PLEASE NOTE:
*
CCNet, 030/2000 - 10 March 2000
-------------------------------
QUOTE OF THE DAY
"What is surprising is the
reversion from a benign to a violent
solar system about 500 million years
ago. This work opens up a new
field that tells us something about the
history of our solar
system that was totally unanticipated.
Until now we did not
realize how peculiar the past 500
million years has been."
--Richard A.
Muller, UC Berkeley, 9 March 2000
(1) LUNAR IMPACT RATE HAS INTENSIFIED IN THE PAST
500 MILLION YEARS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) COSMIC CATASTROPHES & TERRESTRIAL EVOLUTION
SpaceDaily, 9 March 2000
(3) RESEARCHERS OVERCONCERNED ABOUT HUMAN-CAUSED EXTINCTIONS
Andrew Yee <ayee@nova.astro.utoronto.ca>
(4) FURTHER DOUBTS ABOUT GREENHOUSE SCARE: ATMOSPHERIC
POLLUTION MAY CANCEL OUT GLOBAL WARMING
Andrew Yee <ayee@nova.astro.utoronto.ca>
(5) THE END IS NIGH - AGAIN
SPACE.COM, 9 March 2000
(6) BRITISH ROCKETRY ORAL HISTORY PROGRAMME
Dave Wright <L.F.Wright@livjm.ac.uk>
(7) DIVERSITY OF COMETS
R.R. Weissman, CALTECH,JET PROP LAB
(8) NO PERIODICITY IN TERRESTRIAL IMPACT CRATER RECORD
L. Jetsu*) & J. Pelt, UNIVERSITY OF
HELSINKI OBSERVATORY
(9) REDUCED ESTIMATE OF THE NUMBER OF KM-SIZED NEAS
D. Rabinowitz*) E. Helin, K. Lawrence, S.
Pravdo, YALE UNIVERSITY
(10) ASTEROID 1998 KY26 - SPEED OF ROTATION
Richard TAYLOR <richard.taylor3@virgin.net>
(11) VOLCANO MAY CHILL
Bob Kobres <bkobres@uga.edu>
=============
(1) LUNAR IMPACT RATE HAS INTENSIFIED IN THE PAST
500 MILLION YEARS
From Andrew Yee <ayee@nova.astro.utoronto.ca>
University of California-Berkeley
Contact: Robert Sanders, (510) 642-6998, rls@pa.urel.berkeley.edu
NEWS RELEASE: 03/09/00
Meteoroid bombardment of moon has intensified in past 500 million
years, coinciding with blossoming of life on Earth
BERKELEY -- A new chronology of meteoroid impacts on the moon
shows
some surprising correlations with major biological events on
Earth.
By dating minute glass beads thrown out by impacts over the
millennia, scientists at the University of California, Berkeley,
and
the Berkeley Geochronology Center have not only confirmed
expected
intense meteor activity 4 to 3.5 billion years ago, when the
large
lunar seas or maria were formed, but have discovered another peak
of
activity that began 500 million years ago and continues today.
The tapering off of the first peak of activity, which probably
included many large comets and asteroids, coincides with the
earliest
know evidence of life on Earth. The second and ongoing peak,
which
from the evidence seems to have been mostly smaller debris, began
around the time of the great explosion of life known as the
Cambrian.
"The first life on Earth arose just after this real
crescendo around
3.5 billion years ago," said Paul R. Renne, adjunct
professor of
geology and geophysics at UC Berkeley and director of the
Berkeley
Geochronology Center. "Maybe life began on Earth many times,
but the
meteors only stopped wiping it out about 3 billion years
ago."
The more recent and ongoing activity is even more intriguing.
"It's not surprising that the impacts tapered off about 3
billion
years ago. The solar system was just getting cleaned up,
primarily by
Jupiter and the Sun," said Richard A. Muller, a professor of
physics
at UC Berkeley and a research physicist at Lawrence Berkeley
National
Laboratory. "What is surprising is the reversion from a
benign to a
violent solar system about 500 million years ago.
