A BOOK WRITTEN FOR PhD FELLOWS IN THE MAIN, BUT SOME PARTS LIKE THIS SECTION CAN BE UNDERSTOOD BY US NON-PhD PEOPLE - Keith Hunt
RARE EARTH
Why
Complex Life
Is Uncommon in
the Universe
Peter D. Ward and Donald Brownlee
Praise for Rare Earth
"... brilliant and courageous ... likely to cause a revolution in thinking."
—William J. Broad
The New York Times "Science Times"
"A pleasure for the rational reader .. . what good books are all about ..."
—Associated Press
"If Ward and Brownlee are right it could be time to reverse a process that has been going on since Copernicus."
—The Times (London)
"Although simple life is probably abundant in the universe, Ward & Brownlee say, 'complex life—animals and higher plants—is likely to be far more rare than is commonly assumed'."
—Scientific American, Editor's Choice
"... a compelling argument [and] a wet blanket for E.T. enthusiasts ..."
—Discover
"Peter Ward and Donald Brownlee offer a powerful argument... ."
—The Economist
"[Rare Earth] has hit the world of astrobiologists like a killer asteroid .
—Newsday
"A very good book."
-Astronomy
The notion that life existed anywhere in the universe besides Earth was once laughable in the scientific community. Over the past thirty years or so, the laughter has died away .... [Ward and Brownlee] argue that the recent trend in scientific thought has gone too far .... As radio telescopes sweep the skies and earth-bound researchers strain to pick up anything that might be a signal from extraterrestrial beings, Rare Earth may offer an explanation for why we haven't heard anything yet."
“a sobering and valuable perspective ..."
-Science
"Movies and television give the (optimistic) impression that the cosmos is teeming with civilizations. But what if it isn't? …. Life elsewhere in the universe may never reach beyond microbes, which, the authors note, could be much more widespread than originally believed."
—Sky & Telescope
"It's brilliant…. courageous…. It's rare in literature and science that a stance goes so far against the grain."
—Dr. Geoffrey W. Marcy Extra-solar planet discoverer University of California at Berkeley
"It's a thought that grips most everyone who stares into the unfathomable depths of a star-speckled night: Is there anybody out there? The odds, say Peter Ward and Don Brownlee, are probably more remote than you think."
—The Seattle Times
"Alien life is more likely to resemble the stuff you scrub off the tiles in your shower than Klingons, Wookies or Romulans, say Ward and Brownlee."
—Popular Mechanics
"Ward and Brownlee have taken an issue that is much in the public domain and treated it thoughtfully and thoroughly, but with a lightness of touch that draws the reader on …. Rare Earth is an excellent book for both specialists and non-specialists."
—The Times Higher Education Supplement (UK)
"A provocative, significant, and sweeping new book ... Rare Earth is a fast-paced, thought-provoking read that I gobbled like popcorn. It's one of those rare books that is at once delightful, informative, and important: an end-of-the-millennium synthesis of science that tackles the central question of our past, place, and destiny."
—Northwest Science & Technology
“…..well thought out and intriguing….”
-Icarus
“….. a startling new hypothesis ….. Highly recommended."
—Library Journal
"Rare Earth will surely appeal to those who would dare to disagree with icons Carl Sagan and George Lucas."
—San Gahriel Valley Tribune
"... a timely, entirely readable account."
—Toronto Globe and Mail
"... a stellar example of clear writing ..."
—American Scientist
“….. thought-provoking and authoritative…..”
—Physics Today
"In this encouraging and superbly written book, the authors present a carefully reasoned and scientifically statute examination of the age-old question—'Are we alone in the universe?' Their astonishing conclusion that even simple animal life is most likely extremely rare in the universe has many profound implications. To the average person, staring up at a dark night sky, full of distant galaxies, it is simply inconceivable that we are alone. Yet, in spite of our wishful thinking, there just may not be other Mozarts or Monets."
