PLEASE NOTE:
*
CCNet 89/2001 - 17 August 2001
------------------------------
Dear CCNet member,
Welcome back. I hope you have had a nice and
refreshing summer break.
Please feel free to get back in touch now that I have returned
from my
vacation. A compilation of research findings which appeared in
the media
during the last 3 weeks will be posted next week.
Best regards
Benny J Peiser
-------------------------------------
"A dark, lifeless object less than half as massive as Earth
careens
around a newborn Sun. It is one of many planet-sized bodies
hoping for a
long career. But its orbit is shaky. It's future grim. It is a
character
actor on the grand stage of the solar system, a player of great
ultimate
consequence but one destined to never see its name in lights.
This doomed
"protoplanet" travels a path that crosses the orbits of
similar objects and,
ultimately, cannot last. Eventually, the nameless protoplanet
meets up
with a fledgling Earth. It is not a head-on collision, but rather
a
glancing blow. The impact imparts what astronomers call angular
momentum into the system. It sets Earth to spinning on its axis
and creates
a Moon that would go round and round the host planet for billions
of
years."
--Robin Canup's description of the Moon-forming impact,
Space.com, 16 August 2001
"The model we propose is the least restrictive impact
scenario,
since it involves only a single impact and requires little or no
modification of the Earth-Moon system after the Moon-forming
event."
--Robin M. Canup, SwRI Space Studies Department, 15 August
2001
"Our model requires a smaller impactor than previous models,
making
it more statistically probable that the Earth should have a Moon
as large
as ours."
--Erik Asphaug, University of California, Santa Cruz, 15
August 2001
"The new study strengthens just one theory of how the Moon
might
have formed. Other scientists have suggested that the Moon
developed
elsewhere in the solar system and was captured by Earth. An even
more
remote possibility is that the Earth and Moon condensed together
out
of the material that formed the solar system. Another idea is
that
gravitational interactions between the Earth, the Sun, and other
developing
planets simply tore Earth apart and the Moon formed from this
debris. But
the majority of researchers prefer the impact theory. And though
a
similar impact would be extremely unlikely today, it was a fairly
common
occurrence back when the solar system was forming."
--Rob Britt, Space.com, 15 August 2001
(1) WORLDS IN COLLISION: US RESEARCHERS SIMULATE MOON-FORMING
IMPACT
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) WHEN WORLDS COLLIDE
Nature, 16 August 2001
(3) MOON MAKING MADE EASY
Nature Science Update, 16 August 2001
(4) 24 HOURS OF CHAOS: THE DAY THE MOON WAS MADE
Space.com, 15 August 2001
(5) HOW THE MOON WAS MADE
BBC News Online, 15 August 2001
(6) DIAMONDS IN THE SKY: CLUES TO EARLY HISTORY OF SOLAR SYSTEM'S
OLDEST
DIAMONDS
Andrew Yee <ayee@nova.astro.utoronto.ca>
==================
(1) WORLDS IN COLLISION: US RESEARCHERS SIMULATE MOON-FORMING
IMPACT
>From Andrew Yee <ayee@nova.astro.utoronto.ca>
Southwest Research Institute (SwRI)
SwRI(TM), UCSC researchers identify the Moon-forming impact
Boulder, Colorado, August 15, 2001 -- The "giant
impact" theory, first
proposed in the mid-1970s to explain how the Moon formed, has
received a
major boost as new results demonstrate for the first time that a
single
impact could yield the current Earth-Moon system.
Simulations performed by researchers at Southwest Research
InstituteTM
(SwRI) and the University of California at Santa Cruz (UCSC) show
that a
single impact by a Mars-sized object in the late stages of
Earth's formation
could account for an iron-depleted Moon and the masses and
angular momentum
of the Earth-Moon system. This is the first model that can
simultaneously
explain
these characteristics without requiring that the Earth-Moon
system be
substantially modified after the lunar forming impact. The
findings appear
in the August 16 issue of Nature.
