PLEASE NOTE:


*

CCNet DIGEST, 29 October 1998
-----------------------------


(1) DEFINING THE EFFECTS OF SUB-CRITICAL COSMIC IMPACTS
    Richard Taylor <richard.taylor3@virgin.net

(2) GLASS IN THE SAND
    Jeff Wynn <jwynn@usgs.gov>

(3) THE DAY THE SANDS CAUGHT FIRE
    Scientific American
    http://www.sciam.com/1998/1198issue/1198wynn.html

(4) ESTIMATION OF IMPACT ENERGY AND FREQUENCY
    Michael Paine <mpaine@tpgi.com.au>

(5) TINY BUG IN ANTARTICA HOLDS CLUES TO MASSIVE EXTINCTION -
    BUT INTERPRETATION OF FINDINGS ARE RATHER QUESTIONABLE
    Andrew Yee <ayee@nova.astro.utoronto.ca>


======================================
(1) DEFINING THE EFFECTS OF SUB-CRITICAL COSMIC IMPACTS

JOINT DISCUSSION MEETING OF THE ROYAL ASTRONOMICAL SOCIETY, THE BRITISH
INTERPLANETARY SOCIETY & THE GEOLOGICAL SOCIETY

From Richard Taylor <richard.taylor3@virgin.net
 
RAS/BIS/GS Parallel Discussion Meeting - 11th December 1998, Geological
Society, Lecture Theatre, Burlington House, London, 10:30 to 15:30

Subject:"Defining the effects of sub-critical cosmic impacts on the
         Earth"

General Notes and Definition of  a Sub-Critical Cosmic Impact on the
Earth
 
The term sub-critical is used to imply that the impact event is of a
size and magnitude significantly below that capable of causing a global
mass extinction but one large enough to disrupt seriously the
pre-existing environment. For the purposes of guidance we define a
sub-critical impact as an event sufficiently large to interdict up to
an area approximating to a quarter-hemisphere of the globe.
 
Whereas the effects produced by a global mass extinction impact event
are almost entirely independent of the composition and physical nature
of the impactor and target the situation is not the same when we come
to consider the effects of sub-critical impactor.
 
With sub-critical impact events the energy released is not the sole
determinant of the outcome, of the effects produced. The overall
environmental consequences of such an impact can be amplified or
attenuated significantly by the composition and physical nature of the
Earth's surface at ground-zero - the point of impact. Thus the regional
physical geology and geography of Earth may give rise to surprisingly
different outcomes and the environmental and climatic effects produced
by a sub-critical impact may be expected to show also characteristics
that are related to the location of the point of impact.
 
The difficulties in trying to establish whether or not smaller impact
events have produced significant effects of the evolution of life and
perhaps even on the time scale of human history are considerable. In
spite of the size of the KT boundary event collecting the evidence to
prove the impact hypothesis was not an easy or simple matter. Almost
two decades of detailed and painstaking  multi-disciplinery research
was necessary to establish the connexion between the many separate
lines of evidence which indicated that a wide range of catastrophic
global changes had occurred 65 million years ago at the KT boundary.
 
In the case of sub-critical impacts even the strongest evidence is by
definition constrained to smaller regions of the Earth's surface and
global signatures of the event, where they exist, will usually be weak
or ill defined. Indeed, some quite sizeable identified impact events
seem to have left no discernable effects either in the immediate
surroundings of the impact zone or on a more wide-spread or general
scale. Is this lack of evidence real or only apparent? In such cases,
in the absence of clear supporting direct evidence it may be possible
to search for and identify coincident but apparently un-associated
events and research them to try and establish whether there may, or may
not, be any underlying connexion between them. [For an interesting
example of this kind see Gersonde, Kyte et al., Nature, 390,
357-363,(1997) concerning a late Pliocene ocean impact at 72S:85W dated
2.15 Myr BP.]
 
If the effects of a sub-critical impact are likely be modulated by
geological and geographical characteristics at specific ground-zero
point locations we can classify them roughly as follows:
 
1. Antarctic and Arctic Polar Ice-caps - For a GZ on either these
regions the effects of the displacement and dispersal of polar ice will
differ for the two hemispheres as the SP ice cap is more largely on
land and Antartic continent is surrounded mainly by ocean, whereas the
NPC is floating on a continent surrounded ocean. Climatic effects
produced can also be expected to be significantly different
  
2. Desert and Sandy Desert Regions - the nature of the particulate
materials and whether the desert is hot and arid or dry and cold may be
significant
 
3. Deep, Intermediate and Shallow (continental-shelf) Oceans.
 
4. Northern cis-polar latitude permafrost regions - possible a
cataclysmic climatic effects by the release of entrained greenhouse
gases.
 
