporting evidence for plate


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Figure 7. The record of the impact flux on the Moon. Comparison of the number of craters on dif-ferent geologic units can provide a relative time scale of events. Return of samples from well-knownplaces on regional units is required to provide the basis for the absolute time scale. The absolute timescale derived from Apollo and Luna samples shows that in the first few hundred million years oflunar history, the flux was extremely high, decreasing exponentially between 4 and 3 billion yearsago. 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 ofsamples from well known units on other planets will provide the exact time scale for those bodies inthe future. The number of craters on the vertical axis refers to craters larger than 1 km in diameterper million square kilometer area.


Figure 8. Tectonic deformation in the mountains of Venus. In this view of a huge dome in the FreyaMontes region of Ishtar Terra, numerous tectonic features are testimony to the intense deformationaccompanying the creation of this and adjacent tessera terrain. Ringing the dome to the east and westare broad folds caused by shortening and contraction. On top of the dome are seen a set of intersectingextensional structures (graben) indicating that the dome underwent stretching and collapse. Width ofthe image is about 75 km. NASA Magellan radar image.

tectonics! No globe-encircling system of plate boundaries, no evidence for somevery young surfaces (where plates were forming), and no evidence for older sur-faces (where mature plates were being subducted and destroyed). The distributionof impact craters could not be distinguished from a completely random one, andthe global density of craters was so low as to suggest that the surface was onlyseveral hundred million years, not billions of years, old.


How could this be? How could this contrast so much with the Earth, where theyoung average age (Figure 3) is a combination of the older continental surfacesand the very young ocean basins? Geophysicists set about to try to understand thisand one of the models they came up with was radically different than our previousthinking. In the absence of plate tectonics, could the vertical buildup of crust ona one-plate planet, if it continues long enough on a large body like Venus, lead toperiodic density inversion, vertical foundering of the outer layer, and catastrophicresurfacing 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: inthis view, periods in Venus history are alternatively plate-tectonic dominated, andone-plate-planet dominated, and at present we are in a one-plate phase. A thirdalternative is that plate tectonics previously characterized the surface of Venus,constantly destroying old terrain and producing new, but that due to continuingheat 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 nowand 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 firmconsensus has emerged. But the richness of the alternatives has opened our eyes toseveral new ways of thinking about the history of the Earth. Could there be majorchanges in the style of global tectonic activity with time? Could global changes andlong-term loss of heat from the interior be episodic rather than monotonic? Couldthe abundance of water on Earth be a critical factor in plate tectonics?

Widely varying volcanic eruption styles are seen on the planets, providing in-sight 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 to40,000 times the annual output of Kilauea volcano on Hawaii. Venus displays hugeflows (Figure 9) that have resurfaced thousands of square kilometers in very shortperiods of time. These types of eruptions, uncommon on Earth today, may explainthe nature, origin and associations of rock types seen in past history. Indeed, themassive outpourings of lava on Venus are now thought to have put so much gas intothe atmosphere that surface temperatures increased substantially. As this thermalwave passed into the crust, the style of global tectonic activity is thought to havebeen influenced. Imagine, atmosphere changes causing changes in the style ofdeformation of planetary crusts!

Massive edifices on Mars rise to over 20 km height (Figure 10) and dwarf thepuny Hawaii. The stable one-plate planets can build these large edifices over long-lived sources or hot spots, and lay out the complete sequence of deposits withtime. This is in contrast to the Earth, where such volcanoes are smaller, formed inproduction-line-like manner, moved laterally away from the source, and then aresubducted and destroyed.

Determination of the composition of planetary atmospheres has revealed theunusual nature of Earth’s, and its link to the evolution of life. The atmosphere ofVenus and Mars is predominantly carbon dioxide in contrast to the nitrogen and


Figure 9. Huge bright and dark lava flows in the Lada Terra region of Venus converge on a lowpoint in a north–south ridge, and pour through this saddle and out into the surrounding plain. Theseflows have traveled almost 700 km eastward from their source region. The total area of the flowfield exceeds 500,000 km2, similar to some ancient flood basalt provinces on Earth. NASA Magellanradar image.


