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(meteorobs) Excerpts from "CCNet 95/2001 - 30 August 2001"
Mainly "marginally topical" (asteroid- or meteorite-related) items in this issue
of CCNet. But note Item (14)! I was not clear whether the "significant large
particle population near 1 AU" might be observable as a class (or classes) of
Sporadic meteors - but I believe the quoted sizes should be in the range of
visible-light producing meteors? If so, this might be a fascinating amateur
data-mining (or visual/video observing) project...
Clear skies,
Lew Gramer <owner-meteorobs@atmob.org>
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet 95/2001 - 30 August 2001
Date: Thu, 30 Aug 2001 11:40:38 +0100
CCNet 95/2001 - 30 August 2001
------------------------------
(1) DAMN THOSE DAMOCLOIDS
Duncan Steel <D.I.Steel@salford.acdot uk>
(2) COMET-BOUND SPACECRAFT SAVED AGAIN
Space.com, 28 August 2001
(3) METEORITE HUNT IS ON IN COLORADO
Ron Baalke <baalke@jpl.nasadot gov>
(4) EXPERTS GET LOTS OF AID IN COLORADO METEORITE HUNT
Ron Baalke <baalke@jpl.nasadot gov>
[...]
(8) DEVELOPMENT OF A TARGET MARKER FOR LANDING ON ASTEROIDS
Sawai S, et al.
[...]
(10) PHOTEMETRIC OBSERVATIONS OF [ROTATIONS OF] 9 NEOS
Szabo GM, et al.
(11) GROUNDBASED INVESTIGATIONS OF ASTEROID 9969
Lazzarin M, et al.
(12) DETERMINATION OF ASTEROID MASSES
Michalak G
(13) DYNAMICAL CAUSES OF ASYMMETRY IN THE ARRANGEMENTS OF
GAPS IN THE ASTEROID BELT
Markeev AP
(14) THE SIZE-FREQUENCY DISTRIBUTION OF THE ZODIACAL CLOUD
Grogan K, et al.
(15) THE ROTATION AXIS OF THE CENTAUR 5145 PHOLUS
Farnham TL
(16) THE PUZZLING CASE OF THE NYSA-POLANA FAMILY
Cellino A, et al.
(17) FORMATION OF MESOSIDERITES
Edward RDS, et al.
[...]
==============================================================================
(1) DAMN THOSE DAMOCLOIDS
>From Duncan Steel <D.I.Steel@salford.acdot uk>
Dear Benny,
In CCNet dated 2001 August 28 you carried an interesting item from
astronomy.com by Vanessa Thomas concerning the newly-discovered 'Damocloid'
2001 OG108, likely the largest known Earth-crossing asteroid. I'd like to
make a few comments about the significance of this and similar objects; that
is, Earth-crossing asteroids on Halley-type orbits. The high inclination of
this object (80 degrees) adds weight to the conviction that it may well be
an extinct (or dormant) comet. That in turn leads to an idea that the albedo
is low (a few percent) and so the size in somewhat larger than would be
calculated based on typical asteroidal albedos, as Ted Bowell suggested: 15
km or maybe more. To tie this down it would be useful if thermal infra-red
observations could be made as the object get closer to the Sun, rendering
the albedo and hence the size. Similarly a cometary nature - cometary
outgassing activity, I mean - might be indicated by high spatial resolution
imaging showing a coma, or high spectral resolution observations showing
emission lines.
Let me compliment the LONEOS team on their interesting discovery. There is
more to it than that, though. Two Damocloids are now known with
Earth-crossing orbits, this one and 1999 XS35, the latter being 1-2 km in
size (absolute magnitude H=17.2). Both have been found by LONEOS. Given that
LONEOS discovers only a small fraction of all NEOs (due to the high
productivity of LINEAR, NEAT and Spacewatch, rather than any fault of
LONEOS), this requires an explanation unless one puts it down to chance.
