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Re: (meteorobs) Tunguska bolide & Beta taurid stream
At 01:07 PM 6/26/99 EDT, George wrote:
> GeoZay>>Also when I consider the lack of meteorites from cometary meteor
>showers, they seem too
> fragile to get to the lower levels of the atmosphere to produce sonic booms
> as the smaller yearly tunguska-like explosions do.<<
>
> G. Kelly>>A good point. Has anyone, anywhere, to anyone's knowledge, ever
>recovered a meteorite that could definitely be linked to a meteor shower?
> >>
>
>This jogs another thought. There has been a number of multi station
>photographic Fireball networks in existence....also some no longer operating
>that has detected a number of Fireballs that has dropped recovered
>meteorites. From these photographs, the orbits of recovered meteorites has
>been determined. Has any of these ever indicated that the objects came from
>an area other than the asteroid belt? Has any multi station fireball
>photographs where meteorites haven't been recovered and were not part of a
>recognized meteor shower been shown to come from anywhere other than the
>asteroid belt? If some where detected, roughly what percentage of Asteroidal
>origin vs something that could be considered short orbit comet parentage?
>GeoZay
I hate it when questions like this pique my interest... I typically lose a
whole morning surfing the NASA Astrophysics Data System
(http://adswww.harvarddot edu/) researching the answer that I "think" I know!
Fireballs photographed from multiple sites are broken down into four major
types according to their penetration depths into the atmosphere, ablation
coefficient, density, and terminal velocity: Type I fireballs, the ordinary
chondrites; Type II fireballs, the carbonaceous chondrites; Type IIIa
fireballs, high density cometary material; and Type IIIb fireballs, low
density cometary material. Recognize that there is a bit of a disconnect
here: the classifications are largely theoretical, since the direct
comparison of well-observed fireballs and their eventual meteoritic product
has been extremely limited. They are very well established by the
comparison of the physical characteristics of "found" meteorites and the
inferred physical characteristics of fireball events in the atmosphere;
still, the set of well observed falls is vanishingly small.
There have only been three recoveries from the major photo networks:
Pribram, Innisfree, and Lost City, all of which were ordinary chondrites
produced by Type I fireballs from asteroidal orbits. Ceplecha (Ceplecha
Z., 1988, Bull. Astr. Inst. Czech., 39, 221) in an analysis of photographic
and television data from multiple stations for 3624 sporadic meteors found
that 38% of fireballs come from cometary orbits (11% from highly eccentric
orbits typical of new comets), but most of the fireballs (62%) originate at
asteroidal orbits. The standard explanation is that the denser asteroidal
material is more likely to survive an atmospheric transit than "weak"
cometary material.
The picture of the origin of each type is well and truly muddied by all the
comet-asteroid transition objects that have been discovered recently,
attempts to correlate other sources of data on interplanetary dust
particles (IDPs) with fireball and meteorite data, and evidence for the
existence of groups of meteorite-producing asteroidal fragments - meteorite
streams.
Halliday et al (Meteoritics, vol. 25, June 1990, p. 93-99) found evidence
among 89 potentially meteorite-producing MORP and Prarie Network events for
four separate "streams" of asteroidal fragments.
Jakes and Padevet ("Meteorites, Bolides and Comets: A Tale of
Inconsistency", Meteoritics, vol. 27, no. 3, volume 27, page 238) compare
photo network data with recovered meteorites, and examine two extreme
hypotheses: A, that only dense (Type I and II) fireballs produce
meteorites, or B, that all four types (I, II and the less dense types IIIa
and IIIb) produce meteorites. Hypothesis A fits the observed data best if
it is assumed that all four types tend to fragment at the same rate in the
atmosphere; Hypothesis B fits the observed data best if it is assumed that
the less dense bodies fragment more rapidly (which I think is supported
well in the literature elsewhere).
They conclude, with reference to hypothesis B: "Correlation of bolide
properties with meteorite falls could well be accommodated by the
hypothesis B in which each bolide type has a meteorite equivalent. This
has, however, some "outrageous" implications: comets may carry chondrites,
icy dust balls do not produce fireballs; the extremely primitive
carbon-rich particles represented by the IDPs do not form larger discrete
bodies (fireballs) of "asteroidal" size; and the asteroid belt is a mixture
of "native fractionated old bodies" together with the captured comets.
Hypothesis B therefore contradicts the "established" scheme of the asteroid
belt in which the non-differentiated meteorites (CI, CM, and CV) form the
outer part of the asteroid belt, whereas the fractionated metamorphosed and
igneous meteorites characterize the inner asteroids."
H. Campins and T.D Swindle have published ("Are There Cometary
Meteorites?", 29th Annual Lunar and Planetary Science Conference, March
16-20, 1998, Houston, TX, abstract no. 1004) a list of traits that they
believe should be characteristic of a cometary-origin meteorite:
1. Rare (as rare as carbonaceous chondrites)
2. Dark (about 5% geometric albedo)
3. Weak (~10^7 dynes/cm^2)
4. High porosity, low density
5. Highly unequilibriated Fe/(Mg+Fe) in silicates
6. Nearly Solar elemental abundances
7. High abundance of C, N, and organics
8. Anhydrous silicates
9. More likely than asteroidal meteorites to contain interstellar grains
with peculiar isotopic ratios
10. Likely not to have chondrules
11. Unremarkable cosmic ray exposure ages (~10^7 years)
12. and 13. Chemical differences due to cosmic ray exposure.
As is typical in the complex system we inhabit, the answer is not easy, and
not yet entirely clear. That is what keeps us all interested though, no?
JB
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