[meteorite-list] Fewer large asteroids than as predicted

[Sterling K. Webb]

And now, from the pessimists...

A fine theory and simulation. A shame it doesn't
correspond in any way to reality. The impact rate
on terrestrial planets starts out very high at the
beginning of the solar system, declines for half
a billion years then rises again in a final flurry of
accretion and clean-up, at 3.9 billion years ago.
The rate declines in a fairly uniform manner
thereafter, but about 600-to-400 million years
ago, it suddenly climbs to a level not seen since
3.4 billion years ago, and it is STILL at that
higher level today.

This is from the actual impact record, not having-
fun-with-your-computer-model science kit. The
data is from impact spherules in the lunar regolith
and the graph of the impact rates throughout
time can be found on page 660 of:
http://muller.lbl.gov/papers/lunar_impacts_Nemesis.pdf

More information here:
http://www.bibliotecapleyades.net/ciencia/ciencia_nemesis08.htm

True, this researcher thinks it might be proof of
a big comet infall due to an undiscovered companion
small star to the Sun, but so what? It might be, but
the immediate mechanism is likely to be asteroidal
breakup.

The timing matches perfectly with the breakup of
the L-chondrite parent body, the largest asteroidal
breakup in the past few billion years or so. The
evidence and timing for that is presented here:
http://www.psrd.hawaii.edu/Mar04/fossilMeteorites.html

At about 480 million years ago (Ordovician times),
the meteorite fall rate on the Earth was 100 TIMES
GREATER than it is today, during the peak of that
breakup episode.

The history of the formation of the 50 or so asteroidal
families unambiguously identified does not match this
pretty little theory in the least. Reality is contingent,
accidental, and randomly chaotic, and NOT "well
described with a logarithmic decay law."



Sterling K. Webb
--------------------------------------------------------------------------
----- Original Message ----- 
From: "Robert Verish" <bolidechaser at yahoo.com>
To: "Meteorite-list Meteoritecentral" 
<meteorite-list at meteoritecentral.com>
Sent: Saturday, September 26, 2009 10:04 AM
Subject: [meteorite-list] Fewer large asteroids than as predicted


And now a word from our optimistic colleagues:

<http://arxiv.org/abs/0909.3875>

Dynamical erosion of the asteroid belt and implications for large 
impacts in the inner solar system

Authors: David A. Minton, Renu Malhotra
(Submitted on 21 Sep 2009)

Abstract:
The cumulative effects of weak resonant and secular perturbations by the 
major planets produce chaotic behavior of asteroids on long timescales. 
Dynamical chaos is the dominant loss mechanism for asteroids with 
diameters D > 10 km in the current asteroid belt. In a numerical 
analysis of the long term evolution of test particles in the main 
asteroid belt region, we find that the dynamical loss history of test 
particles from this region is well described with a logarithmic decay 
law. In our simulations the loss rate function that is established at t 
= 1 My persists with little deviation to at least t = 4 Gy.

Our study indicates that the asteroid belt region has experienced a 
significant amount of depletion due to this dynamical erosion - having 
lost as much as ~50% of the large asteroids - since 1 My after the 
establishment of the current dynamical structure of the asteroid belt. 
Because the dynamical depletion of asteroids from the main belt is 
approximately logarithmic, an equal amount of depletion occurred in the 
time interval 10-200 My as in 0.2-4 Gy, roughly ~30% of the current 
number of large asteroids in the main belt over each interval.

We find that asteroids escaping from the main belt due to dynamical 
chaos have an Earth impact probability of ~0.3%. Our model suggests that 
the rate of impacts from large asteroids has declined by a factor of 3 
over the last 3 Gy, and that the present-day impact flux of D > 10 km 
objects on the terrestrial planets is roughly an order of magnitude less 
than estimates currently in use in crater chronologies and impact hazard 
risk assessments.
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