EARTH SCIENCES - Notable Research and Discoveries Part 3

earth ScienceS the structure and composition of the upper mantle. But the extreme pressure and temperature of Earth’s interior probably set limits on how far down people can ever drill. Although these limits constrain scien-tists’ reach, simulations and laboratory experiments, coupled with seis-mology, extend all the way to the center of the planet. Another approach to the study of Earth’s depths has recently gained interest. This approach involves extending geology’s reach not only to Earth’s center but also throughout the entire solar system. The solar sys-tem evolved and gave birth to the Sun and the planets about 4.5 billion years ago. Astronomical and geological evidence provide clues about this event, which involves an enormous, swirling cloud of dust and gas that eventually aggregated into the Sun and planets. Yet the details are not at all clear. Studying the birth and evolution of the solar system will help scientists understand how the system’s bodies formed, which would also help explain their present structure. For example, Earth and the planet Mars have many similarities. Mars is smaller, having a radius a little more than half that of Earth, and has a density of about 73 percent the value of Earth’s density. Its orbit is about 1.5 times larger than Earth’s orbit, which places it about 45 million miles (72 million km) farther away from the Sun on average. Probes launched in the United States and other nations have reached Mars, orbiting the planet and in some cases landing on its surface, map-ping the terrain and analyzing soil chemistry. Although seismic data from Mars is not yet available, density and gravity measurements of Mars suggest it has a core similar to Earth’s, although perhaps contain-ing a higher percentage of lighter elements. No probes or spacecraft have yet been sent to retrieve samples from Mars, but scientists have a rare but valuable opportunity to study mate-rial from this planet. Meteorites—rocks from space—sometimes land on Earth. Many of these meteorites come from leftover debris from the solar system’s formation, but some of these rocks display chemical compositions indicating that they came from Mars. (Violent collisions or other activity ejected these rocks from the surface of Mars with suf-ficient speed to escape the planet’s gravity.) Only a few dozen of the thousands of meteorites found on Earth are from Mars, but these rocks provide insights as well as informative comparisons with Earth. In 1996 a team of researchers at the National Aeronautics and Space Adminis-tration (NASA) announced that they had found fossils in one of these eploring earth’s Depths Martian meteorites—which indicates life evolved on Mars—but their results are controversial. Alex N. Halliday and R. Bastian Georg of the University of Oxford in Britain, along with colleagues at the University of California, Los An-geles, and the Swiss Federal Institute of Technology Zurich, recently studied a variety of meteorites. The researchers focused on silicon, the second most common element in Earth’s crust and an abundant element throughout the solar system. Silicon, like other elements, has different isotopes—atoms that have the same number of protons (which speci-fies the element) but a varying number of neutrons. Although isotopes tend to have similar chemical properties, they possess different masses, which gives them slightly different physical properties. When Halliday, Georg, and their colleagues compared silicon isotopes in Earth and Mars material, they found that Earth silicates have a greater proportion of heavier isotopes (isotopes with more neutrons). This evidence sug-gests that Earth and Mars may have formed under different conditions and may have distinctly different cores. The researchers published their report, “Silicon in the Earth’s Core,” in a 2007 issue of Nature. Further studies of astronomical material, perhaps including samples retrieved from future space missions, will enhance knowledge of the so-lar system and all of its planets, including Earth. As science reaches out acrossthe vast distances of space, scientists are also probing deeper into the very heart of the planet. Exploring Earth’s depths is a science whose frontiers range from the great heat and pressure of the planet’s core to the space probes that travel among the planets. CHRonoloGy 1 The Italian researcher Filippo Cecchi (1822–87) builds one of the first seismometers, although the instrument is not very sensitive. 1 The U.S. government establishes the USGS. 1 The German scientist Emil Wiechert (1861–1928) hypothesizes that Earth contains a metal core sur-rounded by a rocky mantle. 0 earth ScienceS 1 Wiechert improves upon seismometer technology, building an instrument that can record throughout an earthquake episode. 10 The British seismologist Richard D. Oldham (1858–1936) analyzes seismic waves to show that part of Earth’s core is liquid. 10 The Croatian researcher Andrija Mohorovičić (1857–1936) analyzes seismic waves and finds the Mohorovicic discontinuity, which separates Earth’s crust and mantle. 