There are of course various other Damocloids with perihelia outside the
terrestrial orbit (such as 1996 PW found by NEAT, and 1997 MD10, 1998 QJ1,
2000 AB229, 2000 DG8, 2001 QF6 and (20461) 1999 LD31 found by LINEAR, and
(15504) 1999 RG33 found by the Catalina Sky Survey, and 2000 VU2 found by
W.K. Yeung, plus 2000 HE46 found by LONEOS: see
http://cfa-www.harvarddot edu/iau/lists/Others.html), but the fact remains that
it is LONEOS that has turned up the two Earth-crossers amongst them all.
Let us step back a decade. After we found Damocles at the Anglo-Australian
Observatory in 1991 (it was actually Rob McNaught who spotted it, as 1991
DA, on a photographic plate taken using the UK Schmidt Telescope), we
predicted that awaiting discovery there must be many dark asteroids on
elongated, comet-like orbits crossing the path of the Earth. It is nice to
see that prediction being borne out at last, even if they do pose a worrying
danger to us. (5335) Damocles itself has a present orbit that comes sunwards
only so far as to just overlap the aphelion distance of
Mars, but its orbital instability is obvious (see, for example, D.J. Asher,
M.E. Bailey, G. Hahn & D.I. Steel, 'Asteroid 5335 Damocles and its
implications for cometary dynamics,' Monthly Notices of the Royal
Astronomical Society, 267, 26-42, 1994), and so the name given it should
not be surprising to anyone knowing the story of the Sword of Damocles.
Based solely on the existence of Damocles, David Asher and I suggested that
perhaps ten percent of the large-body (>1-km) impact hazard might be due to
dark Halley-type asteroids:
D.I. Steel & D.J. Asher, 'The past and future orbit of (extinct
comet?) 1991 DA,' pp.65-73 in Periodic Comets (eds. J.A. Fernandez & H.
Rickman), Universidad de la Republica, Montevideo, Uruguay, 1992.
Damocles itself, though, is large (H=13.3, much the same as the H=13.0 of
2001 OG108). Asher and I were working on the assumption that some form of
usual mass distribution would rule among Damocloids, with much larger
numbers of smaller objects yet unseen. Both the LONEOS discoveries in
question are rather larger than the norm for present NEO discoveries, 2001
OG108 very much so. One might then suggest that this is because LONEOS has a
small aperture (limiting discoveries to brighter objects), but its wide
field explains why the other programmes have not turned up
similar objects. However, if indeed there are many smaller (<2-km)
Earth-crossing Damocloids then one might expect the search projects using
larger apertures (LINEAR, NEAT, Spacewatch) to have turned some of them up.
One wonders, then, whether they do exist; or is there some systematic
selection effect that is stopping the other searches from detecting them?
Let me caution again, though, that this is based on a statistic of just two
objects. On the other hand, all the Damocloids known are large (bigger than
2 km, most much larger).
The existence or otherwise of a significant population of (say) 0.3 to 2 km
dark asteroids in Halley-type orbits is an important one from the
perspective of planetary defence. For the sake of argument, let us assume
that the above given proportion of the impact hazard due to these Damocloids
(10 percent) is broadly correct. There are arguments over the proportion due
to active comets, long-period or otherwise, but again 10 percent is in the
right ballpark. Together those make 20 percent, leaving 80 percent as being
the contribution of asteroids in inner solar system orbits. I won't worry
about renormalisation: the figures are hazy, and if we knew the real figures
then we'd know a lot more than we do now.
The point from that is that the NASA Spaceguard goal (of 90 percent of inner
solar system asteroids larger than 1 km), which many people have criticized
as being not ambitious enough, may in fact be overdoing it. Think carefully
about the figures. Once one has found three-quarters
of such objects then one has 60 percent of the hazard tied up (that is,
three-quarters of 80 percent), with 20 percent of the hazard still available
for attack by present means (80 minus 60): the brighter asteroids in inner
solar system orbits, being found by relatively small telescopes. But that 20
percent is equalled by the 20 percent of the hazard (by my assumed figures)
posed by Damocloids and active comets: objects on moderate-to long-period
orbits that must be found on their apparition of impact when they are still
at least beyond Saturn, and so in essence all are dark. This requires a
deep, at least annual search of the whole sky with apertures of at least
three metres, and preferably more. That is what needs to be done to make the
biggest inroads into the hazard, once 75 percent of the larger inner solar
system Earth-crossing asteroids have been found. The time is upon us.
