The Origin of the Moon
Simulation from A.G.W. Cameron
Copyright © 1997 by Academic Press |
|
Scientists date origin of moon in Earth's "big bang"
ANN ARBOR -- University of Michigan geochemists have made the most accurate
estimate yet of the age of our moon and discovered that it formed later
in the development of the solar system than many scientists believed --
almost certainly as the result of a collision between Earth and another
planet at least as large as Mars.
The interplanetary "big bang" between the Earth and another object
occurred about 50 million years after the start of the solar system, according
to Alexander N. Halliday, U-M professor of geological sciences.
In a study published in the Nov. 7 issue of Science, U-M scientists
Der-Chuen Lee and Halliday, with Gregory A. Snyder and Lawrence A. Taylor
of the University of Tennessee, explain how they analyzed isotopes of tungsten
in rock samples from the lunar surface to unlock the secrets of the moon's
origin.
"Our data indicate the moon formed within the time window of 4.52
billion to 4.50 billion years ago. The tungsten isotopic composition of
the moon is consistent with the hypothesis that the moon was derived from
the Earth itself, or from a large object colliding with the Earth which
had a similar chemical composition," Halliday said.
"Simulations of the giant impact indicate phenomenally high temperatures
of more than 10,000 degrees K., which triggered planet-wide mixing and
melting of the rocky material in the young planet Earth," said Der-Chuen
Lee, a U-M postdoctoral research fellow in geological sciences. "The heat
and energy associated with the moon's formation were also responsible
for producing its magma oceans."
Scientists believe the planets in our solar system began forming
about 4.57 billion years ago from a huge cloud of interstellar gas, dust
and debris leftover from the birth of the sun. The Earth and other rocky
planets in the inner solar system built up gradually over millions of years
as their gravitational pull attracted larger and larger chunks of material
from the cloud.
Halliday and Lee used a technique called multiple-collector, inductively-coupled
plasma mass spectrometry to measure extremely small amounts of tungsten
isotopes in 21 lunar samples. "Since hafnium-182 decays into tungsten-182
with a half-life of 9 million years, it is possible to determine relative
ages of materials based on their isotopic ratios," Halliday said.
The research project was funded by the U.S. Department of Energy,
NASA, the National Science Foundation and the University of Michigan. Gregory
A. Snyder and Lawrence A. Taylor of the University of Tennessee's Planetary
Geosciences Institute were research collaborators and co-authors on the
paper.
The University of Michigan
News and Information Services
412 Maynard
Ann Arbor, Michigan 48109-1399
Contact: Sally Pobojewski
Phone: (313) 647-1844
E-mail: pobo@umich.edu
News Release: November 10, 1997 (8)
Questions & Answers
Subject:
Re: [ASTRO] Scientists Date Origin Of Moon In Earth's 'Big Bang
Date:
Thu, 13 Nov 1997 09:32:12 +0100
From:
Antoni Parra <aparra1@pie.xtec.es>
To:
astro list <astro@lists.mindspring.com>
Jega Arulpragasam:
I'm not clear how this works. How is the 'original' ratio
of hafnium-182
to tungsten-182 known? Without this information, it seems
to me, that we
cannot determine how much of the original hafnium has decayed.
Tony Parra:
The 'original' ratio of Hafnium-182/Tungsten-182
is 1/0. All
tungsten-182 present in moon rocks comes
from Hafnium disintegration.
Subject:
Re: [ASTRO] Scientists Date Origin Of Moon In Earth's 'Big Bang
Date:
Sat, 15 Nov 1997 15:17:14 +0100
From:
Antoni Parra <aparra1@pie.xtec.es>
To:
astro list <astro@lists.mindspring.com>
Jega Arulpragasam:
Thanks, Tony. This would certainly enable an age determination
of the
sample from the current isotope ratios -- as would
any KNOWN 'original'
Hf-182:W-182 ratio, other than 0:1. :-) However,
what I'm looking for
is how is it known (or why is it assumed) that the original ratio
was
1:0? (See my original question.)
For example, if the material of the Moon were knocked off the
Earth by a
Mars-sized body after the Earth had been around for awhile, wouldn't
the
current isotope ratios measure the age of the Earth rather than
that of
the Moon, even using the same underlying assumption about the
'original' ratio?
Tony Parra:
Most minerals found in magmatic rocks 'choose'
atoms from the liquid
phase when they are crystalizing. Crystallization
is a selective
process. If a zircon contains uranium in
some amount *and no lead*, all
Pb produced by disintegration would appear
'in the wrong place'.
Subject: Re: tungsten
isotopes (fwd)
From: Alex
Halliday <anh@umich.edu>
To:
Antoni Parra <aparra1@pie.xtec.es>
Tony Parra:
I would like to make some questions:
In the November 10, 1997 News Release,
you said: "Since hafnium-182 decays into tungsten-182 with a half-life
of 9 million years, it is possible to determine relative ages of materials
based on their isotopic ratios"
1. Wich minerals contain hafnium-182?
2. Which is the original ratio hafnium-182
/ tungsten-182 when the crystal lattice closes?
1/0 perhaps?