"This work opens up a new field that tells us something
about the
history of our solar system that was totally unanticipated. Until
now
we did not realize how peculiar the past 500 million years has
been."
UC Berkeley graduate student Timothy S. Culler, along with Renne,
Muller and Timothy A. Becker, laboratory manager at the Berkeley
Geochronology Center, report their findings in the March 10 issue
of
the journal Science.
Though all the Berkeley researchers agree on the new impact
chronology for the moon, they have their own ideas about its
implications.
Renne, for example, leans toward the theory that interstellar
dust
seeded the Earth with organic molecules, from water to amino
acids,
that were incorporated into life on Earth during the past 500
million
years.
"Life already here would suddenly have a new stimulus, a
greater need
to evolve quickly and more raw material to do it," Renne
said.
"Impacts would have to be really, really big and really,
really
frequent to be deleterious to life on Earth, and it's clear that
the
flux over the past 500 million years has been relatively small
objects. We don't see a lot of young large craters on the moon.
We've
come to accept the idea that impacts are strictly bad news for
life
on Earth, but now that's not so clear."
Culler, the graduate student who originated the project under the
supervision of Muller and Renne, sees the intense meteor activity
as
evidence that large meteor impacts played a major role in the
evolution and extinction of life.
"It shows that large impacts may have been more frequent in
the last
500 million years, creating more extinctions, like the comet or
asteroid that wiped out the dinosaurs 65 million years ago,
" Culler
said. "Even a number of smaller impacts can have a
disastrous effect
on the atmosphere and cause mass extinctions."
Muller too emphasizes the role impacts have played in the history
of
life on Earth. It's not surprising that the recent intense period
of
meteor activity coincides with the rapid radiation of life on
Earth,
he said.
"We're only beginning to realize the role played by
catastrophe in
the evolution of life," he said. "When it comes to
survival of the
fittest, it's not only the ability to compete with other species
that
counts, but also the ability to survive occasional catastrophe.
That
requires complexity and flexibility."
Muller has proposed several controversial theories about the
solar
system, including that the sun has an unseen companion star, one
he
calls Nemesis, that orbits the sun every 26 million years and
periodically knocks comets out of their orbits, sending them
hurtling
toward the inner solar system. He also has proposed that periodic
climate changes are the result of the Earth's orbit periodically
tilting up out of the orbital plane of the planets and
intersecting a
cloud of dust, debris and meteoroids.
The current research was suggested by Muller in 1991, in part as
a
way to determine whether the moon's impact record shows evidence
of a
26 million-year cycle. Muller hit upon the idea of
argon-40/argon-39
dating of lunar spherules as a way to get a more precise
chronology
of the intensity of bombardment of the moon and, by implication,
the
Earth.
"I realized that we didn't have to go to the individual
craters in
order to determine their age, because the craters sent samples to
us," Muller said. "We could obtain samples of hundreds
of different
craters from just one location, without having the expense of
going
back to the moon. This idea is likely to open up a completely new
round of lunar analysis."
Spherules are mostly basaltic glass, Culler said, created when a
meteor hits the surface and generates intense heat that melts the
rock and splatters it outward. As droplets of molten rock fall
back
to the surface they quickly cool to form a glass, much like
obsidian.
Culler, Becker and Renne analyzed 155 beads from one gram of
lunar
soil picked up in 1971 by Apollo 14 from the Fra Mauro formation
-- a
lunar highland bordering Mare Imbrium. The mineral composition of
each bead was determined with a microprobe before it was laser
melted
and the argon gas captured for isotopic analysis.
Contrary to assumptions, they found that the cratering rate on
the
moon has not been constant over its history. Approximately twice
as
many impacts occurred between 4 and 3 billion years ago as
occurred
between 2 and 1/2 billion years ago. About 500 million years ago
the
intensity of impacts increased nearly to what it was at the peak
of
activity 3.2 billion years ago.
Though the dating method was not sensitive enough to reveal a 26
million-year cycle in the impact record, "these findings fit
in
nicely with the Nemesis theory," Muller said. "I think
most of the
debris came from perturbations in the outer solar system by
Nemesis."