5. Impact into particular mineral strata with the release of acid
aerosols or other atmosphere contaminents - eg. sulphuric acid, carbon
dioxide etc.
 
6. Forested regions - risk of fire storm and associated effects.
 
7. Impact in major inhabited areas - e.g.: Continental USA,  Western
Europe, etc.
         
On the otherside of the study of SCI's are topics like evaluating:-
 
The Environmental Self-Restoration Processes  - assessing the Earth's
natural capacity and resilience for recovery under the differing
outcomes of impacts in different ground-zero locations.
 
Restoration Ecology - as a technique for accelerating and/or producing
environmental recovery in tha case where an area of human habitation is
effected.
 
Impact Prevention - the least costly technological Solution?  -
assessing of the costs of inaction versus prevention by changing the
orbital trajectory of the potential impactor.

PROGRAMME

Meeting Organizers: Dr David Hughes, Dr Julian Hiscox & Richard L.S.
Taylor     

Timed Programme
Morning:

First Session - Nature and Frequency of Small Impactors
Chairman: Dr Julian Hiscox

10:30 a.m. Dr Matthew Genge - NHM London
           "The implications of meteorites and micrometeorites for the
            nature of sub-critical impactors"

10:45 a.m. Jonathan Tate - Spaceguard UK
          "The frequency of SCI impactors with diameters in the 0.1
           to 1 km range"

11:00 a.m. Dr David Hughes - University of Sheffield
           "The cratering rate of planet Earth."

Second Session -
Geological, Climatic Evidence
Chairman: Dr David Hughes

11:15 a.m. Professor I.J. Smalley & Ian Jefferson - Nottingham Trent
           "Sedimentological consequences of sub-critical impacts in
            sandy deserts or loess regions"

11:30 a.m. Professor N. Fedoroff - Institut National Agronomique, France
           "Registration of abrupt events in loess:  ~ 67000 yrs BP
            transition reveals unusual attributes."

11:50 a.m. Professor C. Vita-Finzi - UCL
           "Seasonal enhancement and seismic triggering by impacts."
 
12:05 p.m.  Prof N.C. Wickramasinghe, M.K. Wallis and D.H. Wallis - UC
            Cardiff
            "Climate Switches Induced by Stratospheric Dust Loading."

12:20 p.m.   Discussion

12:35 p.m.    LUNCH

Afternoon

Third Session
Evidence for Evolutionary and Recent Environmental Effects
Chairman: Professor Ian J. Smalley

1:35 p.m.  Dr N. MacLeod - NHM London
           "Identifying instances of past environmental change and their
           causal mechanisms."

1:55 p.m.  Dr Julian Hiscox - IAH Compton
           "Possible biochemical/bioevolutionary consequences of SCI
           events"

2:10 p.m.  Dr. Marie-Agnes Courty -  National Centre for Scientific
           Research, France
           "Recognition of instantaneous soil collapse at 3950 BP
           throughout the Middle East in response to a blast wave, wild
           fires and heavy rains caused by an extra-terrestrial event."

2:30 p.m.  Dr.B. J. Peiser - Liverpool John Moores University
           "Current research on Holocene impact events and its
           implications for impact rate calculations."

2:50 p.m.  Dr Victor Clube - University of Oxford
           "Sub-cometary and sub-asteroidal impacts: historical
           considerations"

Fourth Session:
Wider Implications of Impacts
Chairman: Professor Claudio Vita-Finzi

3:10 p.m.  Professor Neville Price - formerly UCL
           "Evidence for impact as a signifincant and periodic
           geological process."

3:25 p.m. Close

3:30 p.m. Tea at Saville Row
Followed by A&G Meeting

======================
(2) GLASS IN THE SAND

From Jeff Wynn <jwynn@usgs.gov>
To Bob Kobres <bkobres@arches.uga.edu>

Thank you for your kind comment in the CCNet Digest. I read it all the
time and was surprised to see the paper mentioned. The original article
was modified substantially by the editor; and I disclaim any "Indiana
Jones" characteristics. It's just that I'm adventurous, and
consequently seem to get into trouble with wrecks and snakes all the
time. The editor apparently gathered this from several phone
conversations.
 
