PLEASE NOTE:
*
CCNet DIGEST, 28 August 1998
----------------------------
(1) TUNGUSKA & THE ASTEROID HYPOTHESIS
Luigi Foschini <L.Foschini@fisbat.bo.cnr.it>
(2) PLANETARY DEFENSE & THE DANGERS OF NUCLEAR POLLUTION IN
SPACE
C.S.L. Keay, UNIVERSITY OF NEWCASTLE
(3) A STUDY OF HILDA ASTEROIDS
M. Dahlgren et al., ASTRONOMICAL OBSERVATORY
UPPSALA
(4) SEARCHING FOR BRIGHT KUIPER BELT OBJECTS
M.J.I. Brown & R.L. Webster, UNIVERSITY OF
MELBOURNE
(5) THE SPLITTING OF HALLEY'S COMET
D.B. Chen et al., CHINESE ACADEMY OF SCIENCE
(6) COSMIC RAY IRRADIATION OF KUIPER BELT COMETS
J.F. Cooper et al., NASA
(7) SPIELBERG & ARMAGEDDON MAKE DEEP IMPACT
Scott Manley <spm@star.arm.ac.uk>
(8) SNOWBALL IN HELL: EARTH WAS COMPLETELY COVERED BY ICE
Andrew Yee <ayee@nova.astro.utoronto.ca>
======================
(1) TUNGUSKA & THE ASTEROID HYPOTHESIS
From Luigi Foschini <L.Foschini@fisbat.bo.cnr.it>
Dear Friends and Colleagues,
I would like to inform you that a preprint is now available on
the
WEB at http://xxx.lanl.gov/abs/astro-ph/9808312:
Title: A solution for the Tunguska event
Author: Luigi Foschini
Comments: 7 pages, no figures. Submitted to Astron. Astrophys
Abstract: This letter presents a possible solution for the
Tunguska
event of June 30th, 1908. The solution, obtained starting from
seismic data and improving fragmentation models, strengthen the
asteroidal hypotesis for the Tunguska cosmic body. An improvement
of
fragmentation models is also proposed in order to reduce
inconsistencies with observational evidences.
At the WEB address you can download files in several formats (PS,
PDF, DVI). However, if you have any problem, please do not
hesitate
to contact me.
Greetings,
Luigi
==================
(2) PLANETARY DEFENSE & THE DANGERS OF NUCLEAR POLLUTION IN
SPACE
C.S.L. Keay: Pollution potentials in interplanetary space.
ADVANCES
IN SPACE RESEARCH, 1998, Vol.21, No.11, pp.1603-1606
UNIVERSITY OF NEWCASTLE, DEPT PHYS, CALLAGHAN, NSW 2308,
AUSTRALIA
Members of several Commissions in the Planetary Systems Division
of
the International Astronomical Union have become increasingly
concerned at the harmful potentials of a number of proposed space
missions, particularly those intending to utilise either nuclear
or
kinetic energy disruption of comets and small asteroids. This
concern
has led to the establishment of a Working Group on the Prevention
of
Pollution in Interplanetary Space, which involves six IAU
Commissions. The aim of the Working Group is to identify
sensitive
issues in the interplanetary environment and draw attention to
the
harmful consequences of ignoring them, rather than adopting a
legalistic regulatory approach. (C) 1998 COSPAR. Published by
Elsevier Science Ltd.
=====================
(3) A STUDY OF HILDA ASTEROIDS
M. Dahlgren*), J.F. Lahulla, C.I. Lagerkvist, J. Lagerros, S.
Mottola,
A. Erikson, M. Gonano Beurer, M. DiMartino: A study of Hilda
asteroids
- V. Lightcurves of 47 Hilda asteroids. ICARUS, 1998, Vol.133,
No.2,
pp.247-285
*) ASTRONOMICAL OBSERVATORY, BOX 515, S-75120 UPPSALA, SWEDEN
The results of a photometric survey of Hilda asteroids are
presented.
We have obtained 224 lightcurves of 47 Hilda asteroids. A wide
span of
rotation periods have been found, ranging from 2.851 to 31 h.
However,
indications of rotation periods longer than 100 h were found for
two
objects. The largest lightcurve amplitude found is 1.38 mag, and
13
additional objects have observed amplitudes greater than or equal
to
0.30 mag. Improved absolute magnitudes have been obtained for 20
objects as well as new V-R color indices for 17 of these.