For the future, Renne says, it is "critical to launch new
lunar
sampling missions targeted to areas rich in potassium," in
order to
confirm the results and probe further back into the moon's
history.
The project was funded by the Ann and Gordon Getty Foundation,
through the Berkeley Geochronology Center and Richard Muller.
NASA
provided the lunar samples.
IMAGE CAPTION:
[http://www.urel.berkeley.edu/urel_1/CampusNews/PressReleases/releases/impactfoto.html]
Scanning electron microscope picture of a glass spherule brought
back
from the moon by Apollo 11. In the upper left side of the
spherule
can be seen a miniature crater (a zap pit) caused by an extremely
small impact on the spherule. Within the zap pit is a central
glassy
area melted by the impact. The fragmented area around the glassy
pit
-- the spall zone -- was caused by shockwaves from the miniature
impact. The zap pit is about 100 microns across; the spherule
itself
is about 250 microns in diameter. Magnification is 320X. PHOTO
CREDIT: Tim Culler/UC Berkeley
========
(2) COSMIC CATASTROPHES & TERRESTRIAL EVOLUTION
From SpaceDaily, 9 March 2000
http://www.spacedaily.com/spacecast/news/life-00h1.html
Apollo Lunar Rocks Reveal Importance Of Impact Events On
Evolution
Berkeley - March 9, 2000 - A new chronology of meteoroid impacts
on
the moon shows some surprising correlations with major biological
events on Earth.
By dating minute glass beads thrown out by impacts over the
millennia, scientists at the University of California, Berkeley,
and
the Berkeley Geochronology Center have not only confirmed
expected
intense meteor activity 4 to 3.5 billion years ago, when the
large
lunar seas or maria were formed, but have discovered another peak
of
activity that began 500 million years ago and continues today.
The tapering off of the first peak of activity, which probably
included many large comets and asteroids, coincides with the
earliest
known evidence of life on Earth. The second and ongoing peak,
which
from the evidence seems to have been mostly smaller debris, began
around the time of the great explosion of life known as the
Cambrian.
"The first life on Earth arose just after this real
crescendo around
3.5 billion years ago," said Paul R. Renne, adjunct
professor of
geology and geophysics at UC Berkeley and director of the
Berkeley
Geochronology Center. "Maybe life began on Earth many times,
but the
meteors only stopped wiping it out about 3 billion years
ago."
FULL STORY at: http://www.spacedaily.com/spacecast/news/life-00h1.html
============
(3) RESEARCHERS OVERCONCERNED ABOUT HUMAN-CAUSED EXTINCTIONS
From Andrew Yee <ayee@nova.astro.utoronto.ca>
University of California-Berkeley
Contact: Robert Sanders, (510) 642-6998, rls@pa.urel.berkeley.edu
NEWS RELEASE: 3/9/00
New study suggests humans will not live long enough to see Earth
recover from a mass extinction
BERKELEY -- The Earth needs, on average, about 10 million years
to
recover from a mass extinction of the planet's species, far
longer than
most scientists thought, according to a new study by scientists
at the
University of California, Berkeley, and Duke University.
Moreover, the recovery time is the same whether the global
die-off
involves the loss of most life on Earth or wipes out far fewer
species.
This unexpected finding has major implications for the Earth's
fate as
human activity threatens species around the globe.
"People have argued that we only have to worry about
human-caused
extinctions if we do something that causes the loss of 80 or 90
percent
of species on the planet," said UC Berkeley environmental
scientist
James W. Kirchner.
"Our analysis shows that even if the human impact is much
smaller than
that -- 20 or 30 or even 50 percent of species -- it's still
going to
take 10 million years for the Earth to recover. That is
well past the
expected life span of the human species, or even of the genus
Homo."
"Extinctions caused by humans don't have to be large to have
an effect
that reverberates in the ecosystem for tens of millions of
years," said
paleontologist Anne Weil, a former doctoral student at UC
Berkeley and
now a research associate in the Department of Biological
Anthropology
and Anatomy at Duke University.
