PLEASE NOTE:
*
CCNet DIGEST, 25 February 1999
--------------------------------------------------------------
The fully indexed archive of the CCNet, from February 1997 on,
can be found at http://abob.libs.uga.edu/bobk/cccmenu.html
---------------------------------------------------------------
QUOTE OF THE DAY
"The most likely
explanation is that [the object] is a chip
off the Moon" (Brian
Marsden, in New Scientist 27 February)
(1) THERE'S A MYSTERY OBJECT NOT FAR FROM THE EARTH
Andrew Yee <ayee@nova.astro.utoronto.ca>
(2) LEONID MAC WORKSHOP
Peter Jenniskens <peter@max.arc.nasa.gov>
(3) MORE ON ASTEROID 8793 THOMASMULLER
Andrew Yee <ayee@nova.astro.utoronto.ca>
(4) GENE SHOEMAKER'S POSTHUMOUS PAPER ON THE CONTROVERSIAL
AUSTRALITES OF PORT CAMPBELL
Eugene M. Shoemaker and H. Ralph Uhlherr, U.S.
Geological Survey
(5) MARTIAN METEORITE ALH 84001 & THE IMPLICATIONS FOR SIGNS
OF LIFE
C. K. Shearer et al., University of New Mexico
(6) THE AGE OF THE SOUTH RAY LUNAR IMPACT CRATER
Otto Eugster, University of Bern
(7) GEOCHEMICAL SIGNALS OF THE MJOELNIR IMPACT CRATER
Henning Dypvik and Moses Attrep, University of
Oslo
(8) THE FUSION CRUSTS OF STONY METEORITES
Matthew J. Genge and Monica M. Grady,The
Natural History Museum
(9) IMPACT SHOCK MELTING AT THE RIES CRATER, GERMANY
Günther Graup, Max-Planck-Institut für
Chemie
(10) THE INTERNAL STRUCTURES AND DENSITIES OF ASTEROIDS
Lionel Wilson et al., University of
Hawaii at Manoa
=======================
(1) THERE'S A MYSTERY OBJECT NOT FAR FROM THE EARTH
From Andrew Yee <ayee@nova.astro.utoronto.ca>
New Scientist
UK CONTACT -- Claire Bowles, New Scientist Press Office, London
Tel: 44-171-2751 or emailclaire.bowles@rbi.co.uk
US CONTACT -- Barbara Thurlow, New Scientist Washington Office
Tel: 202-452-1178 or email newscid@idt.net
EMBARGOED FOR RELEASE: February 24, 1999, 2 p.m. EST
There's A Mystery Object Not Far From The Earth
A CHUNK of rock some 50 metres across has been found circling the
Sun
in an orbit close to Earth's. The object, which was discovered on
10
February by an automated asteroid-hunting telescope in New Mexico
called Linear, is probably a chip off the Moon, say astronomers.
After six nights of observations, Gareth Williams of the
Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts, calculated that it circles the Sun every 1.09
years. Its
nearly circular orbit is just nine million kilometres farther
from the
Sun than the Earth's.
The object's orbit is extremely unusual. Comets and asteroids
that
cross the Earth's orbit normally have eccentric orbits. There is
only
one asteroid-like object, called 1991 VG, that has a similar
orbit to
that of the Earth. When it was discovered, eight years ago,
astronomers
thought it might be a spacecraft that had escaped the Earth's
gravity.
The new object, designated 1999 CG9, is considerably brighter
than 1991
VG, indicating that it is much larger. Brian Marsden of
Harvard-Smithsonian estimates it to be between 30 and 50 metres
across,
too big to be the final stage of a rocket. "The most likely
explanation
is that it's a chip off the Moon," he says.
Although the Moon is small, its low gravity makes it easy to
blast
debris into orbit. "We have seen there are chips off the
Moon," says
Marsden. "Twelve small lunar meteorites have been found on
the Earth."
"If you can shoot things off the Moon, they would continue
to go around
the Sun in an orbit not too different from the Moon,"
Marsden adds. So
far, astronomers do not know the object's composition, which
could cast
light on its origins. However, the astronomers hope to analyse
the
rock's spectrum to see how it compares with that of the Moon.