The original title of the article, by the way, was "Glass in the Sand
-- clues to the physics of a hypervelocity impact."  Unfortunately I
couldn't prevail upon the editor to keep it.  For me, the key point was
how cool it was to figure out - from the clues on the ground - the
sequence of events that took place in just a few minutes in the Rub'
Al-Khali 135 years ago.  The other main point that was subdued by the
editing is that relatively small objects like Wabar and Tunguska are
the probably more immediate threat than "Texas-sized" objects, because
these smaller guys are more common and can be amazingly destructive,
yet are so hard to spot.
 
==Jeff

===================
(3) THE DAY THE SANDS CAUGHT FIRE

From Scientific American
http://www.sciam.com/1998/1198issue/1198wynn.html

A desert impact site demonstrates the wrath of rocks from space

by Jeffrey C. Wynn and Eugene M. Shoemaker

Imagine, for a moment, that you are standing in the deep desert,
looking northwest in the evening twilight. The landscape is absolutely
desolate: vast, shifting dunes of grayish sand stretch uninterrupted in
all directions. Not a rock is to be seen, and the nearest other human
being is 250 kilometers away. Although the sun has set, the air is
still rather warm-50 degrees Celsius-and the remnant of the afternoon
sandstorm is still stinging your back. The prevailing wind is blowing
from the south, as it always does in the early spring.

Suddenly, your attention is caught by a bright light above the
darkening horizon. First a spark, it quickly brightens and splits into
at least four individual streaks. Within a few seconds it has become a
searing flash. Your clothes burst into flames. The bright objects flit
silently over your head, followed a moment later by a deafening crack.
The ground heaves, and a blast wave flings you forward half the length
of a football field. Behind you, sheets of incandescent fire erupt into
the evening sky and white boulders come flying through the air. Some
crash into the surrounding sand; others are engulfed by fire.

Glowing fluid has coated the white boulders with a splatter that first
looks like white paint but then turns progressively yellow, orange, red
and finally black as it solidifies-all within the few seconds it takes
the rocks to hit the ground.. Some pieces of the white rock are fully
coated by this black stuff; they metamorphose into a frothy, glassy
material so light that it could float on water, if there were any water
around. A fiery mushroom cloud drifts over you now, carried by the
southerly breeze, blazing rainbow colors magnificently. As solid rocks
become froth and reddish-black molten glass rains down, you too become
part of the spectacle-and not in a happy way.

Deep in the legendary Empty Quarter of Saudi Arabia-the Rub'
al-Khali-lies a strange area, half a square kilometer (over 100 acres)
in size, covered with black glass, white rock and iron shards. It was
first described to the world in 1932 by Harry St. John "Abdullah"
Philby, a British explorer perhaps better known as the father of the
infamous Soviet double-agent Kim Philby. The site he depicted had been
known to several generations of roving al-Murra Bedouin as al-Hadida,
"the iron things."

There is a story in the Qur'an, the holy book of Islam, and in
classical Arabic writings about an idolatrous king named Aad who
scoffed at a prophet of God. For his impiety, the city of Ubar and all
its inhabitants were destroyed by a dark cloud brought on the wings of
a great wind. When Philby's travels took him to the forbidding Empty
Quarter, his guides told him that they had actually seen the destroyed
city and offered to take him there. Philby gladly accepted the offer to
visit what he transliterated in his reports as "Wabar," the name that
has stuck ever since.

What he found was neither the lost city of Ubar nor the basis for the
Qur'anic story. But it was certainly the setting of a cataclysm that
came out of the skies: the arrival of a meteorite. Judging from the
traces left behind, the crash would have been indistinguishable from a
nuclear blast of about 12 kilotons, comparable to the Hiroshima bomb.
It was not the worst impact to have scarred our planet over the ages.
Yet Wabar holds a special place in meteor research. Nearly all known
hits on the earth have taken place on solid rock or on rock covered by
a thin veneer of soil or water. The Wabar impactor, in contrast, fell
in the middle of the largest contiguous sand sea in the world. A dry,
isolated place, it is perhaps the best-preserved and geologically
simplest meteorite site in the world. Moreover, it is one of only 17
locations-out of a total of nearly 160 known impact structures-that
still contain remains of the incoming body.

