Origin and Evolution of Earth Research Questions for a Changing Planet (2008) / Chapter Skim
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1 Origins
1 Origins
Pages 7-34
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From page 7... ...
Although these questions ing Earth suitable for life and how life arose. Pursuing are longstanding, the answers are only now emerging these questions leads us to fundamental issues about from new insights provided by astronomy, isotopic how stars and planets form and evolve and to questions chemistry, Solar System exploration, and advanced about how the modern Earth works, from the inner- computing.
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From page 8... ...
astronomical observations of planets forming have not Astronomical observations of star-forming regions and yet been made. young stars, together with hydrodynamic models of star Observations of planets around other nearby stars formation, support the conclusion that stars -- includ- with masses similar to the Sun indicate that planet ing the Sun -- form by the gravitational collapse of a formation is a common outcome of star formation, but molecular cloud core composed of materials manufac- no star has yet been observed with a system of planets tured and reprocessed in many earlier generations of that looks anything like the Solar System.
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From page 9... ...
. The two largest circumstellar disk influence the rate or efficiency of outer planets, Jupiter and Saturn, seem to fit this model planet formation.
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, a kilometer in size is better understood: they are less whereas beyond the asteroid belt, temperatures were affected by the presence of gas than are smaller pieces, low enough for ices (i.e., water, methane, ammonia) and their subsequent evolution is governed by mutual containing more volatile elements to have condensed, gravitational attractions.
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From page 11... ...
Jupiter and Saturn also sent objects from the the Sun (for elements other than H, He, Li, C, N, O, asteroid belt either out of the Solar System or spiraling and noble gases) , and some of these same meteorites into the inner-planet region where they became parts contain tiny mineral grains of dust that survived from of the planets forming there or fell into the Sun.
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It has been suggested that the giant planets can Meteorites give us the most detailed information about pluck materials from the asteroid belt region and throw the abundances of these heavier elements. them in toward the Sun.
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Advances in geochronological techniques are beginning to enable the sequence of events to be discerned. The most primitive chondritic meteorites contain inclusions made up of minerals that condense at high temperature from a gas of solar nebula composition.
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From page 14... ...
. Geochemists will play a key role in addressing this question because the relative abundances of elements and isotopes in the different types of presolar grains provide the most specific and detailed data for checking our understanding of how chemical elements are produced in different types of stars (Zinner, 2003)
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From page 15... ...
The relative amounts of refractory elements do not vary much among different classes of the benchmark chondritic meteorites, which is generally taken as a strong argument that Earth is not much different from the meteorites. For the more volatile elements, which evaporate more easily, there are wide and puzzling variations throughout the Solar System.
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From page 16... ...
More is known about the Moon than any terrestrial The differences may reflect deep sequestration of ancient crust planetary body other than Earth because of the rock formed in the early Earth or differences in refractory element samples collected by the U.S. and Soviet lunar missions ratios between Earth and chondritic meteorites.
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From page 17... ...
Another difference is that Earth has water, and the chemical complementarity of the dark lunar as well as other volatile species and oxidized (ferric) basaltic lowlands and the bright highland rocks -- led iron; the Moon has virtually no water and all of its iron to enormous advances in theories of planet formation.
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From page 18... ...
Astronomical observations Assuming that the Moon formed as the result of a giant from increasingly powerful telescopes have added a new impact, the impact would have erased the existing rock dimension to models of star and planet formation, as record, adding enough heat to turn Earth into a mostly have studies of asteroids, comets, and other planets via molten ball, probably to the very surface of the planet. spacecraft.
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The green 3000 1000 house effect and tidal heating maintain the magma ocean for 2 million years. Surface Temperature Liquid Water When the magma surface freezes over, 2000 surface temperature drops quickly and Reservoir Size (bars)
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would start to crystallize when the surface temperature For the Hadean Earth a runaway greenhouse state could dropped to about 1700 K and would be completely solid theoretically coexist with a magma surface provided that sufficient at about 1400 K According to one model (Figure 1.9)
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At this critical juncture the level of uncertainty is re- A mild fractionation of the oxygen isotopes suggests that many zircons formed in melts that incorporated rocks that reacted with doubled. The acid oceans should then chemically attack liquid water (Mojzsis et al., 2001; Wilde et al., 2001)
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From page 22... ...
