PLEASE NOTE:
*
CCNet, 06/2000 - 18 January 2000
--------------------------------
QUOTE OF THE DAY
"About ten ice balls with weights
ranging from less than one
to almost five kilograms have reportedly
fallen from the sky
in Spanish cities in the last seven
days. A team from the
Consejo Superior de Investigaciones
Científicas (CSIC) is
studying the subject: the only known
result so far is that
the ice seems to be in an extremely pure
form. One of the
hypothesis suggested by the researchers
is that the ice balls
may come from a comet. If this is true,
similar balls should
have fallen in recent days in other
areas of the world."
-- Josep
Corbella, La Vanguardia, Barcelona
(1) FIRST NEAR IMAGE OF ASTEROID EROS
http://near.jhuapl.edu/news/flash/00jan17_1.html
(2) ICE FROM THE SKY
Josep Corbella <jcorbella@vanguardia.es>
(3) FOSSILS & THE K/T BOUNDARY
Andrew Glikson <geospectral@spirit.com.au>
(4) KABOOM!
Duncan Steel <D.I.Steel@salford.ac.uk>
wrote:
(5) IAU COLLOQIUM ON COSMIC DUST
Robert Hawkes <rhawkes@mta.ca>
(6) LABORATORY IMPACTS ON SIMULATED E/KBOS
E.V. Ryan et al., PLANETARY SCI INST
(7) ORIGIN, AGING & DEATH OF ASTEROID FAMILIES
F. Marzari & P. Farinella, UNIVERSITY OF
PADUA
(8) LARGE CRATERS ON SMALL OBJECTS
P.C. Thomas, CORNELL UNIVERSITY
(9) DENSITY & POROSITY OF STONE METEORITES
G.J. Flynn et al., SUNY COLL PLATTSBURGH
(10) ASTEROID LIGHTCURVE OBSERVATIONS
A.W. Harris et al. CALTECH
(11) LARGE IMPACTS & PLANETARY FORMATION
C.B. Agnor et al., SW RES INST
(12) COSMIC DUST
J.C. Worms et al., ENSPS
(13) YARKOVSKY FORCE ON ASTEROID FRAGMENTS
D. Vokrouhlicky & M. Broz,
CHARLES UNIVERSITY
=============
(1) FIRST NEAR IMAGE OF ASTEROID EROS
From http://near.jhuapl.edu/news/flash/00jan17_1.html
17 January 2000
The photo album of "NEAR's trip to Eros" has a new
page: the first
image from the spacecraft's approach to the asteroid. Taken Jan.
12 with NEAR's Multispectral Imager, the picture was posted today
on the NEAR Web site. More photos will follow in the weeks
leading
up to NEAR's Feb. 14 rendezvous with Eros.
Snapped from 27,200 miles (45,350 kilometers) away, Eros appears
only as a white speck on the black background of deep space.
However, mission navigators use these early images to confirm the
asteroid's location and keep the spacecraft on the right course.
The NEAR team also uses them to measure variations in the light
reflected off Eros, a key to determining the asteroid's exact
rotation.
NEAR is now about 22,500 miles (or 35,300 kilometers) from its
target - closer than the distance at which most weather and
communications satellites orbit the Earth.
=============
(2) ICE FROM THE SKY
From Josep Corbella <jcorbella@vanguardia.es>
About ten ice balls with weights ranging from less than one to
almost five kilograms have reportedly fallen from the sky in
Spanish cities in the last seven days. A team from the Consejo
Superior de Investigaciones Científicas (CSIC) is studying the
subject: the only known result so far is that the ice seems to be
in an extremely pure form. One of the hypothesis suggested by the
researchers is that the ice balls may come from a comet. If this
is true, similar balls should have fallen in recent days in other
areas of the world. And possibly there should also have been an
increase in meteor activity. I would be happy to provide
additional information to anybody interested in the subject and
grateful if somebody could give any data to test the comet
hypothesis.