Figure 10. Perspective view of Olympus Mons, a gigantic volcano on Mars about 600 km in diameterand over 20 km high. Viking Orbiter photomosaic overlain on Mars Orbiter Laser Altimeter altimetricdata. Vertical exaggeration is about twenty times.

oxygen-rich Earth atmosphere. Analysis of ancient Earth rocks and inventory ofcarbon dioxide stored in carbonate rocks (such as limestones) on the Earth showsthat the Earth originally had as much carbon dioxide as other planets, but theevolution of life has changed the atmosphere considerably.

In addition, analysis of the planetary atmospheres and the geological record ofthe planets shows that they have undergone radical changes with time, providingclues to Earth’s history and destiny. The polar caps of Mars have been recognizedfor over a hundred years, but recent observations of the Moon and Mercury revealevidence for volatile-rich polar ice deposits there. Planetary degassing productsand cometary impact debris apparently migrate to the polar cold traps and producedeposits even in the extreme thermal environment of Mercury. The prospect ofsampling the geologic record of volatiles contained in these caps is extremelyexciting. The larger and more accessible polar caps of Mars contain a stratigraphicrecord of many hundreds of layers (Figure 11) which could provide the keys tounderstanding recent climatic change there.

Earth has always been thought of as the water planet. Water dominates the sur-face, is abundant (occurring in glaciers, rivers, lakes and oceans covering almosttwo-thirds of the planet), is extremely significant in weathering, and is thoughtto be essential in the formation and nurturing of life. But recent exploration hasshown that water may have played a very important role on other planets too, andthat oceans may not be the exclusive purview of Earth. Although liquid water is notnow stable under present conditions on Mars, we see evidence for ancient glacialdeposits, streams, rivers, and lakes. Indeed recent evidence is consistent with thepresence 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 nowhave evidence that a global ocean covers the surface and that it is frozen over, butlikely still liquid today below the surface. These new perspectives on environmentshave changed our frame of reference in thinking about the formation and evolu-


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 andgreater and lesser amounts of deposition of ice (bright) and dust (dark). These layers may be relatedto the same kind of obliquity variation in the orbital axis that are responsible for the ice ages onEarth.


Figure 12. Hypothesized position of an ocean in the northern lowlands of Mars in its earlier history. Inthis 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 hugestanding body of water. The Tharsis region is seen as a high on the right. Much of the water mayhave 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, andEuropa, and evidence for significant changes with time, have also provided insightinto 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 reconstructionof 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.


Figure 13. Galileo image of the surface of Europa in the anti- Jovian region. Note the general lack ofimpact craters and the cracked nature of its frozen water-ice surface layer. The dark wedge-shapedarea in the middle of the image is about 15 km wide, and represents the cracking and opening ofthe European crust, much in the way sea-floor spreading operates on Earth. The icy layer seen inthis image likely overlies a global ocean at depth. The width of the image is about 170 km. NASAGalileo image.


• Massive changes in the internal constitution and thermal structure of earlyEarth.

• 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 spread-

ing 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 surfacetemperatures, and perhaps the tectonic style.

− Internal density instabilities causing global changes in volcanism and tecton-ism, and possible resurfacing of the entire planet. Could this initiate platetectonics?

− 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; poten-tial large-scale modification of the atmosphere and the biota at numerous butrandom times.

