PLEASE NOTE:
*
Date sent: Thu, 13 Nov
1997 15:47:28 -0500 (EST)
From:
Benny J Peiser <B.J.PEISER@livjm.ac.uk
Subject:
CC-DIGEST 13/11/97
To:
cambridge-conference@livjm.ac.uk
Priority: NORMAL
CAMBRIDGE-CONFERENCE DIGEST 13 November 1997
(1) L E O N I D S 1 9 9 7
(2) TERRESTRIAL IMPACT CRATERS - THEIR SPATIAL AND
TEMPORAL
DISTRIBUTION AND IMPACTING BODIES
(3) Extraterrestrial impact events: the record in
the rocks and the stratigraphic column
(4) Impact-related clastic injections in the marine
Ordovician
Lockne impact structure, Central Sweden
===================================================================
(1) L E O N I D S 1 9 9 7
From: Juergen Rendtel <jrendtel@aip.de
The message below was sent by Joe Rao to meteorobs. The
information,
however, is of interest for all observers, so we repeat it here.
I just changed the times to UT.
----------------------
from: Joe Rao <Skywayinc@aol.com
On Monday morning, November 17th, the Leonid meteors are
scheduled to
reach maximum. This year is particularly interesting, in
that the
parent comet of this particular meteor swarm -- 55P/Tempel-Tuttle
--
is due to arrive at its perihelion on February 28th of next year.
Because of these circumstances, there has been some discussion
about
the possibility that a meteor storm "could"
occur. In some ways,
the upcoming Leonid shower does indeed bear a similarity to the
comet-Earth geometry that accompanied the Great Leonid Meteor
Storm
of 1799, which was observed from Peru by the Prussian scientist
and explorer, Alexander von Humboldt.
In that particular case, the Earth led the comet to the
descending
node by 116.9 days. Similarly, this year, the comet follows
Earth to
the node by 108 days.
However. . . the respective orbits are much farther apart in
1997 as
opposed to 1799. . . the difference being 0.0048 a.u.
or
approximately 718,000 kilometers. Thus, the odds of a storm are
greatly reduced.
Nonetheless, it will certainly be worthwhile to keep a careful
watch
for any potentially unusual meteor activity; with the parent
comet so
close to its nodal crossing point, there is always the
possibility of
a brief outburst of activity. . . and/or some unusually brilliant
fireballs or bolides. Last year, reports received indicated
meteors
that left luminous trains for in excess of five minutes. One
especially brilliant meteor seen over the Canary Islands, left a
trail that lingered for nearly 30 minutes!
There are two specific time frames to be especially alert to.
One is
when the Earth crosses the comet's node, which is to occur at
13:34
UT on Monday.
[...]
The other time frame is 10:40 UT. This corresponds to the
moment when
the Earth will be passing that part of 55P/Tempel-Tuttle's orbit
which produced the epic meteor storm of 1966. According to Mr.
Peter
Brown of the International Meteor Organization (IMO) some slight
enhanced activity has been noted near this region of space during
the last two Leonid showers. It will be interesting to see
if
anything unusual is again noted this year when the Earth once
again
encounters this region of space.
The major drawback of this year's Leonids will be the bright
light of
the 89% waning gibbous Moon, which will positioned near Orion's
upraised club -- roughly 55 degrees west of the Leonid
radiant. No
doubt a large number of faint meteors will be washed-out by the
Moonlight, but with the hope of sighting some bright fireballs
and
bolides, it may still be worthwhile to get out and observe. Some
may
use the Moon as an excuse not to get out and observe, but this
year's
Leonids may be worth the effort.
Meteors appear to fan-out from the "Sickle" of Leo.
The Sickle rises
out of the east-northeast around midnight and is high toward the
south-southeast by dawn. If you do go out and observe, we'd sure
like
to hear about what you've seen. Good Luck!
Joe Rao
Skywayinc@aol.com
-----------------------------
Obervations under moonlit conditions are normally not
recommended.
However, a reliable information about the time and (approximate)
strength of the Leonid maximum a year ahead of a major peak is of
great interest. In order to reduce the bad influence of the Moon,
arrange your place in such a way that it is somewhere behind a
building or something else. This is difficult as long as the Moon
is
high in the sky, but easier towards the morning. Reports should
be
split in 15...20 minute intervals.
Clear skies and Good Luck!
Jurgen Rendtel
IMO President
======================================================================
(2) R. A. F. GRIEVE & L. J. PESONEN: TERRESTRIAL IMPACT
CRATERS -
THEIR SPATIAL AND TEMPORAL DISTRIBUTION AND IMPACTING BODIES.
EARTH
MOON AND PLANETS, 1996, Vol.72, No.1-3, pp.357-376
GEOLOGICAL SURVEY OF CANADA, OTTAWA, ON K1A OY3, CANADA
The terrestrial impact record contains currently similar to
145
structures and includes the morphological crater types observed
on the other terrestrial planets. It has, however, been severely
modified by terrestrial geologic processes and is biased towards
young (less than or equal to 200 Ma) and large (greater than or
equal
to 20 km) impact structures on relatively well-studied cratonic
areas. Nevertheless, the ground-truth data available from
terrestrial
impact structures have provided important constraints for the
current
understanding of cratering processes. If the known sample of
impact
structures is restricted to a subsample in which it is believed
that
all structures greater than or equal to 20 km in diameter (D)
have
been discovered, the estimated terrestrial cratering rate is 5.5
+/-
2.7 x 10(-15) km(-2) a(-1) for D greater than or equal to 20 km.