One might argue that this is applying a lot of leverage to a handful of
discoveries. The counter-argument is that the spotting of even these few
dark Damocloids is a strong indicator of our lack of knowledge of the
hazardous objects in the solar system. We must know more. We are
dangerously ignorant.
Finally I note that the existence of large objects like 2001 OG108 on
Earth-crossing orbits must lead to a re-assessment of the expected frequency
of impacts of sufficient energy to cause a mass extinction event.
Duncan Steel
==============================================================================
(2) COMET-BOUND SPACECRAFT SAVED AGAIN
>From Space.com, 28 August 2001
http://www.space.com/missionlaunches/ds1_fix_010828-1.html
By Robin Lloyd
Science Editor
NASA engineers have fixed a navigational glitch on the agency's Deep Space 1
(DS1) probe after it was buffeted by solar weather, and the spacecraft
remains on track for an attempted flyby of a comet next month.
The ion-propelled spacecraft lost focus on the stars it uses for navigation
on Tuesday, Aug. 21, sending DS1 somewhat off course for nearly three days.
Engineers suspect that a gust of ionized particles from the Sun blurred the
vision on a science camera DS1 now uses for navigation, throwing the probe
off craft.
If you think of a clock with its hands at noon to represent the correct
pointing direction for the spacecraft, the error caused by the glitch would
amount to pointing the craft at 15 seconds after noon, said DS1 Project
Manager Marc Rayman.
"It wasn't a gross error, but given that the field of view of science camera
is very narrow and you have tight tolerances when flying a spacecraft, that
was a big error for us," Rayman said.
Contrary to some media reports, the trouble was fixed in time for a planned
initial observation of the probe's ultimate goal -- Comet Borrelly, which
DS1 will attempt to fly by on Sept. 22.
Copyright 2001, Space.com
============
(3) METEORITE HUNT IS ON IN COLORADO
>From Ron Baalke <baalke@jpl.nasadot gov>
http://wwwdot denverpost.com/Stories/0,1002,53%257E122416,00.html
Meteoric hunt is on for source of fireball
By Ann Schrader
Denver Post
Tuesday, August 28, 2001 - A fireball that lit up a wide swath of the
Western sky on Aug. 17 was big, noisy - and it's missing.
Not missing, really; it just hasn't been found. Scientists from the Denver
Museum of Nature & Science are on its hot trail, but they need a little help
in pinpointing where it landed in Colorado.
The meteorite hunters have some clues about the blazing piece of celestial
debris that was visible from Idaho to New Mexico: It was an estimated 40
times brighter than a full moon; it may have weighed about a ton when it
entered the atmosphere; it was moving at 11.25 miles per second; and it
boomed and crackled.
>From eyewitness interviews, Murphy and two other museum researchers have
pegged landfall in a 100-mile-square area in the eastern La Garita Mountains
in Saguache County in the Rio Grande National Forest.
Full story here:
http://wwwdot denverpost.com/Stories/0,1002,53%257E122416,00.html
=============
(4) EXPERTS GET LOTS OF AID IN COLORADO METEORITE HUNT
>From Ron Baalke <baalke@jpl.nasadot gov>
http://wwwdot denverpost.com/Stories/0,1002,53%257E124425,00.html
Experts get lots of aid in finding meteorite
By Ann Schrader
Denver Post
Wednesday, August 29, 2001 - From a new round of witnesses to a
self-described "meteorite junkie," hundreds of Coloradans are following the
hunt for the brilliant fireball that slammed into the state on Aug. 17.
More than 150 people called or sent e-mails to the geology department at the
Denver Museum of Nature & Science on Tuesday claiming to have seen the
meteor after The Denver Post published a story about the search for the
space debris. The meteorite is believed to have landed somewhere in a
mountainous 100 square miles between Creede and Saguache.