11 TheGermanresearcherAlfredWegener(1880–1930) proposesthatEarth’scontinentsdrift overtime. 11 The German seismologist Beno Gutenberg (1889– 1960) uses seismic waves to locate the depth of the mantle-core boundary at about 1,800 miles (2,900 km) below the surface. 1 The Danish seismologist Inge Lehmann (1888– 1993) analyzes seismic waves and discovers evi-dence for a boundary between a solid (inner) and liquid (outer) core, which she places at a depth of about 3,200 miles (5,150 km). 1 TheProjectMohole,anattempttodrillintotheMo-horovicic discontinuity, begins. The project would last eight years but fail to attain its primary goal. 1 The Canadian researcher J. Tuzo Wilson (1908–93) proposes the theory of plate tectonics. 10s The Russian scientists drilling in the Kola Peninsu-la reach a depth of 7.6 miles (12.26 km), the deep-est hole ever drilled. 00 The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) begins testing the drilling eploring earth’s Depths 1 vessel Chikyu, capable of drilling 4.3 miles (7 km) into the ocean floor. 00 In an expedition to Nankai Trough, an area of the Pacific Ocean off Japan’s coast that has been the site of numerous earthquakes and tsunamis, scientists aboard Chikyu drill holes ranging from 1,300 feet (400 m) to 4,600 feet (1,400 m) into the seabed. FuRtHER RESouRCES Print and internet Belonoshko, Anatoly B., Natalia V. Skorodumova, Anders Rosengren, and Börje Johansson. “Elastic Anisotropy of Earth’s Inner Core.” Science 319 (February 8, 2008): 797–800. Belonoshko and colleagues suggest that iron in the core adopts a certain crystal pattern called body-centered cubic, in which the atoms form a cube with an atom in the middle. Bjornerud, Marcia. Reading the Rocks: The Autobiography of the Earth. New York: Basic Books, 2006. Bjornerud, a geologist, chronicles the history of Earth as revealed by the rocks and layers that compose it. Starting at the very beginning, at the birth of the solar system, she discusses evolution, plate tectonics, climate change, and many other topics. Brush, Stephen G. Nebulous Earth. Cambridge: Cambridge University Press, 1996. Suitable for advanced readers, this book details the fas-cinating work of the scientists who developed the concepts and prin-ciples of planetary geology and the evolution of the solar system. Crowhurst, J. C., J. M. Brown, A. F. Goncharov, and S. D. Jacobsen. “Elasticity of (Mg,Fe)O Through the Spin Transition of Iron in the Lower Mantle.” Science 319 (January 25, 2008): 451–453. Crowhurst and his colleagues discovered that the properties of certain materials result in a slowing of the speed of seismic waves. Dixon, Dougal. The Practical Geologist: The Introductory Guide to the Basics of Geology and to Collecting and Identifying Rocks. New York: SimonandSchuster,1992.Thisbookintroducesthesubjectofgeology earth ScienceS and focuses on practical applications, such as collecting minerals and making maps. Georg, R. Bastian, Alex N. Halliday, Edwin A. Schauble, and Ben C. Reynolds. “Silicon in the Earth’s Core.” Nature 447 (June 28, 2007): 1,102–1,106. This research indicates that Earth and Mars may have formed under different conditions and may have distinctly different cores. Greenberg, D. S. “Mohole: The Project That Went Awry.” Science 143 (January 10, 1964): 115–119. The sad history of Project Mohole is chronicled here. Hayden, Leslie A., and E. Bruce Watson. “A Diffusion Mechanism for Core-Mantle Interaction.” Nature 450 (November 29, 2007): 709– 711. These researchers have found a mechanism by which metal at-oms in Earth’s core can leak, or diffuse, across layer boundaries. Hernland, John W., Christine Thomas, and Paul J. Tackley. “A Dou-bling of the Post-Perovskite Phase Boundary and Structure of the Earth’s Lowermost Mantle.” Nature 434 (April 14, 2005): 882–886. This paper describes data suggesting the presence of a thin layer around the mantle-core boundary. Japan Agency for Marine-Earth Science and Technology (JAMSTEC). “Chikyu Hakken.” Available online. URL: http://www.jamstec.go.jp/ chikyu/eng/index.html. Accessed May 4, 2009. The English version of JAMSTEC’s Web pages on their Earth Discovery project contains information on the drilling vessel Chikyu and its expeditions, along with the latest findings. Louie, John N. “Earth’s Interior.” Available online. URL: http://www. seismo.unr.edu/ftp/pub/louie/class/100/interior.html. AccessedMay4, 2009. Beautifully illustrated, this essay discusses the structure of the planetandhowgeologistsdiscoveredthisstructure. Mathez, Edmond A., ed. Earth: Inside and Out. New York: New Press, 2001. Written by a team of experts, this highly informative book contains sections on Earth’s evolution, seismic exploration of the in-terior, plate tectonics, analysis of rocks, and climate change. ScienceDaily. “2006 Tectonic Plate Motion Reversal Near Acapulco Puz-zles Earthquake Scientists.” News release, August 6, 2007. Available on-line. URL: http://www.sciencedaily.com/releases/2007/08/07080213 ... - tailieumienphi.vn