PLEASE NOTE:
*
CCNet, 22 September 1999
-----------------------
(1) THE EUROPEAN SCIENCE FOUNDATION'S IMPACT PROGRAMME
Iain Gilmour <I.Gilmour@open.ac.uk>
(2) LONG-TERM DYNAMICS OF BRIGHT BOLIDES
Luigi Foschini <L.Foschini@isao.bo.cnr.it>
(3) URANUS SWARMING WITH DISTANT MOONS
Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
(4) AMATEUR ASTEROID HUNTERS PART OF NEW GOLDEN AGE OF ASTRONOMY
EXPLOREZONE, 20 September 1999
(5) GEOLOGY OF THE UPHEAVAL DOME IMPACT CRATER, UTAH
B.J. Kriens, E.M. Shoemaker, K.E. Herkenhoff,
US GEOLOGICAL SURVEY
(6) GEOCHEMICAL SIGNALS OF THE MJOLNIR IMPACT
H. Dypvik & M. Attrep, UNIVERSITY OF OSLO
(7) IMPACT CRATER COLLAPSE
H.J. Melosh & B.A. Ivanov, UNIVERSITY OF
ARIZONA
(8) IMPACT CRATERS ON GEOLOGICAL UNITS OF NORTHERN VENUS
A.T. Basilevsky et al., VERNADSKY INSTITUTE
(9) LUNAR MULTIRING BASINS & THE CRATERING PROCESS
M.A. Wieczorek & R.J. Phillips
===============
(1) THE EUROPEAN SCIENCE FOUNDATION'S IMPACT PROGRAMME
From Iain Gilmour <I.Gilmour@open.ac.uk>
Dear Benny:
In light of the main theme of discussion, you may be interested
in a
related programme sponsored by the European Science Foundation
(ESF-IMPACT). Details are available on the ESF-IMPACT homepage at
http://psri.open.ac.uk/esf.
IMPACT is a scientific programme of the European Science
Foundation.
Impacts of asteroids or comets on the earth surface have played
an
important role in the evolution of the planet. The ESF IMPACT
programme
is an interdisciplinary programme aimed at understanding impact
processes and their effects on the Earth System, including
environmental, biological, and geological changes, and
consequences for
the biodiversity of ecosystems; it is geared towards
understanding of
the linkage between impact processes and the Earth System, i.e.,
defining and studying the effects of impact events on the
environment,
including atmospheric, climatic, biologic, and geologic effects
and
interactions between these subsystems. Important aspects of
future
research regard also the consequences of the high-energy impact
events for the biodiversity of ecosystems.
Iain
-------------------------------------
Iain Gilmour
Planetary Sciences Research Institute
The Open University
Milton Keynes MK7 6AA
United Kingdom
Tel. +44 190 865 5140
Fax. +44 190 865 5910
====================
(2) LONG-TERM DYNAMICS OF BRIGHT BOLIDES
From Luigi Foschini <L.Foschini@isao..bo.cnr.it>
Dear Benny,
I would like to inform you that the paper:
Long-term dynamics of bright bolides, by L. Foschini, P.
Farinella, Ch.
Froeschlé, R. Gonczi, T.J. Jopek, P. Michel
has been accepted for the publication on Astronomy and
Astrophysics.
You can find a revised preprint on line at:
http://www.fisbat.bo.cnr.it/homepp/dinamica/foschini.html
Cheers,
Luigi
===================
(3) URANUS SWARMING WITH DISTANT MOONS
From Ron Baalke <BAALKE@kelvin.jpl.nasa.gov>
Harvard-Smithsonian Center for Astrophysics Press Release
For Release: September 20, 1999
http://cfa-www.harvard.edu/cfa/ep/uranus92099.html
BUZZING LIKE A BEEHIVE: URANUS NOW SWARMING WITH DISTANT MOONS
Until just a few years ago, many astronomers believed the planet
Uranus was a bit strange. That's because, unlike the other giant
members of the Solar System, Uranus did not appear to have any
so-called irregular satellites, or, distant moons with unusual
orbits. However, recent observations have found what appear to be
three new irregular moons around Uranus, thus suggesting that the
seventh planet from the Sun is just one of the gang after all.