Full story here:
http://wwwdot denverpost.com/Stories/0,1002,53%257E124425,00.html
================
(8) DEVELOPMENT OF A TARGET MARKER FOR LANDING ON ASTEROIDS
Sawai S, Kawaguchi J, Scheeres D, Yoshizawa N, Ogasawara M: Development of a
target marker for landing on asteroids JOURNAL OF SPACECRAFT AND ROCKETS 38
(4): 601-608 JUL-AUG 2001
The proposed asteroid sample return mission MUSES-C calls for a spacecraft
to approach an asteroid, touch down on its surface, and collect samples that
will be returned to Earth. During the touchdown and sampling phase, the
spacecraft will navigate relative to the asteroid surface using optical
target markers placed on the asteroid surface before the final approach. By
using the target marker as a reference point, navigation during the landing
phase will be much more reliable and precise. Because of the microgravity
environment on the asteroid surface, the settling time and dynamics of the
target markers are items of interest. Thus, it is important to design the
target marker with as small a coefficient of restitution as possible to
minimize the settling time, which in turn minimizes the time the spacecraft
must hover above the asteroid surface. To achieve this small coefficient of
restitution, the target marker will be constructed out of a bag with balls
stored internally. On impact, the balls will dissipate energy relative to
each other and, hence, will dissipate the total energy of the target marker.
To better predict the performance of such a target marker, analytical and
numerical investigations are performed that model the motion of a bouncing
target marker across the surface of a rotating asteroid. As a result of the
analysis, some target limits on the target marker coefficient of restitution
are developed. A series of microgravity tests are reported that confirm the
basic design and show that the target value of coefficient of restitution
can be reached.
Addresses:
Sawai S, Univ Michigan, Dept Aerosp Engn, Ann Arbor, MI 48109 USA
Univ Michigan, Dept Aerosp Engn, Ann Arbor, MI 48109 USA
Inst Space & Astronaut Sci, Space Syst Engn Div, Sagamihara, Kanagawa
2298510, Japan
NEC Aerosp Syst Ltd, Engn Dept 1, Syst Div 1, Yokohama, Kanagawa 2248555,
Japan
Copyright ) 2001 Institute for Scientific Information
============
(10) PHOTEMETRIC OBSERVATIONS OF 9 NEOS
Szabo GM, Csak B, Sarneczky K, Kiss LL: Photometric observations of 9
near-earth objects
ASTRONOMY & ASTROPHYSICS 375 (1): 285-292 AUG 2001
We present new CCD observations of nine Near-Earth Asteroids carried out
between February, 1999 and July, 2000. The bulk of the data was acquired
through an R-C filter, while the minor planet 11 405 was observed without
filter. We could determine synodic periods and amplitudes for 5 asteroids,
699: 3.(h)3, 0.(m)18; 1866: 2.(h)7, 0.(m)12; 1999 JD6: 7.(h)68, 1.(m)2; 2000
GK137: 4.(h)84, 0.(m)27; 2000 NM: 9.(h)24, 0.(m)30. Based on observations
taken at different phases, we could infer a phase parameter m of 0.018 +/-
0.005 for 1865 Cerberus. An epoch-method yielded a sidereal period of
0.(d)27024003(5) for this object with retrograde rotation. The remaining 3
objects have only partial coverage, thus no firm conclusion on their synodic
period is possible.
