PLEASE NOTE:


*

CCNet 29/2001 - 23 February 2001
--------------------------------


"New findings provide evidence that Earth's most severe mass
extinction -- an event 250 million years ago that wiped out 90 percent
of the life on Earth -- was triggered by a collision with a comet or
asteroid. Over 90 percent of all marine species and 70 percent of land
vertebrates perished as a result, according to the NASA-funded research
team, led by Dr. Luann Becker of the University of Washington (UW),
Seattle. The team's findings will be published tomorrow in the journal
Science."
--NASA News, 22 February 2001




(1) ASTEROID OR COMET TRIGGERED LARGEST MASS EXTINCTION IN EARTH'S HISTORY
    NASANews@hq.nasa.gov

(2) STOWAWAYS FROM SPACE PROVIDE LINK BETWEEN IMPACT, BIGGEST EXTINCTION IN
EARTH HISTORY
    Andrew Yee <ayee@nova.astro.utoronto.ca>

(3) P/T MASS EXTINCTION
    Michael Paine <mpaine@tpgi.com.au>

(4) SONIC BOOM SHAKES ENGLAND'S EAST COAST
    Ron Baalke <baalke@jpl.nasa.gov>

(5) SOHO ANALYSIES KAMIKAZE COMET
    ESA News, 23 February 2001

(6) NEAR MISSION EXTENDED THROUGH FEBRUARY
    Ron Baalke <baalke@jpl.nasa.gov>

(7) LANDING ON EROS
    Ron Baalke <baalke@jpl.nasa.gov>

(8) JEFF BELLīS EROS THESIS
    David Morrison <dmorrison@arc.nasa.gov>

(9) SEVEN ENCOUNTERS WITH MINOR BODIES IN THE NEXT 5 YEARS
    Daniel Fischer <dfischer@astro.uni-bonn.de>

(10) THE PERTURBER IN THE OORT CLOUD
     Larry Klaes <lklaes@bbn.com>

(11) DUST ON EROS
     S. Fred Singer <singer@sepp.org>

(12) ORIGIN OF IRIDIUM ANOMALIES, THE P/T EXTINCTION AND THE HAWAIIAN CHAIN
     Andrew Glikson <geospectral@spirit.com.au>

(13) ASTEROID OWNERS NOT LIABLE FOR GRAVITATION (& IMPACT DAMAGE)
     Leigh Palmer <palmer@sfu.ca>

=============
(1) ASTEROID OR COMET TRIGGERED LARGEST MASS EXTINCTION IN EARTH'S HISTORY

From NASANews@hq.nasa.gov

Donald Savage
Headquarters, Washington, DC              February 22, 2001
(Phone: 202/358-1547)

Vince Stricherz
University of Washington, Seattle
(Phone:  206/ 543-2580)

RELEASE:  01-23

ASTEROID OR COMET TRIGGERED LARGEST MASS EXTINCTION
IN EARTH'S HISTORY, FORESHADOWING FATE OF DINOSAURS

New findings provide evidence that Earth's most severe mass extinction -- an
event 250 million years ago that wiped out 90 percent of the life on Earth
-- was triggered by a collision with a comet or asteroid.

Over 90 percent of all marine species and 70 percent of land vertebrates
perished as a result, according to the NASA-funded research team, led by Dr.
Luann Becker of the University of Washington (UW), Seattle. The team's
findings will be published tomorrow in the journal Science.

The collision wasn't directly responsible for the extinction but rather
triggered a series of events, such as massive volcanism, and changes in
ocean oxygen, sea level and climate. That in turn led to species extinction
on a wholesale level, according to the team.

"If the species cannot adjust, they perish. It's a survival-of-the-fittest
sort of thing," said Becker, UW acting assistant professor of Earth and
Space Sciences. "To knock out 90 percent of organisms, you've got to
attack them on more than one front."

The scientists do not know the site of the impact 250 million years ago,
when all Earth's land formed a supercontinent called Pangea. However, the
space body left a calling card -- complex carbon molecules called
buckminsterfullerenes, or Buckyballs, with the noble gases helium and argon
trapped inside the caged structure. Fullerenes, which contain at least 60
carbon atoms and have a structure resembling a soccer ball or a geodesic
dome, are named for Buckminster Fuller, inventor of the geodesic dome.

The researchers know these particular Buckyballs are extraterrestrial
because the noble gases trapped inside have an unusual ratio of isotopes,
atoms whose nuclei have the same number of protons but different numbers of
neutrons. Terrestrial helium is mostly helium-4, while extraterrestrial
helium is mostly helium-3.

"These things form in carbon stars. That's what's exciting about finding
fullerenes as a tracer," Becker said. The extreme temperatures and gas
pressures in carbon stars are
perhaps the only way extraterrestrial noble gases could be forced inside a
fullerene, she said.

These gas-laden fullerenes were formed outside the Solar System, and their
concentration in the sedimentary layer at the boundary of the Permian and
Triassic periods means they were delivered by comets or asteroids. The
researchers estimate the comet or asteroid was roughly 3 3/4 to 7 1/2 miles
(6 to 12 kilometers) across, or about the same size as the asteroid believed
responsible for the extinction of the dinosaurs 65 million years ago.

The telltale fullerenes containing helium and argon were extracted from
sites where the Permian-Triassic boundary layer had been exposed in Japan,
China and Hungary. The evidence was not as strong from the Hungary site, but
the China and Japan samples bear strong evidence, Becker said.

The team's work was made more difficult because there are few 250
million-year-old rocks left on Earth since most rocks of that age have been
recycled through the planet's tectonic processes. "It took us two years to
do this research, to try to narrow it down enough so that we could see this
fullerene signature," Becker said.

Scientists have long known of the mass extinction 250 million years ago,
since many fossils below the boundary -- such as trilobites, which once
numbered more than 15,000 species -- diminish sharply close to the boundary
and are not found above it. There also is strong evidence suggesting the
extinction happened very rapidly, on the order of 8,000 to 100,000 years,
which the latest research supports.

