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 Yucatßn, 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
1Ð26;
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 26Ð27; 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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