Addresses:
Kiss LL, Univ Szeged, Dept Expt Phys, Dom Ter 9, H-6720 Szeged, Hungary
Univ Szeged, Dept Expt Phys, H-6720 Szeged, Hungary
Univ Szeged, Astron Observ, H-6720 Szeged, Hungary
Univ Szeged, Dept Opt & Quantum Elect, H-6701 Szeged, Hungary
Univ Szeged, Astron Observ, H-6701 Szeged, Hungary
ELTE Univ, Dept Phys Geog, H-1088 Budapest, Hungary
Copyright ) 2001 Institute for Scientific Information
===============
(11) GROUNDBASED INVESTIGATIONS OF ASTEROID 9969
Lazzarin M, Fornasier S, Barucci MA, Birlan M: Groundbased investigation of
asteroid 9969 Braille, target of the spacecraft mission Deep Space 1
ASTRONOMY & ASTROPHYSICS 375 (1): 281-284 AUG 2001
Asteroid 9969 Braille (1992 KD) was encountered on July 29, 1999 by the Deep
Space 1 mission, the first of NASA's New Millennium Program, launched on
October 24 1998. The data obtained by the space mission seem to indicate a
composition of the object similar to that of Vesta. To complete the
information obtained in the infrared region by the Deep Space 1 mission we
have performed a visible spectroscopic and photometric investigation of the
asteroid respectively with the 1.5 m telescope and the NTT of ESO, La Silla.
The spectrum was obtained in the spectral range 4500-8200 Angstrom and, for
the photometry, BVRI filters were used. In this paper we report the results
of the analysis of the data obtained indicating that, on the basis of our
visible data, the composition of the asteroid may range from V-type to
Q-type, but we observe also a strong similarity to the H-type ordinary
chondrites.
Addresses:
Lazzarin M, Dipartimento Astron, Vic Osservatorio 5, I-35122 Padua, Italy
Dipartimento Astron, I-35122 Padua, Italy
Observ Paris, F-92195 Meudon, France
Copyright ) 2001 Institute for Scientific Information
=================
(12) DETERMINATION OF ASTEROID MASSES
Michalak G: Determination of asteroid masses - II. (6) Hebe, (10) Hygiea,
(15) Eunomia, (52) Europa, (88) Thisbe, (444) Gyptis, (511) Davida and (704)
Interamnia
ASTRONOMY & ASTROPHYSICS 374 (2): 703-711 AUG 2001
New masses of eight asteroids: (6) Hebe, (10) Hygiea, (15) Eunomia, (52)
Europa, (88) Thisbe, (444) Gyptis, (511) Davida, and (704) Interamnia, were
determined. In most cases, the masses were calculated by means of the
least-squares method as the weighted means of the values found separately
from the perturbations on several single asteroids. Encounters suitable for
determination of masses of these asteroids were found from the extensive
search for large asteroidal perturbations exerted by massive asteroids on
4500 numbered minor planets. Most of the encounters found within the search
had never before been used for mass determinations. The masses of Hebe,
Hygiea, Eunomia, Europa, Thisbe, Gyptis, Davida and Interamnia were
determined from perturbations on, respectively, 2, 8, 3, 4, 1, 1, 2 and 3
asteroids. As an outcome of the search for possible perturbers among the 912
largest asteroids, we propose and use correct dynamical models including
important perturbers for all asteroids under consideration. For all new
asteroid masses, the influence of the variation of the masses of asteroids
in the dynamical model is investigated. A discussion on individual mass
determinations is also presented.
Addresses:
Michalak G, Wroclaw Univ Observ, Kopernika 11, PL-51622 Wroclaw, Poland
Wroclaw Univ Observ, PL-51622 Wroclaw, Poland
Copyright ) 2001 Institute for Scientific Information
============
(13) DYNAMICAL CAUSES OF ASYMMETRY IN THE ARRANGEMENTS OF GAPS IN THE
ASTEROID BELT
Markeev AP: Dynamical causes of asymmetry in the arrangement of gaps in the
asteroid belt
ASTRONOMY LETTERS-A JOURNAL OF ASTRONOMY AND SPACE ASTROPHYSICS 27 (7):
475-479 JUL 2001
The centers of the gaps observed in the asteroid belt are displaced toward
Jupiter from their positions that correspond to the exact commensurability
between the mean motions of an asteroid and Jupiter. Using the current
theory of stability and nonlinear oscillations of Hamiltonian systems, we
point out the dynamical causes of this asymmetry. Our analysis is performed
in terms of the plane circular restricted three-body problem. The orbits
that correspond to Poincare periodic solutions of the first kind are taken
as unperturbed asteroid orbits. (C) 2001 MAIK "Nauka/Interperiodica".