—Don Johanson Director, Institute of Human Origins Arizona State University
"A fabulous book! If we're to believe what we see in the movies, extraterrestrials thrive on every world. But this unique book, written by two of the top scientists in the field, tells a different story. As we know it on Earth, complex life might be very rare, and very precious. For those of us interested in our cosmic heritage, this book is a must-read."
—David Levy
Co-discoverer of Comet Shoemaker-Levy
"Ward and Brownlee take us on a fascinating journey through the deep history of our habitable planet and out into space, in the process they weave a compelling argument that life at the level of an animal should be vastly rarer in the universe than life at the level of a lowly bacterium."
—Steven M. Stanley
Author of Children of the Ice Age and Earth and Life Through Time
The Johns Hopkins University
"Microbial life is common in the universe, but multicellular animal life is rare. A controversial thesis, but one that is well-researched and well-defended. A must-read for anyone who is interested in whether life exists beyond Earth." —James Kasting
Pennsylvania State University
CHAPTER 9
The Surprising Importance of Plate Tectonics
Imagine that we have a spacecraft capable of swiftly taking us to each planet in the solar system. Our goal on this voyage is to try to determine what features of Earth are essential to animal life. On our voyage, therefore, we are looking for some clue to why animal life has been able to survive on Earth for a time period approaching a billion years. What are those factors that have fostered diversity on Earth?
[WE MUST REMEMBER THESE FELLOWS DO NOT BELIEVE GENESIS 1— AN AGE BEFORE AND AN AGE AFTER - Keith Hunt]
We begin our celestial survey with Mercury, a cratered world of great heat on the sunlit side and great cold on its dark side of the slowly spinning planet. Yet we quickly find that Mercury is not only free of atmosphere, liquid water, and life but is also volcanically dead. Its surface shows mainly numberless craters of a meteor-ravaged world, scars left by the bombardment by comets and asteroids. In contrast to Earth, in the 4 billion years since that time of stony rain, little of geological importance has happened on this planet. Mercury looks like our Moon.
Next we travel to cloud-covered Venus. Its surface looks curiously young, like the face of a child, yet Venus is the same age as Earth. We find that the crust of Venus appears to have been geologically "resurfaced" in some sort of cataclysmic event that caused its surface to melt sometime in the last billion years. Because of this, the numerous craters we saw on Mercury are less common. But Venus has two other prominent geological features: crustal plateaus and volcanic rises that look something like a string of volcanoes whose cones have been lopped off. There is no animal or plant life and no oceans or liquid water of any kind. The surface of Venus is simply too hot—hot enough to melt solder.
Mars is the next leg of this long voyage, and as we orbit the red and ochre planet, we see astounding volcanoes rising high above a cratered and rock-strewn landscape. These volcanoes (the largest in the solar system) are enormous by Earth standards. But they are relatively few in number— solitary, lonely sentinels dispersed across the planet's face. Curiously, there are no other mountains, no equivalents of the Alps, or even the Appalachians. And there are no seas, no lakes, no rivers, and no liquid water, although many geomorphic features of the planet's surface indicate that water was present here long ago.
With Mars we have finished our survey of the so-called "terrestrial" planets. We have already learned much: No other planet has linear mountain chains.
Now we travel toward the outer regions of the solar system, arriving in the realm of the gas-giant planets.
First we pass Jupiter, with its writhing, multicolored atmosphere and the distinctive Great Red Spot racing around this rapidly rotating colossus of a planet. There are no land features, for there is no distinct planetary surface, no place where the atmosphere ends and land begins. Jupiter is unsuitable for animal life (as we know it, anyway) because it has no solid planetary surface. Perhaps bacteria-like organisms live within its roiling atmosphere, perhaps not. Its satellites, however, might be places where life has arisen and survived, so we swing by each of the four large "Galilean" moons (so named because they were first seen by the great Italian astronomer Galileo): Europa, Callisto, Ganymede, and Io.
