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(meteorobs) Excerpts from "CCNet, 20 October 1999"
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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 20 October 1999
Date: Wed, 20 Oct 1999 11:33:33 -0400 (EDT)
CCNet, 20 October 1999
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[...]
(6) ORBITAL EVOLUTION OF INTERPLANETARY DUST PARTICLES
J.C. Liou et al., GB TECH LOCKHEED MARTIN
(7) SIZE DISTRIBUTION OF ASTEROID FAMILIES
P. Tanga et al., OSSERV ASTRON TORINO
(8) VELOCITY-SIZE RELATIONSHIP OF ASTEROIDS
A. Cellino et al., OSSERV ASTRON TORINO
(9) SIZES AND MASSES OF CHONDRULES
K.E. Kuebler et al., UNIVERSITY OF TENNESSEE
===========================
[...]
(6) ORBITAL EVOLUTION OF INTERPLANETARY DUST PARTICLES
J.C. Liou*), H.A. Zook, A.A. Jackson: Orbital evolution of retrograde
interplanetary dust particles and their distribution in the solar
system. ICARUS, 1999, Vol.141, No.1, pp.13-28
*) GB TECH LOCKHEED MARTIN,MAIL CODE C104,2400 NASA RD 1,
HOUSTON,TX,77058
The orbital evolution of interplanetary dust particles (IDPs) from
both retrograde and prograde Halley-type comets is numerically
simulated. It is found that dust particles nearly always get trapped
into one or more mean motion resonances (MMRs) with giant planets
while in retrograde orbits. Of the 1000 retrograde particles
simulated 116 are trapped in MMRs that last longer than 100,000
years, and the MMRs are typically exterior MMRs of the type p:1 with
Jupiter where p ranges from 1 to 12. We present a simple analysis of
the physical processes involved when an IDP is in a resonance. A
quasistable resonance is maintained by the combined effect of the
direct perturbation from the planet as well as the indirect
perturbation that arises because the Sun is moving around the
Sun-planet barycenter. The direct and indirect perturbations are
often of comparable magnitudes, although one or the other can be
dominant. Of the 1000 retrograde particles simulated, 45 of them
evolved to prograde orbits while in MMRs with a giant planet.
Although the overall distribution in eccentricity and inclination of
10(-7) g dust particles from Halley-type comets is consistent with D.
H. Humes' (1980, J. Geophys. Res. 85, 5841-5852) interpretation of
the Pioneer 10 and 11 data, the spatial density variation is not.
Instead of the constant spatial density derived by Humes, the spatial
density of Halley-type comet dust varies as r(-gamma) with
heliocentric distance r, where gamma ranges from 1.5 to 1.8.
Additional sources of interplanetary dust particles are needed to
account for the discrepancy. (C) 1999 Academic Press.
=============
(7) SIZE DISTRIBUTION OF ASTEROID FAMILIES
P. Tanga*), A. Cellino, P. Michel, V. Zappala, P. Paolicchi, D.
DellOro: On the size distribution of asteroid families: The role of
geometry. ICARUS, 1999, Vol.141, No.1, pp.65-78
*) OSSERV ASTRON TORINO,I-10025 PINO TORINESE,TO,ITALY
The steep slopes of the size distributions of the presently known
asteroid families have long represented a debated problem. The reason
is that it is not easy to reproduce them by the usual modeling
techniques based on the application of standard power-laws as
suggested by laboratory experiments. In this paper, we suggest that
the failures of the previous models were due to the fact that
geometric effects were not taken into account. In other words, the
finite sizes of the parent bodies and the fact that fragments tend to
have convex shapes cannot be disregarded. Following this approach, we
find that steep size distributions are necessarily produced by
fragmentations of the parent bodies. Moreover, we have been able to
reproduce fairly well the observed size distributions of the major
families, and we have also obtained some reasonable constraints on
the original sizes of the parent bodies. Some anomalous mass
depletions, probably due to injection of fragments in nearby
resonances have also been found, not unexpectedly, in a few cases
(Maria, Themis). (C) 1999 Academic Press.
============
(8) VELOCITY-SIZE RELATIONSHIP OF ASTEROIDS
A. Cellino*), P. Michel, P. Tanga, V. Zappala, P. Paolicchi, A.
DellOro: The velocity-size relationship for members of asteroid
families and implications for the physics of catastrophic collisions.