Josep Corbella
Science editor
La Vanguardia
Barcelona, Spain
============
(3) FOSSILS & THE K/T BOUNDARY
From Andrew Glikson <geospectral@spirit.com.au>
Benny,
In his letter of 6.1.00 Bob Johnson queries whether fossils are
preserved naturally, or specifically due to catastrophic events,
with implications to the K-T extinction and similar events.
Preservation of fossil organisms depends on the original
abundance
of shelly organisms and on the physical and chemical conditions
of
the depositional environment, i.e. the sea or lake bottom
current
regime, rates of deposition, texture of sediments, REDOX
condition, and other factors. For example, strong currents rework
fossils, as evidenced by the paucity of fossils in coarse grained
sandstones and conglomerates, as contrasted to their good
preservation in fine-grained clay and carbonate-rich sediments.
Depending on REDOX, the organic component can be oxidized, or can
be reduced under oxygen-starved environment, in the latter case
leaving carbon traces in black (carbon and sulfide-rich)
shales.
Some sediments are formed almost exclusively of fossils, i.e.
foraminiferal limestones and diatomites. Thus, while
fossils are
rarely preserved in river deposits, they can be ubiquitous and
continuously preserved throughout lacustrine and abyssal (deep
sea) sequences. An example of the latter are the "scaglia
blanka"
of the Apennines - representing near-continuous sedimentation and
preservation of planktonic organisms spanning over 150 million
years during the Jurassic and Cretaceous, terminated by the K-T
boundary event. However, catastrophic events are also manifested
in the geological column. For example, immediately above the
Ir-rich K-T boundary clay horizon at Gubbio, Maciano and other
localities is a distinct layer of coarse-grained debris
consisting
of broken shell and coral fragments swept from shallow water
(bentonic) shelf environment by strong currents, possibly
representing the backwash of post-impact tsunamis. Another
example
are Pleistocene mammal bone beds in swamp/marsh environments,
representing floods and other environmental crises.
To directly observe present-day and recent fossilization
processes
visit beach and near-beach shell deposits. A well illustrated
reference book re-extinctions during geological history is by
Steven M. Stanley ("Extinction". Scientific American
Library,
NewYork, 1987) - there are many others excellent text books on
the
subject.
Sincerely Yours
Andrew Glikson
13.1.00
geospectral@spirit.com.au
<mailto:geospectral@spirit.com.au>
=============
(4) KABOOM!
From Duncan Steel <D.I.Steel@salford..ac.uk>
wrote:
Dear Benny,
CCNet readers might like to try the KABOOM! facility made
available by Douglas Hamilton (University of Maryland) at:
http://janus.astro.umd.edu/astro/impact.html
Cheers,
Duncan
==============
(5) IAU COLLOQIUM ON COSMIC DUST
From Robert Hawkes <rhawkes@mta.ca>
Deadline for Electronic Submission of Abstracts to IAU Colloq.
181: Jan. 22. IAU Colloq. 181 Dust in the Solar System and Other
Planetary Systems Apr.10-14 2000 at University of Kent at
Canterbury. For more information:
http://www.ukc.ac.uk/physical-sciences/space
or email M.L.Watts@ukc.ac.uk.
Note the following deadlines:
22 January 2000 ELECTRONIC abstract submission deadline;
15 February 2000 Deadline for Registration forms. Late
registration
rate applies after this date.
See www page for details of abstract submission, registration,
local
arrangements and program.
==============
(6) LABORATORY IMPACTS ON SIMULATED E/KBOS
E.V. Ryan*), D.R. Davis, I. Giblin: A laboratory impact study of
simulated Edgeworth-Kuiper belt objects. ICARUS, 1999, Vol.142,
No.1,
pp.56-62
*) PLANETARY SCI INST,620 N 6TH AVE,TUCSON,AZ,85705
This paper reports on a series of laboratory impact experiments
designed to provide basic data on how simulated Edgeworth-Kuiper
belt
objects (EKOs) fragment in an impact event. In September-October
1997
we carried out 20 low-velocity airgun shots at the Ames Vertical
Gun
Range into porous and homogeneous ice spheres using aluminum,
fractured
ice, and solid ice projectiles. We found that the porous ice
targets
behaved as strongly as solid ice in collision. Energy is
apparently
well dissipated by the void spaces within the target, such that
these
fragile ice structures respond as if they were strong in impacts.
