PLEASE NOTE:


*

CCNet DIGEST, 28 September 1998
-------------------------------

(1) GIACOBINIDS THIS OCTOBER
    Mark Davis <MeteorObs@Charleston.Net>

(2) TSUNAMI FROM ASTEROID IMPACTS
    Michael Paine <mpaine@tpgi.com.au>

(3) DINOSAUR EXTINCTION: IMPACT vs VOLCANOES
    Bernd Pauli <bernd.pauli@lehrer1.rz.uni-karlsruhe.de>

(4) IRIDIUM NEAR K/T FOSSILS?
    Jeff Grossman <jgrossman@usgs.gov>

(5) PRESOLAR DUST GRAINS FROM PRIMITIVE METEORITES
    E. Zinner, WASHINGTON UNIVERSITY

(6) MICROMETEORITES FROM ANTARCTICA
    C. Engrand*) & M. Maurette, UCLA

(7) SCALING IMPACT MELTING & CRATER DIMENSIONS
    M.J.. Cintala*) & RAF Grieve, *)NASA


===============
(1) GIACOBINIDS THIS OCTOBER

From Mark Davis <MeteorObs@Charleston.Net>
[as posted on the meteorobs e-mail list]

******************************
NAMN Notes: October 1998
******************************

Contents:

1. Giacobinids this October...
2. Memories of Giacobinids...
3. Other October Showers...
4. Upcoming Meetings...
5. For more info...

1. Giacobinids this October...

The maximum of this year's Draconid (DRA) shower is on October 8th,
with a radiant at 262 i.e. RA 17h28m, Dec +54, and meteors may be seen
from October 6th to 10th. These meteors are slow, with velocities of
about 20 km/sec. They are referred to as Draconids as they seem to
radiate from the head of the constellation Draco, the dragon.

This shower is also referred to as the Giacobinids, after its parent
body, Comet Giacobini-Zinner, discovered by Giacobini in 1900, and
re-discovered by Zinner in 1913. Giacobini-Zinner is a short period
comet, returning about every 6.61 years. It is interesting to note that
the last year we experienced an outburst of meteors, 1946, the comet
itself was experiencing an outburst in brightness. The meteor shower
component has been observed since 1926. The comet is currently visible
in amateur telescopes at:

21P/Giacobini-Zinner
Date(00UT) R.A. (2000) Dec Mag
09-29 17h16.3m +10d35' 10.7
10-04 17h28.1m +08d42' 10.5
10-09 17h41.1m +06d44' 10.3
10-14 17h55.4m +04d40' 10.0
10-19 18h11.0m +02d28' 9.8
10-24 18h27.8m +00d11' 9.6
10-29 18h46.3m -02d13' 9.4
11-03 19h06.1m -04d43' 9.2

In a normal year, meteor rates from this comet are very low or
non-existent. However, this shower has created brief periods of storm
activity on a number of occasions. This year may be one of those
outburst years, due to the relationship in the position of our earth to
the dense portion of the cometary debris field in space. This happened
last in 1946, and a meteor storm was recorded by visual, radar, and
photographic means. In 1946 reported visual rates were about 4000 per
hour, with radar rates reaching 10,000 per hour.

One interesting feature in 1946 was a 'dip' in activity between 2 parts
of the outburst, indicating a less dense part of the debris field that
we were encountering. About 3/4 of the total outburst occurred within
one hour, and the whole duration was only 3 hours.

As the Giacobinids occur just past full moon this year, shower rates
will be hampered, but could still appear quite extraordinary. Observers
on the west coast of North America seem to be favored, and should
observe as soon as darkness is available on the evening of October 8th.
The peak of the shower is fairly narrow, with storm activity, should it
occur, probably lasting no more than several hours. Nights before and
after the maximum should also be monitored, however, in case of unusual
activity. Even negative results (i.e. little or no activity) are very
useful to record in order to help define this meteor shower better.

2. Memories of Giacobinids...

Memories of the 1946 Giacobinid storm still remain strong in the minds
of those who were fortunate enough to witness the event. One of these
people was Stan Mott, a elderly member of the Royal Astronomical
Society of Canada, who was the recorder on the Giacobinid meteor
expedition headed by the late Dr. Peter Millman. Cathy Hall talked to
Stan, now in his 70's, about some of the details of the expedition, and
his impressions of the meteor storm...

