PLEASE NOTE:
*
CCNet, 049/2000 - 14 April 2000
-------------------------------
"[The Woodleigh impact] structure
is the largest proven impact
structure discovered to date on the
Australian continent and the
fourth largest in the world after
Vredefort in South Africa
(diameter of 300 km), the Sudbury in
Canada (250 km), and the
Chicxulub in the Gulf of Mexico
(diameter of about 170 km). [...]
The size of the Woodleigh impact
structure suggests that it may
have had a significant role in the
tectonic evolution of the
Southern Carnarvon Basin. Apart from
such tectonic implications,
an impact of this size would have caused
catastrophic
environmental effects that, on the
present dating of Woodleigh,
may correlate with one of three
extinction events known in the
fossil record: the Late Devonian
extinction (364 million years),
the end of the Permian (247 million
years), and the end of the
Triassic (214 million years)."
-- Arthur Mory, Robert
Iasky, Andrew Glikson & Franco Pirajno -
Australian discoverers of the Woodleigh impact crater
(1) FOURTH LARGEST IMPACT STRUCTURE DISCOVERED IN AUSTRALIA
Arthur Mory <A.Mory@dme.wa.gov.au>
(2) IMPACT AT HIGH NOON
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
(3) RESEARCHERS REDUCE PLANETARY IMPACT RATES DUE TO
ECLIPTIC COMETS
H.F. Levison et al., SW RESEARCH INSTITUTE
(4) 1996 PW & 1997 SE5: EXTINCT COMETS OR D-TYPE ASTEROIDS?
M.D. Hicks et al., CALTECH,JET PROP LAB
(5) COLLISIONAL EVOLUTION IN THE VULCANOID REGION
S.A. Stern & D.D. Durda, SW RES INST,DEPT
SPACE STUDIES
(6) ESTIMATED ABUNDANCE OF ATENS & IEOS
P. Michel et al., OSSERV ASTRON TORINO
(7) RADAR MICROMETEORS
D. Janches et al., PENN STATE UNIVERSITY
(8) SOFT DISASTERS: SCIENCE 'NOT ENOUGH' TO ALLAY FEARS
BBC News Online, 14 April 2000
(9) AND FINALLY: REDUCED FARTING COULD SAVE WORLD, SCOTTISH
SCIENTISTS CLAIMS
YAHOO! News, 12 April 2000
=============
(1) FOURTH LARGEST IMPACT STRUCTURE DISCOVERED IN AUSTRALIA
From Arthur Mory <A.Mory@dme.wa.gov.au>
Dear Benny,
Your readers may be interested in our paper in Earth and
Planetary
Science Letters, Vol. 177 (1-2) p. 119-128 on the newly
discovered
120-km diameter Woodleigh impact structure in the Carnarvon
Basin,
Western Australia.
(http://www.elsevier.nl/gej-ng/10/18/23/80/25/34/article.html).
Regards
Arthur Mory 1) <a.mory@dme.wa.gov.au>
Robert Iasky 1) <r.iasky@dme.wa.gov.au>
Andrew Glikson 2) <geospectral@spirit.com.au>
Franco Pirajno 1) <f.pirajno@dme.wa.gov.au>
1) Geological Survey of Western Australia, 100 Plain St., East
Perth,
W.A. 6004, Australia
2) Research School of Earth Science, Institute of Advanced
Studies,
Australian National University, Canberra, A.C.T.
0200, Australia
We report a newly discovered multi-ring impact structure on the
Gascoyne Platform of the Southern Carnarvon Basin in Western
Australia.
The centre of the structure (latitude 26º03'25"S, longitude
114º39'50"E) lies on Woodleigh Station (after which it is
named)
approximately 160 km south-southeast of Carnarvon and directly
east of
Hamelin Pool in Shark Bay. The Woodleigh impact structure is not
recognisible from present day topographic features as it is
covered by
flat-lying Cretaceous and Lower Jurassic strata, but it has a
multi-ring gravity signature that is characteristic of impact
structures. The structure is the largest proven impact structure
discovered to date on the Australian continent and the fourth
largest
in the world after Vredefort in South Africa (diameter of 300
km), the
Sudbury in Canada (250 km), and the Chicxulub in the Gulf of
Mexico
(diameter of about 170 km).