Therefore, it would appear that if EKOs are porous, they are not
collisionally weak. Also, our data show that collisional outcomes
for
low-velocity impacts into ice targets depend on the type of
projectile
used as well as the properties of the target. We observed that
the
degree of fragmentation for a given type of target increases as
the
strength of the projectile increases. Aluminum projectiles are
far more
damaging to the target at the same collisional energy than are
solid
ice projectiles, which, in turn, are more damaging than fractured
ice
projectiles. One possible explanation for this behavior is the
variable
depth of penetration of the projectile for the different
cases-stronger
projectiles penetrate more deeply and couple more energy into the
target than do weak projectiles. Based on this, if we assume that
there
has not been significant heating or differentiation in the
Edgeworth-Kuiper (E-K) belt, the most applicable impact strength
for
the low-velocity E-K belt collisions is likely to be that derived
from
similar target/projectile materials impacting each other. The
laboratory data from this analysis indicate that a value for
impact
strength >5 x 10(5) erg/cm(3) is appropriate for porous ice
targets
impacted with solid/porous ice projectiles. (C) 1999 Academic
Press.
========
(7) ORIGIN, AGING & DEATH OF ASTEROID FAMILIES
F. Marzari*) & P. Farinella: Origin, aging, and death of
asteroid
families. ICARUS, 1999, Vol.142, No.1, pp.63-77
*) UNIV PADUA,DIPARTIMENTO FIS,VIA MARZOLO 8,I-35131 PADUA,ITALY
We have investigated the creation and destruction of asteroid
families
using a collisional code which follows the evolution of the size
distributions of both the main-belt background population and
individual families which are produced by collisions within the
population. Starting with a small-mass initial belt, the number
of
families produced by collisional breakup of parent bodies larger
than
100 km and surviving to the present time is in good agreement
with the
observed number of families. Increasing the mass of the initial
belt
increases the number of families in the model; hence the number
of
families that we see today provides a significant constraint on
the
overall collisional history of asteroids, Most families formed by
disruption of large (D > 200 km) parent bodies are still
recognizable
today. Families formed from smaller parent bodies early in Solar
System
history have been eroded away, and there is a general trend in
our
models to have younger ages associated with families formed from
smaller parent bodies, Today about one-third of the overall
population
should be in families, consistent with the findings of V, Zappala
et
al, (1995, Icarus 116, 291-314), (C) 1999 academic Press.
=========
(8) LARGE CRATERS ON SMALL OBJECTS
P.C. Thomas: Large craters on small objects: Occurrence,
morphology,
and effects. ICARUS, 1999, Vol.142, No.1, pp.89-96
CORNELL UNIVERSITY,CTR RADIOPHYS & SPACE RES,ITHACA,NY,14853
Spacecraft imaging data allow an accounting of the occurrence and
characteristics of the largest craters on small asteroids and
satellites. Data covering Phobos, Deimos, Gaspra, Ida, Mathilde,
Vesta,
Amalthea, Thebe, Janus, Epimetheus, Hyperion, and Proteus show
that
similar to 50% of rocky objects support craters with diameters of
1
object mean radius (R-m), and the great majority have more than
two
craters with D > 0.5R(m). The morphology of these craters is
consistent
with gravity (g(-1)) scaling from craters on larger objects, The
amounts of visible peripheral damage, such as fracturing caused
by
these relatively large impacts, are usually small; the best
possible
example (Phobos) may have been assisted by tidal forces. The
occurrence
and characteristics of these craters supports modeling results
that
predict formation in gravity-controlled regimes with very large
relative-sized craters formed before global fragmentation occurs.