Improved
albedos, derived from the new absolute magnitudes, have been
obtained
for 10 Hilda asteroids. The V-R color indices range between 0.38
and
0.49 mag, indicating that only P- and D-type asteroids are
present
among these asteroids, A probable taxonomy was assigned to four
Hilda
asteroids (one P-type and three D-types) which previously were
unclassified, The Hilda Asteroid 3694 Sharon was observed at low
solar
phase angles, and the obtained lightcurves indicate a reduced to
nonexisting opposition effect for this asteroid, One Hilda
asteroid,
2067 Aksnes, may be in a state of precession as no satisfactory
two-maxima and two-minima lightcurve could be fitted to the data.
However, the short damping time scale makes this very unlikely.
This
favours a four-maxima and four-minima lightcurve of 2067 Aksnes,
which
is a better fit to the data than a two-maxima and two-minima
lightcurve. The Hilda Asteroid 3415 Danby has a very short
rotation
period (P = 2.851 +/- 0.003 h), making Danby the fastest spinning
asteroid known within its size range (D similar to 50 km). Due to
its
fast spin some constraints can be set on 3415 Danby's internal
structure and mean density. The assumption that 3415 Danby is a
rubble-pile, with its shape governed by hydrostatic equilibrium,
leads
to an unrealistically high mean density. On the other hand,
assuming
that 3415 Danby has internal strength results in a mean density
larger
than 1.4 g cm(-3). (C) 1998 Academic Press.
===========================
(4) SEARCHING FOR BRIGHT KUIPER BELT OBJECTS
M.J.I. Brown & R.L. Webster: A search for bright Kuiper Belt
objects.
PUBLICATIONS ASTRONOMICAL SOCIETY OF AUSTRALIA, 1998, Vol.15,
No.2,
pp.176-178
UNIVERSITY OF MELBOURNE, SCH PHYS, PARKVILLE, VIC 3052, AUSTRALIA
Since 1992, 60 large Kuiper Belt objects have been detected by
ground-based telescopes. Previous surveys which have detected
objects
have searched approximately 60 square degrees and detected
objects with
magnitudes 20.6 < m(R) < 25.0. However, the luminosity
function of
brighter Kuiper Belt objects is not well determined. The
detection of
brighter objects would improve our ability to determine the
Kuiper Belt
objects' surface composition and provide constraints on the
population
statistics of different formation mechanisms. This paper
describes a
survey of 12.0 square degrees of sky near the ecliptic to a
limiting
magnitude of m(R) similar to 21. A slow moving candidate was
detected
near the magnitude limit of the survey. Copyright 1998, Institute
for
Scientific Information Inc.
====================
(5) THE SPLITTING OF HALLEY'S COMET
D.B. Chen*), L.Z. Liu, A. Gilmore: The splitting of Comet Halley.
ADVANCES IN SPACE RESEARCH, 1998, Vol.21, No.11, pp.1607-1610
*) CHINESE ACADEMY OF SCIENCE, PURPLE MT OBSERV, NANJING 210008,
CHINA
In combination with the authors' previous observation about the
splitting of Comet Halley in March 1986, the events involving the
sharp, straight feature in the antisolar direction observed in
the
head of Comet Halley in 1910 (such as those occurring on May 14,
25
and 31, and June 2) are rediscussed. The analysis leads to the
following scenario: When Comet Halley explodes and splits, a
fragment
jettisoned or thrown off from the nucleus will, after moving in
the
direction of its tail, develop into a mini-comet. Although not
well
developed or permanent, it has its own plasma tail and,
sometimes, a
dust tail. If Bobrovnikoffs definition of a secondary nucleus is
assumed, then the fragment should be considered as a real
secondary
nucleus. It seems that the current idea of a tailward jet
suggested
by Sekanina and Larson is a wrong explanation for the plasma tail
of
a mini-comet and hence the rotation period of 52-53 h for Comet
Halley is doubtful. (C) 1998 COSPAR. Published by Elsevier
Science
Ltd.