Each is somewhat smaller than Earth, and all have frozen surfaces (although Io has active volcanoes). There are no animals or even liquid oceans here, although the frozen ocean of Europa seems a tantalizing possibility for life simply because liquid water may lie deep below its ice-covered surface. Ganymede and Callisto are also likely to harbor subterranean regions of liquid water or brine.
From Jupiter we travel on to the other gas giants of the solar system: Saturn, Uranus, then Neptune. Like Jupiter, each is a great gas ball without any definable surface, but each has smaller, rocky satellites orbiting it, some cratered, some ice-covered. None has animal life, although Saturn's moon Titan does provide an exotic environment with frigid hydrocarbon liquids at its surface and liquid water at warmer depths.
We finally arrive at Pluto, a solid world, but a world without mountains or volcanoes. Like Mercury (the innermost planet of the solar system), frigid, distant Pluto is devoid of volcanic activity.
As we return to Earth from this trip, we ponder what is unique about Earth that may offer us clues to why animal life exists here but not on other planets and their moons in our solar system. A crucial difference, its seems, is Earth's unique possession of liquid water at its surface. Water, the universal solvent, seems indispensable for animal life. Earth has other unique attributes, too, including its oxygen-rich atmosphere and a temperature range that allows liquid water to exist.
Another unique attribute of Earth at first glance seems extraneous to animal life but may indeed be crucial to it: linear mountain ranges. There are, of course, giant mountains elsewhere in the solar system, the tallest being the great volcano Olympus Mons on Mars. Yet such mountains are always single and never occur in chains, unlike most mountains on Earth. There is no equivalent to the Rockies, the Andes, the Himalayas, or the score of other linear mountain chains we are so familiar with. Even at this crude level of observation, oceans, mountain chains, and life make Earth unique in this solar system. Life has had little to do with creating oceans and mountain chains. Yet these features of Earth may have been crucial to the origin of life.
In this chapter, we argue that all three of these precious attributes of Earth are connected in a complex interrelationship. All three, furthermore, may be the result of plate tectonics.
This process, the movement of the planetary crust across the surface of the planet, is found in our solar system only on Earth, and it may be vanishingly rare in the Universe as a whole. It is not the mountains as such that are so important to life on Earth but the process that creates them: plate tectonics.
It may seem odd to think that plate tectonics could be not only the cause of mountain chains and ocean basins but also, and most enigmatically, a key to the evolution and preservation of complex metazoans on Earth. But there are several reasons to consider this view.
First, plate tectonics promotes high levels of global biodiversity. In the last chapter, we suggested that the major defense against mass extinctions is high biodiversity. Here we argue that the factor on Earth that is most critical to maintaining diversity through time is plate tectonics.
Second, plate tectonics provides our planet's global thermostat by recycling chemicals crucial to keeping the volume of carbon dioxide in our atmosphere relatively uniform, and thus it has been the single most important mechanism enabling liquid water to remain on Earth's surface for more than 4 billion years.
Third, plate tectonics is the dominant force that causes changes in sea level, which, it turns out, are vital to the formation of minerals that keep the level of global carbon dioxide (and hence global temperature) in check.
Fourth, plate tectonics created the continents on planet Earth. Without plate tectonics, Earth might look much as it did during the first billion and a half years of its existence: a watery world, with only isolated volcanic islands dotting its surface. Or it might look even more inimical to life, without continents, we might by now have lost the most important ingredient for life, water, and in so doing come to resemble Venus.
Finally, plate tectonics makes possible one of Earth's most potent defence systems: its magnetic field. Without our magnetic field, Earth and its cargo of life would be bombarded by a potentially lethal influx of cosmic radiation, and solar wind "sputtering" (in which particles from the sun hit the upper atmosphere with high energy) might slowly eat away at the atmosphere, as it has on Mars.
What Is Plate Tectonics?