Addresses:
Markeev AP, Russian Acad Sci, Inst Problems Mech, Pr Vernadskogo 101, Moscow
117526, Russia
Russian Acad Sci, Inst Problems Mech, Moscow 117526, Russia
Copyright ) 2001 Institute for Scientific Information
=============
(14) THE SIZE-FREQUENCY DISTRIBUTION OF THE ZODIACAL CLOUD
Grogan K, Dermott SF, Durda DD: The size-frequency distribution of the
zodiacal cloud: Evidence from the solar system dust bands ICARUS 152 (2):
251-267 AUG 2001
Recent observations of the size-frequency distribution of zodiacal cloud
particles obtained from the cratering record on the LDEF satellite are the
latest evidence for a significant large particle population (100-mum
diameter or greater) near 1 AU. Our previous modeling of the Solar System
dust bands, features of the zodiacal cloud associated with the comminution
of Hirayama family asteroids, has been limited by the fact that only small
particles (25-mum diameter or smaller) have been considered. This was due to
the prohibitively large amount of computing power required to numerically
analyze the dynamics of larger particles. The recent availability of
inexpensive, fast processors has finally made this work possible. Models of
the dust bands are created, built from individual dust particle orbits,
taking into account a size-frequency distribution of the material and the
dynamical history of the constituent particles. These models are able to
match both the shapes and amplitudes of the dust band structures observed by
IRAS in multiple wavebands. The size-frequency index, q, that best matches
the observations is approximately 1.4, a distribution in which the surface
area (and hence the infrared emission) is dominated by large particles.
However, in order to successfully model the "ten degree" band, which is
usually associated with collisional activity within the Eos family, we find
that the mean proper inclination of the dust particle orbits has to be
approximately 9.35 degrees, significantly different from the mean proper
inclination of the Eos family (10.08 degrees). (C) 2001 Academic Press.
Addresses:
Grogan K, NASA, Goddard Space Flight Ctr, Code 681, Greenbelt, MD 20771 USA
NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
Univ Florida, Dept Astron, Gainesville, FL 32611 USA
SW Res Inst, Boulder, CO 80302 USA
Copyright ) 2001 Institute for Scientific Information
==================
(15) THE ROTATION AXIS OF THE CENTAUR 5145 PHOLUS
Farnham TL: The rotation axis of the Centaur 5145 Pholus
ICARUS 152 (2): 238-245 AUG 2001
We present observations of the Centaur 5145 Pholus from January 2000 to
August 2000. A rotational lightcurve was assembled from both V and R
measurements, confirming previous period determinations of 0.416 day but
exhibiting an amplitude over twice as high as previously measured. This
lightcurve was then used in conjunction with previously published
lightcurves to solve for a north pole position of lambda (0) = 149 degrees,
beta (0) = +26 degrees (prograde rotation), and a sidereal period of
0.4159256 +/- 0.0000016 day. As part of this analysis, the axial ratios of
Pholus were found to be alb = 1.8 and b/c = 1.0. Color analyses of the data
give an average V - R color of 0.71 +/- 0.03, which is slightly lower than
the 0.75-0.84 range previously seen. This difference, combined with the fact
that we see color variations as a function of rotation, suggest that Pholus'
northern hemisphere contains bluer features than are seen in its southern
hemisphere. (C) 2001 Academic Press.
