-Caveat Lector- ----- Original Message ----- From: Science-Week <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Monday, September 13, 1999 7:11 AM Subject: ScienceWeek BULLETIN September 13, 1999 **************************************************************** SW BULLETIN is a free publication published each Monday by the Editors of SCIENCE-WEEK, the weekly Email research digest. Read SCIENCE-WEEK for complete explications of research news (see the table of contents of the current issue near the end of this file). For information about various ScienceWeek publications see the end of this text or visit the SW website at http://scienceweek.com **************************************************************** SW BULLETIN - September 13, 1999 --------------------------------------------- This Week's Report: On the Origin of the Earth and the Moon --------------------------------------------- ON THE ORIGIN OF THE EARTH AND THE MOON Two of the central questions in planetary and Earth science concern the origin of the Earth and Moon. How did these two bodies form and what forces defined their basic physical structures? ... ... A.N. Halliday and M.J. Drake (2 installations, CH US) present a short review of current research in this area, the authors making the following points: 1) Advances in this field have come mainly with progress in simulating the dynamics of planetary accretion, in measuring isotopes that act as chronometers for early Solar System processes, in analysis of noble gas isotopes that yield clues about the early atmosphere, and in melting experiments at previously unattainable pressures and temperatures. Although a general picture may be emerging, many issues remain hotly debated. 2) Planet formation is believed to begin with sticking and frictional coagulation of dust particles in a gaseous nebula that persists in the *circumstellar disk. The particles grow in size until there is substantial gravitational attraction between kilometer-sized bodies, and these coalesce further. Major collisions between small proto-planets eventually result in objects the size of Earth. 3) The energy of late-stage planet-building impacts would be colossal, sufficient to melt the entire planet. *Magma oceans would be formed, and some volatile elements would escape into space. The most widely accepted theory for the origin of the Moon is that it coalesced from a ring of debris produced by such a late-stage collision between two Earth-forming proto-planets. This "Giant Impact Theory", established over a decade ago, explains the rotational speed of the Earth-Moon system, a critical feature that must be reproduced by any satisfactory model. But in spite of a growing consensus, some researchers are still opposed to the Giant Impact Theory on both dynamical and geochemical grounds. 4) All isotopic data are consistent with Earth being fully formed within 50 to 100 million years after the start of the Solar System. The isotopic record from Moon rocks is consistent with the formation of the Moon at about the same time. 5) The authors conclude: "We have recently come a long way in obtaining hard constraints on the origin of Earth and the Moon. The issues have changed from discussion of whether or not there was a giant Moon-forming impact to debate about the accretion rates of the Earth and the chemical, isotopic, and physical effects of such catastrophic accretionary scenarios." ... ... In a contiguous short review of the same research area, Frank A. Podosek (Washington University St. Louis, US) makes the following points: 1) The age of the Solar System as a whole is easier to determine than the age of Earth. The age of the Solar System is reliably inferred from the age of *refractory element-rich inclusions in meteorites to be approximately 4.57 billion years, thus providing an upper limit to the age of Earth. These inclusions are the oldest known objects in the Solar System, and their content indicates that the Solar System did not exist for more than approximately 1 million years before the inclusions formed. 2) In contrast to these ancient extraterrestrial objects, there are no known terrestrial rocks or minerals whose formation essentially coincides with the formation of Earth, and therefore the age of Earth must be inferred indirectly. Several independent approaches indicate that Earth formed approximately 100 million years later than the Solar System as a whole. 3) All the various isotopic chronometers are intrinsically capable of considerably higher precision, but this precision cannot yet be realized. It is not even clear whether the chronometers are consistent or in conflict with each other. All methods rely on models of varying complexity involving assumptions difficult to verify and parameters difficult to measure. 4) The author concludes: "For testing the giant impact scenario in particular, it would be useful to have a quantitative theory for whether a preexisting atmosphere is lost in the impact, whether preexisting planetary structures (*core, mantle, and crust) are re-equilibrated after such an impact, and how much of the Moon comes from the impactor and how much comes from the target." ----------- A.N. Halliday and M.J. Drake: Colliding theories. (Science 19 Mar 99 283:1861) QY: A.N. Halliday [[EMAIL PROTECTED]] ----------- Frank A. Podosek: A couple of uncertain age. (Science 19 Mar 99 283:1863) QY: Frank A. Podosek [[EMAIL PROTECTED]] ----------- Text Notes: ... ... *circumstellar disk: One of the important discoveries of the 1980s was the existence of circumstellar disks of dust around some stars, the disks apparently replenished by unseen parent bodies such as comets and asteroids. ... ... *Magma: In general, any mass of molten rock. ... ... *refractory: Refractory materials are materials resistant to decomposition by heat, pressure, or chemical attack. The term is most commonly applied to heat resistance. ... ... *core, mantle, and crust: Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. ------------------- Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 4Jun99 ------------------- Related Background: AGE AND ORIGIN OF EARTH'S MOON The most widely accepted theory for the origin of the Earth's moon is that during the late stages of the Earth's accretion an impact with another planet at least the size of Mars occurred, and the impact generated both the hot debris that formed the moon and the angular momentum of the