Weil and Kirchner, a professor of geology and geophysics at UC
Berkeley, report their findings in the March 9 issue of Nature.
During the past half billion years, life on Earth has blossomed
and
crashed many times -- some die-offs the result of geologic
cataclysm,
but most of unknown cause.
Paleontologists have known that the Earth needed a long time to
recover
from large extinctions, such as that which occurred at the end of
the
Permian 250 million years ago, when more than 90 percent of all
species
died out. The debate was over the smaller, background extinctions
that
pepper the fossil record, in which 10 to 20 percent of species
died
out.
"The presumption has been that while big extinctions require
a long
recovery, the biosphere should bounce right back after smaller
extinctions," Kirchner said.
One critical reason for wanting an answer to this question is
that
human activity now is eliminating many species each year in what
some
see as a major, human-caused extinction event.
"We don't know what our current level of extinction is, but
some
biologists estimate that eliminating 90 percent of tropical rain
forests would lead to the extinction of half the species on
Earth,"
Kirchner.
Scientists despaired of an answer to the question because the
fossil
record is full of holes. Kirchner, however, came across an
analysis
technique used in astrophysics that can be used to compare the
rates of
extinction and of evolution in a spotty fossil record.
Using a database compiled by the late University of Chicago
paleontologist Jack Sepkoski, Weil and Kirchner compared the rate
of
extinction of fossil marine organisms with the rate of
evolution or
"origination" over the past 530 million years.
They found that evolution rebuilds biodiversity very slowly after
extinction events. Looking at all the biotic crises during this
period,
they found an average of 10 million years between an extinction
and a
subsequent flourishing of life.
When they eliminated the five mass extinctions from that period,
including the extinction 65 million years ago when the dinosaurs
died
out, the smaller background extinctions also averaged a 10
million-year
recovery period.
"People are really excited about this from an evolutionary
perspective," Weil said. "This shows that long recovery
time is a
previously unrecognized feature of the fossil record, evidence of
an
evolutionary dynamic we didn't suspect before."
Weil said that the long recovery period is perhaps not
surprising.
While a species' extinction does not leave an empty niche, it
destroys
all or part of other niches. For example, it removes
opportunities for
predators, parasites and other organisms that would normally rely
on
that species. Extinction also is a lineage termination, which
removes a
potential ancestor from which other species could evolve.
"Extinction is a double whammy. You not only take out the
ecological
niche as you take out the species, but you take away the
evolutionary
potential for radiation, too," Weil said.
"Diversification takes a long
time to ramp up."
Such complexity may also be the reason that the size of the
extinction
is relatively unimportant in determining the time it takes the
ecosystem to recover. Extinction totally alters the ecosystem,
making
recovery a slow process no matter what the scale.
"The post-extinction ecosystem is not anything like the
pre-extinction
ecosystem, so life will never recover to the same place,"
Weil said.
"Life reaches a new but different plateau."
Kirchner notes that their sobering findings don't necessarily
imply a
stark fate for the Earth.
"It is not preordained that high levels of human-caused
extinction have
to happen," Kirchner said. "Our future depends on what
we choose to do
on a national and international level, as a society. Those
decisions
are critical because they will have very long-lasting
consequences.
"If we deplete Earth's biological diversity, we will leave a
biologically impoverished planet, not only for our children and
our
children's children, but for all the children of our species that
there
will ever be."
The work was supported by grants from the University of
California and
the National Science Foundation.
----
[MODERATOR'S NOTE: I'm sorry, but I don't buy this bizarre scare
story
widely reported in science media around the globe. In the past,
the
recovery from mass extinction events may have indeed lasted, on
average, some 10 million years. I fail to see any 'major
implications'’
this knowledge has in store for our own future. After all, the
inference that today's planet Earth will also need 10 million
years to
"recover" from the current human-caused extinctions is
incoherent in a
number of ways: Firstly, extinct species do not recover after an
extinction event. They simply become extinct - full stop. Quite
frankly, nature doesn't care that much about the Dodos et al.