The Earth-Moon system is unusual in several respects. The Moon
has an
abnormally low density compared to the terrestrial planets
(Mercury, Venus,
Earth, and Mars), indicating that it lacks high-density iron. If
the Moon
has an iron core, it constitutes only a few percent of its total
mass
compared to Earth's core, which is about 30 percent of its mass.
The angular
momentum of the Earth-Moon system, contained in both the Earth's
spin and
the Moon's orbit, is quite large and implies that the terrestrial
day was
only about five hours long when the Moon first formed close to
the Earth.
This characteristic provides a strong constraint for giant impact
models.
Previous models had shown two classes of impacts capable of
producing an
iron-poor Moon, but both were more problematic than the original
idea of a
single Mars-sized impactor in the last stages of Earth's
formation. One
model involved an impact with twice the angular momentum of the
Earth-Moon
system; this would require that a later event (such as a second
large
impact) alter the Earth's spin after the Moon's formation. The
second model
proposed that the Moon-forming impact occurred when Earth had
only accreted
about half its present mass. This required that the Earth
accumulated the
second half of its mass after the Moon formed. However, if the
Moon also
accumulated its proportionate share of material during this
period, it would
have gained too much iron-rich material -- more than can be
reconciled with
the Moon today.
The models developed by SwRI and UCSC use the modeling technique
known as
smooth particle hydrodynamics, or SPH, which also has been used
in previous
formation studies. In SPH simulations, the colliding planetary
objects are
modeled by a vast multitude of discrete spherical volumes, in
which
thermodynamic and gravitational interactions are tracked as a
function of
time.
The new high-resolution simulations show that an oblique impact
by an object
with 10 percent the mass of the Earth can eject sufficient
iron-free
material into Earth-orbit to yield the Moon, while also leaving
the Earth
with its final mass and correct initial rotation rate. This
simulation also
implies that the Moon formed near the very end of Earth's
formation.
"The model we propose is the least restrictive impact
scenario, since it
involves only a single impact and requires little or no
modification of the
Earth-Moon system after the Moon-forming event," says the
paper's lead
author, Dr. Robin M. Canup, assistant director of the SwRI Space
Studies
Department in Boulder.
UCSC Professor Erik Asphaug adds, "Our model requires a
smaller impactor
than previous models, making it more statistically probable that
the Earth
should have a Moon as large as ours."
Modeling lunar formation is important to the overall
understanding of the
origin of the terrestrial, or Earth-like, planets.
"It is now known that giant collisions are a common aspect
of planet
formation, and the different types of outcomes from these last
big impacts
might go a long way toward explaining the puzzling diversity
observed among
planets," says Asphaug.
The Moon is also believed to play an important role in Earth's
habitability
because of its stabilizing effect on the tilt of Earth's
rotational pole.
"Understanding the likelihood of Moon-forming impacts is an
important
component in how common or rare Earth-like planets may be in
extrasolar
systems," adds Canup.
For more information, contact Maria Martinez at (210) 522-3305 or
Dr. Robin
Canup at (303) 546-6856.
Editors: An image and animation of the moon-forming event are
available from
http://www.swri.org/press/impact.htm
=================
(2) WHEN WORLDS COLLIDE
>From Nature, 16 August 2001
http://www.nature.com/nature/fow/010816.html
The dominant theory of how our moon was formed is that it is a
by-product of
a large off-centre collision between the earth and an
approximately
Mars-sized protoplanet. This theory has become the consensus view
as no
other theory reconciles so many of the facts about the Earth and
the Moon.
Until recently it has been impossible to model this theory, but
now new
mathematical algorithms and the availability of the necessary
computing
power are allowing scientists to put the theory to the test.
In this week's Nature Canup and Asphaug present the most
sophisticated model
yet of the formation of the Moon. They use an approach called
smooth-particle hydrodynamics which not only models the shock
physics,
melting and vaporization of the collision but crucially adds self
gravity to
all the model components, thus creating a more realistic
simulation.