(C)
1999 Academic Press.
============
(9) DENSITY & POROSITY OF STONE METEORITES
G.J. Flynn*), L.B. Moore, W. Klock: Density and porosity of stone
meteorites: Implications for the density, porosity, cratering,
and
collisional disruption of asteroids. ICARUS, 1999, Vol.142, No.1,
pp.97-105
*) SUNY COLL PLATTSBURGH,DEPT PHYS,PLATTSBURGH,NY,12901
The grain density, hulk density, and porosity were measured for
19
ordinary chondrite and 5 carbonaceous chondrite meteorites, and
the
grain density was measured for an additional 30 ordinary
chondrite and
7 carbonaceous chondrite meteorites. The unweathered ordinary
chondrites generally have porosities ranging from 5 to 25%, while
the
carbonaceous chondrites have porosities in the range 10 to 30%.
These
high porosities indicate that the asteroidal parent bodies from
which
these chondritic meteorites are derived are porous, or contain
porous
regions. The grain densities of all 49 ordinary chondrites are
greater
than 3.1 g/cc, the grain densities of all 9 anhydrous
carbonaceous
chondrites are greater than 3.0 g/cc, and the grain densities of
the
hydrated carbonaceous chondrites range from 2.5 to 3.0 g/cc.
Comparison
of these grain densities with spacecraft determinations of the
densitiy
of the S-type asteroid Ida, 2.6 g/cc, and the anhydrous C-type
asteroid
Mathilde, 1.3 g/cc, suggests that if these asteroids are composed
of
material similar in mineralogy to that of the stone meteorites,
then
Ida and Mathilde must exhibit significant porosity. Cratering and
collisional disruption are sensitive to the porosity of the
target. The
parent bodies of the ordinary and carbonaceous chondrite
meteorites
appear to contain regions of significant porosity, which may
affect
their compression and compaction during impact events. (C) 1999
Academic Press.
=======
(10) ASTEROID LIGHTCURVE OBSERVATIONS
A.W. Harris*), J.W. Young, E. Bowell, D.J. Tholen: Asteroid
lightcurve
observations from 1981 to 1983. ICARUS, 1999, Vol.142, No.1,
pp.173-201
*) CALTECH,JET PROP LAB,4800 OAK GROVE DR,PASADENA,CA,91109
We present observations of 40 asteroids taken mostly from Table
Mountain Observatory, mostly from 1982, but with a few
observations
from late 1981 and early 1983. Several new or substantially
revised
periods are reported. Perhaps most interesting are observations
of two
asteroids, 288 Glauke and 3288 Seleucus, which are in 'tumbling,'
or
nonprincipal axis rotation, states. Glauke has the longest
rotation
period known, similar to 1200 h. Absolute photometry was
performed on
most objects, allowing us to derive phase relations and absolute
magnitudes, For objects where these results are superior to
previous
values, we have revised the IRAS albedos using our absolute
magnitude
(H) values. (C) 1999 Academic Press.
==============
(11) LARGE IMPACTS & PLANETARY FORMATION
C.B. Agnor*), R.M. Canup, H.F. Levison: On the character and
consequences of large impacts in the late stage of terrestrial
planet
formation. ICARUS, 1999, Vol.142, No.1, pp.219-237
*) SW RES INST,SPACE STUDIES DEPT,1050 WALNUT,SUITE
426,BOULDER,CO,80302
We perform three-dimensional N-body integrations of the final
stages of
terrestrial planet formation. We report the results of 10
simulations
beginning with 22-50 initial planetary embryos spanning the range
0.5-1.5 AU, each with an initial mass of 0.04-0.13M(.).
Collisions are
treated as inelastic mergers. We follow the evolution of each
system
for 2 x 10(8) years at which time a few terrestrial type planets
remain. On average, our simulations produced two planets larger
than
0.5M(.) in the terrestrial region (1 simulation with one m
greater than
or equal to 0.5M(.) planet, 8 simulations with two m greater than
or
equal to 0.5M(.) planets, and 1 simulation with three m greater
than or
equal to 0.5M(.) planets). These Earth-like planets have
eccentricities
and orbital spacing considerably larger than the terrestrial
planets of
comparable mass (e.g., Earth and Venus). We also examine the
angular
momentum contributions of each collision to the final spin
angular
momentum of a planet, with an emphasis on the type of impact
which is
believed to have triggered the formation of the Earth's Moon.