Even if,
as some ecological prophets of doom predict, half of the world's
current species were to become extinct as a result of
deforrestation,
that would not necessarily lead to impoverished living standards
for the
human population. In fact, most human-caused extinctions in the
past
200 years or so have coincided with continuously risinging living
standards and better health conditions for human populations
around the
globe. And lastly, we have just begun to experiment on reviving
the
mammoth by means of genetical engineering. If these and other
programmes that try to breath new life into extinct species
succeed -
and I have little doubt that they will prevail some day - we will
be
able to intervene in and manipulate more favourably the Earth's
biological diversity in the future. In short, there is really
little to
worry about here, as far as I can see.
============
(4) FURTHER DOUBTS ABOUT GREENHOUSE SCARE: ATMOSPHERIC
POLLUTION MAY CANCEL OUT GLOBAL WARMING
From Andrew Yee <ayee@nova.astro.utoronto.ca>
Office of News Services
University of Colorado-Boulder
354 Willard Administrative Center
Campus Box 9
Boulder, Colorado 80309-0009
(303) 492-6431
Contact:
Owen B. Toon, (303) 492-1534
Jim Scott, (303) 492-3114
March 9, 2000
AEROSOL POLLUTANTS IN ATMOSPHERE LIKELY MASKING GREENHOUSE
WARMING
New observations with space-borne instruments by Daniel Rosenfeld
of
the Hebrew University of Jerusalem showing that aerosols
over land can
modify clouds, suppress precipitation and reflect light is a
"huge leap
forward" in understanding the interplay among pollutants,
clouds and
greenhouse warming, said University of Colorado Professor Owen B.
Toon.
A professor in CU-Boulder's Laboratory for Atmospheric and Space
Physics, Toon authored a perspective article in the March 10
issue of
Science magazine tied to a research paper by Rosenfeld in the
same
Science issue on his new findings. Rosenfeld's research shows
that
aerosol particulates from urban and industrial areas modify
clouds over
large land areas, suppress rain and snow and are responsible for
reflecting significant amounts of sunlight back to space.
Toon, who has studied similar modification of clouds over the
ocean and
their effects, said aerosol particulates in the atmosphere are
reflecting sunlight back into space, directly cooling Earth. The
new
findings by Rosenfeld are more evidence that aerosol-cloud
processes
may be diluting and perhaps even canceling out greenhouse
warming, Toon
said.
Unlike greenhouse gases -- which stay in the atmosphere for long
periods and are fairly evenly distributed -- aerosols are more
concentrated near their sources and variable in space and time,
making
it difficult to quantify their impacts. "But their cooling
effect may
be as large as the warming effect created by humans pumping
greenhouse
gases into the atmosphere during the last century," said
Toon, also a
professor in CU-Boulder's Program for Atmospheric and Ocean
Sciences.
"Computer models could not explain the lack of warming that
had been
predicted because they were overlooking the opposing forces due
to
aerosol pollution," Toon said.
In the 1980s, long, bright lines in ocean clouds seen from the
air were
determined to be the tracks of ships, which were sending exhaust
into
the atmosphere, including low-lying cloud decks. Because the
particulates triggered new cloud formation and redistributed
water in
existing clouds, they became brighter and more reflective, Toon
said.
In 1998, Rosenfeld detected similar "pollution tracks"
over land areas
of Earth for the first time. "We had known that aerosol
particulates
emitted by ships can trigger changes in clouds and even their
formation
over the ocean," said Toon. "What Rosenfeld has done in
his latest
paper is to use satellite data to demonstrate a clear, widespread
influence of aerosol pollution on continental precipitation.
"
Rosenfeld used data taken by Advanced Very High Resolution
Radiometers
onboard several U.S. weather satellites and NASA's new TRMM
satellite
for his Science study. The images showed pollution tracks caused
by
urban and industrial activity in areas of Turkey, Australia and
Canada.
They also indicated that clouds making up the pollution tracks
were
prohibiting rain and snow from falling downwind from the sites.