Increased computing power also allows the researchers to add
increased
granularity to their model - their simulation is made up of
20,000
individual particles while earlier models relied on only a few
thousand.
Their results reveal a class of impacts that yield an iron-poor
Moon, as
well as the current masses and angular momentum of the Earth-Moon
system.
This class of impacts involves a smaller - and thus more likely -
object
than previously considered viable, and suggests that the Moon
formed near
the very end of Earth's accumulation.
Featured articles
Planetary science: A new model Moon
JAY MELOSH
The most sophisticated simulations yet of the Moon's birth show
that it
could have been created from an impact of a large body with a
fully formed,
rather than half-formed, Earth.
Nature 412, 694-695 (16 August 2001)
| Full Text | PDF (115 K) |
Origin of the Moon in a giant impact near the end of the Earth's
formation
ROBIN M. CANUP & ERIK ASPHAUG
Nature 412, 708-712 (16 August 2001)
| Summary | Full Text | PDF (289 K) |
Related articles
It's not easy to make the Moon
JACK J. LISSAUER
Theory has it that the Moon grew within a disk of material
splashed out of
the Earth by a body the size of Mars. According to new
calculations,
however, the impacting body was at least twice that size. There
are probably
very many terrestrial planets in our Galaxy, yet the implication
of this and
other simulations is that fewer of them than previously expected
have
Moon-sized satellites.
Nature 389, 327-328 (25 September 1997)
| Full Text | PDF (252 K) |
Lunar accretion from an impact-generated disk
SHIGERU IDA, ROBIN M. CANUP & GLEN R. STEWART
Nature 389, 353-357 (25 September 1997)
| Summary | Full Text | PDF (300 K) |
Nature © Macmillan Publishers Ltd 2001 Registered No. 785998
England
==================
(3) MOON MAKING MADE EASY
>From Nature Science Update, 16 August 2001
http://www.nature.com/nsu/010816/010816-14.html
Moon making made easy
Mars-sized mass implicated by new model for Moon's violent birth.
16 August 2001
JOHN WHITFIELD
It would have been a good day to stay indoors. The surface of the
Earth
melted. A large part of the rocky mantle was hurled into orbit.
But when the
smoke cleared, there was a new Moon - literally - in the sky.
The most sophisticated reconstruction of the Moon's formation so
far
suggests that our satellite resulted from an almighty collision
between a
body roughly the size of Mars and an almost fully formed Earth
about 4.5
billion years ago1.
Robin Canup, of the Southwest Research Institute, Boulder,
Colorado, and
Erik Asphaug, of the University of California, Santa Cruz,
divided the Earth
and its 'impactor' into more than 20,000 units.
Their model is a three-dimensional description of how each unit
interacts
with its neighbours when subjected to stress, melting and the
like, and with
every other unit through gravity. Each simulated collision takes
from a few
days to a week to run on a powerful desktop workstation.
Modelling every possible impact would be the work of several
lifetimes:
"It's very inefficient to try something, see if it works,
try something
else, and so on," says Canup. But with new analytical
techniques, she says,
"we were able to zero in and predict what would give us the
best results".
This left about 30 candidates for the computer to get its teeth
into.
An oblique clash between a Mars-sized impactor and the Earth
"seems to put
just the right mass of material into orbit", Canup says.
This scenario improves on previous models, which, says planetary
scientist
David Stevenson at the California Institute of Technology,
Pasadena,
required "a disturbingly narrow range of conditions".
These models worked
only for a much larger impactor, a much smaller, only partially
formed
Earth, or more than one impact, for example.
The Moon is unlikely to have been chipped off an incomplete Earth
- its core
contains much less iron than the Earth's. It is tricky to explain
how the
Earth could have grown to its present, iron-rich size after the
impact
without the Moon doing the same. Hypotheses that Earth and Moon
grew
alongside one another suffer from the same problem.
