[meteorite-list] NEA ASTEROIDS, ALBEDOS, AND ORIGINS

[Sterling K. Webb]

drtanuki wrote:

> Sterling,
>    A question I have pondered. What if an incoming body has no
> reflectance whatsoever, therefore invisible?  Best, Dirk Ross...Tokyo

    A very good question!  And a worrying one.  The answer to "what if
there is an "invisible" potential impactor" is:  BAD LUCK!

    A body that makes repeated close passes to the Sun in its orbit will
suffer very high surface temperatures. If the body is icy or a comet, it
will "come alive" with the evaporation of its volatiles and brighten,
but a carbonaceous body or even a rocky one will darken, more with each
passage.  The "potato" asteroid will become a "baked potato"!  And
eventually, a "charred" potato.

    Bamberga, in the darkest, low-albedo C-class of carbonaceous
chondrites, has an albedo of less than 0.05.  That's the record low, I
think, but Bamberga is in the Belt and doesn't venture near the Sun.
Its darkness is from its primordial composition.  A similar object that
did approach the Sun, would almost certainly darken down to 0.02 or
0.01, which is the same as saying that it would be functionally
"invisible."  Until the very last minute, that is.

    My first thought was that since a Sun close approacher's orbit would
be anchored by the greatest mass in the solar system, it would be very
imperturbable, but no, it seems Jupiter does a fine job of causing
certain kinds of perturbations, precessing tightly bound solar orbits.
One less comfort.  So, such a body does constitute a hazard that we
can't calculate the likelihood of, and that's worrying.

    Many think (or thought) that some NEA's may be "dead" comets or
baked once volatile bodies, hence structurally weak and subject to
further degradation over time which might mitigate their hazard slightly
-- it would be better if they crumbled to rubble rather than smacking
our little planet, or anybody's little planet.

    But, then I ran across a study carried out by ESA's Infrared Space
Observatory (ISO).  The surfaces of most NEA's examined are rocky and
almost entirely free of small rubble, regolith and dust.

    Why is that important?  Because it strongly implies that NEA's
originate, not in being perturbed into NEA-style orbits, but acquire
these orbits because of recent collisions and mutual impacts that
fragment solid bodies and knock every bit of the loose stuff clean off.

    Thus, the NEA's seem to be former impactors and impactees.  They got
to where they are because of bad luck.  Given the long-term instability
of NEA orbits, their run of bad luck is not over.  Only now, their bad
luck could also be our bad luck...

    I should stress here that the current "professional" belief is very
strong that NEA's get into Near Earth orbits almost entirely through a
long series of perturbations by weak orbital resonances and the action
of the Yarkovsky effect of sunlight on their motion, and NOT by
collisional mechanisms, which are pretty much dismissed as "too rare."

    These are mechanisms that produce a slow and non-dramatic trickle of
outer system bodies to be delivered gently to the inner solar system and
become NEA's.  But that opinion is flatly contradicted by the ISO study
results which show NEA's to be unlike Main Belt asteroids transported
gently with all their regolith and surface junk intact.  NEA's have
undergone some alteration process, almost certain to have been
collisional.

    Another explanation could be that today's NEA's are the depleted
remnant of much larger NEA population in the past, one that was produced
by a really big "recent" collisional event or events.  I put the quotes
around "recent" because I mean astronomically recent, like half a
billion years or less!  That's ten percent of the age of the solar
system or less.  If you're thirty, don't you think of the last three
years of your life as recent?

    That notion, for example, would explain a paradox about Chicxulub
(the dino killer, or not, set free by a California jury, as you see
fit).  Namely, that it's TOO BIG for its "recent" date.  That was one of
the strong objections when the idea of explaining the iridium by impact
was first proposed.  Impacts that large are far rarer than one per
100,000,000 years, more in the one per 1,000,000,000 year class.

    The upper size estimates for Chicxulub would mean that it might have
been the biggest impactor to strike the Earth in several billions of
years, an odd piece of bad luck (if you're a dinosaur). And, recent
research in Cuba on "overturned" sediments suggest it may have been an
even bigger impactor than present estimates. Additionally, the Chicxulub
"crater," instead of being a two or three ring basin as originally
thought, well, it now develops that there are five rings and probable
traces of a sixth.  BIG.

    It's hard to explain how a body the size of Chicxulub could "evolve"
by no other force than weak resonances and the Yarkovsky effect acting
over 4,500,000,000 years until finally one "recent" day it is suddenly
plopped into an Earth intersecting orbit and -- SMACK!

    This is not geology, folks, where the gentle rain wears mighty
mountains away a grain of sand at a time.  We're talking about orbital
shift, big, sudden, dramatic change (with an option on violence).  This
is like the mountains suddenly jumping up and crashing back down on a
new continent far away.  Long runs of small forces (rain) doesn't do
things like that.

    Big bodies, fat chunky asteroids, small moons -- they either last
forever or run into trouble in a big hurry.  The Centaur orbits, between
Saturn and Uranus, where the giant 250-mile diameter 2060 Chiron (aka 95
P/Chiron) sleeps, have a dynamic lifetime of about 100,000 years or
less!  Chiron's orbit is a rest stop; it's going to pull out onto the
Interstate again, and soon. Wanna be on the road when it does?  Me
neither.

    As you can see from the naming confusion, we don't know if 2060
Chiron is an asteroid of a comet.  Of course, with a 250 mile diameter,
does it matter? Chicxulub was an asteroid and NOT a comet, by the way.
See:
<http://www.news.uiuc.edu/scitips/02/1025craters.html>

    I never did understand why whenever there's a contradiction in which
a truth may hide, someone always drags in comets and icy bodies, about
which we know so little, as if you could explain one mystery by
assigning it to another mystery.

    Where did Chicxulub come from?  Musta been a comet.

    Why is there no crater at Tunguska?  Musta been a comet.

    Why does the Earth have so much water?  Musta been comets. (They
couldn't find Venus?)

    Why only four tektite producing impacts in 35,000,000 years when
there were hundreds of other impacts in that time that didn't produce
any tektites?  Musta been a comet.

    And so on...

    Not knocking comets, you understand, just the way they get stuck
with the blame for everything...


Sterling K. Webb
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