In three grueling expeditions to the middle of the desert, we have
reconstructed the sequence of events at Wabar. The impact was an
episode much repeated throughout the earth's geologic and biological
history. And the solar system has not ceased to be a shooting gallery.
Although the biggest meteors get most of the attention, at least from
Hollywood, the more tangible threat to our cities comes from smaller
objects, such as the one that produced Wabar. By studying Wabar and
similarly unfortunate places, researchers can estimate how often such
projectiles strike the earth. If we are being shot at, there is some
consolation in knowing how often we are being shot at.

One has to wonder how Philby's Bedouin guides knew about Wabar, which
is found in the midst of a colossal dune field without any landmarks,
in a landscape that changes almost daily. Even the famously tough
desert trackers shy away from the dead core of the Empty Quarter. It
took Philby almost a month to get there. Several camels died en route,
and the rest were pushed to their limits. "They were a sorry sight
indeed on arrival at Mecca on the ninetieth day, thin and humpless and
mangy," Philby told a meeting of the Royal Geographical Society on
his return to London in 1932.

Otherworldly

When he first laid eyes on the site, he had become only the second
Westerner (after British explorer Bertram Thomas) to cross the Empty
Quarter. He searched for human artifacts, for the remains of broken
walls. His guides showed him black pearls littering the ground, which
they said were the jewelry of the women of the destroyed city. But
Philby was confused and disappointed. He saw only black slag, chunks of
white sandstone and two partially buried circular depressions that
suggested to him a volcano. One of his guides brought him a piece of
iron the size of a rabbit. The work of the Old People? It slowly dawned
on Philby that this rusty metal fragment was not from this world.
Laboratory examination later showed that it was more than 90 percent
iron, 3.5 to 5 percent nickel and four to six parts per million
iridium-a so-called sidereal element only rarely found on the earth
but common in meteorites.

The actual site of the city of Ubar, in southern Oman about 400
kilometers (250 miles) south of Philby's Wabar, was uncovered in 1992
with the help of satellite images [see "Space Age Archaeology," by
Farouk El-Baz; Scientific American, August 1997]. Wabar, meanwhile,
remained largely unexplored until our expeditions in May 1994, December
1994 and March 1995. The site had been visited at least twice since
1932 but never carefully surveyed.

It was not until our first trip that we realized why. One of us (Wynn)
had tagged along on an excursion organized by Zahid Tractor
Corporation, a Saudi dealer of the Hummer vehicle, the civilian version
of the military Humvee. To promote sales of the vehicle, a group of
Zahid managers, including Bill Chasteen and Wafa Zawawi, vowed to cross
the Empty Quarter and invited the U.S. Geological Survey mission in
Jeddah to send a scientist along. This was no weekend drive through the
countryside; it was a major effort requiring special equipment and two
months of planning. No one had ever crossed the Empty Quarter in the
summer. If something went wrong, if a vehicle broke down, the caravan
would be on its own: the long distance, high temperatures and irregular
dunes preclude the use of rescue helicopters or fixed-wing aircraft.

An ordinary four-wheel-drive vehicle would take three to five days to
navigate the 750 kilometers from Riyadh to Wabar. It would bog down in
the sand every 10 minutes or so, requiring the use of sand ladders and
winches. A Hummer has the advantage of being able to change its tire
pressure while running. Even so, the expedition drivers needed several
days to learn how to get over dunes. With experience, the journey to
Wabar takes a long 17 hours. The last several hours are spent crossing
the dunes and must be driven in the dark, so that bumper-mounted
halogen beams can scan for the unpredictable 15-meter sand cliffs.

Our first expedition stayed at the site for a scant four hours before
moving on. By that time, only four of the six vehicles still had
working air conditioners. Outside, the temperature was 61 degrees C
(142 degrees Fahrenheit)-in the shade under a tarp-and the humidity was
2 percent, a tenth of what the rest of the world calls dry. Wynn went
out to do a geomagnetic survey, and by the time he returned he was
staggering and speaking an incoherent mixture of Arabic and English.
Only some time later, after water was poured on his head and cool air
was blasted in his face, did his mind clear.

Zahid financed the second and third expeditions as well. On our
weeklong third expedition, furious sandstorms destroyed our camp twice,
and the temperature never dropped below 40 degrees C, even at night. We
each kept a two-liter thermos by our beds; the burning in our throats
awoke us every hour or so.

Shocking Rock

The Wabar site is about 500 by 1,000 meters in size. There are at least
three craters, two (116 and 64 meters wide) recorded by Philby and the
other (11 meters wide) by Wynn on our second expedition. All are nearly
completely filled with sand. The rims we now see are composed of
heaped-up sand, anchored in place both by "impactite" rock-a bleached,
coarse sandstone-and by large quantities of black-glass slag and
pellets. These sandy crater rims are easily damaged by tire tracks.
There are also occasional iron-nickel fragments.