Small step or giant leap?
It's good news that the best model gives the most plausible
result, says
Stevenson. But this will not be the last word on the subject. The
models
still have their limitations, he says: "They may not be
capturing all the
dynamics of the impact correctly."
More computer power will help, but the problem will remain that
giant
impacts are inherently improbable, and the Solar System's chaotic
nature
makes it impossible to re-run history and get the same outcome.
"None of the
scenarios for the Moon's formation is highly likely,"
Stevenson says.
Another problem, says space scientist Sarah Dunkin, is that
"we still don't
know what the Moon is made of". Dunkin, who works at the
Rutherford Appleton
Laboratory, Didcot, England, is advising Europe's SMART-1 mission
set to
reach the Moon at the end of 2004. By probing the Moon's
composition, this
should give clues to its origin. "The whole Moon is a scar
of its birth,"
she says.
References
Canup, R. M. & Asphaug, E. Origin of the Moon in a giant
impact near the end
of the Earth's formation. Nature, 412, 708 - 712, (2001).
© Nature News Service / Macmillan Magazines Ltd 2001
============
(4) 24 HOURS OF CHAOS: THE DAY THE MOON WAS MADE
>From Space.com, 15 August 2001
http://www.space.com/scienceastronomy/solarsystem/moon_making_010815-1.html
By Robert Roy Britt
Senior Science Writer
For 25 years, scientists have pondered a theory that the Moon was
created
when an object the size of Mars crashed into Earth less than 100
million
years after the Sun was born, some 4.6 billion years ago. The
general idea
has been run through the paces and massaged into shape and is now
the
favored explanation.
But attempts to model cousins of that theory on computers
generate
inexplicable side effects.
In one of two leading computer models, Earth was creamed while it
was still
gathering mass, during a brief time when it was only half its
current size.
All the rocky inner planets are thought to have formed this way,
a method
rapid accumulation of matter called runaway growth.
But if the Moon was carved out during Earth's growth phase, then
it would
have been around when Earth continued bulking up by swallowing
tremendous
numbers of large asteroids. Some of these iron-rich rocks would
have hit the
Moon, too. Yet the iron is not there.
In the other model, the aggressor was three times as massive as
Mars and
created an excess of rotation in the Earth-Moon system that
simply doesn't
exist today.
Now researchers have harnessed the latest in computing power to
provide the
most detailed model ever made of the cosmic scene that supposedly
created
the Moon. The result, a 3-D animation of the blast and subsequent
chaos, is
comforting. It shows that the Moon could have formed when a
Mars-sized
object hit a fully formed Earth.
The collision would have given Earth its spin, defined what we
now call an
equator, and put enough material into orbit at the right distance
from Earth
to allow the formation of a satellite that generations would
later swoon
over.
24 hours of chaos
Robin Canup of the Southwest Research Institute has been modeling
the Moon's
formation for eight years. On previous studies, she has worked
with William
Ward and Alastair Cameron, who represent one of two separate
research groups
that developed the original impact theory back in the mid-70s.
(William K.
Hartmann and Donald R. Davis were the other team.)
As Canup knows, all ideas about how the Moon formed must contend
with one
important fact: The Moon contains very little iron. Earth, on the
other
hand, is loaded with iron, the bulk of it tied up in the planet's
core.
So the Moon is thought to have been pieced together by the bits
that got
blown off the upper layers of Earth, as well as the outer
portions of the
object that hit Earth.
Canup's latest effort, produced with the help of Erik Asphaug of
the
University of California, Santa Cruz, is like a small scene in a
blockbuster
disaster movie -- the first 24 hours of time in the epic calamity
that made
the Moon. It is detailed in the Aug. 16 issue of the journal
Nature.
The model treats the debris created by the collision as more than
20,000
computational lumps, or particles, all of which are given their
own gravity
to play with as the cataclysm unfolds.
