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(meteorobs) Excerpts from "CCNet, 030/2000 - 10 March 2000"


------- Forwarded Message

From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 10 March 2000
Date: Fri, 10 Mar 2000 11:57:43 -0500 (EST)

CCNet, 030/2000 - 10 March 2000
-------------------------------

(1) LUNAR IMPACT RATE HAS INTENSIFIED IN THE PAST
    500 MILLION YEARS
    Andrew Yee <ayee@nova.astro.utorontodot ca>

[...]

(7) DIVERSITY OF COMETS
    R.R. Weissman, CALTECH,JET PROP LAB

[...]

(10) ASTEROID 1998 KY26 - SPEED OF ROTATION
     Richard TAYLOR <richard.taylor3@virgindot net>=20

[...]

=========================================================

(1) LUNAR IMPACT RATE HAS INTENSIFIED IN THE PAST
    500 MILLION YEARS

>From Andrew Yee <ayee@nova.astro.utorontodot ca>

University of California-Berkeley

Contact: Robert Sanders, (510) 642-6998, rls@pa.ureldot berkeleydot edu

NEWS RELEASE: 03/09/00

Meteoroid bombardment of moon has intensified in past 500 million=20
years, coinciding with blossoming of life on Earth

BERKELEY -- A new chronology of meteoroid impacts on the moon shows=20
some surprising correlations with major biological events on Earth.
By dating minute glass beads thrown out by impacts over the=20
millennia, scientists at the University of California, Berkeley, and=20
the Berkeley Geochronology Center have not only confirmed expected=20
intense meteor activity 4 to 3.5 billion years ago, when the large=20
lunar seas or maria were formed, but have discovered another peak of=20
activity that began 500 million years ago and continues today.

The tapering off of the first peak of activity, which probably=20
included many large comets and asteroids, coincides with the earliest=20
know evidence of life on Earth. The second and ongoing peak, which=20
from the evidence seems to have been mostly smaller debris, began=20
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=20
3.5 billion years ago," said Paul R. Renne, adjunct professor of=20
geology and geophysics at UC Berkeley and director of the Berkeley=20
Geochronology Center. "Maybe life began on Earth many times, but the=20
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=20
years ago. The solar system was just getting cleaned up, primarily by=20
Jupiter and the Sun," said Richard A. Muller, a professor of physics=20
at UC Berkeley and a research physicist at Lawrence Berkeley National=20
Laboratory. "What is surprising is the reversion from a benign to a=20
violent solar system about 500 million years ago.

"This work opens up a new field that tells us something about the=20
history of our solar system that was totally unanticipated. Until now=20
we did not realize how peculiar the past 500 million years has been."

UC Berkeley graduate student Timothy S. Culler, along with Renne,=20
Muller and Timothy A. Becker, laboratory manager at the Berkeley=20
Geochronology Center, report their findings in the March 10 issue of=20
the journal Science.

Though all the Berkeley researchers agree on the new impact=20
chronology for the moon, they have their own ideas about its=20
implications.

Renne, for example, leans toward the theory that interstellar dust=20
seeded the Earth with organic molecules, from water to amino acids,=20
that were incorporated into life on Earth during the past 500 million=20
years.

"Life already here would suddenly have a new stimulus, a greater need=20
to evolve quickly and more raw material to do it," Renne said.=20
"Impacts would have to be really, really big and really, really=20
frequent to be deleterious to life on Earth, and it's clear that the=20
flux over the past 500 million years has been relatively small=20
objects. We don't see a lot of young large craters on the moon. We've=20
come to accept the idea that impacts are strictly bad news for life=20
on Earth, but now that's not so clear."

Culler, the graduate student who originated the project under the=20
supervision of Muller and Renne, sees the intense meteor activity as=20
evidence that large meteor impacts played a major role in the=20
evolution and extinction of life.

"It shows that large impacts may have been more frequent in the last=20
500 million years, creating more extinctions, like the comet or=20
asteroid that wiped out the dinosaurs 65 million years ago, " Culler=20
said. "Even a number of smaller impacts can have a disastrous effect=20
on the atmosphere and cause mass extinctions."

Muller too emphasizes the role impacts have played in the history of=20
life on Earth. It's not surprising that the recent intense period of=20
meteor activity coincides with the rapid radiation of life on Earth,=20
he said.

"We're only beginning to realize the role played by catastrophe in=20
the evolution of life," he said. "When it comes to survival of the=20
fittest, it's not only the ability to compete with other species that=20
counts, but also the ability to survive occasional catastrophe. That=20
requires complexity and flexibility."

Muller has proposed several controversial theories about the solar=20
system, including that the sun has an unseen companion star, one he=20
calls Nemesis, that orbits the sun every 26 million years and=20
periodically knocks comets out of their orbits, sending them hurtling=20
toward the inner solar system. He also has proposed that periodic=20
climate changes are the result of the Earth's orbit periodically=20
tilting up out of the orbital plane of the planets and intersecting a=20
cloud of dust, debris and meteoroids.

The current research was suggested by Muller in 1991, in part as a=20
way to determine whether the moon's impact record shows evidence of a=20
26 million-year cycle. Muller hit upon the idea of argon-40/argon-39=20
dating of lunar spherules as a way to get a more precise chronology=20
of the intensity of bombardment of the moon and, by implication, the=20
Earth.