Geologists can deduce that a crater was produced by meteorite
impact-rather than by other processes such as erosion or volcanism-by
looking for signs that shock waves have passed through rocks. The
impactite rocks at the Wabar site pass the test. They are coarsely
laminated, like other sandstones, but these laminations consist of
welded sand interspersed with ribbonlike voids. Sometimes the layers
all bend and twist in unison, unlike those in any other sandstone we
have ever seen. The laminations are probably perpendicular to the path
taken by a shock wave. Moreover, the impactite contains coesite, a form
of shocked quartz found only at nuclear blast zones and meteorite
sites. X-ray diffraction experiments show that coesite has an unusual
crystal structure, symptomatic of having experienced enormous
pressures.

The impactite is concentrated on the southeastern rims and is almost
entirely absent on the north and west sides of the craters. This
asymmetry suggests that the impact was oblique, with the incoming
objects arriving from the northwest at an angle between 22 and 45
degrees from the horizontal.

The two other types of rock found at Wabar are also telltale signs of
an impact. Iron-nickel fragments are practically unknown elsewhere in
the desert, so they are probably remnants of the meteorite itself. The
fragments come in two forms. When found beneath the sand, they are
rusty, cracked balls up to 10 centimeters in diameter that crumble in
the hand. Daniel M. Barringer, an American mining engineer who drilled
for iron at Meteor Crater in Arizona early this century, called such
fragments, which occur at several iron-meteor sites, "shale balls."

When the iron fragments are found at the surface, they are generally
smooth, covered with a thin patina of black desert varnish. The largest
piece of iron and nickel is the so-called Camel's Hump, recovered in a
1965 expedition and now displayed at King Saud University in Riyadh.
This flattened, cone-shaped chunk, weighing 2,200 kilograms (2.43
tons), is probably a fragment that broke off the main meteoroid before
impact. Because the surface area of an object is proportional to its
radius squared, whereas mass is proportional to the radius cubed, a
smaller object undergoes proportionately more air drag. Therefore, a
splinter from the projectile slows down more than the main body; when
it lands, it may bounce rather than blast out a crater.

The other distinctive type of rock at Wabar is the strange black glass.
Glassy rock is often found at impact sites, where it is thought to form
from molten blobs of material splattered out from the crater. Near the
rims of the Wabar craters, the black glass looks superficially like
Hawaiian pahoehoe, a ropy, wrinkled rock that develops as thickly
flowing lava cools. Farther away, the glass pellets become smaller and
more droplike. At a distance of 850 meters northwest of the nearest
crater, the pellets are only a few millimeters across; if there are any
pellets beyond this distance, sand dunes have covered them. When
chemically analyzed, the glass is uniform in content: about 90 percent
local sand and 10 percent iron and nickel. The iron and nickel appear
as microscopic globules in a matrix of melted sand. Some of the glass
is remarkably fine. We have found filigree glass-splatter so fragile
that it does not survive transport from the site, no matter how well
packaged.

The glass distribution indicates that the wind was blowing from the
southeast at the time of impact. The wind direction in the northern
Empty Quarter is seasonal. It blows from the north for 10 months of the
year, sculpting the huge, horned barchan sand dunes. But during the
early spring, the wind switches direction to come from the southeast.
Spring is the desert sandstorm season that worried military planners
during the Gulf War; it coincides with the monsoon season in the
Arabian Sea. All year long, the air is dead still when the sun rises,
but it picks up in the early afternoon. By sunset it is blowing so hard
that sand stings your face as you walk about; on our expeditions, we
needed swim goggles to see well enough to set up our tents. Around
midnight the wind drops off
again.

Curtains

Black material and white-the Wabar site offers little else. This
dichotomy suggests that a very uniform process created the rocks. The
entire impact apparently took place in sand; there is no evidence that
it penetrated down to bedrock. In fact, our reconnaissance found no
evidence of outcropping rock (bedrock that reaches the surface)
anywhere within 30 kilometers.

From the evidence we accumulated during our expeditions, as well as
from the modeling of impacts by H. Jay Melosh and Elisabetta Pierazzo
of the University of Arizona, we have pieced together the following
sequence of events at Wabar.