Addresses:
Farnham TL, Univ Texas, Dept Astron, Austin, TX 78712 USA
Univ Texas, Dept Astron, Austin, TX 78712 USA
Copyright ) 2001 Institute for Scientific Information
==============
(16) THE PUZZLING CASE OF THE NYSA-POLANA FAMILY
Cellino A, Zappala V, Doressoundiram A, Di Martino M, Bendjoya P, Dotto E,
Migliorini F: The puzzling case of the Nysa-Polana family ICARUS 152 (2):
225-237 AUG 2001
The Nysa-Polana dynamical family is one of the most complex groupings
identified in the asteroid main belt. It has a generally twofold structure
and is located in a region of the belt showing an unusual abundance of the
fairly rare F-type taxonomic class. We present here new spectra of 22 family
members, observed at ESO (European Southern Observatory) and at CASLEO
(Complejo Astronomico El Leoncito, Argentina). We find evidence that the
family is actually formed by two distinct groupings mutually overlapping in
the space of orbital proper elements. The first group of objects, consisting
of dark asteroids including several F-type members, can be named after its
least-numbered member, 142 Polana. The second group consists of S-type
asteroids and should be named after its most plausible least-numbered
member, 878 Mildred. Some detailed physical analysis of the two groupings
can be already attempted at this stage, including a reasonable
reconstruction of the original ejection velocity field for the Mildred
family. However, we face here a number of intriguing problems that deserve
further analysis. For instance, it is not clear how to interpret the role of
the two largest nominal members of the clan, 44 Nysa and 135 Hertha, as well
as the presence of several large F-type asteroids apparently not related,
but very close to the clan. Moreover, recent discoveries of common spectral
features indicating hydrated minerals in the spectra of Nysa and Hertha add
complexity to an already puzzling scenario. (C) 2001 Academic Press.
Addresses:
Cellino A, Osservatorio Astron Torino, I-10025 Pino Torinese, Italy
Osservatorio Astron Torino, I-10025 Pino Torinese, Italy
Observ Paris, DESPA, F-92190 Meudon, France
Univ Nice, F-06108 Nice, France
Copyright ) 2001 Institute for Scientific Information
=================
(17) FORMATION OF MESOSIDERITES
Edward RDS, Haack H, Love SG: Formation of mesosiderites by fragmentation
and reaccretion of a large differentiated asteroid METEORITICS & PLANETARY
SCIENCE 36 (7): 869-881 JUL 2001
We propose that mesosiderites formed when a 200-400 km diameter asteroid
with a molten core was disrupted by a 50-150 km diameter projectile. To test
whether impacts can excavate core iron and mix it with crustal material, we
used a low-resolution, smoothed-particle hydrodynamics computer simulation.
For 50-300 km diameter differentiated targets, we found that significant
proportions of scrambled core material (and hence potential mesosiderite
metal material) could be generated. For near-catastrophic impacts that
reduce the target to 80% of its original diameter and about half of its
original mass, the proportion of scrambled core material would be about 5
vol%, equivalent to similar to 10 vol% of mesosiderite-like material. The
paucity of olivine in mesosiderites and the lack of metal-poor or
troilite-rich meteorites from the mesosiderite body probably reflect biased
sampling. Mesosiderites may be olivine-poor because mantle material was
preferentially excluded from the metal-rich regions of the reaccreted body.
Molten metal globules probably crystallized around small, cool fragments of
crust hindering migration of metal to the core. If mantle fragments were
much hotter and larger than crustal fragments, little metal would have
crystallized around the mantle fragments allowing olivine and molten metal
to separate gravitationally. The rapid cooling rates of mesosiderites above
850 degreesC can be attributed to local thermal equilibration between hot
and cold ejecta. Very slow cooling below 400 degreesC probably reflects the
large size of the body and the excellent thermal insulation provided by the
reaccreted debris. We infer that our model is more plausible than an earlier
model that invoked an impact at similar to1 km/s to mix projectile metal
with target silicates. If large impacts cannot effectively strip mantles
from asteroidal cores, as we infer, we should expect few large eroded
asteroids to have surfaces composed purely of mantle or core material. This
may help to explain why relatively few olivine-rich (A-type) and metal-rich
asteroids (M-type) are known. Some S-type asteroids may be scrambled
differentiated bodies.
Addresses:
Edward RDS, Univ Hawaii Manoa, Sch Ocean & Earth Sci & Technol, Hawaii Inst
Geophys & Planetol, Honolulu, HI 96822 USA
Univ Hawaii Manoa, Sch Ocean & Earth Sci & Technol, Hawaii Inst Geophys &
Planetol, Honolulu, HI 96822 USA
NASA, Lyndon B Johnson Space Ctr, Houston, TX 77058 USA
Copyright ) 2001 Institute for Scientific Information
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