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(meteorobs) Excerpts from "CCNet, 6 March 2000"
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From: Benny J Peiser <b.j.peiser@livjm.acdot uk>
To: cambridge-conference@livjm.acdot uk
Subject: CCNet, 6 March 2000
Date: Mon, 6 Mar 2000 09:55:37 -0500 (EST)
CCNet, 28/2000 - 6 March 2000
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[...]
(8) ANALYSIS OF NANOGRAM-SIZED ASTROMATERIALS
M.E. Zolensky et al., NASA,LYNDON B JOHNSON SPACE CTR
(9) PLANET & ASTEROID FORMATION IN THE PRESENCE OF GIANT PLANETS
S.J. Kortenkamp & G.W. Wetherill, CARNEGIE INST WASHINGTON
(10) LONG-TERM DYNAMICS OF BRIGHT BOLIDES
L. Foschini et al., UNIVERSITY OF TRIESTE
(11) ELEMENTAL COMPOSITION OF VENUS, EARTH, MARS & CHONDRITES
Y.T. Chuburkov, JOINT INST NUCL RES, RUSSIA
========================================================
(8) ANALYSIS OF NANOGRAM-SIZED ASTROMATERIALS
M.E. Zolensky*), C. Pieters, B. Clark, J.J. Papike: Small is beautiful: =
The analysis of nanogram-sized astromaterials. METEORITICS & PLANETARY=20
SCIENCE, 2000, Vol.35, No.1, pp.9-29
*) NASA,LYNDON B JOHNSON SPACE CTR,PLANETARY SCI =20
BRANCH,HOUSTON,TX,77058
The capability of modem methods to characterize ultra-small samples=20
is well established from analysis of interplanetary dust particles=20
(IDPs), interstellar grains recovered from meteorites, and other=20
materials requiring ultra-sensitive analytical capabilities. Powerful=20
analytical techniques are available that require, under favorable=20
circumstances, single particles of only a few nanograms for entire=20
suites of fairly comprehensive characterizations. A returned sample=20
of > 1000 particles with total mass of just 1 mu g permits=20
comprehensive quantitative geochemical measurements that are=20
impractical to carry out in situ by flight instruments.. The main goal=20
of this paper is to describe the state-of-the-art in microanalysis of=20
astromaterials. Given that we can analyze fantastically small=20
quantities of asteroids and comets, etc., we have to ask ourselves,=20
how representative are microscopic samples of bodies that measure a
few to many kilometers across? With the Galileo flybys of Gaspra and=20
Ida, it is now recognized that even very small airless bodies have=20
indeed developed a particulate regolith. Acquiring a sample of the=20
bulk regolith, a simple sampling strategy, provides two critical=20
pieces of information about the body. Regolith samples are excellent=20
bulk samples because they normally contain all the key components of=20
the local environment, albeit in particulate form. Furthermore,=20
because this fine fraction dominates remote measurements, regolith=20
samples also provide information about surface alteration processes=20
and are a key link to remote sensing of other bodies. Studies=20
indicate that a statistically significant number of nanogram-sized=20
particles should be able to characterize the regolith of a primitive=20
asteroid, although the presence of larger components (e.g.,=20
chondrules, calcium-aluminum-rich inclusions, large crystal=20
fragments, etc.) within even primitive meteorites (e.g., Murchison)=20
points out the limitations of using data obtained from nanogram-sized=20
samples to characterize entire primitive asteroids. However, the most=20
important asteroidal geological processes have left their mark on the=20
matrix, because this is the finest-grained portion and therefore most=20
sensitive to chemical and physical changes. Thus, the following=20
information can be learned from this fine grain size fraction alone:=20
(1) mineral paragenesis; (2) regolith processes; (3) bulk=20
composition; (4) conditions of thermal and aqueous alteration (if=20
any); (5) relationships to planets, comets, meteorites (via isotopic=20
analyses, including O); (6) abundance of water and hydrated material;=20
(7) abundance of organics; (8) history of volatile mobility; (9)=20
presence and origin of presolar and/or interstellar material. Most of=20
this information can be obtained even from dust samples from bodies=20
for which nanogram-sized samples are not truly representative. Future=20
advances in sensitivity and accuracy of laboratory analytical=20
techniques can be expected to enhance the science value of nano- to=20
microgram-sized samples even further. This highlights a key advantage=20
of sample returns-that the most advanced analysis techniques can=20
always be applied in the laboratory and that well-preserved samples=20
are available for future investigations. Copyright 2000, Institute=20
for Scientific Information Inc.