An impact origin is indicated by: a central core of uplifted
granitoid
basement probably less than 25 km in diameter, which displays
shock-induced planar deformation features in quartz (the single
most
diagnostic criterion for an impact origin), pervasive diaplectic
vitrification of feldspar and penetrative pseudotachylite
veining, and
an inner ring syncline containing a ~70 m thick thermally
modified
diamictite overlain by ~380 m of Lower Jurassic lacustrine
deposits. An
outermost diameter of 120 km, defined by gravity, magnetic and
surface
drainage, indicates a ring fault that sharply intersects the
NS-striking regional structure. At the centre of the basement
uplift
shock metamorphosed granitoid was intersected at a depth of 171 m
- at
least 1800 m higher than the gravity-modelled level of regional
basement. Pseudotachylite veins systems within the shocked
granitoid
are strongly enriched in Al, Ca, Mg, Ni, Co, Cr, V and S, and
depleted
in K and Si, suggesting chemical fractionation attendant on shock
volatilisation, enrichment by an injected and volatilised
meteoritic
component, and potentially of sulphide mineralisation.
The impact age is constrained by overlying Lower Jurassic strata,
reworked Early Permian palynomorphs, Late Devonian (Frasnian/
Fammenian
boundary) K/Ar ages derived from the base of the lacustrine
section,
and deformed Lower Devonian and older units.
The size of the Woodleigh impact structure suggests that it may
have
had a significant role in the tectonic evolution of the Southern
Carnarvon Basin. Apart from such tectonic implications, an impact
of
this size would have caused catastrophic environmental effects
that, on
the present dating of Woodleigh, may correlate with one of three
extinction events known in the fossil record: the Late Devonian
extinction (364 million years), the end of the Permian (247
million
years), and the end of the Triassic (214 million years).
============
(2) IMPACT AT HIGH NOON
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
NEAR image of the day for 2000 April 13
http://near.jhuapl.edu/iod/20000413/index.html
For people who live in mountain valleys, daylight can be a
short-lived
experience. Mountains to the east make the Sun rise later
in the
morning, while mountains to the west make the Sun set
earlier in
the evening. The brevity of daytime in a valley is accentuated
during
the shortened days of winter.
The floor of the 5.5-kilometer (3.4-mile) diameter
impact
crater that dominates one face of Eros experiences the
ultimate in short daylight hours. Three factors
conspire to make
this true. Firstly, Eros rotates rapidly, once
every 5.27 hours.
Secondly, the crater's 0.9-kilometer (0.56-mile) high
walls tend to
block direct sunlight from the floor, even while the
outside of
the crater is illuminated. Thirdly, during the current season
on
Eros, the Sun never makes it high in the sky in this location.
This NEAR Shoemaker image, taken April 1, 2000, from a
range of 209
kilometers (130 miles), caught the crater near local noon with
the Sun
highest in the sky. On that day, a hardy astronaut
standing at the
bottom of the crater would have experienced 1 hour
and 45 minutes of
daylight.
-----------------
NEAR image of the day for 2000 April 12
http://near.jhuapl.edu/iod/20000412/index.html
The Record of What Came Before
The most common, pervasive, and scientifically informative
landform on Eros is impact craters. This image from NEAR
Shoemaker, taken March 20, 2000, from a range of 206
kilometers
(128 miles), shows a highly cratered region of the
asteroid. The
whole scene is 7.4 kilometers (4.6 miles) across. Craters are
formed
by the explosive impacts of smaller asteroid
fragments, which
constantly rain onto the surface over
the eons. A freshly
exposed surface will have fewer craters than a surface
exposed to
space for a longer time, because it hasn't experienced the rain
of
projectiles for as long a duration. The great
number of impact
craters in the region shown here shows that it
has been an
extremely long time since this region was wiped clean of
craters,
or "resurfaced," by a geologic
process such as the chipping
off of part of the asteroid. Also, the much greater
abundance of
smaller craters than larger craters says that small
asteroid
fragments have impacted Eros much more frequently than large
asteroid fragments.
--------------------------------------------------------
Built and managed by The Johns Hopkins University Applied Physics
Laboratory, Laurel, Maryland, NEAR-Shoemaker was the first
spacecraft
launched in NASA's Discovery Program of low-cost, small-scale
planetary missions. See the NEAR web site for more details
(http://near.jhuapl.edu).