This
rate estimate is equivalent to that based on astronomical
observations of Earth-crossing bodies. These rates are a factor
of
two higher, however, than the estimated post-mare cratering rate
on
the moon but the large uncertainties preclude definitive
conclusions
as to tile significance of this observation. Statements regarding
a
periodicity in the terrestrial cratering record based on
time-series
analyses of crater ages are considered unjustified, based on
statistical arguments and the large uncertainties attached to
many
crater age estimates. Trace element and isotopic analyses of
generally siderophile group elements in impact lithologies,
particularly impact melt rocks, have provided the basis for the
identification of impacting body compositions at a number of
structures. These range from meteoritic class, e.g., C-1
chondrite,
to tentative identifications, e.g., stone?, depending on the
quality
and quantity of analytical data. The majority of the
identifications
indicate chondritic impacting bodies, particularly with respect
to
the larger impact structures. This may indicate an increasing
role
for cometary impacts at larger diameters; although, the data base
is
limited and some identifications are equivocal. To realize the
full
potential of the terrestrial impact record to constrain the
character
of the impact flux, it will be necessary to undertake additional
and
systematic isotopic and trace element analyses of impact
lithologies
at well-characterized terrestrial impact structures..
=========================================================================
(3) R. A. F. Grieve: Extraterrestrial impact events: the
record in
the rocks and the stratigraphic column. PALAEOGEOGRAPHY
PALAEOCLIMATOLOGY PALAEOECOLOGY, 1997, Vol.132, No.1-4, pp.5-23
GEOLOGICAL SURVEY OF CANADA, OTTAWA, ON K1A 0Y3, CANADA
The known terrestrial impact record is a biased sample of a
much
larger population of impact events. The biases are due to the
modifying effects of terrestrial geologic processes, coupled with
incomplete searches for impact structures and impact-related
materials, Terrestrial impact structures have the same basic
forms as
impact craters on the other planets of the inner solar system
but,
because of post-impact modification by terrestrial geologic
process,
are recognised by the occurrence of shock metamorphic effects. In
some cases; siderophile anomalies have been identified in impact
lithologies and have been used to estimate the composition of the
impacting body, Similar shock metamorphic effects and a
siderophile
anomaly in K-T boundary materials are indicative of a major
impact
event, which has been correlated with the formation of the
Chicxulub
structure: Mexico. Evidence of a small number of other impacts
occur
in the stratigraphic record, most commonly as tektite or
microtektite
horizons. In some cases they are known to be accompanied by
geochemical anomalies, In other cases a number of Ir anomalies
have
been reported in the stratigraphic record but there is no
confirmatory evidence that they are due to impact, The majority
of known impact events in the stratigraphic record are from
relatively recent geologic time. Logic dictates, however, that
many more impacts must be recorded ill terrestrial sediments
and model calculations indicate that relatively small impacts
(D greater than or equal to 20 km) have the potential to cause
atmospheric blow-out and, thus, global dispersion of some of
the impact products. Geochemical detection, however, of such
events
may not be easy; in some cases because of relatively small
absolute
signals against the background of the daily infall of cosmic
material. In addition, non-chondritic bodies may result in no
appreciable geochemical anomaly. In view of this, any claim to a
geochemical signature of impact in the stratigraphic record
should be
accompanied by a physical search for impact materials; although,
in
the case of impacts into oceanic crust, this too will be
difficult.
Given the K-T experience, however, and the fact that large-scale
impact on Earth is a natural consequence of the character of the
solar system, the potential of impacts to provide local and
global
marker horizons can not be ignored. Similarly, the fact that
impacts
may have the potential to result in shortterm biologic or
climatic
excursions can not be dismissed arbitrarily, when considering the
causes of such phenomena as stable isotope anomalies in the
stratigraphic record.
======================================================================
(4) E. F. F. Sturkell & J. Ormo: Impact-related clastic
injections in
the marine Ordovician Lockne impact structure, Central Sweden.
SEDIMENTOLOGY, 1997, Vol.44, No.5, pp.793-804
UNIVERSITY OF STOCKHOLM, DEPARTMENT OF GEOLOGY, S-10691
STOCKHOLM,
SWEDEN
Clastic injections generated in connection with the formation
of
impact craters show many similarities to injections created by
other
geological processes. However, circumstances such as their
position
relative to the impact structure and the evidence of forceful
processes indicate an impact origin. The Ordovician Lockne impact
structure was formed in a marine environment with both
sedimentary
(Cambrian and Ordovician) and underlying crystalline
(Proterozoic)
target rocks. Sea water played a substantial part in the
cratering
process, especially in the modification of the newly formed
crater as
the water surged back into the structure. In the Lockne area
elastic
dykes and sills have long been known and have earlier been
interpreted as neptunian dykes and conglomerates. So far seven
cases
of dykes and sills are known in the area. In this work these are
interpreted as elastic injections formed in connection with the
Lockne impact. The elastic injections occur in the crystalline
basement and the sedimentary sequence. The material in the
injections
comes from all local lithologies (both sedimentary and
crystalline)
but the sedimentary sequence dominates as a source. The dykes and
sills were injected simultaneously with the fracturing and
dilation
of the host rock in the cratering process, and occur at different
stratigraphic levels. In some dykes, clasts from the host rock
wall
can be fitted back to their original position; the clasts are
slightly rotated and surrounded by exotic material. Quartz grains
with planar deformation features were observed in the injected
material. Most of the sills within the bedded Ordovician
limestone
are restricted to marry beds, except for the feeder dykes which
cut
through the overlying beds. This circumstance demonstrates how
the
decompression has opened the strata along weaker layers and that
the
underpressure created subsequently sucked the material down.
Laminar
flow is a conspicuous internal structure in the dykes and sills
and
indicates viscous flow of injected material. The lamination in
the
injected material is parallel to the walls in each case. The
material
was lithified prior to the event and was crushed, mobilized in a
water/sediment slurry and injected as dykes and sills.