Stan traveled with Dr. Millman's group by plane to North Bay, Ontario
to observe, as the weather looked like it was going to be cloudy in
Ottawa. Apparently, it did clear off in Ottawa, but they had decided
not to take any chances. There were 4 observers, and Stan as recorder.
The sky conditions were good, and the temperature a bit cold. They used
heavy blankets and chairs, no sleeping bags, and in Stan's words,
looked like 'Tibetan monks studying the stars for omens'...

All the meteor recording was done by hand - no tape recorders! Stan
said the rates kept him 'very busy'. He said there were just so many...
that for every meteor he recorded for the group members, that he
probably saw about 6 himself. However, he said he couldn't really stop
to watch the sky a lot - as he was the recorder. He said it 'really did
look like a shower', and that 'the meteors were coming fast and
furious, with several at any instant'.

There were so many meteors that they just started watching specific
areas, like the head of Draco. "It looked like the eyes were just
winking" Stan said, with all the many point meteors. When asked if
there were lots of both bright and faint meteors, he said that most
seemed to be about magnitude 2... but then added that they gave up on
anything fainter than about magnitude 3! He said there were a mixture
of long and short meteors, and that some had trains. Most of the
meteors appeared to be white in color.

How did they manage to record the meteors with so many coming down?
Stan replied 'poorly' and then smiled...

Another friend in Ottawa also observed the 1946 Giacobinids. Mary
Henderson, then a girl of about 16, watched the display from the
countryside just east of Ottawa. She had first noticed the shower from
the driveway of her house in the city, and got her father to drive her
out into the country. This was the first astronomical event she had
ever taken note of, so was not familiar with the normal data that one
would want to record.

Having since observed meteors seriously, however, she has been able to
give some comments on the 1946 storm. She said the sky was dark and
clear in Ottawa. The meteors seemed shorter than Perseids, or that was
the impression she remembers. There were a mixture of magnitudes. She
doesn't recall whether there were trains or not, as she didn't know
what a train was at the time. When asked if there was any color to the
meteors, or if they just were mainly white in color, she said that she
'didn't realize that stars had any color' at the time, so no, did not
note any in the meteors.

She watched for several hours, and in her words, 'was just totally
overwhelmed at the marvelous display'. When asked if she noticed any
lull in activity, she said no, she had no impression of any lull. They
were just coming down 'so fast and furious'.

Mary went on after that to become a summer student at the Dominion
Observatory in Ottawa in the summer of 1951, and was given the project
in the summer of 1952 of helping Dr. Millman analyze photographs of the
Giacobinids taken by a news photographer in Chicago...

3. Other October Showers...

The Orionids (ORI) are the major shower of October, and are a reliable
yearly shower, with a ZHR, zenithal hourly rate, of about 20 meteors
per hour with the naked eye. The maximum is on October 21st, with a
radiant at 095 i.e. RA 06h20m, Dec +16. Shower members can be seen from
about October 2nd to November 7th. The meteors are fast, at about 66
km/sec, and have as their parent body, Comet Halley.

The Orionids are an excellent shower for new observers. The rates
remain high for at least a couple days, and many of the meteors have
trains left behind them. In recent years, some increased numbers of
brighter meteors have also been seen. The conditions are excellent this
year as the shower is close to new moon. At the maximum, the radiant is
near the left foot of Gemini, in the top left part of the constellation
of Orion.

There are a number of minor showers in October as well. Minor showers
have much weaker rates, usually only several meteors per hour, even on
their best night, so extra care must be taken when observing them.

The October Arietids (OAR) reach a maximum on October 8th, near full
moon. The radiant is at RA 02h08m, Dec +08. The meteors are slow, at
about 28 km/sec. They can be seen all month, but the rates are very
low.

The epsilon Geminids (EGE) reach maximum on October 18th, close to new
moon. The radiant is at 102 i.e. RA 06h48m, Dec +27. These are fast
meteors, at about 70 km/sec. They can be seen in the last half of
October, from about the 14th to the 27th. The rates are also low, only
about 2 meteors per hour, on the maximum night. The parent body is
believed to be either Comet Ikeya 1964VIII, or Comet
Nishikawa-Takamizawa-Tago 1987III.

Lastly, the Leo Minorids (LMI) reach a very weak maximum on October 22nd at
RA 10h48m, Dec +37, with fast meteors, about 62 km/sec. Their activity
period, besides being extremely weak, is also very short, from about
October 21st to 23rd.