Author: Jeff Hecht, Boston
New Scientist issue 27th Feb 99
PLEASE MENTION NEW SCIENTIST IF YOU USE THIS ARTICLE - THANK YOU
=======================
(2) LEONID MAC WORKSHOP
From Peter Jenniskens <peter@max.arc.nasa.gov>
Second announcement
Leonid MAC Workshop / NASA-Ames Research Center / April 12-15,
1999
-------------------------------------------------------------------
This is to remind you that the deadline for registration for the
Leonid MAC Workshop is coming up on:
March 1.
Automatic registration: http://leonid.arc.nasa.gov/workshops/
Purpose of the workshop is to review the first results of the
1998
Leonid campaigns, both ground-based and airborne campaigns,
discuss the
relevant science, and make recommendations for the upcoming
campaigns
in November 1999 (in time to help the necessary preparations).
Researchers from the fields of meteor physics, atmosphere
science,
planetary astronomy, and astrobiology are invited to attend, as
well as
those concerned with the satelite impact hazard of meteor storms.
The
meeting is international and open to amateurs.
We anticipate an exciting 3-day meeting that is very
interdisciplinary
in nature and that will set the stage for the organisation of
next
year's campaigns, our last chance to witness a meteor storm in
our
lifetime.
Sincerely,
Dr. Peter Jenniskens
LOC Leonid MAC Workshop (chair)
==================
(3) MORE ON ASTEROID 8793 THOMASMULLER
From Andrew Yee <ayee@nova.astro.utoronto.ca>
European Space Agency
Press Information Note No 02-99
Paris, France 17 February 1999
Asteroid named after ESA astronomer, as a reward for his
discoveries
If you want an asteroid named after you, make a valuable
scientific
contribution to the study of these rocky mini-planets of the
Solar
System. That is what 32 year old ESA astronomer Thomas Mueller
did,
and now the International Astronomical Union has rewarded him by
giving the name 'Thomasmuller' to asteroid number 8793.
"It's been a
great surprise", Mueller said. He is part of the team
working at
ESA's Infrared Space Observatory (ISO) Data Centre at
Villafranca,
Spain, and his research is mostly based on ISO data (see footnote
on
ISO).
Mueller's scientific work has proved to be crucial in describing
the
physical properties of an asteroid. But that is not all: it will
improve the performance of the next millennium's infrared space
telescopes, since these instruments will be able to check the
accuracy of their observations by using the new data on
asteroids.
Mueller has therefore shown that asteroids can be very useful
tools
for astronomy.
"Many astronomers tend to look at asteroids as boring
objects. I've
always disagreed. Take the infrared sky: if our eyes were able to
see
in the infrared, and of course if we could get rid of the Earth's
atmosphere which is opaque to most infrared radiation, we would
see
hundreds of asteroids sparkling and very few stars. I love this
idea", he says.
Thomas got his reward for his dedication on 10 December, when his
collaborator Johan Lagerros, of the Astronomiska Observatoriet in
Uppsala, Sweden, surprised him with the news about
"Thomasmuller". "I
didn't expect it, I didn't even know I had been proposed!",
the ESA
astronomer exclaimed.
The proposal to the International Astronomical Union (IAU) came
from
astronomer Claes-Ingvar Lagerkvist, also at the Astronomiska
Observatoriet, who is entitled to name the 47 asteroids he has
discovered throughout his life. Asteroid 8793 was one of them.
The
name "Thomasmuller" was immediately accepted by the
IAU.
There are at present almost 10,000 asteroids with a known orbit,
of
which a few hundred are named after a living astronomer. (See
"How to
name a minor planet" at
http://cfa-www.harvard.edu/iau/info/HowNamed.html).
The orbit is the
minimum every discoverer needs to know to get an object
catalogued,
and often it is all that is known about an asteroid. That is the
case
with "Thomasmuller".