Previously, it was thought that any asteroid or comet collision would leave
strong evidence of the element iridium, the signal found in the sedimentary
layer from the time of the dinosaur extinction. Iridium was found at the
Permian-Triassic boundary, but not nearly in the concentration as from the
dinosaur extinction. Becker believes that difference is because the two
space bodies that slammed into Earth had different compositions.

Members of the research team are Becker; Robert Poreda and Andrew Hunt from
the University of Rochester, NY; Ted Bunch of the NASA's Ames Research
Center, Moffett Field, CA; and Michael Rampino of New York University and
NASA's Goddard Institute of Space Sciences, New York. Funding for the
research was provided by NASA's Astrobiology and Cosmochemistry programs and
the National Science Foundation.

Images are available at:
http://www.washington.edu/newsroom/news/images/extinct

=========
(2) STOWAWAYS FROM SPACE PROVIDE LINK BETWEEN IMPACT, BIGGEST EXTINCTION IN
EARTH HISTORY

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

University of Rochester
Rochester, New York

CONTACT:
Tom Rickey, (716) 275-7954

February 22, 2001

STOWAWAYS FROM SPACE PROVIDE LINK BETWEEN IMPACT, BIGGEST EXTINCTION IN
EARTH
HISTORY

An asteroid or comet similar to the one that wiped out the dinosaurs smacked
into Earth 251 million years ago, triggering the biggest extinction in Earth
history. The findings by scientists from the University of Washington,
University of Rochester, NASA, and New York University are published in the
Feb. 23 issue of Science and provide the strongest evidence yet that an
impact played a role in the extinction known as "the Great Dying."

The impact of the asteroid or comet, estimated to be anywhere from six to 12
kilometers wide, would have released unimaginable fury. "The impact of a
bolide of this size releases an amount of energy that is basically about 1
million times the largest earthquake recorded during the last century. It
was like a magnitude 12.0 earthquake on the Richter scale," says Robert
Poreda, associate professor of earth and environmental sciences at the
University of Rochester and one of the authors.

The impact and rapid extinction occurred simultaneously with some of the
most extensive volcanic activity the world has ever seen: More than 1.6
million cubic kilometers of lava, enough to cover the entire planet with 10
feet of lava had it spread evenly around the globe, oozed out of the ground
in Siberia in a relatively short amount of time, less than 1 million years.

"It was the proverbial blast from the double-barreled shotgun," Poreda says.
"We're not sure of all the environmental consequences, but with both the
impact and with the volcanic activity, we do know that Earth was not a happy
place. It may be that the combined effects of impact and volcanism are
necessary to cause such a tremendous extinction."

The evidence for the impact comes in the form of cosmic stowaways, helium
and argon molecules formed elsewhere in the universe that survived a journey
through space and crashed into Earth as part of the impact. The molecules
were encased in carbon molecules known as buckyballs, which are big enough
to hem in small gas molecules and hardy enough to survive a massive impact
between a bolide (asteroid or comet) and Earth.

By making sensitive measurements of different forms or isotopes of the gases
locked within the carbon cages, the scientists determined that the ratios of
helium and argon molecules are characteristic of meteorites and comets and
must have been formed in space.

The gas measurements were made by Poreda and Rochester post-doctoral
associate Andrew Hunt, who were part of a team headed by Luann Becker of the
University of Washington. Becker says that it's unlikely that the collision
was directly responsible for the extinction; rather, it may have triggered a
series of events, such as the volcanic activity and changes in sea level and
climate, that wiped out more than 90 percent of marine animals and about 70
percent of land vertebrates.

"If the species cannot adjust, they perish. It's a survival-of-the-fittest
sort of thing," Becker says. "To knock out 90 percent of organisms, you've
got to attack them on more than one front."

Several scientists have suggested how extensive volcanic activity could
contribute to a large extinction. Volcanoes release tons of sediment and
ash, as well as massive amounts of carbon dioxide, a greenhouse gas, into
the atmosphere. The dust could have blocked out sunlight across the Earth,
preventing plant photosynthesis and causing food chains to collapse. Or,
carbon dioxide could have trapped the sun's heat, sending temperatures on
Earth soaring.

The coupling of a massive impact and widespread volcanic activity also
occurred in the more widely publicized extinction event that wiped out the
dinosaurs 65 million years ago, at the K/T (Cretaceous/Tertiary) boundary.
Scientists generally agree that an impact on Mexico's Yucatan Peninsula
played a role in that extinction. Simultaneously, there was a dramatic
outpouring of lava in present-day India, in a complex known as the Deccan
Traps. Geologists have shown that both that eruption and the Siberian Traps,
both known as flood basalt volcanism, originated as a plume from the Earth's
mantle deep beneath the Earth's crust.

"These two extinctions are like bookends for the age of the dinosaurs,"
Poreda says. "The P/T boundary helped to usher in the age of the dinosaurs,
and the K/T boundary snuffed it out.

"There has been lots of flood basalt volcanism over time, and many impacts,
but these impact events caused major extinctions. Both coincided with
periods of heavy volcanic activity. It's possible that you need both the
impact trigger and the major eruption of flood basalts to tip the Earth over
the edge, to really put the ecosystem under stress," says Poreda.

The paper in Science is the latest in a series by Becker, Poreda, and
colleagues that exploits buckyballs, or fullerenes, to learn more about our
universe. Buckyballs are molecules made of carbon atoms that link together
in the shape of a soccer ball, forming a tiny cavity where molecules of such
light elements as helium can nestle. Becker has developed methods to extract
the molecules from rock samples, then sends the fullerenes -- which usually
form a small blob akin to ear wax -- to Poreda for analysis. He and his
colleagues in the Rare Gas Laboratory use a sophisticated gas spectrometer
to measure different isotopes of elements like helium, argon, and xenon. The
samples cited in the Science paper came from the Permian-Triassic (P/T)
boundary in China, Japan, and Hungary.