4. Upcoming Meetings...

The Asteroids, Comets, Meteors 1999 Conference is being held July
26-30, 1999 at Cornell University, near Ithaca, in New York State.
Details are available at their website:
http://scorpio.tn.cornell.edu/ACM/. You can also leave your name and
address, to be contacted with more information. A number of North
American amateurs are planning to attend...

5. For more info...

Contact:
Mark Davis, MeteorObs@charleston.net
Mt. Pleasant, South Carolina, USA
Coordinator, North American Meteor Network

And check out:
NAMN home page:
http://medicine.wustl.edu/~kronkg/namn.html

Here's to 'Clear Skies' for October!...

October 1998 NAMN Notes co-written
by Mark Davis and Cathy Hall

====================================
(2) TSUNAMI FROM ASTEROID IMPACTS

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

I have added two new sections to my Tsunami web page
http://www1.tpgi.com.au/users/tps-seti/spacegd7.html. The first
discusses the effects of a Poisson distribution and how one "Tunguska"
event in the last 200 years (if that is the case) has the same
probablity as 2 events, even though the average interval between events
might be 100 years (see also Duncan Steel's book) The second provides a
sensitivity analysis of the estimated chance of the East Coast of
Australia being struck by a tsunami generated by a 50m asteroid
(varying the average interval between events from 50 yrs to 500 yrs and
the radius at which a 10m tsunami is formed from 1,000km to 5,000km).

Michael Paine
NSW Co-ordinator,
The Planetary Society Australian Volunteer Co-ordinators

================
(3) DINOSAUR EXTINCTION: IMPACT vs VOLCANOES

From Bernd Pauli <bernd.pauli@lehrer1.rz.uni-karlsruhe.de>

I unearthed another interesting and controversial K/T abstract in
Meteoritics 25-4, 1990, A412-413:

M e t e o r i t i c versus  v o l c a n i c  events at the Cretaceous /
Tertiary boundary - An Australian perspective. F.L. Sutherland. Division
of Earth Sciences, The Australian Museum, 6-8 College Street, Sydney,
NSW, 2000, Australia.

A review of the end Cretaceous extinction debate (Sutherland, 1988)
concluded that the cause 'was a coincidence of both impact and volcanic
cycles coming together, but not necessarily the first directly causing
the other.' New literature expands such views on dual, but uncoupled
causes.

Arguments for cosmic body impact are reinforced by:

(1) organic molecular results on soot from global fires
(2) extra-terrestrial amino acids at the boundary
(3) ages of 65-67 Ma for Manson Crater, USA, a suitable source for
    shocked quartz
(4) iridium anomalies at older impacts, eg., Lake Acraman, South
    Australia.

Arguments for volcanic fallout are supported by:

(1) Ir enrichment in volcanic dust bands in Antarctic blue ice
(2) multiple Ir anomalies across the Bavarian boundary
(3) similar smectite clays at the Danish Ir anomaly and at higher levels
    which lack Ir
(4) basaltic 'feldspars' at that boundary
(5) stronger confirmation for rapid Deccan eruptions at the boundary
(6) sharp Ir anomalies related to volcanism at other American extinction
    events
(7) differences in platinum group element patterns between northern and
    southern hemispheres, weakening an exclusive global impact.

Sufficient eruptives to give observed Ir levels at the boundary are
critical for volcanic arguments.  Deccan basalts give a large hot spot
source, particularly as early eruptives seem more mafic.  Another is
proposed for NE Australia. The 65 Ma Coral Sea spreading rift of
1500-2000 km diameter approaches plume sizes for flood basalts. Its
link to east Australian hot spots was criticised on absolute motion
modelling, but several southern hemisphere hot spot tracks show similar
discrepancies. The great coincidence in size and shape of the structure
to hot spot distribution is preferred for correlation. Deccan, Coral
Sea, Cameroon Line and possibly Louisville hot spots provide major
southern 65 Ma volcanism to give a 'basaltic' geochemical pattern in
the New Zealand boundary layer.  Greenland (Kap Washington Voleanics)
and Hawaii probably contribute to 65 Ma northern volcanism. Global
correlations in hot spot distributions and lower mantle features
suggest plume sources enriched in Ir. Thus, 65 Ma volcanism seems
adequate for boundary Ir anomalies, allowing for secondary enhancements.

