[meteorite-list] THEORIES OF THE ORIGIN OF TEKTITES, Part Three
From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat Feb 18 02:02:19 2006
Part Three of
THEORIES OF THE ORIGIN OF TEKTITES
Leaving the Volcanic Tektite Exhibition Hall, we enter the spacious
7. The Terrestrial Expulsion and Re-Entry Theory. Kuiper, 1953, 1954. Famous
for much better ideas than this one, Kuiper proposed that at the peak of
accretional heating in the formation of the Earth, impacts ejected silicate
terrestrial materials into solar orbits, which material was later re-melted
by close passages of the Sun and then ultimately collided with the Earth,
producing tektites. Harold Urey immediately performed a dynamic analysis
which showed that a very close collection of very small bodies cannot remain
in association while in orbit in the Solar System but are quickly disrupted
and dispersed everywhere, scattered by tidal forces. The unexpected result
of this was to destroy the very-long-held notion of comets as "flying gravel
banks" as impossible and shove Fred Whipple's "dirty snowball" theory into
the starring role it enjoys today. So, even bad ideas can be fruitful.
8. The Terrestrial Impactite Tektite Theory. This is the currently
prevailing contemporary theory and was first proposed by Spencer in 1933,
followed by Urey, 1953 and 1957; Khan, 1947 (anti-matter impacts only);
Barnes, 1961; Cohen, 1961; and Gentner et al, 1962. Despite many
difficulties with terrestrial impact theories, the theory has been regarded
as "proven" since the 1982 publication by Shaw and Wasserburg, "Age and
provenance of the target materials for tektites and possible impactites as
inferred from Sm-Nd and Rb-Sr systematics." It is now officially orthodoxy.
The Semi-Extra-Terrestrial Pavilion has an equally spacious
Entirely-Extra-Terrestrial Wing, which we will visit next.
9. Meteoritic Origin of Tektites. Lecroix and Seuss in 1932 proposed
tektites to be the shed ablative melt from the glassy crust of an as-yet
undiscovered meteorite type. But Goldschmidt in 1921 had already explained
tektites as just another type of meteorites by postulating a tektite-like
parent body that tektites came from, an idea expanded by Linck, 1926, and by
Paneth in 1940.
This idea may be new to the reader, but it cleanly and neatly eliminates ALL
problems in explaining tektites and wins the Occam's Razor Award in the
bargain. It's simple and obvious; just as there are iron parent bodies for
iron meteorites and stony parent bodies for stone meteorites, there's a
tektite parent body for tektite "meteorites." All the many problems faced by
other tektite theories vanish completely with this theory. It is merely the
classic hypothesis of meteoritics, now regarded as slam-dunk fact, applied
to tektites; what's the problem, then?
Well, it's too damned neat, that's what. There are only four falls of true
tektites in 34,000,000 years, each presumably the result of a large impactor
composed of that unique tektite material. It must be a tremendously rare
material that exists only in the form of large objects. Why are there no
little tektite impacts, why no occasional small tektite falls, why are no
single tektite object discovered as meteorites are, why no sign of these
tektite parent bodies in asteroidal surveys, and many other why's? leap to
mind as well.
Well, you could cook up separate answers to each of these questions (I could
anyway), but it rapidly begins to sound like special pleading. The chief
reason why the perplexed don't just rush en masse to this most simple
explanation is, oddly, the improbability that so simple an explanation could
solve such a complex problem.
10. The Direct Cometary Origin of Tektites. This was first proposed by Suess
in 1951, that tektites are formed from devolatilized cometary residues. In
the same year Lyttleton in response proposed a mechanism whereby comets
could form and direct streams of tektites to the Earth in a close cometary
passage, which was based on the gravel-bank theory of comets which no one
now believes in (openly, but listen to the talk of rubble-pile objects both
cometary and asteroidal). There is no reason to think that the composition
of comets resembles the composition of tektites. However, a comet sample
could change that overnight, but they seems awfully wet to be the source of
ultra-dry tektites. Direct impact of a comet would be included in the
Terrestrial Impact theories, though.
11. The Unique Extra-Terrestrial Theories of Tektite Origins: Belot, 1933,
suggested that tektites derived from a former satellite of the Earth that
fell to Earth in stages. This notion re-appears in 1985 from John O'Keefe;
see No. 14 below. Denaeyer, 1944, concluded that tektites derived from the
glassy crust of a planetoid, some planetoid. Washington and Adams, 1951,
wrote that tektites derived from the "original glass skin of a cosmic body."
Stair, 1964, proposed that tektites derived from the crust of a lost planet
located between Jupiter and Mars (reading too much bad science fiction).
Cassidy, 1956, thought tektites derived from a self-melting radioactive
planetoid with an acid crust (worrying about The Bomb, are we?). Barnes,
1957, a leading and important tektite scientist, one of the first, briefly
entertained the notion that tektites were fragments of a single body of
tektite material that has since been destroyed by collisions. Well, it's
Continued in Part Four
Sterling K. Webb
Received on Sat 18 Feb 2006 02:02:11 AM PST