Geologists of the eighteenth and nineteenth centuries had little difficulty understanding the origin of volcanoes: Hot magma from deep within the planet rose to the surface regions and spewed forth lava, ash, and pumice to form a cone. Understanding how nonvolcanic mountains and mountain ranges could form, however, was more problematic. Countless hypotheses were proposed. These included buckling of the crust as a result of sediment loading (where the weight of slowly accumulating sediment finally causes the crust to crack in linear fashion), shrinking of the planet (causing ridges to form as on a dried prune), and an expanding Earth (where the expansion creates mountain ranges).
In 1910 American geologist Frank B. Taylor proposed a radically new idea: The drifting of continents caused the great mountain chains. This heresy was immediately decried by nearly all other geologists and geophysicists, who could envision no mechanism by which such "drift" could occur.
Taylor's hypothesis, however, kindled a spark of interest that would not die. Soon other scientists began toying with the idea and searching for supporting evidence. The most dogged of the new converts was a German meteorologist named Alfred Wegener, who from 1912 until his death in 1930 on Arctic ice was obsessed with the idea. Drawing on evidence from geology and geophysics, Wegener was the first to show how the fit of various coastlines supported the idea that all the continents were once united in a single "supercontinent." He was also the first to use paleontological evidence to support this claim: He argued that the presence of similar fossil species on land masses now widely separated could have come about only if the various continents had once been in contact. He convinced some other geologists that continents did and do drift, although the majority remained skeptical until the 1960s.
The greatest obstacle to the idea (and the rallying point of all "anti-drifters") was the seeming absence of any sort of reasonable, underlying mechanism. How could the massive continents "float" over the surface of the planet's stony surface? The answer to this question, it was eventually discovered, lies in the different phase states of Earth's uppermost layers, known as the crust and upper mantle, and the presence of thermal convection in these regions.
Scottish geologist Arthur Holmes first proposed that the upper mantle acts like boiling water, producing large moving "cells" of material. Deep below the surface, the fluid, hot material composing the upper mantle is heated and begins to rise, as it rises it cools, and eventually it begins to flow parallel to the planet's surface. When it cools sufficiently, it sinks again. Holmes proposed that where it rises, the convection cells might rupture the rigid, solid crust and then carry it along, piggyback fashion, in those regions where the mantle moves parallel to the surface.
The outlandish ideas behind the early theory of continental drift were eventually shown to be correct.
Evidence came from many sources, including paleontological data and even the fit of the continental coastlines, as first proposed by Wegener. Yet the two most powerful lines of evidence for plate tectonics (another term for continental drift) came from fields unknown to Wegener: From the study of paleomagnetics, which allowed the reconstruction of ancient continental positions, and from oceanographic studies of the ocean floor, which revealed the presence of enormous underwater volcanic centers, areas where the sea floor literally pulls away from itself.
We know now that all continents are masses of relatively low-density rock embedded in a ground mass of more dense material. The low-density rocks have the average composition of granites, whereas the higher-density rocks that make up the ocean crust are basaltic in composition. Because granite is less dense than basalt, the granite-rich continents essentially "float" on a thin (relative to Earth's diameter) bed of basalt. Earth scientists like to use the analogy of an onion, the thin, dry, and brittle onion skin is the crust, sitting atop a concentric globe of higher-density, wetter material. Continents are like thin smudges of slightly different material embedded in the onion skin. Unlike an onion, however, Earth has a radioactive interior and constantly generates great quantities of heat as the radioactive elements, entombed deep within in the planet, break down into their various isotopic byproducts. As this heat rises toward the surface, it creates gigantic convection cells of hot, liquid rock in the mantle, just as Arthur Holmes envisioned. Like boiling water, the viscous upper mantle rises, moves parallel to the surface for great distances (all the while losing heat), and then, much cooled, settles back down into the depths. These gigantic convection cells carry the thin, brittle outer layer—known as plates—along with them. Sometimes this outermost layer of crust is composed only of ocean bed, sometimes, however, one or more continents or smaller land masses are trapped in the moving outer skin.