"It puzzles me the fact that we know nothing about
"Thomasmuller" but
its orbit and luminosity, although it was discovered in 1979 and
it
has been observed 43 more times", Mueller confessed. (see
also
Official homepage of asteroid 'Thomasmuller':
http://www.astro.uu.se/planet/asteroid/astdiv/8793.html)
A hard task
It has been precisely this scarce knowledge on asteroids that has
added value to the work of Mueller and Lagerros. They have
developed
a very accurate model to predict the brightness of asteroids at
any
given time, a type of data that will be useful in the calibration
of
future infrared space telescopes such as ESA's FIRST (due for
launch
in 2007).
Predicting the brightness of an asteroid is a very hard task.
Asteroid sizes range from a few to one thousand kilometres. They
have
very irregular shapes and spin quickly, approximately once every
five
to ten hours. Their infrared brightness is caused by heating due
to
sunlight, and this heating strongly depends on the surface
material
and the surface structures, like craters, and the irregular
shape.
So, to know the infrared brightness of an asteroid at any given
time,
one has to find out which "face" it is showing to the
Sun and to the
observer, as well as its internal thermal behaviour and its
distance
to the Earth.
The researchers set up a large database with 650 individual
observations, including those from the spectrophotometer ISOPHOT
on
board ISO, of the "top-ten" main asteroids. They were
then able to
model the egg-like shape of the ten asteroids, and to predict
exactly
how much energy they would be emitting at any given time. They
could
then derive the brightness with a very high accuracy.
The model has been tested against ISO observations. As Mueller
explains, "We compared hundreds of observations and
predictions one
by one, and all test cases confirmed the quality of our model. We
can
now predict flux densities from mid-infrared to submillimetre
wavelengths for all ten asteroids, including thermal and
lightcurve
effects."
A further application of this work is that it is now possible to
infer diameters and albedos (the ability to reflect light) for
all
asteroids observed by the infrared with unprecedented accuracy.
The model applications are not only limited to ISO observations:
all
infrared measurements from ground, airborne or space telescopes
can
now be used to improve our knowledge about asteroids.
The description of the model and the technique has been published
in
the scientific journal "Astronomy and Astrophysics"
("Asteroids as
far-infrared photometric standards for ISOPHOT", by T.G.
Mueller and
J.S.V. Lagerros, Vol 338, p340-352, 1998).
Footnote on ISO
ISO was put into orbit in November 1995, by an Ariane 4 launcher
from
the Guiana Space Centre, Europe's Spaceport in Kourou, French
Guiana.
As an unprecedented observatory for infrared astronomy, able to
examine cool and hidden places in the Universe, ISO successfully
made
almost 30,000 scientific observations. These are now available to
the
scientific community via the archive at the ISO Data Centre
(IDC),
located at ESA's Satellite Tracking Station in Villafranca
(Spain).
For more information and ISO pictures:
Thomas Mueller at the ISO Data Centre at Villafranca (Spain):
Tel: +34 91 8131100
e-mail: tmueller@iso.vilspa.esa.es
ESA Public Relations Division:
Tel: +33(0)1.53.69.71.55 Fax: +33(0)1.53.69.76.90
Visit the ISO web sites (hi-res images available) at:
http://www.iso.vilspa.esa.es
http://sci.esa.int/iso
==================
(4) GENE SHOEMAKER'S POSTHUMOUS PAPER ON THE CONTROVERSIAL
AUSTRALITES OF PORT CAMPBELL
Eugene M. Shoemaker* and H. Ralph Uhlherr: Stratigraphic
relations of
australites in the Port Campbell embayment, Victoria. METEORITICS
&
PLANETARY SCIENCE 34, May (1999)
*) U.S. Geological Survey and Lowell Observatory, Flagstaff,
Arizona
86001, USA; e-mail address: cshoemaker@flagmail.wr.usgs.gov
In the Port Campbell Embayment of Victoria, australites have been
found
in situ in channel deposits of the Hanson Plain Sand of Pliocene
and
Pleistocene age. The large majority of the australites, however,
occur
as a lag deposit at the basal contact of the Sturgess Sand of
late
Pleistocene and Holocene age and are spatially correlated with
ferruginous sandstone clasts that are derived from the Hanson
Plain
Sand. Some of the tektites are imbedded in or bonded to the
ferruginous
sandstone clasts, but most are found as individual tektite
fragments. A
few percent of the tektites have nearly perfectly preserved,
complete
aerodynamically shaped forms. The sandstone clasts and associated
tektites have been reworked from the much older underlying Hanson
Plain
and have been locally concentrated in the lag deposit. Some
tektites
also occur at higher levels in the Sturgess Sand, almost
invariably in
association with stone flakes, exotic stones transported by the
aborigines, and, locally, with middens of mollusc shells.