In the coming decades, these concepts derived from the last forty years of SolarSystem exploration will be folded into our ongoing reconstruction of the missingchapters in Earth history to produce a radically different picture of the formativeyears of our own home planet. In the words of T. S. Elliot, “We will not cease fromexploration, and the end of all of our exploring will be to arrive where we startedand 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 scalesof 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 ofwhich provides important missing information and perspective on the other, andtogether reveal the fabric of the history of the Solar System. Our perception oflife has changed radically from the photosynthesis-based, mammalian-dominatedtree, leading inexorably to humans, the ultimate evolutionary achievement. Life isnow seen everywhere on Earth, even in the most extreme environments, from high-temperature deep-sea volcanic vents, to the deepest, hottest mines, deriving energy


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-temperatureenvironments. Assessment of the terrestrial biota underlines again that bacteria arephenomenally important in terms of sheer numbers, biomass, consistency and sur-vivability. Mars rocks may contain microfossils. Rocks can be readily transportedfrom planet to planet throughout geological history, and the interiors of rocks aresurvivable environments for microorganisms.

Rather than the crowning achievement of organic evolution, humans are nowseen as just another marginal species. Bacteria rule. Evolution is not directed, andcertainly not directed toward us. Evolution is a random process involving mul-tiple chance and chaotic events. Life may have arrived on Earth from Mars insidemeteorites almost 4 billion years ago. Humans may owe their day in the Sun to achance impact that terminated a successful global population of reptiles and pavedthe way for opportunistic mammals. Humans could easily suffer the same fate asthe dinosaurs. Bacteria would still rule.

We have had just a few years to enjoy this recently-acquired Solar System-centered perspective. Ongoing and future exploration and perspectives will placeour Solar System in the context of all of the others that are now known to existaround other stars, and the many more soon to be discovered. The continuing retreatfrom terracentrism and human specialness is underlined by the incredible diversityof solar system arrangements that have been encountered in the last few years. Ofcourse, 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 examinedour past perceptions in understanding Earth and the cosmos. These assessmentsshow some progress, but abundant mistakes, wrong turns, and prejudices. Therecord demonstrates that human thinking is almost by definition, limited in spaceand time (Figure 14). Indeed, limited thinking in space and time is a fundamentalcharacteristic of almost all animal species, and is almost certainly an inheritancefrom our genetic forebearers. Our daily lives are so dominated by short-term localevents 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 ourability to probe over greater distances and longer time scales, and understand whatwe 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 toomany years ago we were the center of the cosmos and the star atop the tree oflife. Now we are not. But this should not be a threat. The past tells us that wehave a very exciting ride ahead as we probe to ever greater dimensions of spaceand time, explore outer and inner space, and perhaps other dimensions as well.


Figure 14. The density of humans occupying different parts of the space and time scales. Mosthumans 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 eventhose occupy it only for a short period of time. The decisions that are made and the actions that aretaken in the lower left part of the diagram may have immense consequences for the upper right handportion. Similarly, inaction in this area, and inattention to the broader scales of space and time, canhave equally important effects. Modified from D. H. Meadows et al., The Limits to Growth, UniverseBooks, New York, 1974.

In fact, early in this millennium it will likely be conclusively shown that the veryexploration ethic that we think of as uniquely human, is instead a manifestation ofthe survival of species. Of course, this is not so obvious to us as yet in species otherthan ourselves.

The importance of maintaining an open mind to new and seemingly hereticalideas cannot be understated. The very things that bind us perceptually to a smallarea of space and time (Figure 14) also make us threatened by and resistant to newand changing ideas. The socialization and acculturation processes work heavilyagainst individual and creative thinking. We must recognize this and strive to over-come these effects. But progress is clearly being made and the times are very, veryexciting. And the future is as promising as it is incomprehensible. Several years


ago, a student entered my office in utter frustration, having been unable to find athesis topic on which no previous work had been done. In answer to her query, mycolleague 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 theirwritings: 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.


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.

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Lunar And Planetary Perspectives On The Geological History Of The Earth · 2013-07-25 · LUNAR AND PLANETARY PERSPECTIVES ON THE GEOLOGICAL HISTORY OF THE EARTH JAMES W. HEAD III