========================================================
(9) PLANET & ASTEROID FORMATION IN THE PRESENCE OF GIANT PLANETS
S.J. Kortenkamp*) & G.W. Wetherill: Terrestrial planet and asteroid=20
formation in the presence of giant planets - I. Relative velocities=20
of planetesimals subject to Jupiter and Saturn perturbations. ICARUS,=20
2000, Vol.143, No.1, pp.60-73
*) CARNEGIE INST WASHINGTON,DEPT TERR MAGNETISM,5241 BROAD BRANCH=20
RD,WASHINGTON,DC,20015
We investigate the orbital evolution of 10(13)- to 10(25)-g=20
planetesimals near 1 AU and in the asteroid belt (near 2.6 AU) prior=20
to the stage of evolution when the mutual perturbations between the=20
planetesimals become important. We include nebular gas drag and the=20
effects of Jupiter and Saturn at their present masses and in their=20
present orbits. Gas drag introduces a size-dependent phasing of the=20
secular perturbations, which leads to a pronounced dip in encounter=20
velocities (V-enc) between bodies of similar mass. Planetesimals of=20
identical mass have V-enc similar to 1 and similar to 10 m s(-1)=20
(near 1 and 2.6 AU, respectively) while bodies differing by similar=20
to 10 in mass have V-enc similar to 10 and similar to 100 m s(-1)=20
(near 1 and 2.6 AU, respectively). Under these conditions, growth,=20
rather than erosion, will occur only by collisions of bodies of=20
nearly the same mass. There will be essentially no gravitational=20
focusing between bodies less than 10(22) to 10(25) g, allowing growth=20
of planetary embryos in the terrestrial planet region to proceed in a=20
slower nonrunaway fashion. The environment in the asteroid belt will=20
be even more forbidding and it is uncertain whether even the severely=20
depleted present asteroid belt could form under these conditions.
The perturbations of Jupiter acid Saturn are quite sensitive to
their semi-major axes and decrease when the planets' heliocentric=20
distances are increased to allow for protoplanet migration. It is=20
possible, though not clearly demonstrated, that this could produce a=20
depleted asteroid belt but permit formation of a system of=20
terrestrial planet embryos on a similar to 10(6)-year timescale,=20
initially by nonrunaway growth and transitioning to runaway growth=20
after similar to 10(5) years. The calculations reported here are=20
valid under the condition that the relative velocities of the bodies=20
are determined only by Jupiter and Saturn perturbations and by gas=20
drag, with no mutual perturbations between planetesimals. If, while=20
subject to these conditions, the bodies become large enough for their=20
mutual perturbations to influence their velocity and size evolution=20
significantly, the problem becomes much more complex. This problem is=20
under investigation. (C) 2000 Academic Press.
========================================================
(10) LONG-TERM DYNAMICS OF BRIGHT BOLIDES
L. Foschini*), P. Farinella, C. Froeschle, R. Gonczi, T.J.Jopek,=20
P. Michel: Long-term dynamics of bright bolides. ASTRONOMY AND=20
ASTROPHYSICS, 2000, Vol.353, No.2, pp.797-812
*) UNIVERSITY OF TRIESTE,DIPARTIMENTO ASTRON,VIA TIEPOLO 11,I-34131=20
TRIESTE,ITALY
We have integrated backward and forward in time the orbits of 20 very=20
bright bolides (with visual magnitude brighter than -10) over a time=20
span of 5 Myr or more. The sample was mainly selected among events=20
observed during the period between 1993 and 1996, but we have=20
included also three older, particularly interesting events (Abee,=20
1952; Glanerbrug, 1990; and EN220991, 1991). For a large part of the=20
sample, the orbit is known with sufficient accuracy from the=20
reduction and analysis of photographic data. However, there are also=20
some cases in which lower-accuracy orbital data were derived from=20
other techniques, such as visual, seismic, and radar observation. For=20
these events we have used two or three alternative initial orbits,=20
consistent with the existing uncertainty. The results of our=20
integrations show a great diversity of orbital evolution patterns,=20
consistent with the behaviour of larger near-Earth objects. The most=20
frequent fate (42% of the cases) is solar collision, followed by=20
hyperbolic ejection (17%), and the average dynamical lifetime is of=20
the order of 10 Myr. Three bolides either have initially or achieve=20
later Aten-type or Q <1 AU orbits, similar to the fraction of such=20
objects in the near-Earth asteroid population. Only 2 bolides have a=20
clear comet-like dynamical behaviour dominated by Jovian encounters,=20
although ablation properties indicate that the fraction of very weak=20
bolides is probably higher. Copyright 2000, Institute for Scientific=20
Information Inc.
========================================================
(11) ELEMENTAL COMPOSITION OF VENUS, EARTH, MARS & CHONDRITES
Y.T. Chuburkov: Comparison of elemental composition of Venus, Earth,=20
Mars, and chondrites in the light of the Mendeleev periodic law.=20
RADIOCHEMISTRY, 1999, Vol.41, No.5, pp.434-440
*) JOINT INST NUCL RES,FLEROV LAB NUCL REACT,DUBNA 141980,RUSSIA
The fraction of free neutral atoms No for all elements in the=20
protoplanet nebula was determined with the account of their abundance=20
and physicochemical properties. A linear dependence of the ratio of=20
nonvolatile and volatile elements in chondrites and igneous rocks of=20
the Earth on No was obtained. The Mendeleev periodic law was used to=20
prove the occurrence of magnetic separation of elements in the=20
protoplanet nebula. To this end the concentration ratios of analogous=20
elements with different No in the matters of Venus, Earth, Mars, and=20
chondrites were compared. The data obtained demonstrate the=20
occurrence of magnetic separation of elements in the protoplanet=20
nebula and show that the Shergotty and Tunguska meteorites by their=20
relative elemental composition are genetically related to Mars and=20
asteroids, respectively. Copyright 2000, Institute for Scientific=20
Information Inc.
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