In 1996 Becker, Poreda and colleagues discovered that fullerenes found in a
huge impact crater near Sudbury, Ontario came from space nearly two billion
years ago and arrived on Earth intact. Last year they showed that even more
complex carbon molecules, with as many as 200 atoms, had survived an impact
from space at the same time as an impact wiped out the dinosaurs at the K/T
boundary.

Both papers showed that it's possible for comets and meteorites to deliver
organic compounds to Earth, adding credence to the theory that early life on
Earth was somehow seeded from space with complex carbon compounds.
Scientists say it's possible that carbon cages might provide a template or
skeleton for other molecules, and the gases they carried to Earth might
somehow make up part of our atmosphere.

The project was funded by NASA and the National Science Foundation. Also
contributing to the research were Theodore Bunch of NASA Ames Research
Center in California, and Michael Rampino of New York University.

================
(3) P/T MASS EXTINCTION

From Michael Paine <mpaine@tpgi.com.au>

Dear Benny,

You are probably putting together several items about the announcement of
evidence of a huge impact at the time of the P/T extinction. The recent
items on CCNet have been timely.
Rob Britt from Space.com has put together a great article at
http://www.space.com/scienceastronomy/planetearth/impact_extinction_010222.html

Arthur C Clarke should find it interesting since it links impacts,
extinctions and buckyballs! (I have just finished reading an excellent
collection of his essays in "Greetings carbon-based bipeds")

regards
Michael Paine

===========
(4) SONIC BOOM SHAKES ENGLAND'S EAST COAST

From Ron Baalke <baalke@jpl.nasa.gov>

http://www.mirror.co.uk/shtml/NEWS/P23S5.shtml

COAST HIT BY BOOM
The Mirror (London)
February 22, 2001

A SONIC boom has been blamed for tremors which shook buildings and rattled
windows along a 25-mile stretch of coastline yesterday.

Residents from Scarborough to Whitby in Yorkshire told of a "bang, a boom
and a rumble" for two seconds at 11.40am.

Experts ruled out an earthquake, saying it was probably a sonic boom caused
by an aircraft or even a meteor.

Seismologist Glenn Ford said: "All the evidence points to a sonic boom,
which can travel hundreds of miles if not impeded."

Investigators have contacted military officials.

=========
(5) SOHO ANALYSIES KAMIKAZE COMET

From ESA News, 23 February 2001
http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=26188

A comet that fell into the Sun on 7 February was tracked by two different
instruments on the ESA-NASA SOHO spacecraft, enabling scientists to
characterize it quite precisely. This was just one of nearly 300 comets
discovered by SOHO since 1996, thanks mainly to the privileged view of the
sky around the Sun given by the visible-light coronagraph LASCO. On this
occasion SOHO's ultraviolet coronagraph UVCS also observed the comet
repeatedly. It gave valuable additional information, both about the comet
and about the solar wind close to the Sun.

More at: http://sci.esa.int/content/news/index.cfm?aid=1&cid=1&oid=26188

==========
(6) NEAR MISSION EXTENDED THROUGH FEBRUARY

From Ron Baalke <baalke@jpl.nasa.gov>

http://near.jhuapl.edu/news/flash/01feb22_1.html

          NEAR Extended Through February
          February 22, 2001

          NASA has given the go-ahead for the NEAR mission to
          collect data from the surface of Eros through Feb. 28,
          tacking four days onto an extension granted after the
          NEAR Shoemaker spacecraft's historic landing on the
          asteroid last week.

          The extension gives NEAR Shoemaker's gamma-ray
          spectrometer additional time to observe the elemental
          composition on and below Eros' surface, and the NEAR
          team at least two more opportunities to download this
          information through NASA's heavily used Deep Space
          Network of antennas.

          "This allows us to build a much better sample," says
          Jacob Trombka, of NASA's Goddard Space Flight Center,
          team leader for NEAR Shoemaker's X-ray/Gamma-ray
          Spectrometer. "The longer you accumulate data the more
          you can reduce the uncertainty of your results. When you
          look at a little bit of data you see clues, but when you
          get more data down you can be a bit more definitive."

          Touching down on Eros certainly hasn't kept NEAR
          Shoemaker from touching base with NEAR scientists. The
          spacecraft has returned readings from its magnetometer,
          and today mission team members are downloading the
          latest information from the gamma-ray spectrometer.

          The gamma-ray instrument can measure elemental
          composition to a depth of about 4 inches, and is much
          more sensitive on the surface than it was in orbit.
          Mission engineers fine-tuned the device last week to
          account for things it hadn't encountered in orbit; it
          operates at a higher temperature near the surface, for
          example, because it can no longer radiate heat into
          space.

          "We optimized the instrument for collecting science in
          its new environment," says John Goldsten, of the Johns
          Hopkins Applied Physics Laboratory (APL), lead engineer
          for the gamma-ray spectrometer. "Now that we know how
          well it's operating . . . the data we expect from here
          on is prime science data."

          While Trombka says they'll need months to interpret that
          data, it won't take nearly as long for mission
          scientists to get a clearer picture of the asteroid's
          magnetic properties - or lack thereof. NEAR Shoemaker's
          magnetometer found no evidence of magnetism at its
          landing site. Having returned data from the surface, the
          instrument's work is done and it has been turned off.

          "We already knew there was no global magnetic field, but
          measuring this close dramatically increased our ability
          to see if there was evidence for localized 'hot spots'
          on the surface," says Brian Anderson, magnetometer
          instrument scientist at APL. "The landing site shows no
          evidence for an intrinsic magnetic field. Since the
          sensor is only two meters above the surface this is a
          pretty definitive measurement."