=================
(6) COSMIC RAY IRRADIATION OF KUIPER BELT COMETS
J.F. Cooper*), E.R. Christian, R.E. Johnson: Heliospheric cosmic
ray
irradiation of Kuiper belt comets. ADVANCES IN SPACE RESEARCH,
1998,
Vol.21, No.11, pp.1611-1614
*) NASA, HUGHES STX CORP, GODDARD SPACE FLIGHT CTR, CODE 632,
GREENBELT, MD, 20771
Irradiation by galactic cosmic ray particles at energies above
0.1
GeV has been shown to be a significant source of energy for
chemical
modification of ices in Oort Cloud comets. However, these ions
have
minimal efficiency for surface modification, since their energy
is
deposited over depths of many meters, and they must act over the
lifetime of the Solar System to produce an appreciable
radiation-induced mantle. Recent measurements by the Voyager and
Pioneer spacecraft, now moving outward from the Sun beyond 40 -
60
A.U. into the Kuiper belt region, have found a radially
increasing
intensity of the ''anomalous'' cosmic ray component, consisting
of
interstellar hydrogen and heavier ions accelerated at energies up
to
10(2) MeV/nucleon at the solar wind termination shock at 65 - 100
A.U. from the Sun. Significant intensities of the anomalous
cosmic
ray ions are expected throughout the inner Kuiper belt region
beyond
the termination shock and out to the heliopause. These ions would
significantly affect mantle formation on Kuiper Belt comets at
column
depths less than 10 grams/cm(2) in the outer surface layer of
material accessible to direct and remote sensing measurements.
(C)
1998 COSPAR. Published by Elsevier Science Ltd.
====================
(7) SPIELBERG & ARMAGEDDON MAKE DEEP IMPACT
From Scott Manley <spm@star.arm.ac.uk>
People might be interested to know that both Deep Impact and
Armageddon have made it into the top 50 grossing films in History
-
and Armageddon looks set to climb higher in coming weeks.
Currently the films also are the biggest grossing films to be
released this year (Titanic was premiered in 1997.....).
RANK
USA
OVER WORLD "" TITLES
(1998-1998)
045 $ 185.0 $ 143.5 $
328.5 Armageddon (1998)
050 $ 140.1 $ 180.5 $
320.6 Deep Impact (1998)
We at Armagh have noted this trend and have made plans -
http://szyzyg.arm.ac.uk/~spm/armaged.html
;-)
Please not that my views don not bear any relationship to those
of
the observaotory and are probably at odds for much of the
time....
Scott Manley (aka Szyzyg)
==============
(8) SNOWBALL IN HELL: EARTH WAS COMPLETELY COVERED BY ICE
From Andrew Yee <ayee@nova.astro.utoronto.ca>
Harvard University
27 August 1998
Earth Was Completely Covered by Ice, Geologists Say
First Large Animals Appeared After Melting
By William J. Cromie, Gazette Staff
Seven hundred million years ago, Earth's oceans were completely
frozen over. No rivers flowed, no rain or snow fell. Life,
limited to
simple plants and bacteria at the time, became severely depleted.
But inside Earth, the activity that leads to surface volcanism
continued. Volcanoes belched carbon dioxide and other gases into
the
air. Carbon dioxide accumulated for millions of years, preventing
heat from escaping into space (the greenhouse effect), and
producing
a global warming that eventually melted the ice.
Between 750 million and 570 million years ago, this icehouse to
greenhouse cycle occurred several times. Glaciers turned Earth
into a
"snowball" that stayed unmelted for millions of years
until volcanic
gases finally freed it.
At least that's what the rocks in southwest Africa tell Harvard
geologists. "It's staggering to think that such events are
not only
possible in theory, but actually occurred at a critical turning
point
in Earth's history," says Paul Hoffman, Sturgis Hooper
Professor of
Geology. "The first diverse fossils of large animals appear
soon
after the last snowball glaciation. There are reasons to believe
that
this is no mere coincidence."
For three billion years preceding the snowball glaciations, life
was
confined to algae, bacteria, and other simple organisms. Only the
most
adaptable of these creatures survived the global freezeovers.
Soon
after the last ice ages ended, about 565 million years ago, large
animals with cells formed into tissues and organs suddenly began
to
appear. They included the ancestors of many groups of animals
still
alive.
Hoffman spent six summers examining rock formations in Namibia,
Africa, where the rock record of ice ages and global warmings is
clearly preserved. "Things didn't start falling into place
until last
December," he recalls. "What excites me about the
snowball idea is
that it provides a common explanation for many notable geological
features in that period of Earth history, any one of which is
puzzling when approached in isolation."
What do other scientists think about Hoffman's reading of the
rocks?
"They either love it, or they hate it," he replies.
"We've had both
support and stiff challenges, both of which have led to a
stronger
theory. At this point, the remaining questions appear minor
compared
to all the things that the idea explains."
A detailed report of the theory and evidence that supports it
appears
in Friday's issue of the journal Science. It was written by
Hoffman;
along with Alan Kaufman, a former Harvard post-doctoral fellow
now at
the University of Maryland; Galen Halverson, a graduate student
working with Hoffman; and Harvard geochemist Daniel Schrag.
Do Worms Keep Us Warm?