Since each cloud droplet must form on a pre-existing particle,
additional aerosols in clouds like sulfates or sulfuric acid
increase
the number of water droplets in clouds, said Toon. Because
temperatures
and atmospheric motions driving cloud formation control the mass
of
water condensing in the clouds, the droplets formed on aerosol
particulates tend to be smaller in size.
Clouds harboring smaller droplets have larger surface areas,
making
them more reflective and sending more sunlight back to space,
said
Toon. Because of their diminutive size, the chances of the
droplets
coagulating into raindrops large enough to fall as precipitation
are
greatly diminished.
A typical cloud droplet, nearly microscopic, can travel little
more
than an inch through dry air before evaporating. "About 1
million cloud
droplets must collide and coalesce in order to form a
precipitation-sized drop," said Toon, who noted that a
typical raindrop
-- which is about the size of a rice grain -- can fall a mile
before
evaporating.
The rate at which droplets collide and coalesce depends on their
size
and the number of other similar droplets in their path, he said.
Normal-sized droplets in an unperturbed cloud would sweep up
about 64
times the volume of air containing other droplets than would
droplets
half that size inside a polluted cloud. This would make the
aerosol-filled cloud much less likely to rain, said Toon.
One of Rosenfeld's satellite images shows a pollution track
emanating
from a mining and smelting company in Flin-Flon, Manitoba.
Another
shows pollution tracks from several sources near Istanbul,
Turkey. A
third image shows a track originating in the vicinity of a brown
coal
power plant, the world's largest smelter and refinery, a huge
cement
plant and a major oil refinery near Adelaide, South Australia.
Other areas of the world are at least as tainted with aerosols.
But
pollution tracks from huge, nearly adjacent cities such as
those in
the Northeast United States, for example, are virtually invisible
because of the perpetual pollution that hangs in the atmosphere,
he
said.
"Rosenfeld's work points to locales where in situ
observations should
be made to pinpoint the mechanisms by which pollution affects
clouds,"
Toon concluded in the Science article. "Such knowledge may
allow us to
estimate how widespread the aerosol interaction with cloud
precipitation may be in our globally polluted world."
================
(5) THE END IS NIGH - AGAIN
From SPACE.COM, 9 March 2000
http://www.space.com/science/solarsystem/planet_alignment_000309.html
Doomsayers charge that the gravitational effects of an upcoming
planetary alignment will wreak havoc on Earth. Astronomers call
the
idea absurd.
FULL STORY at
http://www.space.com/science/solarsystem/planet_alignment_000309.html
==========
(6) BRITISH ROCKETRY ORAL HISTORY PROGRAMME
From Dave Wright <L.F.Wright@livjm.ac.uk>
The British Rocketry Oral History Programme
Annual Conference at Charterhouse School, April 6/7th
Co- Sponsored by the British Interplanetary Society
and with Support from Matra Marconi Space
The conference offers a dozen academic papers on a variety of
topics.
There will also be a witness seminar with individuals who worked
on
the Black Knight programme in the 50s and early 60s discussing
their
experiences. The conference is a convivial setting with a chance
to
learn about Britain’s impressive scientific and engineering
contribution in Aerospace. Those enjoying the conference will be
a
mix of Scientists, Engineers, Historians, academics and
enthusiasts.
Papers include:
Robert Baker - The University of London - The Wilson Government
Space
Policy.
Doug Bateman - ex-DERA - WE 177.
Dr John Becklake - A summary of the history of British Rockets up
to
1945.
John Bonser - Waverider research.
Dr. Ben Cole - The personalities of the Blue Streak cancellation
1960.
Dr. Wayne Cocroft - English Heritage - Industrial Archaeology.
Roy Dommett CBE - Chief Consultant DERA - Black Knight re-entry
experiments.
Terry Dike - ex AVRO - Blue Steel the formative years.
Guy Finch - University of Wales Aberystwyth - Red Cat and the
origins
of Blue Steel.
John Harlow MBE - ex Westcott- The Delta engine.
Duncan Lunan - Terrence Nonweiler and Waverider research.