Northern hemisphere impact for some boundary contribution seems
sustained, but only a few smallish potential craters are recognised.
The multiple Ir anomalies require spaced impacts.  Suggestions that
large hot spots are triggered by impacts require multiple strikes in
the southern hemisphere. A claim for a shock origin for lamellar quartz
below Deccan basalts is generally considered unproven. Coincidental,
but independent, northern impacts and largely southern hot spot
outbursts seem to fit present data. Recent ideas stress the improbable
in Earth's evolution.  Simultaneous smaller cratering impacts, larger
hot spot inducing impacts and massive volcanism, however, may represent
an overkill for the extinction boundary.

Whatever the causes, the biologic record for Cretaceous extinctions
seems to be stepped, with decimations increasing amongst terrestrial
animals, land plants and marine organisms respectively.  Dinosaur
extinction may stagger beyond the boundary event. A demise due to
decrease in atmospheric oxygen is not confirmed by further studies of
air bubbles in Mesozoic and Tertiary ambers.

Reference: Sutherland F.L. (1988) J. Proc. Roy. Soc. (NSW 121, 123-164).

Best wishes,

Bernd

=======================
(4) IRIDIUM NEAR K/T FOSSILS?

From Jeff Grossman <jgrossman@usgs.gov>

I have seen several good talks on this subject by paleobiologists from
the Smithsonian, most recently one by Dr. Kay Behrensmayer.  It is true
that vertebrate fossils have not been found in association with Ir-rich
boundary layers. But, this is probably not due to global "immolation"
of the corpses. Dinos must have been dying and falling into swamps and
rivers right up to the time of the terminal Cretaceous impact, so there
ARE probably a "normal" number of fossils from this time in sedimentary
rocks somewhere.  The problem is that large vertebrate fossils are very
rare, and the bone-beds needed to ensure a good date are even rarer. 
Given the restricted areas in which we can look for fossils from the
late Cretaceous, and the poor odds of finding animals of any age, no
less a particular age, it is not at all surprising that we haven't
found any right at the boundary layer. You have to remember that
dinos were big animals living mostly on land, and they didn't give rise to
extensive fossil deposits as did, e.g., many small marine species.
Similarly, you can't find the Ir layer everywhere (although you can
find it in many places), and you'd have to find your fossils in one of
the places where the boundary layer is well exposed. Moreover, with any
fossil that you do find, there is always the problem of trying to
decide if it was transported, or if the sediments were reworked at a
later time.  Single fossils are always going to present you with this
problem, making interpretation of any find from right near the
boundary difficult.

The bottom line here is that we are limited by very poor statistics. If
bone-beds of dino fossils DO exist right at the boundary, there are
good reasons why we have not found them.

jeff

p.s. This whole discussion reminds me of questions people asked in
1980, after the initial discovery: why don't you find dino fossils
sheared off at the knees by the initial shock waves?


Dr. Jeffrey N. Grossman       phone: (703) 648-6184
US Geological Survey          fax:   (703) 648-6383
954 National Center
Reston, VA 20192, USA

===============
(5) PRESOLAR DUST GRAINS FROM PRIMITIVE METEORITES

E. Zinner: Trends in the study of presolar dust grains from primitive
meteorites. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.4,
pp.549-564

WASHINGTON UNIVERSITY, MCDONNELL CTR SPACE SCI, ST LOUIS, MO, 63130

A series of trends can be discerned in the study of presolar dust
grains from primitive meteorites, and these trends might give us hints
in which direction this new field of astronomy is developing. They
include: (1) a focus on ever smaller components of meteorites; (2) a
shift from the study of the elemental abundances in the solar system to
the study of isotopic abundances; (3) a shift of emphasis from averages
of the isotopic abundances as represented by the whole solar system to
individual isotopic components preserved in circumstellar dust grains;
(4) the preferential study of rare types of presolar dust grains; (5)
the emergence of new technical capabilities for the study of individual
presolar dust grains; examples include isotopic imaging and resonance
ionization mass spectrometry (RIMS); and (6) a shift from a situation
in which grain data confirm previously held theoretical ideas to a
situation in which the experimental data impose new constraints on
theoretical models of nucleosynthesis, stellar mixing and grain
formation in stellar outflows. In other words, the data do not confirm
but drive the theory. An example is the distribution of Si isotopic
ratios in individual mainstream SiC grains for which many different
theoretical explanations have been offered. There are still many
unsolved problems posed by the grain data, the most difficult being the
interpretation of the isotopic ratios of grains with a supernova
signature (evidence for Ti-44 and excesses in Si-28) in terms of
theoretical models of nucleosynthesis and the mixing of supernova
ejecta. Future progress is expected to come from the analysis of larger
numbers of grains, the search for new types of presolar grains, the
analysis of smaller grains and of more elements in a given grain, both
made possible by the increase in sensitivity of ion microprobes and the
extended application of RIMS, from multi-dimensional models of stellar
evolution with enlarged nuclear networks, and from new measurements of
nuclear cross sections. Copyright 1998, Institute for Scientific
Information Inc.