=========
(7) LANDING ON EROS

From Ron Baalke <baalke@jpl.nasa.gov>

http://near.jhuapl.edu/news/sci_updates/01feb20.html

          NEAR Shoemaker Science Update
          Landing on Eros
          February 20, 2001

          On Monday, 12 February 2001, the NEAR spacecraft touched
          down on asteroid Eros, after transmitting 69 close-up
          images of the surface during its final descent. Watching
          that event was the most exciting experience of my life.
          I was asked immediately afterwards how I felt, and I
          mumbled something about being tired and happy, but I
          missed the point. I realized afterward what I should
          have said: it was like watching Michael Jordan on the
          basketball court, when the game is on the line and he is
          in the groove. One miracle after another unfolds, and we
          are left stunned and speechless. When we learned that
          the spacecraft had not only landed on the surface, but
          was still operational, we hardly knew what to think.

          Over the past week, we have started to come to
          our senses again and to appreciate how
          fortunate we are. The final weeks of low altitude
          operations revealed bizarre and surprising aspects of
          surface structures on Eros, including one type of
          feature we noticed for the first time in the very last
          image taken by the spacecraft (the incomplete image
          taken from a height of 120 meters, 2001 Feb 12F ). As we
          discussed previously, there are markedly fewer small,
          fresh craters on Eros than we would expect from our
          experience at the Moon, and an amazing profusion of
          boulders, likewise more than we expected. We do not know
          just what is happening on the surface of Eros to cover
          and/or obliterate craters while making and/or uncovering
          boulders. We have seen many examples of mass motion on
          Eros - loose material sliding downhill - and that is no
          doubt part of the story, but maybe not all of it. We
          also believe that at least some of the bouldery debris
          found on Eros is comprised of ejecta from impacts on
          Eros; some of these ejecta do not escape but fall back
          to the surface.

          Some of the strange features we are beginning to think
          about can be seen in the low altitude images obtained
          during the past few weeks. The new type of feature seen
          in the last image returned ( 2001 Feb 12F ) can be
          found, for example, at the bottom of the image (just
          above the vertical streaks indicating loss of signal),
          to the left of center. It appears to be a collapse
          feature, formed when support is removed from below the
          surface, and it is about the size of one's hand. Other
          strange sights are clusters of boulders (e.g., the upper
          right of 2001 Feb 12E ) - are these cases of
          disintegration in place? - and extremely flat, sharply
          delineated areas in the bottoms of some craters (e.g.,
          the two left panels of 2001 Jan 31 ). The mere existence
          of sharp boundaries, called "contacts", is surprising in
          itself, especially if the entire surface of the asteroid
          is thought to have been blanketed by debris from
          impacts. These boundaries can be incredibly sharp on
          Eros, as evidenced by the last frame, 2001 Feb 12F
          (compare the upper right and lower left of the image).

          The images tell us a tale whose outcome we don't yet
          know, but there is more: the story of Eros's composition
          is likewise still emerging. Our orbital data from the
          x-ray spectrometer showed that the abundances of key
          elements on Eros are very similar to those in the
          undifferentiated meteorites called ordinary chondrites,
          but there was a discrepancy. The abundance of the
          volatile element sulfur is less than we would expect
          from an ordinary chondrite. However, the x-ray spectra
          tell us only about the uppermost hundred microns of the
          surface, and we do not know if the sulfur depletion
          occurs only in a thin surface layer or throughout the
          bulk of the asteroid.

          Fortunately, the spacecraft is now in a position to help
          answer the question (on the surface, that is). The gamma
          ray spectrometer measures the composition to a depth of
          about ten centimeters, and it is much more sensitive on
          the surface than it was in orbit. We are now in the
          process of trying to obtain our best yet gamma ray
          spectrum of Eros. We will try to determine the
          abundances of the volatile element potassium and the
          major element iron from this spectrum, to look harder at
          the match between the compositions of Eros and the
          ordinary chondrites, and to look for evidence for bulk
          depletion of volatiles. The latter would suggest that
          Eros has undergone significant heating (a geologist
          would call it "metamorphism").

          It is sad for me to say, but the gamma ray measurement
          will be the last from NEAR - one more miracle is what we
          ask of this little spacecraft. Its job is almost done,
          but ours is just beginning.

     Andrew Cheng
     NEAR Project Scientist               

=============
(8) JEFF BELLīS EROS THESIS

From David Morrison <dmorrison@arc.nasa.gov>

NEO News (02/20/01) Jeff Bell's Eros Thesis

Dear Friends and Students of NEOs:

This is an unusual edition of NEO News. The NEAR-Shoemaker mission
has been a tremendous success, and the spacecraft continues to
collect data on the surface of the asteroid.  We now know that this
asteroid is a monolithic (or nearly so) rock with relatively
primitive composition, with a surface sculpted by innumerable
impacts.  But the size distribution of the craters on Eros is
different from anything we have seen before, with a remarkable
deficit of craters with diameters below 100 m, as well as a great
many rocks and boulders on the surface.  Most of the interpretations
discussed by the NEAR science team involve the filling in of the
small craters by surface dust (a mobile regolith).  This is the
"orthodox" opinion.  What is printed below is an unorthodox new
interpretation by Jeff Bell of the University of Hawaii, submitted to
the Lunar and Planetary Science Conference (LPSC) to be held in
Houston next month.  I certainly would not claim that Jeff is right,
but I do think his ideas are provocative and interesting, and that
they should contribute to a spirited technical debate about the
geological history of both Near Earth and Main Belt Asteroids.

David Morrison

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

EROS: A COMPREHENSIVE MODEL.  Jeffrey F. Bell,
Hawaii Institute of Geophysics and Planetology, Univ. of
Hawaii, 2525 Correa Rd., Honolulu HI 96822
(bell@pgd.hawaii.edu).

Introduction: The NEAR spacecraft has provided a
variety of information about the planet-crossing S-class
asteroid Eros. However, most interpretations to date
have relied heavily on earlier experience on the Moon
and asteroids which were viewed only distantly during
spacecraft flybys. The close approach of NEAR to Eros
in October 2000 revealed surprising new facts that
suggest that impact processes and regolith evolution on
asteroids are very different from any other planetary
objects.