The incoming object came from the northwest at a fairly shallow angle.
It may have arrived in the late afternoon or early evening, probably
during the early spring. Like most other meteoroids, it entered the
atmosphere at 11 to 17 kilometers per second (24,600 to 38,000 miles
per hour). Because of the oblique angle of its path, the body took
longer to pass through the atmosphere than if it had come straight
down. Consequently, air resistance had a greater effect on it. This
drag force built up as the projectile descended into ever denser air.
For most meteoroids, the drag overwhelms the rock strength by eight to
12 kilometers' altitude, and the object explodes in midair. The Wabar
impactor, made of iron, held together longer. Nevertheless, it
eventually broke up into at least four pieces and slowed to half its
initial speed. Calculations suggest a touchdown velocity of between
five and seven kilometers per second, about 20 times faster than a
speeding .45-caliber pistol bullet.

The general relation among meteorite size, crater size and impact
velocity is known from theoretical models, ballistics experiments and
observations of nuclear blasts. As a rule of thumb, craters in rock are
20 times as large as the objects that caused them; in sand, which
absorbs the impact energy more efficiently, the factor is closer to 12.
Therefore, the largest object that hit Wabar was between 8.0 and 9.5
meters in diameter, assuming that the impact velocity was seven or five
kilometers per second, respectively. The aggregate mass of the original
meteoroid was at least 3,500 tons. Its original kinetic energy amounted
to about 100 kilotons of exploding TNT. After the air braking, the
largest piece hit with an energy of between nine and 13 kilotons.
Although the Hiroshima bomb released a comparable amount of energy, it
destroyed a larger area, mainly because it was an airburst rather than
an explosion at ground level.

At the point of impact, a conelike curtain of hot fluid-a mixture of
the incoming projectile and local sand-erupted into the air. This fluid
became the black glass. The incandescent curtain of molten rock
expanded rapidly as more and more of the meteorite made contact with
the ground. The projectile itself was compressed and flattened during
these first few milliseconds. A shock wave swept back through the body;
when it reached the rear, small pieces were kicked off-spalled off, in
geologic parlance-at gentle speeds. Some of these pieces were engulfed
by the curtain, but most escaped and plopped down in the surrounding
sand as far as 200 meters away. They are pristine remains of the
original meteorite. (Spalling can also throw off pieces of the planet's
surface without subjecting them to intense heat and pressure. The
famous Martian meteorites, for example, preserved their delicate
microstructures despite being blasted into space.)

A shock wave also moved downward, heating and mixing nearby sand. The
ratio of iron to sand in the glass pellets suggests that the volume of
sand melted was 10 times the size of the meteorite-implying a
hemisphere of sand about 27 meters in diameter. Outside this volume,
the shock wave, weakened by its progress, did not melt the sand but
instead compacted it into "insta-rock": impactite.

The shock wave then caused an eruption of the surface. Some of the
impactite was thrown up into the molten glass and was shocked again. In
rock samples this mixture appears as thick black paint splattered on
the impactite. Other chunks of impactite were completely immersed in
glass at temperatures of 10,000 to 20,000 degrees Celsius. When this
happened, the sandstone underwent a second transition into bubbly
glass.

The largest crater formed in a little over two seconds, the smallest
one in only four fifths of a second. At first the craters had a larger,
transient shape, but within a few minutes material fell back out of the
sky, slumped down the sides of the craters and reduced their volume.
The largest transient crater was probably 120 meters in diameter. All
the sand that had been there was swept up in a mushroom cloud that rose
thousands of meters, perhaps reaching the stratosphere. The evening
breeze did not have to be very strong to distribute molten glass 850
meters away.

Fading Away

And when did all this take place? That has long been one of the
greatest questions about Wabar. The first date assigned to the event,
based on fission-track analysis in the early 1970s of glass samples
that found their way to the British Museum and the Smithsonian 
Institution, placed it about 6,400 years ago. Field evidence, however,
hints at a more recent event. The largest crater was 12 meters deep in
1932, eight meters deep in 1961 and nearly filled with sand by 1982.
The southeastern rim was only about three meters high during our visits
in 1994 and 1995. Dune experts believe it would be impossible to empty
a crater once filled.

The Wabar site might have already disappeared if impactite and glass
had not anchored the sand. At least two of the craters are underlaid by
impactite rocks, which represent the original bowl surface before
infilling by sand. We were able to collect several samples of sand
beneath this impactite lining for thermoluminescence dating. The
results, prepared by John Prescott and Gillian Robertson of the
University of Adelaide, suggest that the event took place less than 450
years ago.