Using the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea,
Hawaii,
an international team of astronomers made very careful
observations
over the summer to find these extremely faint objects. If
confirmed,
and tallied with two other irregular satellites discovered in
1997,
Uranus would have 16 regular and five irregular moons, making it
the
most populated planetary satellite system known.
Irregular satellites do not follow the normal, near-circular
orbits
of most satellites, such as the Earth's Moon. Instead, these
irregular objects either travel in highly elliptical orbits, or
follow paths that are severely tipped to the plane of the
planet's
equator.
"The discovery of these irregular satellites is very
important
because it means that Uranus is not some oddball, but rather is
just
like Neptune, Saturn, and Jupiter," says Matthew Holman, a
planetary
scientist at the Harvard-Smithsonian Center for Astrophysics and
a
member of the team that made the discovery. "It might also
help us
better understand how the irregular satellites of the giant gas
planets originated and how they've evolved."
These newly discovered objects are being referred to as
"candidate"
irregular satellites because further observations are necessary
to
absolutely confirm that these bodies are not comets or asteroids
on
planet-encountering orbits. However, based on the data so far,
the
team is confident these are true moons of Uranus.
"Given how these bodies are following the planet exactly, it
is
highly unlikely that these are some sort of Solar System
interlopers," says Brett Gladman of the Observatory of Nice,
France,
and leader of the team. Gladman and his colleague J.J. Kavelaars
of
McMaster University, Canada, were both members of the team that
found
Uranus's first two irregular moons in 1997.
The three new candidate satellites were discovered in a search
using
the world-class wide-field imaging camera, known as CFH12K, which
is
a mosaic of CCD detectors covering a very large patch of sky
(currently 35x28 arcmin, or roughly the area of the full moon).
This
instrument allowed the team to explore more than 90 percent of
the
region around Uranus in which satellite orbits are stable and to
find
these extremely faint objects, which are no more than 20
kilometers
in diameter and orbit Uranus at a distance of 10 to 25
kilometers.
Other members of the discovery team include Jean-Marc Petit and
Hans
Scholl (Observatory of Nice, France), and P. Nicholson and J. A.
Burns (Cornell University.) Follow-up observations were obtained
at
the Mount Palomar 5-meter and Kitt Peak 4-meter telescopes, the
latter in conjunction with D. Davis and C. Neese of the Planetary
Science Institute in Tucson, AZ. Brian Marsden and Gareth
Williams of
the International Astronomical Union's Minor Planet Center
computed
preliminary orbits for the reported objects.
Contact information:
United States:
Matthew Holman, Harvard-Smithsonian Center for Astrophysics,
mholman@cfa.harvard.edu,
617-496-7775,
http://cfa-www.harvard.edu/~mholman
Canada:
J.J. Kavelaars, McMaster University, kavelaars@physics.mcmaster.ca,
905-525-9140 x2716
France:
Brett Gladman, Observatory of Nice, gladman@obs-nice.fr, 011 33
4
9200
3126, http://www.obs-nice.fr/gladman
(English and French versions)
Jean-Marc Petit, Observatory of Nice, petit@obs-nice.fr, 011 33 4
9200 3126
Hans Scholl, Observatory of Nice, scholl@obs-nice.fr, 011 33 4
9200
3041
====================
(4) AMATEUR ASTEROID HUNTERS PART OF NEW GOLDEN AGE OF ASTRONOMY
From EXPLOREZONE, 20 September 1999
http://explorezone.com/archives/99_09/20_asteroid_nz.htm
Amateur asteroid hunters part of new golden age of astronomy
By Allan Coukell for explorezone.com
September 20, 1999
Amateur astronomers can make an important contribution to
asteroid
research, according to Ian Griffin, director of the Auckland
Observatory and Stardome in New Zealand. Griffin, himself a
former
professional astronomer, has established a programme that relies
on
amateurs to search for previously unknown asteroids and comets.