Circumstantial evidence indicates that the aborigines transported
the
tektites found in the upper part of the Sturgess, particularly at
Stanhope Bay. As Port Campbell australites unequivocally occur in
strata much older than the late Pleistocene and Holocene
Sturgess,
there is no longer any conflict between the apparent
stratigraphic age
of the tektites and the middle Pleistocene ages obtained by
various
chronometric methods. © Meteoritical Society, 1999.
===================
(5) MARTIAN METEORITE ALH 84001 & THE IMPLICATIONS FOR SIGNS
OF LIFE
C. K. Shearer*), L. A. Leshin and C. T. Adcock: Olivine in
Martian
meteorite ALH 84001: Evidence for a high-temperature origin and
implications for signs of life. METEORITICS & PLANETARY
SCIENCE 34,
May (1999)
*) Institute of Meteoritics, Department of Earth and Planetary
Sciences, University of New Mexico, Albuquerque, New Mexico
87131-1126;
e-mail address: chearer@unm.edu
Olivine from martian meteorite ALH 84001 occurs as clusters
within
orthopyroxene adjacent to fractures containing disrupted
carbonate
globules and feldspathic shock-glass. The inclusions are
irregular in
shape and range in size from approximately 40 µm to sub-micron.
Some of
the inclusions are elongate and boudinage-like. The olivine
grains are
in sharp contact with the enclosing orthopyroxene, and often
contain
small inclusions of chromite. The olivine exhibits a very limited
range
of composition from Fo65 to Fo66 (n= 25). The (18O values of the
olivine
and orthopyroxene analyzed by ion microprobe range from +4.3 to
+5.3‰
and are indistinguishable from each other within analytical
uncertainty.
The mineral chemistries, oxygen isotopic data, and textural
relationships indicate that the olivine inclusions were produced
at a
temperature greater than 800°C. It is unlikely that the olivines
formed
during the same event that gave rise to the carbonates in ALH
84001,
which have more elevated and variable (18O values, and were
probably
formed from fluids that were not in isotopic equilibrium with the
orthopyroxene or olivine. The reactions most likely instrumental
in the
formation of olivine could be either the dehydration of hydrous
silicates that formed during carbonate precipitation or the
reduction of
orthopyroxene and spinel. If the olivine was formed by either
reaction
during a post-carbonate heating event, the implications are
profound
with regards to the interpretations of McKay et al. (1996).
Due to the
low diffusion rates in carbonates, this rapid, high-temperature
event
would have resulted in the preservation of the fine-scale
carbonate
zoning, while partially devolatilizing select carbonate
compositions on
a sub-micron scale (Brearley, 1998). This may have resulted in
the
formation of the minute magnetite grains that McKay et al (1996)
attributed to biogenic activity. © Meteoritical Society, 1999.
