SETI ScienceWeek BULLETIN September 13, 1999


Bob Cutter (bcutter@teal.csn.net)
Tue, 14 Sep 1999 16:04:11 -0600


>X-Sender: lklaes@pobox3.bbn.com
>Date: Tue, 14 Sep 1999 16:47:07 -0400
>To: Benny J Peiser <b.j.peiser@livjm.ac.uk>,
> "Dr. H. Paul Shuch" <n6tx@setileague.org>, bcutter@teal.csn.net
>From: Larry Klaes <lklaes@bbn.com>
>Subject: ScienceWeek BULLETIN September 13, 1999
>X-UIDL: 937340841.003
>
>>From: "Science-Week" <prismx@scienceweek.com>
>>Organization: Science-Week
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>>Date: Mon, 13 Sep 1999 08:22:23 -0600
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>>Subject: ScienceWeek BULLETIN September 13, 1999
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>>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
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>>
>>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 [halliday@erdw.ethz.ch]
>>-----------
>>Frank A. Podosek: A couple of uncertain age.
>>(Science 19 Mar 99 283:1863)
>>QY: Frank A. Podosek [fap@levee.wustl.edu]
>>-----------
>>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 [dclee@umich.edu]
>>(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 [ida@geo.titech.ac.jp]
>>(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]
>>
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