The most tantalizing evidence for a recent date is the Nejd meteorites,
which were recovered after a fireball passed over Riyadh in either 1863
or 1891, depending on which report you believe. The fireball was said
to be headed in the direction of Wabar, and the Nejd meteorites are
identical in composition to samples from Wabar. So it is likely that the
Wabar calamity happened only 135 years ago. Perhaps the grandfathers of
Philby's guides saw the explosion from a long way off.

The date is of more than passing interest. It gives us an idea of how
often such events occur. The rate of meteorite hits is fairly
straightforward to understand: the bigger they come, the less
frequently they fall. The most recently published estimates suggest
that something the size of the Wabar impactor strikes the earth about
once a decade.

There are similar iron-meteorite craters in Odessa, Tex.; Henbury,
Australia; Sikhote-Alin, Siberia; and elsewhere. But 98 percent of
Wabar-size events do not leave a crater, even a temporary one. They are
caused by stony meteoroids, which lack the structural integrity of
metal and break up in the atmosphere. On the one hand, disintegration
has the happy consequence of protecting the ground from direct hits.
The earth has relatively few craters less than about five kilometers in
diameter; it seems that stony asteroids smaller than 100 to 200 meters
are blocked by the atmosphere. On the other hand, this shielding is not
as benevolent as it may seem. When objects detonate in the air, they
spread their devastation over a wider area. The Tunguska explosion over
Siberia in 1908 is thought to have been caused by a stony meteoroid.
Although very little of the original object was found on the ground,
the airburst leveled 2,200 square kilometers of forest and set much of
it on fire. It is only a matter of time before another Hiroshima-size
blast from space knocks out a city [see "Collisions with Comets and
Asteroids," by Tom Gehrels; Scientific American, March 1996].

By the standards of known impacts, Wabar and Tunguska are mere dents.
Many of the other collision sites around the world, including the 
Manicouagan ring structure in Quebec, and the Chicxulub site in
Mexico's northern Yucat▀n, are far larger. But such apocalypses happen
only every 100 million years on average. The 10-kilometer asteroid that
gouged out Chicxulub and snuffed the dinosaurs hit 65 million years
ago, and although at least two comparable objects (1627 Ivar and the
recently discovered 1998 QS52) are already in earth-crossing orbits, no
impact is predicted anytime soon. Wabar-size meteoroids are much more
common-and harder for astronomers to spot-than the big monsters.
Ironically, until the Wabar expeditions, we knew the least about the
most frequent events. The slag and shocked rock in the deserts of
Arabia have shown us in remarkable detail what the smaller beasts can
do.


Further Reading

AN ACCOUNT OF EXPLORATION IN THE GREAT SOUTH DESERT OF ARABIA. Harry
St. John B. Philby in Geographical Journal, Vol. 81, No. 1, pages 1đ26;
January 1933.

IMPACT CRATERING: A GEOLOGIC PROCESS. H. J. Melosh. Oxford University
Press, 1989.

"SECRET" IMPACTS REVEALED. J. Kelly Beatty in Sky & Telescope, Vol. 87,
No. 2, pages 26đ27; February 1994.

HAZARDS DUE TO COMETS AND ASTEROIDS. Edited by Tom Gehrels. University
of Arizona Press, 1995.

RAIN OF IRON AND ICE: THE VERY REAL THREAT OF COMET AND ASTEROID
BOMBARDMENT. John S. Lewis. Addison-Wesley Publishing, 1996.


The Authors

JEFFREY C. WYNN and EUGENE M. SHOEMAKER worked together at the U.S.
Geological Survey (USGS) until Shoemaker's death in a car accident in
July 1997. Both geoscientists have something of an Indiana Jones
reputation. Wynn, based in Reston, Va., has mapped the seafloor using
electrical, gravitational, seismic and remote sensing; has analyzed
mineral resources on land; and has studied aquifers and archaeological
sites around the world. He served as the USGS resident mission chief in
Venezuela from 1987 to 1990 and in Saudi Arabia from 1991 to 1995. His
car has broken down in the remote deserts of the southwestern U.S., in
the western Sahara and in the deep forest in Amazonas, Venezuela; he
has come face-to-snout with rattlesnakes, pit vipers and camel spiders.
Shoemaker, considered the father of astrogeology, was among the first
scientists to recognize the geologic importance of impacts. He founded
the Flagstaff, Ariz., facility of the USGS, which trained the Apollo
astronauts; searched for earth-orbit-crossing asteroids and comets at
Palomar Observatory, north of San Diego; and was a part-time professor
at the California Institute of Technology. At the time of his death, he
was mapping impact structures in the Australian outback with his wife
and scientific partner, Carolyn Shoemaker.