There was
an average of two impacts per simulation that contributed more
angular
momentum to a planet than is currently present in the Earth/Moon
system. We determine the spin angular momentum states of the
growing
planets by summing the contributions from each collisional
encounter.
Our results show that the spin angular momentum states of the
final
planets are generally the result of contributions made by the
last few
large impacts, Our results suggest that the current angular
momentum of
the Earth/Moon system may be the result of more than one large
impact
rather than a single impact. Further, upon suffering their first
collision, the planetary embryos in our simulations are spinning
rapidly throughout the final accretion of the planets, suggesting
the
proto-Earth may have been rotating rapidly prior to the
Moon-forming
impact event. (C) 1999 Academic Press.
==============
(12) COSMIC DUST
J.C. Worms*), J.B. Renard, E. Hadamcik, A.C. Levasseur Regourd,
J.F. Gayet: Results of the PROGRA(2) experiment: An experimental
study
in microgravity of scattered polarized light by dust particles
with large size parameter. ICARUS, 1999, Vol.142, No.1,
pp.281-297
*) ENSPS,EUROPEAN SPACE SCI COMM,PARC INNOVAT,BLVD SEBASTIAN
BRANDT,F-67400 ILLKIRCH GRAFFENS,FRANCE
The PROGRA(2) experiment allows measurements under microgravity
conditions of polarization phase curves for clouds of dust
particles.
The sizes that can be used range from a few micrometers to
hundreds of
micrometers; regular or irregular particles can be tested.
Measurements
obtained on compact particles of boron car bide and silicon
carbide are
presented. Results for the silicon carbide, obtained for
different
values of the number densities of the particles, indicate that
values
of the polarization maximum are lower when measured under
microgravity
conditions than on the ground. Although still not fully
demonstrated,
this result could be used to understand why it has been difficult
to
correlate measurements of the positive branch of the polarization
phase
curve done on the ground with actual data on Solar System
objects, (C)
1999 Academic Press.
============
(13) YARKOVSKY FORCE ON ASTEROID FRAGMENTS
D. Vokrouhlicky*) & M. Broz: An improved model of the
seasonal
Yarkovsky force for regolith-covered asteroid fragments.
ASTRONOMY AND
ASTROPHYSICS, 1999, Vol.350, No.3, pp.1079-1084
*) CHARLES UNIVERSITY,INST ASTRON,V HOLESOVICKACH 2,CR-18000
PRAGUE
8,CZECH REPUBLIC
We derive a new analytical solution for the seasonal Yarkovsky
effect,
the mean-motion frequency mode of the recoil force due to
reradiated
sunlight, on a spherical asteroid fragment. The body is assumed
to have
a thin low-conductivity (regolith-like) surface layer, covering a
much
more thermally conductive core. If the penetration depth of the
seasonal thermal wave in the low-conductivity surface material is
larger than the regolith's geometrical thickness, the previous
simplified solution assuming a homogeneous interior of the body
might
lead to wrong estimates on the intensity of the perturbing force.
Our
approach removes this problem and the results indicate: (i) an
increased seasonal mobility of 10-m sized and larger fragments
with an
insulating surface layer, and (ii) a decreased seasonal mobility
of
meter-sized fragments with the same structure. These results may
affect
the accuracy of simulations of meteorite and NEA transport to the
Earth, as well as the dynamical evolution of some real small
asteroids
(e.g. 1566 Icarus). Copyright 2000, Institute for Scientific
Information Inc.
----------------------------------------
THE CAMBRIDGE-CONFERENCE NETWORK (CCNet)
----------------------------------------
The CCNet is a scholarly electronic network. To
subscribe/unsubscribe,
please contact the moderator Benny J Peiser <b.j.peiser@livjm.ac.uk>.
Information circulated on this network is for scholarly and
educational use only. The attached information may not be copied
or
reproduced for any other purposes without prior permission of the
copyright holders. The fully indexed archive of the CCNet, from
February 1997 on, can be found at http://abob.libs.uga.edu/bobk/cccmenu.html