Amateur astronomy is entering "a new golden age," says
Griffin.
“There are probably less professional astronomers working on
asteroid
research than there are staff in your average local McDonald's.
And
that's not enough to keep track of these hundreds of thousands of
objects."
In the 18th and 19th centuries, many of the important discoveries
in
astronomy were made by amateurs. It was the age of the
"gentleman
astronomer." But in our own century, sophisticated and
expensive
technology has often meant that astronomy was left to the
professional. But now, "we're going back to where someone,
if they
are dedicated and willing to invest a bit of money, can make a
real
contribution to science from their own back yard."
Technology aids amateurs
Advances in digital cameras and imaging software mean that even a
public observatory situated in the heart of a city and using a
relatively small telescope can see very, very faint objects.
"The telescope here in Auckland, with our special camera on
the back,
is the same sensitivity as Mount Palomar when it opened back in
1947," Griffin said. "So we can see things as faint as
they were
observing with the biggest telescope in the world just 50 years
ago."
The Auckland group is using a technique called time delay
integration
or drift scanning. Essentially, the technique is to turn off the
telescope drive and allow the telescope to scan across the sky as
the
Earth rotates. The Auckland group scans for 30 minutes, covering
about 7.5 degrees of sky, before they move their telescope back
to
the start position and do another scan. In total, they scan the
region of sky three times. They then superimpose the three
images,
using a computer to blink between them (as in the image above).
Far away stars do not appear to change their position on the
three
images, but asteroids and comets, which are much closer to earth,
do
appear to move. Even so, spotting an asteroid amid thousands of
stars
requires patience and sharp eyes, but, says amateur astronomer
Rebecca Greatrex, who works with Griffin, "for me, this is
the
exciting part."
Quick results
Much to their surprise, the Auckland astronomers found a new
asteroid
the very first night they looked. And the next night, they found
another. When an asteroid is found, its coordinates are sent to
the
minor planet center of the International Astronomical Union to
establish whether it is new, or something that has already been
described.
There are millions of asteroids out there, and so far none has
been
found that poses any risk to Earth in the near future. But, says
Ian
Griffin, it pays to look. "With the discovery of Near Earth
Asteroids
and asteroids that have the potential to collide with the Earth,
suddenly people are beginning to realise that these are things we
should study. We should study them in quite a lot of detail
because
obviously an object 10 miles across crashing into Earth at a
hundred
thousand miles an hours does a considerable amount of
damage." ez
Allan Coukell is a New Zealand science writer and the producer
and
presenter of Eureka!, a Radio New Zealand weekly science
programme.
Copyright 1999, Explorezone
==================
(5) GEOLOGY OF THE UPHEAVAL DOME IMPACT CRATER, UTAH
B.J. Kriens*), E.M. Shoemaker, K.E. Herkenhoff: Geology of the
upheaval dome impact structure, southeast Utah. JOURNAL OF
GEOPHYSICAL RESEARCH-PLANETS, 1999, Vol.104, No.E8,
pp.18867-18887
*) US GEOLOGICAL SURVEY,2255 N GEMINI DR,FLAGSTAFF,AZ,86001
Two vastly different phenomena, impact and salt diapirism, have
been
proposed for the origin of Upheaval Dome, a spectacular scenic
feature in southeast Utah. Detailed geologic mapping and seismic
refraction data indicate that the dome originated by collapse of
a
transient cavity formed by impact. Evidence is as follows:
(I)sedimentary strata in the center of the structure are
pervasively
imbricated by top-toward-the-center thrust faulting and are
complexly
folded as well; (2) top-toward-the-center normal faults are found
at
the perimeter of the structure; (3) clastic dikes are widespread;
(4)
the top of the underlying salt horizon is at least 500 m below
the
surface at the center of the dome, and there are no exposures of
salt
or associated rocks of the Paradox Formation in the dome to
support
the possibility that a salt diapir has ascended through it; and
(5)
planar microstructures in quartz grains, fan-tailed fracture
surfaces
(shatter surfaces), and rare shatter cones are present near the
center of the structure. We show that the dome formed mainly by
centerward motion of rock units along listric faults.