Craters and Boulders: Early crater abundance
curves [1] based on low-resolution NEAR images
suggested a "normal" crater curve in the 1000m to
100m diameter range, i.e. a high crater density, rising
toward saturation levels at the smaller sizes. Later
NEAR images from closer distances have revealed an
extraordinary crater distribution: the crater density
declines sharply below 100m until at 4m diameter the
craters are about 200 times less abundant than expected.
Post-cratering modification seems inadequate to explain
this situation. In particular, any geological process such
as regolith migration that obscures craters should also
obscure boulders; yet the size spectrum of boulders on
Eros shows an inverse correlation with the craters,
being highly biased toward smaller sizes. It seems much
more likely that this "anomalous" crater distribution
reflects the actual size spectrum of incoming projectiles.
This must be very different from the normal inner solar
system projectile size spectrum we see reflected in the
crater records of the Moon, Mercury, and Mars.  I
propose that Eros exhibits the normal cratering function
on main-belt asteroids, previously concealed from us by
the low resolution of the images obtained in distant
flybys of Gaspra, Ida, and Mathilde.

Yarkovsky to the Rescue: The Yarkovsky Effect
[2] is a mechanism for orbital evolution due to asymmetric
emission of thermal IR photons from a rotating
object that is warmer on the "afternoon" region than on
the "morning" region. This effect is strongest for objects
around a few meters in size (the force/mass ratio is too
small for larger objects, while smaller ones cannot
maintain the necessary morning/afternoon thermal
assymetry due to internal conduction). It has been
proposed [3,4] that the Yarkovsky Effect provides an
efficient mechanism for moving meteorites from any
location in the asteroid belt to the narrow Jupiter
resonance zones, from which they are rapidly perturbed
out of the belt.  If so, the asteroid belt should be strongly
depleted in objects smaller than a few meters, which
would naturally produce a strong depletion in craters
smaller than about 100m on all main-belt objects.  Since
the collisional cascade in the belt is prematurely cut off
by the Yarkovsky Effect, even particles too small to be
directly affected will be underabundant.  Since there is
a population of dust derived from asteroids (e.g. the dust
bands associated with the Hirayama families), there
must be some  mechanism for limited replenishment of
very small particles, probably direct generation of dust
from larger objects.

The underabundance of small impactors also provides
a natural explanation for the size spectrum of
boulders on Eros. Boulders are created by ejection from
larger impacts and gradually eroded by smaller ones.
This can be easily seen on the Moon, where fields of
jagged boulders are seen around fresh craters. The
absence of smaller impactors allowed boulders to
accumulate on Eros without being broken up or eroded.
The fact that this unique signature of the main-belt
environment is still visible on Eros implies that it has
undergone little cratering since its orbital evolution
decoupled it from the asteroid belt. In its current orbit,
it should be experiencing a roughly lunar-like bombar-
dment environment.  There is no trace of this late phase
of Eros history on its surface.

Elemental Abundances:  The increasing abundance
of boulders visible in the close-approach images suggests
that at sizes somewhat smaller than the current
resolution limit, the surface may be mostly covered with
rocks. This implies that the published X-ray data [5]
(which samples material to ~100 microns depth) is
mostly  sampling the outer layers of large rocks, not
fine-grained weathered regolith as expected before the
mission [6]. (In meteoritical terms, the data mostly
samples clasts instead of matrix) The XGRS team has
suggested that impact volatilization of  sulfur in the
weathered regolith could account for the grossly
nonchondritic level of sulfur (<1%) observed in the X-
ray data.  It appears more likely that their alternate
hypothesis of early partial differentiation (in which
sulphur is mobilized early) is the correct explanation.
Gamma-ray data, with a sampling depth of ~10cm,
should almost entirely represent weathering-free rock
interiors and resolve this ambiguity.

Color and Spectrum:  A striking "anomaly" on
Eros is the close similarity of all areas on the surface in
color.  While there are many regions of higher albedo in
exactly the locations we expect to find fresh,
unweathered material (steep slopes where downslope motion is
likely), the spectra curves of these areas are very similar
to those of darker (presumably older) areas. In particular,
there is no region that looks at all like ordinary
chondrites, the most publicized meteorite analog for
Eros. The advocates of a primitive Eros have been
forced to argue that "space weathering" is so rapid there
that even the most recent crater interiors, ejected blocks,
and landslides have been weathered almost to maturity.
Little attention has been given to the alternate hypothesis:
that even the oldest surfaces are so young and
immature that they are almost identical to the youngest
craters and slumps. This is the logical result of a
Yarkovsky-controlled main-belt bombardment
environment: an intense (~1000 times lunar) bombardment of
large, low-velocity (~6 km/sec) projectiles constantly
excavating fresh bedrock, and completely swamping the
weathering (impact melting and volatilization) effects
produced by the small component of  high-velocity  dust
entering the belt from other sources (mostly long-period
comets).

Of course, in the time since Eros became decoupled
from the asteroid belt, it has been in a more familiar,
approximately lunar, bombardment environment.  But
spectral weathering effects on the moon  simply do not
occur fast enough to account for the absence of any
chondrite-like areas  on Eros.  Planet-crossing asteroids
are ephemeral phenomena. On average, their survival
time against sun impact or ejection by Jupiter encounters
is 5-10My [7]. From the limited orbital modeling
that has been done for Eros, it appears that it may
currently be in a special dynamical environment that
would lengthen its lifetime to perhaps 50-100My [8].
Even this is about a factor of 10-20 too short. On the
Moon, we know that crater Tycho (age 110My) is almost
pristine spectroscopically, while Copernicus (age
~800My) is well along toward maturity.  Unless Eros
has been specially preserved for ~1By in some "cosmic
lockbox", it has not been out of the belt long enough for
its spectral properties to reflect its new environment.