Earth started to freeze over, Hoffman says, due to a lack of
carbon
dioxide in the air combined with a dimmer sun. Once ice and snow
covered much of the land and ocean, a process called the albedo
effect -- wherein ice and snow reflect the sun's energy back into
space -- kicked in. The more ice and snow, the higher the albedo
and
the colder it gets.
"Albedo tends to drive global change to one extreme [all
ice] or the
other [no ice], causing catastrophic change in either
direction,"
Hoffman explains. But for albedo-driven glaciation to start, it
must
first get cold enough for polar sea ice to expand to the latitude
of
Boston. During the most recent ice age, 18,000 years ago, land
ice
reached Cape Cod, but during summers the Atlantic stayed open as
far
north as Iceland.
The sun also radiated less energy 750 million years ago than it
does
now. Our star works like a nuclear reactor, converting hydrogen
to
helium and releasing energy. As the proportion of helium grows,
the
sun produces more heat. Some 750 million years ago, the solar
furnace
was an estimated 6-7 percent cooler than at present.
The sun, however, did not operate alone. "Over most of
geological
time, varying amounts of carbon dioxide in the air have regulated
Earth's climate through the greenhouse effect," Hoffman
notes. The
gas acts like glass in a greenhouse; it lets light in but
prevents
heat from escaping into space. Carbon dioxide along with other
gases
coming from smokestacks, vehicle exhausts, and burning of
tropical
forests make a major contribution to the present global warming.
During the icehouse part of the greenhouse-icehouse transitions,
a
severe shortage of carbon dioxide in the air came from a loss of
carbon, which entered the ocean and got buried with muddy
sediments
on the ocean floor. These sediments, which were later heated,
compressed and uplifted by the shifting of continents and ocean
floors, are beautifully preserved as rock layers in Namibia.
Chemical testing of these rocks reveals evidence for rapid
removal of
carbon from the atmosphere before the ice ages, then virtually no
removal during the warmings. "At no time since the last
snowball
event do we observe carbon shifts of such magnitude,"
Hoffman
comments.
How come there have been no snowball glaciations since that time?
"We
think we have worms and snails to thank," laughs Hoffman.
They and
many other animals that live on the sea bottom constantly churn
muddy
sediments searching for bits of food. This contributes to the
breakdown of organic forms of carbon and its release into the
water,
then into the air.
"With the advent of bottom-dwelling animals, burial of
organic carbon
became seriously impeded by their feeding activities,"
Hoffman points
out. Before the advent of animals, the rocks in Namibia show thin
layers of undisturbed sediment on the sea bottom. After animals
appeared, the fossil sediments were disrupted by feeding trails,
burrows, and other signs of grazing activity.
But what caused the sudden appearance of such animals? Hoffman
notes
that a leading theory for the creation of new species involves
mass
mortality of organisms, disorganization of genetic material among
the
survivors, then renewed population growth in a different
environment.
"This is just what happened in the icehouse-greenhouse
transitions,"
Hoffman says. "A succession of global glaciations, each
terminated by
intense warming conditions, may be just what the biologists
ordered"
for a sudden evolution of new forms of life.
If he is right, that explains one of the greatest mysteries of
life
on Earth: what caused the first appearance of animal life.
Snowball in Hell
"Whenever we see the top of deposits laid down by glaciers
in
Namibia, they are capped with layers of pure carbonate
rock," says
Hoffman, pointing to a chunk of such rock on his desk. These
limestone caps consist of carbon and calcium that precipitated
out of
sea water. They appear all over the world and have always been a
mystery to geologists.
Daniel Schrag, the Harvard geochemist, maintains that this unique
combination of glacial deposits, sharply capped by carbonates
laid
down in warm water, can be neatly accounted for by the snowball
theory. The high concentrations of carbon dioxide would break the
ice's long grip. As the ice receded, rapid precipitation of
carbonate
from the water would occur.
"It was the most extreme and rapid change on record,"
says Schrag.
"Organisms surviving the deep freeze would immediately have
to face
the heat."
But how did the snowballs get rolling in the first place?
"For 300
million years before the cycles began," Hoffman explains,
"all land
was gathered together in a single supercontinent called
'Rodinia.'
The name comes from the Russian work rodit, which means 'to
beget.'
When Rodinia began to breakup about 750 million years ago, it
begot
smaller continents which created many new continental margins.
These
margins are where most of organic matter, including carbon,
settles
to the ocean bottom and gets buried. The burials speeded up
withdrawal of carbon dioxide from the air and begot the whole
previously unimagined chain of events."
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