================
(3) RESEARCHERS REDUCE PLANETARY IMPACT RATES DUE TO
ECLIPTIC COMETS
H.F. Levison, M.J. Duncan, K. Zahnle, M. Holman, L. Dones:
Planetary
impact rates from ecliptic comets. ICARUS, 2000, Vol.143, No.2,
pp.415-420
*) SW RESEARCH INSTITUTE,SPACE STUDIES DEPT,BOULDER,CO,80302
We have reevaluated the impact rates for the planets from
ecliptic
comets using the integrations in H. Levison and M. Duncan (1997,
Icarus
127, 13-32; LD97), We fmd that the current impact rates on the
giant
planets are actually about four times smaller than LD97's values
due to
differences in methods of calculating the relevant timescales.
The
newly calculated impact rates are listed in Table I. However, if
the
objects leaving the Kuiper belt were primarily on high
inclination
orbits, then the impact rates on the giant planets are larger
than
those in Table I by a factor less than or similar to 2. We
discuss the
dynamics of objects hitting the giant planets in detail,
including
measurements of the impact velocities. We find that 21% of the
objects
that hit Jupiter in our simulations were bound to the planet
before the
impact. The fraction of bound impactors for Saturn is much lower,
Jupiter, Saturn, and Uranus have a significant apex-antapex
asymmetry
for the unbound impactors. (C) 2000 Academic Press.
===========
(4) 1996 PW & 1997 SE5: EXTINCT COMETS OR D-TYPE ASTEROIDS?
M.D. Hicks*), B.J. Buratti, R.L. Newburn, D.L. Rabinowitz:
Physical
observations of 1996 PW and 1997 SE5: Extinct comets or D-type
asteroids?.ICARUS, 2000, Vol.143, No.2, pp.354-359
*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,MS
183-501,PASADENA,CA,91109
The minor planets 1996 PW and 1997 SE5 are two of the few known
asteroids with orbital elements typical of long-period and
Jupiter-family comets and as such represent strong candidates
for extinct cometary nuclei. We obtained filter photometry of
1996 PW
and filter photometry and medium-resolution CCD spectroscopy of
1997
SE5 during their discovery apparitions. We also observed a suite
of
D-type asteroids as possible spectral analogs of cometary nuclei.
Both
1996 PW and 1997 SE5 have moderately red, featureless spectra
typical
of the D-type asteroids, cometary nuclei, and other extinct
cometary
candidates. The photometry for 1997 SE5 was fit by a
triple-peaked
lightcurve with a period of 9.050 +/- 0.005 h and an amplitude of
0.4
magnitude, suggesting a relatively complex and elongated shape.
With
this work, 1997 SE5 and 1996 PW join the ranks of 3552 Don
Quixote and
944 Hildago as established candidates for extinct comet nuclei.
(C)
2000 Academic Press.
===========
(5) COLLISIONAL EVOLUTION IN THE VULCANOID REGION
S.A. Stern*), D.D. Durda: Collisional evolution in the Vulcanoid
region: Implications for present-day population constraints.
ICARUS,
2000, Vol.143, No.2, pp.360-370
*) SW RES INST,DEPT SPACE STUDIES,BOULDER,CO,80302
We explore the effects of collisional evolution on putative
Vulcanoid
ensembles in the region between 0.06 and 0.21 AU from the Sun in
order
to constrain the probable population density and population
structure
of this region today. Dynamical studies have shown that the
Vulcanoid
Zone (VZ) could be populated. However, we find that the frequency
and
energetics of collisional evolution this close to the Sun,
coupled with
the efficient radiation transport of small debris out of this
region,
together conspire to create an active and highly intensive
collisional
environment that depletes any very significant population of
rocky
bodies placed in it, unless the bodies exhibit orbits that are
circular
to similar to 10(-3) or less or highly lossy mechanical
properties that
correspond to a fraction of impact energy significantly less than
10%
being imparted to ejecta. The most favorable locale for residual
bodies
to survive in this region is in highly circular orbits near the
outer
edge of the dynamically stable Vulcanoid Zone (i.e., near 0.2
AU),
where collisional evolution and radiation transport of small
bodies and
debris proceed most slowly. If the mean random orbital
eccentricity in
this region exceeds similar to 10(-3), then our work suggests it
is
unlikely that more than a few hundred objects with radii larger
than 1
km will be found in the entire VZ; assuming the largest objects
have a
radius of 30 km, then the total mass of bodies in the VZ down to
0.1 km
radii is likely to be no more than similar to 10(-6) M+,
<10(-3) the
mass of the asteroid belt. A 0.01-AU-wide ring near the outer
stability
boundary of the VZ at 0.2 AU would likely not contain over a few
tens
of objects with radii larger than 1 km. Despite the dynamical
stability
of large objects in this region (Evans, N. W., and S. Tabachnik,
1999,
Nature 399, 41-43), it is plausible that the entire region is
virtually empty of kilometer-scale and larger objects. (C) 2000
Academic Press.