In a telephone interview, Canup described the day the Moon was
made:
A dark, lifeless object less than half as massive as Earth
careens around a
newborn Sun. It is one of many planet-sized bodies hoping for a
long career.
But its orbit is shaky. It's future grim. It is a character actor
on the
grand stage of the solar system, a player of great ultimate
consequence but
one destined to never see its name in lights.
This doomed "protoplanet" travels a path that crosses
the orbits of similar
objects and, ultimately, cannot last. Eventually, the nameless
protoplanet
meets up with a fledgling Earth.
It is not a head-on collision, but rather a glancing blow. The
impact
imparts what astronomers call angular momentum into the system.
It sets
Earth to spinning on its axis and creates a Moon that would go
round and
round the host planet for billions of years.
The shock of the impact strips material from the outer layers of
Earth and
the impacting object. The mostly iron cores of both bodies meld
into Earth's
core. It is like a compact car merging onto the highway and
colliding with
an S.U.V. -- glass, trim and hubcaps fly, but the two chassis
remain
hopelessly tangled.
All told, about 2 percent of the combined mass of the objects --
mostly
rocky stuff that's largely bereft of iron -- begins to orbit the
Earth.
About half of this eventually becomes the Moon.
Some of the stripped material is heated so fantastically that it
vaporizes
and expands into the surrounding vacuum of space.
"The material that was vaporized expands into a cloud that
envelops the
whole planet," Canup explained.
Meanwhile, a long arm of solid matter is winging its way around
Earth. Some
of it develops into a clump that slams back into the planet. The
rest is
flung into orbit, all pretty much along a plane that mimics the
path of the
incoming object. This plane slices through what is now Earth's
equator, and
it is roughly the same plane along which the Moon orbits.
"The object came in and hit, and that's what set the Earth's
rotation and
what its equator would be," Canup said.
The model assumes Earth was not spinning before the impact,
though it might
have been. If it were already spinning, Canup said the model
could be
tweaked to account for that fact and would still work.
"For the first time, we demonstrated with simulations that a
single impact
can give you an iron-depleted Moon of the right mass, and the
current mass
of the Earth, and the current angular momentum of the Earth-Moon
system,"
Canup said.
Though the model covers only a day's time, Canup said shortly
thereafter the
material in outer regions began to cool. Gradually, small clumps
would have
formed, collided with one another, and grown. Based on other
models, she
said it would have taken between 1 and 100 years to make a Moon
after the
impact.
Case is not airtight
The new model is a significant improvement over previous efforts,
which
treated gravity as an overall issue or worked with no more than
3,000
computational lumps. But it is just one step toward a fuller
understanding
of what really happened.
Jay Melosh, a University of Arizona researcher who is known for
his work in
modeling asteroid impacts, told SPACE.com the new model is an
incremental
step rather than a trailblazing one. And there are outstanding
questions
about some assumptions made.
"Their case is not airtight," Melosh said.
In a review of the work that also appears in Nature, Melosh
argues that the
real promise is in how computers are becoming powerful enough to
handle the
complicated scenario of a such a colossal impact.
"Not only does such a collision involve all the details of
shock physics,
melting and vaporization, but the mutual interactions of all
those hot
fluids squirting around in space have to be taken into
account," Melosh
writes.
He says that as with any attempt to model the Moon formation, the
results
hinge on an incomplete understanding of how the energy, density
and pressure
would affect the material of which the Earth and Moon are
composed.
And Canup acknowledges that there is not, and never will be,
direct physical
remains of the Moon-forming impacter. The ensuing drama was so
hot, and the
characters so well-mixed, that there are no ancient layered
deposits to
provide clues, as are found by people like Melosh who study
smaller and more
recent asteroid impacts.
But Melosh said the prospects for better models are promising. In
fact, he
is working with Canup and Asphaug on ways to refine the new model
to better
account for the shock and fluid dynamics. And he figures others
will soon
use the improved computing power and more capable software
packages to
produce their own scenarios.