=========================
(6) THE AGE OF THE SOUTH RAY LUNAR IMPACT CRATER
Otto Eugster: Chronology of dimict breccias and the age of South
Ray
crater at the Apollo 16 site. METEORITICS & PLANETARY SCIENCE
34, May
(1999)
Physikalisches Institut, University of Bern, Sidlerstrasse 5,
3012
Bern, Switzerland; e-mail address: eugster@phim.unibe.ch
We report the noble gas isotopic abundances of five dimict
breccias and
one cataclastic anorthosite that were collected at the Apollo 16
landing site. Orbital and surface photographs indicate that rays
from
South Ray crater, an almost 1 km wide young crater in the Cayley
plains,
extend several km from their source into the area that was
sampled by
the Apollo 16 mission. Previous studies have shown that South Ray
crater formed 2 Ma ago and that a large number of rocks might
originate
from this cratering event. Based on cosmic-ray produced nuclei we
find
that the six rocks investigated in this work yield the same lunar
surface exposure age. Using literature data we recalculate the
exposure
ages of additional 16 rocks with suspected South Ray crater
origin and
obtain an average exposure age of 2.01±0.10Ma. In particular,
all nine
dimict breccias, a type of rock essentially restricted to the
Apollo-16
area consisting of anorthosite and breccia phases, dated until
now
yield an average ejection age of 2.06±0.17 Ma. We conclude that
they
must originate from the Cayley formation or from bedrock
underlying the
Cayley plain. We determined the gas retention ages for the dimict
breccias based on the 40K-40Ar and U,Th-136Xe dating methods:
rock
64425 yields a 40K-40Ar age of 3.96 Ga and rock 61016 a
U,Th-136Xe age
of 3.97 Ga. These results, together with 39Ar-40Ar ages, obtained
by
other workers for rocks 64535 (3.98 Ga) and 64536 (3.97 Ga) show
that
the dimict breccias formed 3.97 Ga ago. © Meteoritical Society,
1999.
==================
(7) GEOCHEMICAL SIGNALS OF THE MJOELNIR IMPACT CRATER
Henning Dypvik*) and Moses Attrep, Jr.: Geochemical signals of
the Late
Jurassic, marine Mjølnir impact. METEORITICS & PLANETARY
SCIENCE 34,
May (1999)
*) Department of Geology, University of Oslo, P.O.Box 1047,
Blindern,
N-0316 Oslo, Norway; e-mail address: henning.dypvik@geologi.uio.no
Of the only seven submarine impact craters that have been found
globally, the Mjølnir Crater is one of the best preserved and
retails
crater and ejecta. Geochemical studies (organic pyrolysis: Rock
Eval,
major elements, Co, Cr, Ir, Ni, Pb, Rb, Sr, Th, U, V, Zr, Y) of
the IKU
(Institute for Petroleum Research) core 7430/10-U-01, which is
located
about 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
Mjølnir 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 iridium)
from the
crater rim and deeper levels to the core site. Normal marine
depositional conditions were established a short time after the
crater
collapsed. © Meteoritical Society, 1999.
===================
(8) THE FUSION CRUSTS OF STONY METEORITES
Matthew J. Genge*) and Monica M. Grady: The fusion crusts of
stony
meteorites: Implications for the atmospheric reprocessing of
extraterrestrial materials. METEORITICS & PLANETARY SCIENCE
34, May
(1999)
*) Department of Mineralogy, The Natural History Museum, Cromwell
Road,
London SW7 5BD, Great Britain; e-mail address: mjg@nhm.ac.uk
Fusion crusts develop on all meteorites during their passage of
the
atmosphere but have been little studied. We have characterised
the
textures and compositions of the fusion crusts of 73 stony
meteorites
to identify the nature of meteorite ablation spheres (MAS) and
constrain the processes operating during the entry heating. Most
chondrite fusion crusts are porphyritic and dominated by olivine,
glass
and accessory magnetite whereas those of the achondrites are
mainly
glassy. Chondrite fusion crusts contain sulphide droplets with
high-Ni
contents (>55 wt%). The partially melted substrate of OCs,
underlying
the outer melted crusts, are dominated by silicate glass and
composite
metal, sulphide and Cr-bearing Fe-oxide droplets that form as
coexisting immiscible liquids. Enstatite chondrite substrates
contain
Cr- and Mn- bearing sulphides. The substrates of the CCs comprise
a
sulphide-enriched layer of matrix. The compositions of melted
crusts
are similar to those of the bulk meteorite. Differences from
whole
rock, however, suggest that three main processes control their
chemical
evolution: 1) the loss and reaction of immiscible Fe-rich
liquids, 2)
mixing between substrate partial melts and bulk melts of the
melted
crust, 3) the loss of volatile components by evaporation and
degassing.
Data from fusion crusts suggests that MAS produced at low
altitude have
compositions within the range of those of silicate-dominated
cosmic
spherules (CSs) that are formed by the melting dust particles.