Doug Millard - Science Museum - Black Prince.
Gill Staerck - London University - Defence Policy in the
Black
Knight period.
Dick Stratton - Project Manager - SR 53 Rocket powered
interceptor..
On the afternoon of the 7th there will be a forum on Space
Heritage
and Education issues. Groups with an interest in Aerospace
Heritage
and/or Education will have a chance to discuss their activities.
We
believe that this will be a chance to learn about a wide range of
exciting initiatives. For further details:- L.F.WRIGHT@LIVJM.AC.UK
or
phone 0151-281-1134
==========
(7) DIVERSITY OF COMETS
R.R. Weissman: Diversity of comets: Formation zones and dynamical
paths. SPACE SCIENCE REVIEWS, 1999, Vol.90, No.1-2, pp.301-311
*) CALTECH,JET PROP LAB,DIV EARTH & SPACE SCI,MAIL STOP
183-601,4800
OAK GROVE DR,PASADENA,CA,91109
The past dozen years have produced a new paradigm with regard to
the
source regions of comets in the early solar system. It is now
widely
recognized that the likely source of the Jupiter-family
short-period
comets (those with Tisserand parameters, T > 2 and periods: P,
generally < 20 years) is the Kuiper belt in the ecliptic plane
beyond
Neptune. In contrast, the source of the Halley-type and
long-period
comets (those with T < 2 and P > 20 years) appears to be
the Oort
cloud. However, the corners in the Oort cloud almost certainly
originated elsewhere, since accretion is very inefficient at such
large heliocentric distances. New dynamical studies now suggest
that
the source of the Oort cloud comets is the entire giant planets
region from Jupiter to Neptune, rather than primarily the
Uranus-Neptune region, as previously thought. Some fraction of
the
Oort cloud population may even be asteroidal bodies formed inside
the
orbit of Jupiter. These comets and asteroids underwent a complex
dynamical random walk among the giant planets before they were
ejected to distant orbits in the Oort cloud, with possible
interesting consequences for their thermal and collisional
histories.
Observational evidence for diversity in cometary compositions is
limited, at best. Copyright 2000, Institute for Scientific
Information Inc.
===================
(8) NO PERIODICITY FOUND IN TERRESTRIAL IMPACT CRATER RECORD
L. Jetsu*) & J. Pelt: Spurious periods in the terrestrial
impact
crater record. ASTRONOMY AND ASTROPHYSICS, 2000, Vol.353, No.1,
pp.409-418
*) UNIVERSITY OF HELSINKI OBSERV,POB 14,HELSINKI 00014,FINLAND
We present a simple solution to the controversy over periodicity
in
the ages of terrestrial impact craters and the epochs of mass
extinctions of species. The first evidence for a 28.4 million
year
cycle in catastrophic impacts on Earth was presented in 1984. Our
re-examination of this earlier Fourier power spectrum analysis
reveals that the rounding of the impact crater data distorted the
Monte Carlo significance estimates obtained for this cycle. This
conclusion is confirmed by theoretical significance estimates
with
the Fourier analysis, as well as by both theoretical and Monte
Carlo
significance estimates with the Rayleigh method. We also apply
other
time series analysis methods to six subsamples of the currently
available more extensive impact crater record and one sample of
mass
extinction epochs. This analysis reveals the spurious
'human-signal'
induced by rounding. We demonstrate how the data rounding
interferes
with periodicity analysis and enhances artificial periodicities
between 10 and 100 million years. Only integer periodicities
connected to irregular multimodal phase distributions reach a
significance of 0.001 or 0.01. We detect no real periodicity in
the
ages of terrestrial impact craters, nor in the epochs of mass
extinctions of species. Copyright 2000, Institute for Scientific
Information Inc.