"More studies of this kind will be published in the
not-too-distant future,"
Melosh said.
Other ways the Moon might form
The new study strengthens just one theory of how the Moon might
have formed.
Other scientists have suggested that the Moon developed elsewhere
in the
solar system and was captured by Earth. An even more remote
possibility is
that the Earth and Moon condensed together out of the material
that formed
the solar system.
Another idea is that gravitational interactions between the
Earth, the Sun,
and other developing planets simply tore Earth apart and the Moon
formed
from this debris.
But the majority of researchers prefer the impact theory. And
though a
similar impact would be extremely unlikely today, it was a fairly
common
occurrence back when the solar system was forming.
"The last stages of planetary accumulation were very
violent," Melosh said.
"An event of this type within 100 million years of the birth
of the solar
system is not rare at all."
The Moon is not the only result of this chaos. In fact, the
present spacing
between planets "evolved by a sort of natural selection
involving the demise
of intervening objects whose orbits were not so stable,"
Melosh said.
Copyright 2001, Space.com
===========
(5) HOW THE MOON WAS MADE
>From BBC News Online, 15 August 2001
http://news.bbc.co.uk/hi/english/sci/tech/newsid_1493000/1493095.stm
A new computer simulation of how the Moon was formed indicates it
is younger
than previously thought.
The simulation, the most sophisticated yet, sees a Mars-sized
body hitting
the almost fully-formed Earth around 4.5 billion years ago,
ejecting debris
which then formed the Moon.
The result is a happy one for scientists, because older
simulations did not
fit too well with observed reality.
Robin Canup of the Southwest Research Institute, Boulder,
Colorado, US and
Erik Asphaug of the University of California, Santa Cruz, were
able to use
much more powerful computers than their predecessors.
Easier job
Now they have a much easier job matching up the model to the
current orbits
and compositions of the Earth and its satellite.
As Jay Melosh of the University of Arizona explains in the
journal Nature,
the key has been not just adding raw computer power, but refining
the
simulation.
"It has taken squadrons of physicists in the United States,
Russia and
elsewhere nearly 50 years to come up with computers and
three-dimensional
computer codes that can adequately treat the effects of impacts
and
explosions under relatively simple conditions in which
self-gravity is not
important.
"Adding self-gravity to these codes therefore posed a
formidable challenge,"
he writes.
Three dimensions
The new model is the highest-resolution computer model so far of
the birth
of the Moon.
It takes into account in three dimensions both the thermodynamic
effects of
another planet hitting the Earth and the gravitational
interactions between
all the pieces which were dislodged.
Previous simulations have left scientists conjecturing a much
bigger
impacting planet, a much smaller Earth or even more than one
collision.
They pointed, too, to an earlier collision, when the Earth was
less fully
formed.
Had this really taken place, the Moon would have probably been
much more
iron-rich than it really is.
Details of the research are published in the journal Nature.
Copyright 2001, BBC
==============
(6) DIAMONDS IN THE SKY: CLUES TO EARLY HISTORY OF SOLAR SYSTEM'S
OLDEST
DIAMONDS
>From Andrew Yee <ayee@nova.astro.utoronto.ca>
Press and Public Relations Department
Max Planck Society for the Advancement of Science
Munich, Germany
Contact:
Dr. Ulrich Ott
Max Planck Institute for Chemistry, Mainz
Phone: +49 6131 305-366
Fax: +49 6131 305-575
E-mail: ott@mpch-mainz.mpg.de
August 09, 2001
Clues to early history of Solar System's oldest diamonds
Simulating implantation of noble gases into terrestrial diamond
grains,
scientists from the Karpov Institute for Physical Chemistry
(Moscow, Russia)
and the Max Planck Institute for Chemistry (Mainz, Germany) infer
a sequence
of events in the early life of presolar diamonds in meteorites,
the most
common form of stardust available for laboratory study (Nature,
August 9,
2001).