========================
(6) MICROMETEORITES FROM ANTARCTICA

C. Engrand*) & M. Maurette: Carbonaceous micrometeorites from
Antarctica. METEORITICS & PLANETARY SCIENCE, 1998, Vol.33, No.4,
pp.565-580

*) UCLA, DEPT EARTH & SPACE SCI, LOS ANGELES, CA, 90095

Over 100 000 large interplanetary dust particles in the 50-500 mu m
size range have been recovered in clean conditions from similar to 600
tons of Antarctic melt ice water as both unmelted and partially
melted/dehydrated micrometeorites and cosmic spherules. Flux
measurements in both the Greenland and Antarctica ice sheets indicate
that the micrometeorites deliver to the Earth's surface similar to
2000x more extraterrestrial material than brought by meteorites.
Mineralogical and chemical studies of Antarctic micrometeorites
indicate that they are only related to the relatively rare CM and CR
carbonaceous chondrite groups, being mostly chondritic carbonaceous
objects composed of highly unequilibrated assemblages of anhydrous and
hydrous minerals. However, there are also marked differences between
these two families of solar system objects, including higher C/O ratios
and a very marked depletion of chondrules in micrometeorite matter;
hence, they are ''chondrites-without-chondrules.'' Thus, the parent
meteoroids of micrometeorites represent a dominant and new population
of solar system objects, probably formed in the outer solar system and
delivered to the inner solar system by the most appropriate vehicles,
comets. One of the major purposes of this paper is to discuss
applications of micrometeorite studies that have been previously
presented to exobiologists but deal with the synthesis of prebiotic
molecules on the early Earth, and more recently, with the early history
of the solar system. Copyright 1998, Institute for Scientific
Information Inc.

===============
(7) SCALING IMPACT MELTING & CRATER DIMENSIONS

M.J. Cintala*) & RAF Grieve: Scaling impact melting and crater
dimensions: Implications for the lunar cratering record. METEORITICS &
PLANETARY SCIENCE, 1998, Vol.33, No.4, pp.889-912

*) NASA, JOHNSON SPACE CTR, PLANETARY SCI BRANCH, CODE SN2, HOUSTON,
   TX, 77058

The dimensions of large craters formed by impact are controlled to a
large extent by gravity, whereas the volume of impact melt created
during the same event is essentially independent of gravity. This
''differential scaling'' fosters size-dependent changes in the dynamics
of impact-crater and basin formation as well as in the final
morphologies of the resulting structures. A variety of such effects can
be observed in the lunar cratering record, and some predictions can be
made on the basis of calculations of impact melting and crater
dimensions. Among them are the following: (1) as event magnitude
increases, the volume of melt created relative to that of the crater
will grow, and more will be retained inside the rim of the crater or
basin. (2) The depth of melting will exceed the depth of excavation at
diameters that essentially coincide with both the inflection in the
depth-diameter trend and the simple-to-complex transition. (3) The
volume of melt will exceed that of the transient cavity at a cavity
diameter on the order of the diameter of the Moon; this would arguably
correspond to a Moon-melting event. (4) Small lunar craters only rarely
display exterior flows of impact melt because the relatively small
volumes of melt created can become choked with clasts, increasing the
melt's viscosity and chilling it rapidly. Larger craters and basins
should suffer little from such a process. (5) Deep melting near the
projectile's axis of penetration during larger events will yield a
progression in central-structure morphology; with growing event
magnitude, this sequence should range from single peaks through
multiple peaks to peak rings. (6) The minimum depth of origin of
central-peak material should coincide with the maximum depth of
melting; the main central peak in a crater the size of Tycho should
have had a preimpact depth of close to 15 km. Copyright 1998, Institute
for Scientific Information Inc.


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