Alex Halliday:
Dear Tony,
The tungsten isotopic composition of a material will be a function
of when it formed, its Hf/W ratio and the Hf/W ratios and tungsten isotopic
compositions of the materials it formed from. So it could be quite
complicated. The half-life is sufficiently short that one could only
use this technique for establishing the first 100 million years of solar
system history. Early solar system 182Hf would have behaved chemically
like other isotopes of Hf. In general Hf is enriched in the same minerals
as those which are rich in Zr. In an igneous rock the Hf will behave
as an "incompatible element" unable to substitute readily during melting
and crystallization unless zircon (ZrSiO4) forms, and will wind up enriched
in the minerals from the last liquids to crystallize. So the "original
ratio" is directly proportional to the Hf/W weight or molar ratio of the
mineral and the time that has elapsed. It will not be 1 because there will
be initial W already present. It will not be zero because there will be
182Hf present.
Hope this helps.
Alex
Subject: Re: [ASTRO] Scientists
Date Origin of Moon
Date:
Sat, 22 Nov 1997 14:48:31 -0500 (EST)
From: Truman
P Kohman <tk11+@andrew.cmu.edu>
To:
astro@lists.mindspring.com
Astro listers,
Admittedly, Ron Baalke's report [1997
November 11] was somewhat vague as to the hafnium-tungsten dating technique
used by Lee, Halliday,
Snyder, and Taylor [Science 278, 1098-1103, 1997 November 7]
on lunar samples and its application to the time of formation of the Moon.
The analogy to the 238U-206Pb dating
method suggested by Leigh Palmer [1997 November 13] is not relevant here,
because 238U is an extant radionuclide. That and other methods based
on primary (extant) natural radioactivity (U235-207Pb, 232Th-208Pb, 87Rb-87Sr,
40K-40Ar, 147Sm-143Nd, 187Re-187Os) date chemical fractionation events
back from the present. 182Hf, like 129I, 244Pu, 26Al, 53Mn, and 146Sm,
is an extinct natural radionuclide, so of course it has no
present abundance.
It is also not correct that the initial
182Hf/182W was 1/0 and that all 182W present in Moon rocks comes from Hf
disintegration [Antoni Parra, 1997 November 13]. Common W containing
182W was certainly present originally in all samples, and an essential
part of the analysis is the determination of how much 182W was initially
present.
The analysis involves the determination
of the isotopic composition of the W in each sample, particularly the 182W/184W
ratio, as well as the Hf/W ratio, expressed as 180Hf/184W, in the same
samples. Samples with high Hf/W will generate more W182 relative
to that initially present than samples with low Hf/W. A plot is made
of 182W/184W versus 180Hf/184W. Points for samples with the same
chronologic history (time of fractionation of Hf and W from the parent
matter in their formation) should fall on a straight line, called an isochron.
The slope of the isochron gives the 182Hf/180Hf ratio at the time of fractionation,
and the intercept gives the initial 182W/184W ratio of the commmon W present.
Some of the lunar samples (including
an Antarctic lunar meteorite) studied by Lee at iii al. had essentially
chondritic W isotopic composition, meaning no excess 182W was present.
However, many of them had statistically significant excesses of 182W, indicating
that 182Hf was still "alive" at the time of their Hf-W separation.
Most of the points scattered on the isochron plot, indicating additional
fractionation subsequent to the major fractionation which "set the clock".
The best isochron was obtained from three volcanic "orange glass" samples.
The slope corresponded to an initial 182Hf/180Hf of 5.9 +/- 2.3 X 10^-6
and the intercept corresponded to essentially chondritic initial 182W/184W.
The chronologic interpretation is not
straightforward, but depends on assumptions regarding the nucleosynthesis
of r-process nuclides (including 182Hf) and the time thereof relative to
the time of formation of the Solar System including the Earth, and assumptions
about the nature and stratigraphic time of the major fractionation.
Lee et iii al. do not discuss nucleosynthesis in this paper, but simply
state that the bulk Solar-System initial 182Hf/180Hf ratio was 2.4 +/-
0.6 X 10^-4 (no explanation or justification, but references to two previous
papers by Lee and Halliday; this may be the theoretical ratio at the time
of r-process synthesis in a nearby supernova, which may have triggered
the condensation of the Solar Nebula, with a time lapse of a few Ma or
less.)
Among various models of the Earth-Moon
system, Lee et iii al. favor an impact of a Mars-sized body on the Earth
leading to a largely molten Moon with Hf/W enriched relative
to the chondritic parent material of the Earth and possibly also the impactor.
Qualitatively, the presence of live 182Hf indicates that this event occurred
less than ~10 times the half-life of 182Hf (9 Ma), or ~90 Ma. The
orange-glass isochron slope is stated to yield an isochron age of ~54 Ma
(6 182Hf half-lives). Ages calculated from individual samples range
from 40 to 67 Ma, the mean being 53 +/- 4 Ma.
The authors conclude, "On the basis
of these data, we estimate that the Moon formed and differentiated ~50
Ma after the start of the Solar System, within the time window 4.52 to
4.50 Ga [ago]." This seems to imply that they regard the age of the
Solar System to be 4.56 +/- 0.01 Ga, based on dating of primitive meteorites
by primaty natural radioactivity methods.
Truman
Truman P. Kohman, Department of Physics,
FAX: 412/681-0648
Carnegie-Mellon University, Pittsburgh, PA 15213 USA
Internet: tk11+@andrew.cmu.edu Phone: 412/268-8865, 412/561-8343
Graphic image: http://www.rahul.net/resource/astro-list/kohman.htm
Subject:
Re: [ASTRO] Scientists Date Origin of Moon
Date:
Mon, 24 Nov 1997 14:00:50 +0100
From:
Antoni Parra <aparra1@pie.xtec.es>
To:
astro@lists.mindspring.com
Nice explanation, Truman.
My point of view was too simplistic.
I must apologize! :-)
Tony