Under the pressures and temperatures encountered at depths many kilometers beneath Earth's surface, the familiar rocks of our crust act in ways very different from what we are used to. Victor Kress of the University of Washington pointed out that all but a tiny fraction of the upper mantle is entirely solid. Yet it acts like a liquid in certain ways, most significantly in its "convection": the process whereby a liquid, when heated, flows upward and then across the top of its container. The mantle convects in the manner of a liquid only because the movement is so slow, and the temperatures so high, that individual crystals have time to deform in response to stress. The upper mantle is a hot, highly compressed mass of crystal that acts like a very viscous liquid.
The "plates" of plate tectonics are composed of all of the crust and a thin section of mantle that underlies it, which together act as a relatively rigid composite layer. Plates are of varying thickness, and their "bottoms" are thought by many scientists to coincide with the 1400°C isotherm (a region where the rock is heated to that very high temperature at which mantle rock material melts into a plastic-like medium). Another way of visualizing the plate foundation is to recognize that this region is characterized by much decreased viscosity. The difference in viscosity between the overlying plate and the underlying region of lowered viscosity is highly important in plate tectonics. It allows the relatively rigid crust to slip as a unit over the zone of high viscosity. Plates composed of oceanic crust and mantle are about 50-60 kilometers thick, whereas the plates with continental crust average about 100 kilometers in thickness.
Let's begin our examination of the plate tectonic process with ocean basins.
The crust we find lining the bottom of the world's oceans is largely made up of basalt, the same type of volcanic rock that makes up the Hawaiian Islands. This material originates within the deeper mantle region of Earth, it ascends along the rising zones of the convection cells. As this hot, dense mantle material rises toward the surface, it moves into regions of ever lessening pressure, because the weight of overlying material decreases. A lower-density liquid separates from the higher-density mantle material, rising to the surface as the "lava" we are familiar with from so many movies of erupting volcanoes. The magma enters a huge crack in the surface of the planet formed by the pulling apart of two plates and solidifies into basaltic ocean crust. It too begins to move away from the "spreading center" where it first lithified, and more new magma wells up to take its place—an endless conveyor belt.
The basalt produced in the spreading centers has a much different composition from its "parent," the mantle material rising along the limbs of the convection cells. Because it contains a much higher percentage of silica atoms, it is much lower in density than the mantle material. The basalt has differentiated from the parent material (which, when occasionally found on the surface, has the name peridiotite). This differentiation from a peridiotite composition to a basaltic composition is the final step of oceanic crust formation.
Continents, however, have an even lower density than the oceanic crust. The recipe for their creation requires a further step in this arcane lithic cooking: the formation of the rock types granite and andesite. The characteristic speckled appearance of both of these rocks, compared to the more somber, chocolate to black color of basalt, comes from their containing even more of the white (and low-density) silica. The major step in forming continental crust is thus the differentiation of granite from material of a basaltic composition. This process takes place in several steps, but the key ingredient is water, and the key mechanism is called subduction.
Over many millions of years, oceanic crust moves away from its birthplace, the spreading centers, all the while being carried piggyback on the convecting mantle beneath it. Like all journeys, however, this long ride must eventually end, the oceanic crust cannot expand forever. The basalt has cooled through time, and even more significantly, it has gained some heavy freeloaders—piles of dense igneous rock known as gabbro that attach to the base of the basalt. The basalt now just barely floats, and as it cools, it gets heavier. Given any good excuse, it simply sinks, descending as deep as 650 kilometers. Eventually, then, the convection cell begins its downward journey back into the deep mantle, and when it does, it carries its veneer of oceanic crust back down with it, at regions called subduction zones.
Subduction zones (see Figure 9.1) are long, linear regions where oceanic crustal material is driven deep into Earth, not so much by being pushed down as sinking down through gravity. It is near and parallel to these subduction zones that linear mountain ranges are constructed. The mountains form partly as a by-product of the collision of two plates, which causes buckling and crumpling of the leading edges, and partly by the upward movement of hot magma, which eventually solidifies into granites and other magmatic rocks parallel to the subduction zones. The Cascade Mountains of Washington State are an example, the still-active peaks, such as Mt. Baker, Mt. Rainier, and Mt. St. Helens, are direct evidence of the power and importance of subduction in creating mountain ranges. Most of the world's volcanoes and mountain chains are found near these subduction zones (or where ancient subduction zones used to operate), further testimony to the fundamental link between subduction and mountain building. That mountain chains are not found on other planets or moons of our solar system is clear evidence that only Earth now has plate tectonics.