Earth-moon system. In geology, the mantle of a planet or moon is the layer that lies between the crust and the core. Chondrites are a type of stony meteorite consisting of an agglomeration of millimeter-sized globules (chondrules) that are thought to be unchanged since the original condensation out of the nebula from which the sun and solar system formed, and "chondritic" is the term used to describe a rock composition similar to that of chondrites, which implies an age of 4.2 to 4.5 billion years. The term "radiogenic", on the other hand, is used to describe a rock composition apparently resulting from varying isotope decays, and the oldest radiogenic compositions on Earth have been dated at 3.6 to 3.8 billion years. A hafnium-tungsten chronometer is not an actual instrument but a method of radiometric age determination using the isotope ratios of the elements hafnium and tungsten. Hafnium is lithophilic (silicate-loving), which means it tends to associate with chondritic materials, while tungsten is siderophilic (metal- loving), which means it tends to associate with metal cores, and using these differing affinities of these elements, one can attempt a construction of the age and origin of the moon by analysis of moon rock samples and comparisons with Earth rocks. Lee et al (4 authors at 2 installations, US) report a study of the age and origin of the moon with the hafnium-tungsten chronometric method. The tungsten isotopic compositions of 21 lunar samples were found to range from chondritic to slightly radiogenic. The authors suggest this heterogeneity is probably the result of late radioactive decay within the moon itself, and that the moon formed 4.52 to 4.50 billion years ago and its mantle has since remained poorly mixed. QY: Der-Chuen Lee [[EMAIL PROTECTED]] (Science 7 Nov 97) (Science-Week 28 Nov 97) ------------------- Related Background: THE ORIGIN OF EARTH'S MOON The large impact hypothesis of the origin of the Earth's moon is the current consensus view. The essential idea is that the moon formed from debris ejected into a disk around Earth by the impact of a large body. A version of this is that Earth and its moon were created more or less simultaneously by the collision of two large planetesimals, the resultant large body becoming Earth, and the ejected debris formed the moon. What is accepted by nearly everyone is that an accretion disk of debris was the first stage of the moon's formation. Shigeru Ida et al (Tokyo Institute of Technology, JP; University of Colorado Boulder, US) have evidently now provided the most detailed simulation calculations of lunar growth in an impact-generated accretion disk. Using direct N-body simulations, they show that a single dominant moon can grow from such a disk within a year, but to satisfy the present angular momentum and mass constraints on the analysis, the impacting body must have been at least twice as massive as Mars, and had to provide the resultant system with a few times more angular momentum than it now possesses. There is presently no explanation for the subsequent loss of angular momentum, and the required massive size of the impacting object is also puzzling. Although this is apparently the best set of simulation calculations to date, the authors emphasize that further simulation modeling is needed [*Note #1]. QY: S. Ida [[EMAIL PROTECTED]] (Nature 25 Sep) (Science-Week 10 Oct 97) ----------- Text Notes: ... ... *Note #1: Accretion is considered an important factor in the evolution of stars, planets, and comets. The essential idea is the coalescence of small particles in space as a result of collisions, and the gradual formation of larger bodies from smaller ones as a result of gravitational attraction. An accretion disk is a disk of gas or particles in orbit around an object, the disk formed by inflowing matter. A simulation of the sort mentioned in the report involves computational solutions of the dynamical equations for the history of a chosen mass of particulate matter initially ejected from a larger body. By solving the equations for the mathematical model, one can follow the evolution of the accretion disk and the agglomeration that forms the final orbiting satellite. The study mentioned here was first presented at a meeting of the American Astronomical Society in July, and here is part of the related SCIENCE-WEEK (1 Aug 97) report: Until the 1980s, there were three extant theories, with no data available to support or refute any of them. The Fission Hypothesis proposed that the moon broke away from a rapidly spinning proto-Earth after the proto-Earth's differentiation, the moon forming from iron-poor crust. But the moon rocks in hand have been found to differ chemically from those of Earth. Also, if the proto-Earth had been spinning fast enough to break up, the present Earth-moon system should contain a great deal more angular momentum than is observed. The Fission Hypothesis therefore had to be abandoned. The Condensation Hypothesis was based on the idea that the Earth and the moon condensed simultaneously from the same cloud of material in the solar nebula. This hypothesis did not survive because analysis of moon rocks has shown the Earth and the moon have greatly different densities and compositions. The Capture Hypothesis proposed that the moon was formed elsewhere in the solar system and later "captured" by Earth. This hypothesis was always the least popular because it required too many coincidental events. Thus, after the mid-1980s, there was no satisfactory theory of the moon's origin. During the past decade, a new idea gradually developed, the Large-Impact Hypothesis, the idea of which is that the moon formed from debris ejected into a disk around the Earth after a major collision of the Earth with another large body about 4.5 billion years ago, the other body a planet perhaps as large as Mars. The Large-Impact Hypothesis is at present the consensus theory in planetary science. ----------- [SW Bulletin 13 Sep 99] **************************************************************** If you have questions or comments about SW BULLETIN, send Email to: [EMAIL PROTECTED] Claire Haller, Managing Editor **************************************************************** What you are now reading is SW BULLETIN, a free publication sponsored by ScienceWeek. SW BULLETIN is published on Mondays and delivered only via Email. 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