"I realized that we didn't have to go to the individual craters in=20
order to determine their age, because the craters sent samples to=20
us," Muller said. "We could obtain samples of hundreds of different=20
craters from just one location, without having the expense of going=20
back to the moon. This idea is likely to open up a completely new=20
round of lunar analysis."

Spherules are mostly basaltic glass, Culler said, created when a=20
meteor hits the surface and generates intense heat that melts the=20
rock and splatters it outward. As droplets of molten rock fall back=20
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=20
soil picked up in 1971 by Apollo 14 from the Fra Mauro formation -- a=20
lunar highland bordering Mare Imbrium. The mineral composition of=20
each bead was determined with a microprobe before it was laser melted=20
and the argon gas captured for isotopic analysis.

Contrary to assumptions, they found that the cratering rate on the=20
moon has not been constant over its history. Approximately twice as=20
many impacts occurred between 4 and 3 billion years ago as occurred=20
between 2 and 1/2 billion years ago. About 500 million years ago the=20
intensity of impacts increased nearly to what it was at the peak of=20
activity 3.2 billion years ago.

Though the dating method was not sensitive enough to reveal a 26=20
million-year cycle in the impact record, "these findings fit in=20
nicely with the Nemesis theory," Muller said. "I think most of the=20
debris came from perturbations in the outer solar system by Nemesis."

For the future, Renne says, it is "critical to launch new lunar=20
sampling missions targeted to areas rich in potassium," in order to=20
confirm the results and probe further back into the moon's history.

The project was funded by the Ann and Gordon Getty Foundation,=20
through the Berkeley Geochronology Center and Richard Muller. NASA=20
provided the lunar samples.

IMAGE CAPTION:
[http://www.ureldot berkeleydot edu/urel_1/CampusNews/PressReleases/releases/i=
mpactfoto.html]

Scanning electron microscope picture of a glass spherule brought back=20
from the moon by Apollo 11. In the upper left side of the spherule=20
can be seen a miniature crater (a zap pit) caused by an extremely=20
small impact on the spherule. Within the zap pit is a central glassy=20
area melted by the impact. The fragmented area around the glassy pit --
the spall zone -- was caused by shockwaves from the miniature=20
impact. The zap pit is about 100 microns across; the spherule itself=20
is about 250 microns in diameter. Magnification is 320X. PHOTO=20
CREDIT: Tim Culler/UC Berkeley

=========================================================

(7) DIVERSITY OF COMETS

R.R. Weissman: Diversity of comets: Formation zones and dynamical=20
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=20
   OAK GROVE DR,PASADENA,CA,91109

The past dozen years have produced a new paradigm with regard to the=20
source regions of comets in the early solar system. It is now widely=20
recognized that the likely source of the Jupiter-family short-period=20
comets (those with Tisserand parameters, T > 2 and periods: P,=20
generally < 20 years) is the Kuiper belt in the ecliptic plane beyond=20
Neptune. In contrast, the source of the Halley-type and long-period=20
comets (those with T < 2 and P > 20 years) appears to be the Oort=20
cloud. However, the corners in the Oort cloud almost certainly=20
originated elsewhere, since accretion is very inefficient at such=20
large heliocentric distances. New dynamical studies now suggest that=20
the source of the Oort cloud comets is the entire giant planets=20
region from Jupiter to Neptune, rather than primarily the=20
Uranus-Neptune region, as previously thought. Some fraction of the=20
Oort cloud population may even be asteroidal bodies formed inside the=20
orbit of Jupiter. These comets and asteroids underwent a complex=20
dynamical random walk among the giant planets before they were=20
ejected to distant orbits in the Oort cloud, with possible=20
interesting consequences for their thermal and collisional histories.=20
Observational evidence for diversity in cometary compositions is=20
limited, at best. Copyright 2000, Institute for Scientific=20
Information Inc.

=========================================================
* LETTERS TO THE MODERATOR *
=========================================================

(10) ASTEROID 1998 KY26 - SPEED OF ROTATION

>From Richard TAYLOR <richard.taylor3@virgindot net>=20

Dear Benny,

Andrew Yee's report of the confirmatory observations of the 'rapid'=20
rotation of 1998 KY26 at 10.7 minutes and a stated ~30 metre diameter=20
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=20
spin rate is taken as being unusually fast. Here we would suggest that=20
the spin rate is inexplicably slow. If we take the shape of 1998 KY26=20
as a sphere the equatorial circumpherence is ~95 metres which is=20
equivalent to an equatorial velocity of ~0.15 metres per second. If we=20
assume that the spheroidal shape is the result of impacts with other=20
asteroidal or meteoritic bodies it appears that these must have largely =
been low-incidence collisions, otherwise so small a body wiould have=20
been fragmented and dispersed.

This suggests that the rate of spin of 1998 KY26 would be derived from=20
collisions and could be expected to spin very much faster - perhaps by=20
a factor of one to several orders of magnitude as any grazing impacts=20
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=20
asteroids have much lower rates of soin than might be expected for=20
collision origin. What has slowed them? Or is there some other=20
explanation for their low angular momentum density?

Richard Taylor

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