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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
----------------------

[...]

(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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