==================
(9) REDUCED ESTIMATE OF THE NUMBER OF KM-SIZED NEAS
D. Rabinowitz*) E. Helin, K. Lawrence, S. Pravdo: A reduced
estimate of
the number of kilometre-sized near-Earth asteroids. NATURE, 2000,
Vol.403, No.6766, pp.165-166
*) YALE UNIV,DEPT PHYS,266 WHITNEY AVE,JWG 552,NEW HAVEN,CT,06511
Near-Earth asteroids are small (diameters < 10 km), rocky
bodies with
orbits that approach that of the Earth (they come within 1.3 AU
Of
the Sun). Most have a chance of approximately 0.5% of colliding
with
the Earth in the next million years. The total number of such
bodies
with diameters > 1 km has been estimated to be in the range
1,000-2,000, which translates to an approximately 1% chance of a
catastrophic collision with the Earth in the next
millennium(1,2).
These numbers are, however, poorly constrained because of the
limitations of previous searches using photographic plates. (One
kilometre is below the size of a body whose impact on the Earth
would
produce global effects(3).) Here we report an analysis of our
survey
for near-Earth asteroids that uses improved detection
technologies.
We find that the total number of asteroids with diameters > 1
km is
about half the earlier estimates. At the current rate of
discovery of
near-Earth asteroids, 90% will probably have been detected within
the
next 20 years. Copyright 2000, Institute for Scientific
Information
Inc.
=============================
* LETTERS TO THE MODERATOR *
=============================
(10) ASTEROID 1998 KY26 - SPEED OF ROTATION
From Richard TAYLOR < richard.taylor3@virgin.net
>
Dear Benny,
Andrew Yee's report of the confirmatory observations of the
'rapid'
rotation of 1998 KY26 at 10.7 minutes and a stated ~30 metre
diameter
spheroidal shape the result of collisions with other asteroids
raises a
question that appears to have been overlooked.
Most of the comments I have seen focus on the short day-length
and the
spin rate is taken as being unusually fast. Here we would suggest
that
the spin rate is inexplicably slow. If we take the shape of 1998
KY26
as a sphere the equatorial circumpherence is ~95 metres which is
equivalent to an equatorial velocity of ~0.15 metres per second.
If we
assume that the spheroidal shape is the result of impacts with
other
asteroidal or meteoritic bodies it appears that these must have
largely
been low-incidence collisions, otherwise so small a body wiould
have
been fragmented and dispersed.
This suggests that the rate of spin of 1998 KY26 would be derived
from
collisions and could be expected to spin very much faster -
perhaps by
a factor of one to several orders of magnitude as any grazing
impacts
would be at velocities of kilometres per second rather than
centimetres
per second the observed equatorial velocity for 1998 KY26.
Thinking about this it suggests that all the small irregularly
shaped
asteroids have much lower rates of soin than might be expected
for
collision origin. What has slowed them? Or is there some other
explanation for their low angular momentum density?
Richard Taylor
================
(11) VOLCANO MAY CHILL
From Bob Kobres < bkobres@uga.edu
>
RE: SOMETHING ELSE (NOT) TO WORRY ABOUT: VOLCANO MAY CHILL GLOBAL
TEMPERATURES
http://abob.libs.uga.edu/bobk/ccc/cc030800.html
Benny I think the parenthetic NOT is misplaced here. Volcanoes
can
be, and have been, hard on human health. Unfortunately about the
only
thing we are apt to be able to do to mitigate this type of
inevitable
perturbation of the biosphere is to ensure that enough vittles
are
available to augment what might be several years of rough
growing. If
we actually did keep such reserves, THEN there would be no reason
to
worry about volcanic outbursts--that is--unless you happened to
live
in close proximity to one of these hard to predict things.
http://www.abcnews.go.com/sections/science/DyeHard/dyehard.html
http://bigmac.civil.mtu.edu/home/classes/ce459/public/p14/climate.html
http://ddelene.homepage.com/volcano_climate.html
http://volcano.und.edu/vwdocs/volc_images/europe_west_asia/laki.html
Interestingly, it seems likely that the erratic 1785 weather in
Georgia (USA), as well as the haze that prompted the natives to
gather at their Stonehenge-like circle, was caused by the Laki
eruption. See:
http://abob.libs.uga.edu/bobk/yamacuta.html
Later.
bobk
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