Copyright 1998, Scientific American

=====================
(4) ESTIMATION OF IMPACT ENERGY AND FREQUENCY

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

Being interested in the uncertainty about the energy and frequency of
small (<1km) impacts, I have added error bars to the conventional
log-log plot of impact energy vs typical interval between impacts.
The results are graphed at

http://www1.tpgi.com.au/users/tps-seti/Spacegd2.html#kineticenergy

For each asteroid diameter the error bars approximately span an order of
magnitude in both directions. The assumptions are set out on the web
page.

Michael Paine, TPS Australian Volunteer Co-ordinators

========================
(5) TINY BUG IN ANTARTICA HOLDS CLUES TO MASSIVE EXTINCTION -
    BUT INTERPRETATION OF FINDINGS RATHER QUESTIONABLE

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

Office of News & Public Affairs
Vanderbilt University
Nashville, Tennessee

Contact:
Beth Fox
Vanderbilt University
(615) 343-NEWS (6397)
beth.fox@vanderbilt.edu
or
Sandra Rush
Geological Society of America
(416) 585-3706 or (416) 585-3707


Tiny bug in Antarctica holds clues to massive extinction

The mass extinction that killed the dinosaurs was small compared to the
one that happened 250 million years ago, an extinction that occurred at
a rate that some scientists say is on par with today's. A Vanderbilt
researcher has discovered clues to what may have caused the earlier
extinction, an extinction that killed off almost 90 percent of the
organisms on earth.

Professor of Geology Molly Miller says that a small burrowing insect
that lived in Antarctica survived that worldwide extinction and can
provide clues to its cause.

Miller will present her discoveries at the annual meeting of the
Geological Society of America in Toronto Oct. 28.

Miller's research focuses on trails and burrows she discovered in
sandstone deposits while she was in Antarctica in the fall of 1997. The
sandstone deposits were in large riverbeds that flowed near the South
Pole at that time. From the markings on the sandstone, she deduced that
several types of marks were made by the same kind of animal, probably
an insect whose tough legs left the characteristic markings.

The burrows and trails in the sandstone are present both before and
after the extinction, showing that the insect apparently survived, in
contrast to animals that disappeared forever.

"This is significant because this is the only major extinction that
affected the insects. Even when the dinosaurs were killed, the insects
survived. But in the extinction that occurred 250 million years ago,
when many of the insects died, these river burrowers just kept going."

What exactly these survivors looked like is a bit of a mystery; Miller
envisions them as resembling the modern-day burrowing mayflies that are
used for bait in fly-fishing.

No one knows why -- or even if -- these creatures eventually died off.
They may be related to insects living today, said Miller, whose
research is funded by the National Science Foundation.

There are several theories as to why these animals survived when so
many disappeared. "Burrowing in riverbeds and feeding on dead plant
material perhaps buffered them from disastrous environmental changes,"
Miller said. "Also, living near the South Pole may have required them
to be more adaptable than those living at lower latitudes."

"This extinction was much more devastating than what killed the
dinosaurs. The animals living in the ocean were particularly hard hit,
as were many insects. To figure out the cause, we must look at how
different ecosystems were affected."

Theories abound over what caused the extinction: from massive changes
in the composition of the air, to changes in the oceans' chemistry, to
major climate changes. Miller's research on the little bug in the
Antarctic riverbeds is just one piece of the larger puzzle. "We're
going to find the answer to what happened by picking up little small
bits one at a time. This is just one of those small pieces."

What is known is that when approximately 70 to 90 percent of the life
forms on earth were killed off, it happened relatively quickly -- just
like what seems to be happening today [sic].

"The period of extinction was fairly brief -- about a million years or
so. Compare that to the period of extinction we're in now. In the past
10,000 years, many large mammals and birds have become extinct,
probably because of human activity. It is likely that each year many
species, especially insects that have never been studied, will become
extinct. Some of them probably would be beneficial to humans, but
they're gone before we can know," Miller said.

"To our human senses, this is an incomprehensible amount of time. But to
wipe out such a large number of organisms during a fairly short period of
time is really a catastrophe like what happened 250 million years ago.

"10,000 years from now, people are going to look back at this period and
be shocked at how quickly it all disappeared -- if people are even still
around."

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