Outcrop-scale
folding and upturning of beds, especially common in the center,
are
largely a consequence of this motion. We have also detected some
centerward motion of fault-bounded wedges resulting from
displacements on subhorizontal faults that conjoin and die out
within
horizontal bedding near the perimeter of the structure. The
observed
deformation corresponds to the central uplift and the encircling
ring
structural depression seen in complex impact craters. Copyright
1999,
Institute for Scientific Information Inc.
==============
(6) GEOCHEMICAL SIGNALS OF THE MJOLNIR IMPACT
H. Dypvik*) & M. Attrep: Geochemical signals of the late
Jurassic,
marine Mjolnir impact. METEORITICS & PLANETARY SCIENCE, 1999,
Vol.34,
No.3, pp.393-406
*) UNIVERSITY OF OSLO,DEPT GEOL,POB 1047,N-0316 OSLO,NORWAY
Of the only seven submarine impact craters that have been found
globally, the Mjolnir crater is one of the best preserved and
retains
crater and ejecta. Geochemical studies (organic pyrolysis using
the
Rock Eval technique and XRF analysis for major, minor, and trace
elements) of the Institute for Petroleum Research (IKU) core
7430/10-U-01 that was taken from a drillhole located similar to
30 km
north-northeast of the crater rim show gradual establishment of
anoxic sea floor conditions through the late Jurassic. These
poorly
ventilated water conditions were overturned due to the Mjolnir
impact
event. Waves and currents transported impact glass (which is now
partly weathered to smectite) into the depositional area where
the
drillhole is located. The succeeding crater collapse transported
impact material (e.g., shocked quartz and Ir) from the crater rim
and
deeper levels to the core site. Normal marine depositional
conditions
were established a short time after the crater collapsed.
Copyright
1999, Institute for Scientific Information Inc.
=============
(7) IMPACT CRATER COLLAPSE
H.J. Melosh*) & B.A. Ivanov: Impact crater collapse. ANNUAL
REVIEW OF
EARTH AND PLANETARY SCIENCES, 1999, Vol.27, pp.385-415
UNIVERSITY OF ARIZONA,LUNAR & PLANETARY LAB,TUCSON,AZ,85721
The detailed morphology of impact craters is now believed to be
mainly caused by the collapse of a geometrically simple,
bowl-shaped
'transient crater.' The transient crater forms immediately after
the
impact. In small craters, those less than approximately 15 km
diameter on the Moon, the steepest part of the rim collapses into
the
crater bowl to produce a lens of broken rock in an otherwise
unmodified transient crater. Such craters are called ''simple''
and
have a depth-to-diameter ratio near 1:5. Large craters collapse
more
spectacularly, giving rise to central peaks, wall terraces, and
internal rings in still larger craters. These are called
''complex''
craters. The transition between simple and complex craters
depends on
1/g, suggesting that the collapse occurs when a strength
threshold is
exceeded. The apparent strength, however, is very low: only a few
bars, and with little or no internal friction. This behavior
requires
a mechanism for temporary strength degradation in the rocks
surrounding the impact site. Several models for this process,
including acoustic fluidization and shock weakening, have been
considered by recent investigations. Acoustic fluidization, in
particular, appears to produce results in good agreement with
observations, although better understanding is still needed.
Copyright 1999, Institute for Scientific Information Inc.
=============
(8) IMPACT CRATERS ON GEOLOGICAL UNITS OF NORTHERN VENUS
A.T. Basilevsky*), J.W. Head, M.A. Ivanov, V.P. Kryuchkov: Impact
craters on geologic units of northern Venus: Implications for the
duration of the transition from tessera to regional plains.
