[meteorite-list] Asteroids, Comets, and 536 AD

[Sterling K. Webb]

Hi, to all parties in the wrangle,

    It's been fascinating, but it's wandered a long 
way from 536 AD (or 534 or 537, whenever). Some 
points that got lost along the way:

1. We have no clear definition or concept of what 
exact differences there may be, if any, between 
"comets" and some "asteroids." We used to think
that comets were majority ices but recovered
cometary particles are majority silicates. There
are Main Belt comets in the "Asteroid" Zone, and
NEA's occasionally develop a coma. There are all 
the kinds of object we know about, and then there 
are the kinds of object we don't know about. In a 
word, we are still too ignorant to argue about one 
versus the other as if we were certain.

2. The highest danger (and the smallest likelihood)
is the first-time in-coming long-period comet, not
something we could do much about, so it's useless
to argue about them. In this department, our best
defense is feel lucky and keep whistling.

3. The original cometary hypothesis for specifically
the 534-545 AD events is not for the cases that have
been argued for and against, here on the list and in
the academic pissing contests, but for a "swarm" of
cometary objects resulting from the breakup of a
larger body.
    This hypothesis was put forth in the 1970's by
Napier and Clube, who wrote two very bad books 
about the mechanism they proposed. (Some scientists
should not try to write "pop science" books. Good
idea -- bad books.)
    What they proposed was that every 50,000 or 
100,000 years (or million years; take your pick) some 
large fresh cometary body got perturbed into the inner 
solar system where, in its new "Apollo-like" orbit, it 
underwent rapid breakup from the warming and collisions, 
resulting in a large number of "asteroidal" and "cometary" 
fragments that continued the "cascade" breakup, creating 
a short but violent era in which the inner solar system 
is turned into a shooting gallery.
    In other words, instead of a low, steady rate of 
impacts, we would exposed to long peaceful eras
punctuated by short stretches like the last round of
the big fight in a Rocky movie. We would become
accustomed for a long time to this abnormally low rate 
of impacts (the usual odd meteoroid) and then be 
terrified by a brief 100-fold increase in such events
(or maybe 1000-fold). 
    So, scribes from the "unlearned" eras with breakup
swarms would draw celestial maps full of dragonoid
portents and fiery signs in the skies by night and day,
and the rational scholars of more peaceful times would
hypothesize an orderly "clockwork" universe. (Oddly
enough, just what we have in our own historic record...)

4. Abbott finds rock vapor spherules and marine 
microfossils in the ice as proof of impact. Larson 
find sulfates in the ice which he thinks are proof of 
major volcanic events. Both find evidence of a world-
wide event that probably took place in the tropics. 
The rocks underlying most shallow coastal seas in 
the tropics are carbonates and sulfates; an impact 
there would produce ALL  THREE markers. 
    There are a slew of papers on atmospheric 
sulfate release in destructive amounts from impacts, 
particularly as relates to Chicxulub and the devastating 
acid rains that followed it, if you want to follow this up. 
On the other hand, a marine caldera explosion would 
also account for the markers. But such a volcanic 
event only 1500 years ago would leave far more 
evidence behind than would a shallow-water impact 
crater. 
    Neither set of findings proves or dis-proves 
either hypothesis at this point.
    Another point is that if the ice retains a record
of rock vapor spherules, marine microfossils, and
sulfates, where is the ash? Ice core ash has been 
recovered from presumably smaller volcanic events
that were more ancient, like Thera.
    Just on the general run of evidence, it seems to 
me that evidence for impact just continues to pile 
up, however slowly, but additional evidence for 
the alternative hypotheses remains elusive. Where 
was that huge volcano? The problem for the volcano 
explanation is that the evidence keeps suggesting 
a bigger and bigger event, requiring of course a 
bigger volcano. Another problem is that large 
volcanic events are notoriously hard to date with 
any precision (plus or minus a century), even if 
you can find the evidence.
    
5. On the other hand, the "comet-swarm" hypothesis,
being an intermittent cause for brief episodes of high 
impact rates, makes that whole hypothesis harder to 
prove (or dis-prove either). 
    There are some persuasive outside arguments for the 
Napier-Clube (and later Duncan Steele) cometary debris 
mechanism: the Zodiacal Light. The Zodiacal dust is 
rapidly dispersed. Without regular replenishment, it would
be gone in short order. We see no source of replenishment
that would account for more than a few percent of what
is needed, yet the dust persists. The breakup of a comet
getting trapped in the inner system every now and then 
would account for it.
    As usual, there are arguments about how much Zodiacal 
dust there is, arguments about how fast it is dispersed, 
arguments about how much is contributed by known
bodies, and so forth. It's a difficult and developing field
of research. Infrared studies show dust bands that are
associated with young asteroidal families, the biggest
obvious source of dust, but they only account for a minor
fraction of the dust, suggesting that the rest is from 
somewhere (or something) else.

6. It always makes me want to grind my teeth when
someone speaks of a search strategy and says 1000
objects of an "expected" 1100 have been found. Yes,
I understand the assumptions on which such estimates
are made, often correctly, but when you're talking about
"proof," a search is finished when you search until you
don't find any more. Until then, you don't know, you 
only assume.
    When you've got all them rocks (or iceballs) rounded 
up and there's a red plastic tag in every ear, let me know...


Sterling K. Webb