Summary:   The history of Eros may be summarized as:
1) Condensation and accretion of a larger
parent body; 2) Heating and limited partial melting; 3)
Migration of the sulfur-rich melt to another region of
the parent body;  4) Progressive collisional fragmentation
of the parent body in which the current shape and
surface of Eros was produced; 5) Intense cratering in
the asteroid belt by a projectile population strongly
depleted in small objects by the Yarkovsky Effect; 6)
Recent perturbation onto a planet-crossing orbit; 7)
Short exposure to more lunar-like bombardment and
solar-wind environments which have had insufficient
time to significantly alter the surface.

References:

[1] Veverka J. et al. (2000) Science, 289, 2088-2097.
[2] Opik E. J. (1951) Proc. R. Irish Acad., 54, 165.
[3] Hartmann W. K. et al. (1997) LPS XXVIII,  517.
[4] Farinella P. et al. (1998) Icarus, 132, 378.
[5] Trombka J. I. et al. (2000) Science, 289, 2101.
[6] Bell J. F. (1997) LPS XXVIII, 83-84.
[7] Gladman et al. (1997) Science, 277, 197.
[8] Michel et al. (1998) Astron. J., 116, 2023.

-----------------------------------------------------

ADDED COMMENTS AFTER NEAR LANDING.  (Jeffrey F. Bell, Univ. of Hawaii)

       The model for Eros described in my LPSC abstract (above) has
been fully confirmed by the high resolution images acquired during
the "landing phase" of the NEAR mission on Feb. 12.  These reveal a
surface dominated by rocks, so many in some areas that the surface is
nearly saturated with cm- to m- sized rocks.  There is no sign of
small craters or "zap pits" on the rocks, and most of them appear
angular and un-eroded, indicating that the population of mm- to cm-
sized projectiles in the asteroid belt is still depleted well below
the 1 to 100 meter size range directly affected by the Yarkovsky
Effect.  This suggests that "filling-in" of the size spectrum below
the Yarkovsky Gap by further collisional evolution is negligible.
Furthermore, it is now clear that a large fraction of the X-ray
photons detected by the XGRS instrument came from solid rock
surfaces, not fine-grained "weathered" regolith.  The >90% depletion
of sulfur abundance relative to chondrites observed by this
instrument cannot be explained by weathering processes and must be
intrinsic to the bedrock of Eros, implying a significant degree of
partial melting and migration of S (and Fe?) to some other part of
the Eros Parent Body.

================
(9) SEVEN ENCOUNTERS WITH MINOR BODIES IN THE NEXT 5 YEARS

From: Daniel Fischer <dfischer@astro.uni-bonn.de>

Dear Benny,

CCNet readers bracing for the first science results from the first asteroid
lander might appreciate the following (not that) little list: In the next 5
or so years there will be, if everything works out, no fewer than seven
encounters of spacecraft with comets and asteroids.
All the following missions are fully funded, though only 2 of the 6 have
already been launched (the others will follow in 2002 to 2004):

2001 Sept. 22  Comet    Borrelly   Deep Space One (simple flyby)
2003 Nov.  12  Comet    Encke      CONTOUR        (simple flyby)
2004 Jan.   1  Comet    Wild 2     Stardust       (coma sample return)
2005 July   3  Comet    Tempel 1   Deep Impact    (big mass impact)
2005 Sept. XX  Asteroid 1998 SF36  Muses-C        (sample return)
2006 June  18  Comet    S.-W. 3    CONTOUR        (simple flyby)
2006 July  11  Asteroid Otawara    Rosetta        (simple flyby)

All dates are from the respective mission homepages or press releases (in
http://www.astro.uni-bonn.de/~dfischer/mirror/219.html you can find links to
all of them in a sidebar of the NEAR stories).

There are more scheduled flybys in 2008 (CONTOUR & Rosetta again) - and in
2011 we'll then have Rosetta as the first comet orbiter and eventually its
RoLand as the first comet lander (though both might well be beaten by a
clever Discovery mission - it's still 10 years to go).

All the missions listed above are funded by civilian space agencies (NASA,
ISAS and ESA) - but there were also two NEA missions under consideration in
the 1990's, Clementine 2 by the BMDO and NEAP by the company SpaceDev. The
former seems to have disappeared completely after the 1997 death of its
chief-scientist-to-be Gene Shoemaker, and the latter is apparently in limbo:
NEAP will be launched "within the next 3-5 years", according to
http://www.spacedev.com/missions/neap.htm .

Daniel Fischer

============
(10) THE PERTURBER IN THE OORT CLOUD

From Larry Klaes <lklaes@bbn.com>

Feb. 21, 2001

Changing orbit is simple, really

By Billy Cox
A FLORIDA TODAY column

The distance from Earth to the Sun - roughly 93 million miles - is known as
an astronomical unit. So when somebody says Jupiter is five AUs out,  that
adds up to, well, whatever. And if somebody says something is 30,000 AUs
away, you feel like the straight man in "Revenge of the Nerds
IV," so you zone out and switch channels.

But somewhere out there, at around 30,000 AUs, along the outer band of a
massive cosmic debris field called the Oort Cloud, something very big and
very weird is going on. Because it hasn't been recorded visually, it only
can be inferred, like black holes. Whatever it is, the thing
appears to be warping the orbital patterns of comets. Scientists on opposite
sides of the Atlantic Ocean who've measured the sucker's effects are calling
it The Perturber.

FULL STOR at
http://www.floridatoday.com/news/columnists/cox/022101cox.htm

----------------------------
* LETTERS TO THE MODERATOR *
----------------------------

(11) DUST ON EROS

From S. Fred Singer <singer@sepp.org>

Dear Benny

I have read informal reports quoting Joe Veverka, a leading planetary
researcher at Cornell, that there is something strange about the
distribution of dust accumulation in craters on Eros.