GEOPHYSICAL RESEARCH LETTERS, 1999, Vol.26, No.16, pp.2593-2596
*) VERNADSKY INSTITUTE,KOSYGIN ST 19,MOSCOW 117975,RUSSIA
Using Magellan SAR images and the Schaber el al, [1998] crater
data
base we examined impact craters in the area north of 35 degrees N
and
determined the geologic units on which they are superposed. The
crater density of the regional plains with wrinkle ridges (Pwr)
was
found to be very close to the global average and thus the mean
surface age of the plains is close to the mean surface age of the
planet (T). About 80 to 97% of the craters superposed on a
composite
unit that includes materials of Tessera terrain (Tt), Densely
fractured plains (Pdf), Fractured and ridged plains (Pfr), and
Fracture Belts (FB), also postdate the regional plains. Thus, the
time interval between the formation of these older units and
emplacement of the regional plains (Delta T) should be
geologically
short, from a few percent to about 20% of T, or approximately 40
to
150 m.y. This means that in the area under study, volcanic and
tectonic activity in the beginning of the morphologically
recognizable part of the geologic history of Venus (about the
last 750 m.y.) was much more active than in the subsequent time.
Copyright 1999, Institute for Scientific Information Inc.
================
(9) LUNAR MULTIRING BASINS & THE CRATERING PROCESS
M.A. Wieczorek*) & R.J. Phillips: Lunar multiring basins and
the
cratering process. ICARUS, 1999, Vol.139, No.2, pp.246-259
*) WASHINGTON UNIVERSITY,DEPT EARTH & PLANETARY SCI,1
BROOKINGS
DR,BOX 1169,ST LOUIS,MO,63130
Numerous studies of the lunar gravity field have concluded that
the
lunar Moho is substantially uplifted beneath the young multiring
basins. This uplift is presumably due to the excavation of large
quantities of crustal material during the cratering process and
subsequent rebound of the impact basin floor. Using a new
dual-layered crustal thickness model of the Moon, the excavation
cavities of some nearside multiring basins (Grimaldi and larger,
and
younger than Tranquillitatis) were reconstructed by restoring the
uplifted Moho to its preimpact location. The farside South
Pole-Aitken (SPA) basin was also considered due to its importance
in
deciphering lunar evolution. Restoring the Moho to its preimpact
position beneath these basins resulted in a roughly parabolic
depression from which the depth and diameter of the excavation
cavity
could be determined. Using these reconstructed excavation
cavities,
the basin-forming process was investigated. Excavation cavity
diameters were generally found to be on the small side of most
previous estimates (for Orientale the modeled excavation cavity
lies
within the Inner Rook Ring). Additionally, with the exception of
the
three largest basins (Serenitatis, Imbrium, and South
Pole-Aitken)
the depth/diameter ratios of the excavation cavities were found
to be
0.115 @ 0.005, a value consistent with theoretical and
experimental
results for impact craters orders of magnitude smaller in size.
The
three largest basins, however, appear to have significantly
shallower
depths of excavation compared to this trend. It is possible that
this
may reflect a different physical process of crater formation
(e.g.,
nonproportional scaling), special impact conditions, or
postimpact
modification processes. The crustal thickness model also shows
that
each basin is surrounded by an annulus of thickened crust. We
interpret this thickened crust as representing thick basin ejecta
deposits, and we show that the radial variation in the thickness
of
these deposits is consistent with scaling laws obtained from
small-scale experimental studies. If multiring basins ever
possessed
a terraced main crater rim, this terraced zone may be presently
unrecognizable at the surface due to the emplacement of ejecta
deposits that exceed a few kilometers in thickness exterior to
the
excavation cavity rim. We also show that the interiors of many
basins
were superisostatic before mare volcanism commenced. Those basins
that were closest to approaching a premare isostatic state lie
close
to or within an anomalous geochemical province rich in
heat-producing
elements. (C) 1999 Academic Press.
----------------------------------------
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