==========
(6) ESTIMATED ABUNDANCE OF ATENS & IEOS
P. Michel*), V. Zappala, A. Cellino, P. Tanga: Estimated
abundance of
Atens and asteroids evolving on orbits between Earth and Sun.
ICARUS,
2000, Vol.143, No.2, pp.421-424
*) OSSERV ASTRON TORINO,I-10025 PINO TORINESE,ITALY
The so-called Aten asteroids, evolving on orbits with semimajor
axis a
< 1 AU and aphelion distance Q > 0.983 AU, spend most of
their time
inside Earth's orbit. Currently, they account for about 7% of the
observed near-Earth asteroid population and 13% of the
Earth-crossing
one. However, observational biases play against their discovery;
thus
the present number of Atens is probably severely underestimated.
Another still unobserved population of asteroids, which we call
IEO
(inner-Earth objects), could also exist and evolve entirely
inside
Earth's orbit. Orbital numerical integrations of known source
populations of Earth-crossers indicate that the real fraction of
Atens
and IEOs in the Earth-crossing population depends on the
considered
objects' diameters and could be close to 20% considering only the
multikilometer bodies. Moreover, the fraction of IEOs could be as
large
as half that of Atens. Since these asteroids can also impact
Earth,
their threat should seriously be taken into account. (C) 2000
Academic
Press.
==========
(7) RADAR MICROMETEORS
D. Janches*), J.D. Mathews, D.D. Meisel, V.S. Getman, Q.H. Zhou:
Doppler studies of near-antapex UHF radar micrometeors. ICARUS,
2000,
Vol.143, No.2, pp.347-353
*) PENN STATE UNIVERSITY,COMMUN & SPACE SCI LAB,316 EE
EAST,UNIVERSITY PK,PA,16802
A 'radar micrometeor' is the radar-scattering signature from the
free
electrons in the plasma generated by entry of a dust-sized
meteoroid
into the atmosphere. We report the first direct Doppler
measurements,
made using the Arecibo Observatory 430-MHz radar, of the
so-called
meteor head echo. Our observations demonstrate that this region
is
moving with the speed of the meteoroid as determined from the
meteor
head-echo altitude-time trajectory and that this radar return is
distinct spatially and in velocity from the much more commonly
observed
trail echo. We also report the first observations of near-antapex
micrometeors which are characterized by the very slow atmospheric
speeds expected from low-ecliptic-inclination objects entering
the
atmosphere from behind Earth's orbital path. Of the 32 meteors
observed
during four early evening hours of observations on 10 January
1997,
velocities were determined for 18 of the meteors of which 7 were
at or
just below Earth escape velocity (11.2 km/s), We give
heliocentric
orbits for the 11 meteor events with speeds greater than the
escape
velocity and present a detailed analysis of these orbital
parameters
and their possible origins. One particle was determined to be
interstellar: a preliminary analysis indicates that the ecliptic
coordinates of the radiant relative to the local standard of rest
(LSR)
(with the solar motion relative to the nearby stars removed) are
lambda
= 43.02 degrees, beta = -43.28 degrees, V = -25.11 km/s or, in
system
II galactic coordinates l(II) = 219.8 degrees, b(II) = -52.4
degrees, V
= -25.1 km/s. (C) 2000 Academic Press.
===============
(8) SOFT DISASTERS: SCIENCE 'NOT ENOUGH' TO ALLAY FEARS
From The BBC News Online, 14 April 2000
http://news.bbc.co.uk/hi/english/sci/tech/newsid_711000/711753.stm
By environment correspondent Alex Kirby
In highly technological societies, governments can no longer rely
on
science alone to win consent for potentially risky new
developments,
according to social scientists.
The scientists say more crises of public trust, like those over
mad cow
disease (BSE) and genetically-modified (GM) food, are almost
inevitable.
They think likely candidates for new concern are the safety of
mobile
telephones, and the effects of chemicals which disrupt human
hormones.
And British Nuclear Fuels, now embroiled in a dispute over the
falsification of data at its Sellafield plant in north-west
England,
could be another.
The scientists, from the Global Environmental Change Programme
(GECP)
at the University of Sussex, are speaking at a conference on
sustainability.