ICARUS, 1999, Vol.141, No.1, pp.79-95
*) OSSERV ASTRON TORINO,I-10025 PINO TORINESE,TO,ITALY
An extensive analysis of the size-ejection velocity relationship for
members of several of the most important asteroid families identified
in the Main Belt is presented. We have found a well defined behavior,
with smaller fragments having on the average higher ejection
velocities. The results provide useful constraints to current models
of catastrophic breakup processes and lead also to a new estimate of
the transition limit in largest remnant/parent body mass ratio,
distinguishing cratering, and shattering regimes. Moreover, we have
now available a practical method for estimating fragment ejection
velocities in interasteroid collisional events. This can be easily
implemented in numerical models of the collisional evolution of the
asteroid belt. In particular, it should be possible to undertake a
more quantitative assessment of the efficiency of collisional events
in the Main Belt as the sources of near Earth asteroids of different
sizes. (C) 1999 Academic Press.
============
(9) SIZES AND MASSES OF CHONDRULES
K.E. Kuebler*), H.Y. McSween, W.D. Carlson, D. Hirsch: Sizes and
masses of chondrules and metal-troilite grains in ordinary
chondrites: Possible implications for nebular sorting. ICARUS, 1999,
Vol.141, No.1, pp.96-106
*) UNIVERSITY OF TENNESSEE,DEPT GEOL SCI,KNOXVILLE,TN,37996
Intergroup variation in chondrule sizes is commonly attributed to
mass or aerodynamic sorting in the solar nebula, a concept that has
recently been extended to other chondrite constituents (metal
chondrules, metal-troilite grains). Both sorting mechanisms are
dependent on grain density and size. Because metal chondrules and
metal-troilite grains have smaller average sizes than their
coexisting chondrule populations, the assumption of mass equivalence
has been made and invoked in support of nebular sorting. We present
the results of a quantitative comparison of the sizes, masses, and
aerodynamic stopping times of chondrules and metal-troilite grains
from three ordinary chondrites: Kelly (LL4), Bjurbole (L/LL4), and
Hammond Downs (H4). Chondrule volumes were determined from corrected
thin-section measurements, and metal-troilite grain volumes were
estimated from X-ray tomographic images. Chondrule and metal-troilite
grain populations have similar masses in Hammond Downs but are
dissimilar in Bjurbole and Kelly. The difference in the average
particle aerodynamic stopping times of the chondrules and
metal-troilite grains (15% for Hammond Downs, 16% for Bjurbole, 32%
for Kelly) are much smaller than differences in their average masses
(28% for Hammond Downs, 73% for Bjurbole, 82% for Kelly). The
observed ranges in mass of the two populations are relatively narrow
in Hammond Downs and are wider in Bjurbole and Kelly. Furthermore, in
all three meteorites the observed range in mass of the chondrules is
narrower than that of the corresponding metal-troilite grain
populations. It appears that the chondrules were sorted more
efficiently than the metal-troilite grains. Our results agree
with the idea that aerodynamic stopping times vary with particle size
and density and disagree with sorting only by mass. While the average
stopping times (expressed as r(p) p(s)) of the two populations
correlate better than their average masses, the percent difference in
(r(p) p(s)) between the two populations (Hammond Downs, 15%;
Bjurbole, 16%; and Kelly, 32%) is greater than that between the
chondrules and metal chondrules previously reported for the Acfer 059
carbonaceous chondrite (W. R. Skinner and J. M. Leenhouts 1993, Proc.
Lunar Planet. Sci. Conf. 24th, 1315-1316). We attribute this to the
small and irregular shapes of the metal-troilite grains, although
thermal metamorphism may have also affected the metal-troilite grain
data. For these reasons, our results are at variance with the concept
of nebular mass sorting but may be in agreement with aerodynamic
sorting models. Our results are consistent with the hypothesis that
chondrule sorting is related to the phenomena of metal-silicate
fractionation. However, these data are only preliminary. Final
interpretation should be reserved until more meteorites can be
analyzed, the effects of thermal metamorphism on metal grain sizes
quantified, and software capable of true three-dimensional analysis
developed. (C) 1999 Academic Press.
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