Epsilon units represent deviabased on a logical scenario, now known to be incor- tions in the 182W/184W ratio of Earth relative to the meteorites, rect, that Earth first accreted into a more or less ho- measured in parts per 10,000. The greater epsilon 182W value of Earth relative to chondritic meteorites indicates that Earth's rocky mogeneous globe (a mixture of both silicates and iron portion formed when 182Hf was still alive, which produces 182W, metal)
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From page 23... ...
How Did Earth's Earliest Crust Form and What Became of It? A central question about the Hadean Earth concerns event on core formation and metal-silicate differentia- the nature of its crust and whether, in the absence of tion remain open questions.
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From page 24... ...
The large, dark lunar "seas," or maria, are huge impact basins that formed mostly between 4,000 and 3,900 Ma and are evidence of a late heavy meteorite bombardment that would also have affected Earth (see Box 1.5)
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From page 25... ...
The recent observation that every could cause the disappearance of a large preexisting Earth sample measured is enriched in 142Nd compared continental crust, given that low-density ancient crust to chondritic meteorites suggests very early formation is preserved on the Moon. Rather, vigorous internal of a crustal component enriched in incompatible ele- convection is more likely responsible for the demise of ments (such as the light rare-earth elements)
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From page 26... ...
The biggest would have been comparable to Vesta or Pallas, the largest asteroids now in the asteroid belt. Whether these impacts marked the tail end of a sustained bombardment dating back to the accretion of the planets or whether they record a catastrophic event, such as a sudden influx of planetesimals to the inner Solar System due to rapid migration of the giant planets (e.g., Gomes et al., 2005)
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From page 27... ...
A critical component of understanding an age of planetary exploration, the origin of life is also Hadean climate is our knowledge of atmospheric an astrobiological issue, currently investigated on Mars, processes, but despite the advanced state of models where a sedimentary record of earliest planetary history for the modern Earth atmosphere, our understanding is preserved, and potentially across the wider stretch of of radically different types of planetary atmospheres Universe where planets have been detected. is still rudimentary.
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From page 28... ...
Experimental approaches to prebiotic chemistry must be framed in terms of environments likely to have formed life's incubators, and only Earth scientists can inform us about the physical and chemical characteristics of these settings. A Search for Clues in the Laboratory We have understood for more than half a century that modern laboratory experiments can shed light on our search for life's beginning.
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From page 29... ...
clusters. Understanding the roles these metals might have played in prebiotic chemistry is a geological quesEarth scientists are trying to answer this question by tion whose answer depends on how the metals were discombining field and laboratory studies of the planet's tributed in primitive Earth environments.
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From page 30... ...
Carbon isotopic abundances in 3,500 million A second important question flows from the first: year old sedimentary rocks are similar to those found When did life arise on our planet? Paleontologists and in much younger deposits, suggesting that a biologibiogeochemists have long agreed that the origin of life cal carbon cycle was established early in our planet's preceded the deposition of minimally metamorphosed history.
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From page 31... ...
tional wisdom that terrestrial life arose on Earth prior The same uncertainties confound investigations of to 3,500 Ma. Explanations that do not involve biology larger scale features of sedimentary rocks that may have have been proposed for micron-scale carbon-bearing been imported by organisms, as well as molecular or structures previously interpreted as Earth's oldest isotopic features of ancient organic matter that might microfossils, for stromatolites, and for carbon isotopic reflect biological processes.
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32 ORIGIN AND EVOLUTION OF EARTH a b c FIGURE 1.18 (a) Fossil of a eukaryotic microorganism preserved in ca.
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From page 33... ...
Stratigraphers, paleontologists, biogeoWith continuing planetary exploration, Earth scientists chemists, and geochronologists can provide sharper will be able to establish with greater certainty whether constraints on when life arose and the metabolic life could have originated elsewhere in our Solar Sys- character of early organisms. Geochemists focused tem -- and even whether organisms could have become on both crustal differentiation and low-temperature established on Earth by meteoritic transfer from an- reactions can build an improved sense of redox condiother planet.
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