Now, I haven't examined any data or seen any pictures yet. But I would
speculate that we may be dealing here with an interesting phenomenon of
electrostatic dust transport.  I discussed it in papers in Icarus around
1960 +/-- a year or two.  [I don't have the papers at hand.]

The idea is the following: An airless body in interplanetary space will
carry an electric charge.  [It was originally thought to be negative
--because of ambipolar diffusion (Spitzer, Whipple)--but I determined that
the photoelectric effect from solar UV was important enough to make
it positive.]

A dust particle sitting on the body's surface would have the same potential
but carry a negligible charge. However, once it is kicked up (by an impact)
and moves beyond the Debye shielding distance, its electric charge increases
and affects its motion in the body's gravity field.

Since the ratio of electrostatic to gravity force depends strongly on size
of the dust particle, there will be a strong fractionation effect. Particles
in a certain size interval will be able to hop over large distances before
returning to the surface.  The tiniest particles will escape.

In addition, there will be interesting effects if the particle enters the
body's shadow and loses its positive charge.  On the Moon, there are regions
in perpetual shadow. I don't know the situation on Eros, but we should look
for the effect.

Of course, once we land astronauts on Phobos and Deimos, we will be able to
carry out the detailed measurements that are required.

On to Mars!

Fred

============
(12) ORIGIN OF IRIDIUM ANOMALIES, THE P/T EXTINCTION AND THE HAWAIIAN CHAIN

From Andrew Glikson <geospectral@spirit.com.au>

Dear Benny,

The suggested genetic connection between a hypothetical Permian-Triassic
boundary impact and the Hawaiian hot spot track, based on iridium anomalies
(H. Burchard, CCNet, 15-02-01), and the queried connection between cometary
impacts and the iridium anomalies along the Cretaceous-Tertiary boundary (J.
Tatum, CCNet, 16-02-01) need to be placed in perspective in view of (1) the
significance of Ir anomalies; (2) the onset of the Hawaiian volcanic chain,
and (3) the origin of the P-T boundary extinction.

Anomalous concentrations of iridium can result from several factors: (1)
contamination of crustal rocks by asteroid impacts; (2) Ir-bearing basalts
and volcanic volatiles, such as measured at Kilauea, Hawaii; and (3) Ir and
other platinum group elements (PGE) absorbed by marine algae
and organic matter present in black shale.

However, PGE abundances in chondrites are more than two orders of magnitude
higher than in the Ir-enriched volcanics.  Compare the mean PGE levels in
C1-chondrites (Pd ~ 490 ppb; Re - 35 ppb; Os ~ 510 ppb; Ir ~ 450 ppb) with
those in typical basalts (Pd ~ 0.6 ppb; Re ~ 1.0 ppb; Os ~ 0.1 ppb; Ir ~
0.05 ppb) (cf. Chou, 1978, 9th Lunar Sci. Planet. Conf., 219-230; McDonough
and Sun, 1995, Chemical Geology, 120, 223-253). PGE abundances in impact
fallout deposits, although diluted, are still commonly higher by an order of
magnitude compared to volcanic rocks, for example <7 ppb on the K-T boundary
at Gubbio.

Moreover, the ratios between the volatile PGE (Pd, Re) and refractory PGE
(Os, Ir) effectively discriminate between meteoritic and terrestrial
materials thanks to the preferential enrichment of volatile PGE in crustal
rocks relative to the mantle during magmatic fractionation.  For
example, typical crustal Pd/Ir ratios (~12) and Re/Os ratios (~10) are
higher by over an order of magnitude compared to typical meteoritic Pd/Ir
ratios (~1.0) and Re/Os ratios (~0.07).  Isotopic discriminants between
terrestrial and meteoritic materials include high 187Os/188Os ratios and
intermediate 52Cr/53Cr ratios.

Both primitive Hawaiian basalts and volcanic volatiles show typical
terrestrial-type PGE abundances and ratios, for example high Pd/Ir ratios
and Re/Os ratios (Bennet et al., 2000, Earth Planet. Sci. Lett. 183,
513-526), and are therefore highly unlikely to represent signatures of
meteoritic components.  Significantly, the oldest volcanics dated at the
northwest extremity of the Emperor-Hawaii chain are about 65 Ma - namely at
the K-T impact boundary ... Therefore, neither the PGE geochemistry nor
structural evidence of which I am aware connect the Emperor-Hawaiian chain
with the Siberian Norilsk plateau basalts of Permian-Triassic boundary age
(248-251 Ma). 

Evidence for impact coinciding with the P-T boundary is offered by (1) the
42 km-diameter Araguinha impact structure, Brazil (247 Ma) and by
impact-shocked quartz grains (containing planar deformation features) within
sediments in Antarctica and eastern Australia (Retallack et al., 1998.
Geology, 26:979-982).  However, it is not clear whether these impacts were
large enough to trigger the mass extinction across the P-T boundary.

Referring to iridium contents of comets (Tatum, CCNet 16-02-01), whereas Ir
levels in chondrites, believed to form the bulk of main belt asteroids, are
more than 2 orders of magnitude higher than in terrestrial rocks, little is
known about the composition of stony fragments believed to exist within
comets.  Even where such stony material may be exposed at cometary surfaces,
the low level abundances (parts-per-billion) of Ir can hardly be expected to
be detected in cometary spectra.

Sincerely

Andrew Glikson
Australian National University
20-02-2001

----------------
(13) ASTEROID OWNERS NOT LIABLE FOR GRAVITATION (& IMPACT DAMAGE)

From Leigh Palmer <palmer@sfu.ca>

Graham Richard Pointer <grp1@st-andrews.ac.uk> wrote:

>My office-mates and I were trying to work out the legal implications - if
>Eros crashed into the Earth, could those affected sue this company for the
>damage caused? Any lawyers out there?