Meteorite ablation spheres produced at high altitude probably
have
compositions very different from bulk meteorite and will resemble
CSs
derived from coarse-grained precursors. © Meteoritical Society,
1999.
==================
(9) IMPACT SHOCK MELTING AT THE RIES CRATER, GERMANY
Günther Graup: Carbonate-silicate liquid immiscibility upon
impact
melting, Ries Crater, Germany. METEORITICS & PLANETARY
SCIENCE 34, May
(1999)
Max-Planck-Institut für Chemie, Abteilung Geochemie, Postfach
3060,
D-55020 Mainz, Germany
The 24-km-diameter Ries impact crater in southern Germany is one
of the
most studied impact structures on Earth. The Ries impactor struck
a
Triassic to Upper Jurassic sedimentary sequence overlying
Hercynian
crystalline basement. At the time of impact (14.87 ± 0.36 Ma;
Storzer
et al., 1995), the 350 m thick Malm limestone was present only to
the S
and E of the impact site. To the N and W, the Malm had been
eroded
away, exposing the underlying Dogger and Lias. The largest
proportion
of shocked target material is in the impact melt-bearing breccia
suevite. The suevite had been believed to be derived entirely
from the
crystalline basement. Calcite in the suevite has been interpreted
as a
post-impact hydrothermal deposit. From optical inspection of 540
thin
sections of suevite from 32 sites, I find that calcite in the
suevite
shows textural evidence of liquid immiscibility with the silicate
impact melt. Textural evidence of liquid immiscibility between
silicate
and carbonate melt in the Ries suevite includes: carbonate
globules
within silicate glass, silicate globules embedded in carbonate,
deformable and coalescing carbonate spheres within silicate
glass,
sharp menisci or cusps and budding between silicate and carbonate
melt,
fluidal textures and gas vesicles in carbonate schlieren, a
quench
crystallization sequence of the carbonate, spinifex textured
quenched
carbonate, separate carbonate spherules in the suevite
mineral-fragment-matrix, and inclusions of mineral fragments
suspended
in carbonate blebs. Given this evidence of liquid immiscibility,
the
carbonate in the suevite has, therefore, like the silicate melt a
primary origin by impact shock melting. Evidence of
carbonate-silicate
liquid immiscibility is abundant in the suevites to the SW to E
of the
Ries crater. The rarer suevites to the W to NE of the crater are
nearly
devoid of carbonate melts. This correspondence between the
occurrence
of outcropping limestones at the target surface and the formation
of
carbonate melt, indicates that the Malm limestones are the source
rocks
of the carbonate impact melt. This correspondence shows that the
suevites preserve a compositional memory of their source rocks.
From
the regional distribution of suevites with or without immiscible
carbonate melts, it is inferred that the Ries impactor hit the
steep
Albtrauf escarpment at its toe, in an oblique impact from the
north.
© Meteoritical Society, 1999.
====================
(10) THE INTERNAL STRUCTURES AND DENSITIES OF ASTEROIDS
Lionel Wilson, Klaus Keil*) and Stanley J. Love: Report: The
internal
structures and densities of asteroids. METEORITICS &
PLANETARY SCIENCE
34, May (1999)
*) Hawaii Institute of Geophysics and Planetology, University of
Hawai'i at Manoa, Honolulu HI 96822; e-mail address:
keil@kahana.pgd.hawaii.edu
Four asteroidal bodies (the martian satellites Phobos and Deimos
and
the main-belt asteroids 243 Ida and 253 Mathilde) have now been
the
subjects of sufficiently close encounters by spacecraft that the
masses
and sizes and, hence, the densities of these bodies can be
estimated to
~10%. All of these asteroids are significantly less dense than
most
members of the classes of meteorites identified as being
compositionally most nearly similar to them on the basis of
spectral
characteristics. We show that two processes can act,
independently or
in concert, during the evolutionary histories of asteroids to
produce a
low bulk density. One of these processes is the result of one or
more
impact events and can affect any asteroid type, whereas the other
can
occur only for certain types of small asteroids which have
undergone
aqueous alteration. © Meteoritical Society, 1999.
----------------------------------------
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