A most exciting event in the study of meteorites has been the
discovery that
many primitive meteorites contain stardust -- grains of presolar
origin,
older than the Solar System itself. Formed as circumstellar
grains around
dying stars, and having survived all subsequent events in the
interstellar
medium and in the Solar System, they carry information about the
processes
by which chemical elements are created in stars
(nucleosynthesis). In turn,
from our concepts about nucleosynthesis we can infer the stellar
sources of
the grains. Complementing previous analyses of stardust diamonds
by the Max
Planck group, scientists from the Karpov Institute of Physical
Chemistry and
from the Max Planck Institute for Chemistry have studied the
introduction of
diagnostic trace elements into terrestrial analog diamonds. The
results are
used to draw conclusions about the early history of "stellar
diamonds"
(Nature, August 9, 2001).
Presolar grains known to be present in meteorites are thermally
and
chemically extremely stable minerals such as diamond, graphite,
silicon
carbide, corundum (aluminum oxide) and silicon nitride. Although
discovered
first and by far the most abundant (ca. 1 per mill by weight in
the most
primitive meteorites), the diamonds are the least understood, and
their very
identification as being presolar is based on the isotopic
composition of
trace elements they carry. Noble gases have played a special role
among
these trace elements, and it is primarily the unusual isotopic
composition
of xenon -- ca. 100% enrichment of the lightest and heaviest
isotopes --
which suggests that they came from supernova explosions.
How, when, and where introduction of xenon and other trace
elements occurred
may provide crucial information on formation and early history of
the
diamond grains, and there are strong indirect arguments that
introduction
was by implantation of ions. To test the viability of the
process, a
simulation experiment was performed: a noble gas mixture
consisting of
helium, argon, krypton and xenon ions with an energy of 700
electronvolts
was implanted into a layer of terrestrial nanodiamonds of similar
size as
the presolar nanodiamonds (extremely small, only a few nanometer;
see
figure), and after irradiation the release of the implanted noble
gases was
studied. Surprisingly, release as a function of temperature was
bimodal,
with one peak in the 200-700 C range and another one above 1000
C. This
situation -- after a single implantation -- at first glance is
similar to
the case of the meteoritic nanodiamonds, there is a complication,
however.
In the case of the "stellar" diamonds differences in
isotopic composition
demand that at least two different events must have been involved
in the
introduction of noble gases: isotopically unremarkable noble
gases are
released primarily at low temperature, gases of presumably
supernova origin
at higher temperature.
If indeed, ion implantation is the mechanism by which trace
elements were
introduced into stardust diamonds and if, as the simulation study
suggests,
ion implantation results in the gases being located in two
different sites
within the diamonds of different thermal stability, the following
sequence
of events seems required:
* formation of diamonds presumably by chemical vapor deposition;
* irradiation of the diamonds (or a subfraction of them) with
supernova trace elements;
* loss of the less retentively held fraction of implanted
supernova material;
* irradiation of the diamonds at some later time (or of a
different
subfraction at an unspecified time) with trace elements of
commonplace
isotopic composition, possibly in the interstellar medium
or the early
Solar System;
* no more exposure to elevated temperature for any significant
length of time
(e.g. no more than ca. 10,000 years at more than 100 C).
A second important information from the implantation study is
that the more
retentively sited gases are isotopically fractionated relative to
the
starting composition. How this may have affected the inferred
abundance and
isotopic composition of the supernova implants into the stardust
diamonds
and how important the resulting changes are for the inferred
nuclear
processes remains to be worked out in detail.
IMAGE CAPTION: [ http://www.mpg.de/news01/news0114.htm
(208KB)]
Presolar diamond grain observed by transmission electron
microscopy. High
resolution image -- obtained by F. Banhart (then at Max Planck
Institute for
Metal Research, Stuttgart) -- shows the crystallographic[III]
planes
(distance 0.206 nanometer) of a typical-sized grain.
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