Volcanoes occur along subduction zones because by the time (which may be millions of years after its formation) that oceanic crust reaches a subduction zone and begins to descend, it is of slightly different composition from when it was created in the spreading centers. As the basalt created in spreading centers moves away from its birthplace, water is gradually added to the crystal structures of key minerals—in other words, the basalt becomes hydrated. Over long millennia, seawater works its way down through many cracks and crevices of the oceanic crust and reacts chemically through the addition of water molecules to the crustal lattices of minerals making up the basalt. Water-poor minerals actually incorporate significant amounts of water in their structure. The newly hydrated minerals have a lower melting point than nonhydrated minerals, so as the oceanic basalt descends in the subducting slab, the hydrated, silicate-rich minerals making up the basalt melt, and the liquid that is produced rises back toward the surface. This water leads to a decrease in the melting temperature of the overlying mantle rock that now surrounds it, creating liquid magma where one would otherwise expect to find only solid rock. This magma, when eventually cooled, becomes the rocks we call andesite and granite, and its rise back to the surface is a key force in producing new mountains and the line of volcanoes we find along subduction zones.
But the crucial aspect of these volcanoes is that they are made up of magma of lower density than the basalt that parented them, and in this way, a new, lower-density rock type is created. This rock starts out as andesite (named after the Andes Mountains) and becomes part of the continental crust. Because andesite and granite (which is created in similar fashion) are so rich in silicate mineral, they are less dense than basalt. They become the backbone of the continents—and their flotation devices! With andesite-and granite-rich cores, continents can float on a sea of basalt.
They can never be sunk in subduction zones. Continents cannot be destroyed (though they can be eroded). They can be split and fragmented, to drift from place to place, but their basic volume cannot be reduced. Through time, in fact, the number of continents on Earth has seemingly increased.
One of the most important findings about Earth history is that since the formation of our planet, the total area of oceanic plates has gradually diminished as the area of continental plates has grown (see Figure 9.2). This seems counterintuitive, because the oceans are continuously enlarging as a result of sea floor spreading. Yet as we have just seen, ocean crust can sink (and be remelted back to magma in the process), whereas the lighter continental crust remains afloat like a cork on this sea of basalt.
Furthermore, the continents enlarge through the process of mountain building, for the volcanoes lining subduction zones and many continental edges receive vast quantities of granitic and andesitic magma. Geologist David Howell, in his book Principles of Terrane Analysis, estimates that the volume of continents increases by between 650 and 1300 cubic kilometers of rock each year. This estimate is for the modern day, and some geologists believe continental volume increased more rapidly in the past, especially early in Earth history, when plate tectonic processes may have occurred much faster than they do now because more heat emanated from the early Earth.
Plates thus intersect with each other in three ways: at the spreading centers (where new magma reaches the surface along enormous linear cracks, such as the mid-Atlantic ridge), areas where plates grind by each other side by side (such as the San Andreas Fault of California), and regions where plates collide— the subduction zones— which are associated with linear chains of active volcanoes [such as the Cascades and the Aleutian Islands].
Why Is Plate Tectonics Important to Life?
The rate of continental growth is of major importance to life and its ecosystems. The majority of Earth's biodiversity is today found on continents, and there is no reason to believe that this relationship has changed over the last 300 million years. As continents have grown through time, they have affected global climate, including the planet's overall albedo (its reflectivity to sunlight), the occurrence of glaciation events, oceanic circulation patterns, and the amount of nutrients reaching the sea. All of these factors have biological consequences and affect global biodiversity.
TO BE CONTINUED
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