Graham isn't thinking big enough. Since gravitation is universal, *any*
celestial body owned by a private party is responsible to some degree for
the trajectory of an asteroid that causes damage on Earth. If the objects in
question have been orbiting for billions of years then there is no doubt
that the influence was crucial; it was. I will be glad to appear (for a fee)
as an expert witness to defend this claim.

Leigh Palmer

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*

U.S. PLAN TO BUILD GIANT NEO TELESCOPE UPSTAGES UK INITIATIVE:
THE RACE TO SEARCH FOR SMALL IMPACTORS IS ON

From David Morrison <dmorrison@arc.nasa.gov>

NEO News (02/23/01) New Survey Telescope Proposed

Dear Friends & Students of NEOs:

A new survey telescope is being proposed that could extend the
Spaceguard Survey of NEOs down to objects 300 m in diameter. It would
operate roughly ten times faster, and reach roughly ten times
fainter, than current NEO searches. However, NEO searches are only
one of several justifications for this telescope, called the
Large-aperture Synoptic Survey Telescope (LSST).

As many of you know, the United States astronomical community,
working through the National Research Council of the National Academy
of Sciences, provides each decade a prioritized list of desirable new
projects to NASA and the NSF. The recommendations of these Astronomy
and Astrophysics Survey Committees (as they are called) have
established the roadmap for development of astronomical facilities in
the United States, both in orbit and on the ground. All of the
well-known big observatories, from the Hubble Space Telescope and
Chandra X-ray Observatory to the 8-m-class optical-infrared
telescopes and the VLA and VLBA for radio astronomy, were among the
high-priority recommendations from past Survey Committees. It is
therefore with considerable interest that we find a recommendation in
the just-published report of the most recent Survey Committee for a
telescope to search for NEOs. The goal of cataloging 90 percent NEOs
larger than 300 m within one decade is one of the primary objectives
of the proposed Large-aperture Synoptic Survey Telescope (LSST).
Following are excerpts from the report "Astronomy and Astrophysics in
the New Millennium" just published by the National Research Council.

I note that the UK's recommended program for dealing with the impact
threat also includes extending the current NEO surveys from 1 km down
to 300 m. The UK has proposed construction of a 3-m telescope to
begin such a search. However, reaching reasonable completeness to 300
m in a timely manner will probably require an international
consortium of several such 3 m telescopes. The proposers of the LSST
apparently have concluded that a single telescope of greater than 6 m
aperture can carry out the NEO survey to 90 percent completeness
within a decade, as well as meeting its other astrophysics goals. I
have not yet seen a detailed plan for how either the proposed UK 3-m
telescope or the proposed US 6-m telescope would be used to
accomplish the NEO survey goals.

David Morrison

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

Large-aperture Synoptic Survey Telescope: Quotes from the NRC
Astronomy & Astrophysics Survey Committee (2001)

The Large-aperture Synoptic Survey Telescope (LSST) is a
6.5-meter-class optical telescope designed to survey the visible sky
every week down to a much fainter level than that reached by existing
surveys. It will catalog 90 percent of the Near-Earth Objects larger
than 300 meters and assess the threat they pose to life on Earth. It
will find some 10,000 primitive objects in the Kuiper Belt, which
contains a fossil record of the formation of the solar system. It
will also contribute to the study of the structure of the universe by
observing thousands of supernovae, both nearby and at large redshift,
and by measuring the distribution of dark matter through
gravitational lensing. All the data will be available through the
[proposed] National Virtual Observatory, providing access for
astronomers and the public to very deep images of the changing night
sky. [The estimated cost] of the LSST is $170 million.  (p 10-11)

By surveying the visible sky every week to a much fainter level than
can be achieved with existing optical surveys, LSST will open a new
frontier in addressing time-variable phenomena in astronomy. This
6.5-m-class optical telescope will detect 90 percent of the
Near-Earth Objects larger than 300 meters within a decade, and will
enable assessment of the potential hazard each poses to Earth. . .
(p 38-39)

The Near-Earth Objects (NEOs) are asteroids with orbits that bring
them close to the Earth. The orbits of many NEOs actually cross that
of the Earth, making NEOs an impact threat to our planet.
Extrapolations from existing data suggest that about 1000 NEOs are
larger than 1 km in diameter, and that between 100,000 and 1 million
are larger than 100 m. . . . it is estimated that the probability of
an NEO larger than 300 m will strike the Earth during this century is
[only] about 1 percent. Nonetheless, it behooves us to learn much
more about these objects. Over a decade, the LSST will discover 90
percent of the NEOs larger than 300 m, providing information about
the origin of these objects in the process. . . (p 58-61)

With its huge array of detectors, LSST will collect more than a
trillion bits of data per day, and the rapid data reduction,
classification, archiving, and distribution of these data will
require considerable effort. The resulting database and data-mining
tools will likely form the largest non-proprietary data set in the
world and could provide a cornerstone for the National Virtual
Observatory. (p 108)

Study of the history of collisions of asteroids and comets with Earth
provide the framework for understanding cataclysmic climate changes
over geological time scales. While far rarer now than during the
first billion years of the solar system's history, collisions of
comets and asteroids with planets still take place. On Earth, such
collisions can produce dramatic environmental events, from giant
tidal waves to Earth-girdling dust clouds that can alter climate for
centuries and in some cases lead to mass extinctions of species.
Astronomers now have the tools to detect comets and Earth-crossing
asteroids of size sufficient to threaten human civilization and to
assess the threat of such a collision. (p 154)

+++++++++++++++++++++++++++++++++++++++++++

NEO News is an informal compilation of news and opinion dealing with
Near Earth Objects (NEOs) and their impacts.  These opinions are the
responsibility of the individual authors and do not represent the
positions of NASA, the International Astronomical Union, or any other
organization.  To subscribe (or unsubscribe) contact
dmorrison@arc.nasa.gov.  For additional information, please see the
website: http://impact.arc.nasa.gov.  If anyone wishes to copy or
redistribute original material from these notes, fully or in part,
please include this disclaimer.



CCCMENU CCC for 2000

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