Destroying trust
They classify the BSE and GM crises as examples of "soft
disasters",
which kill few people or even none, but destroy public trust in
politicians.
A GECP report last year said the current approach to such risk
issues
"would inevitably lead to political and environmental crises
that
emerge only slowly but at great cost to society".
The GECP team, which is funded by the Economic and Social
Research
Council, says soft disasters require new public policy
approaches.
"While enhanced scientific information is important to
inform debates,
it is not enough on its own to determine the best way forward.
"Research shows that values and subjective factors are
central to
decisions on such issues, so a new, more transparent and
participative
style of decision-making is required.
"Scientific institutions need to develop their ability to
respond
rapidly to emerging risk issues.
Endemic disasters
"There is also an urgent need to develop more sophisticated
science
that is capable of monitoring and establishing cause and effect
around
highly complex environmental risk factors."
The GECP scientists say soft disasters "are endemic in
highly
technologically advanced risk societies. Governments therefore
need to
develop much better capabilities to handle these issues."
They say ordinary people are not as ignorant about the issues
involved
in debates like the one over GM foods as politicians are inclined
to
believe.
This is partly because science cannot provide definitive answers
about
the safety of such new technologies, something which people know
from
their experience.
Yet public trust is central to effective decisions on risk
issues,
although it is both difficult to establish and fragile to
maintain.
The director of the GECP, Dr Frans Berkhout, told BBC News
Online: "The
central, co-ordinating parts of government such as the Cabinet
Office
seem to have grasped the issue.
Few practical changes
"They understand both the importance of soft disasters and
the changes
needed in how they make decisions.
"Other parts of government are not doing so well. They often
acknowledge the value of our analysis, but have made few attempts
to
tackle the issues in practical ways.
"This is not surprising, as soft disasters are
extraordinarily
difficult to handle. There are no magic bullets."
The GECP is calling for "a new style of governance in which
scientific
evidence plays an important but not dominant role. Public values
should
inform the questions asked of the science rather than being
addressed
as a token 'add-on'."
Copyright 2000, BBC
========
(9) AND FINALLY: REDUCED FARTING COULD SAVE WORLD, SCOTTISH
SCIENTISTS CLAIMS
From YAHOO! News, 12 April 2000
http://dailynews.yahoo.com/h/nm/20000412/sc/science_warming_2.html
Scientists: New Animal Feed Can Cut Global Warming
LONDON (Reuters) - Scientists in Scotland said on Wednesday they
have
come up with a novel method of reducing global warming.
By adding a bacteria supplement to feed for farm animals such as
sheep
and cattle, they said it was possible to cut the amount of
methane, a
major greenhouse gas, the animals produce when they break wind.
Preliminary tests of the method, which will be presented at a
conference in France next month, have been promising. "We
discovered
that we could cut the amount of methane produced by almost 50
percent
by adding the bacteria Brevibacillus parabrevis," Dr. Jamie
Newbold,
of the Rowett Research Institute in Aberdeen, said in a
statement.
When Newbold tested the feed in sheep the amount of methane each
animal
produced per day fell by an average of about four liters.
If the method is incorporated on a worldwide scale, Newbold
estimates
it could reduce methane output by about six percent.
MORE HOT AIR at
http://dailynews.yahoo.com/h/nm/20000412/sc/science_warming_2.html
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*
CCNet-ESSAY, 14 April 2000
--------------------------
DISCOVERY OF THE WORLD'S FOURTH LARGEST
IMPACT, AND THE TALE
OF TWO CRATERS
By Andrew Glikson <andrew.glikson@anu.edu.au>
Research School of Earth Science,
Australian National University
Canberra, ACT 0200
As Eugene and Carolyn Shoemaker knew so well during their 13
years-long
(1984-1997) exploration in the Australian outback, for every
eventually-proven extraterrestrial impact crater there are
numerous
false alarms. The question is often raised whether any particular
circular feature, be it a lake, a geological dome, or a
geophysical
anomaly, may be of impact origin, even though positive
identification
depends on the occurrence of diagnostic criteria for shock
metamorphism, i.e. planar deformation features (in quartz,
feldspar or
zircon), shatter cones, impact melt, contribution by meteoritic
components including siderophile elements (Ni, Co, Cr, V) and
platinum
group elements (PGE) anomalies, and other criteria. Compounding
the
enthusiasm of crater hunters is the geo-centric philosophy which
lingers among many geologists, denying the reality and the
essential
role of asteroid and comet impacts in Earth history.
. . .[continued]