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Re: Mars Global Images
Jamie Platt schrieb:
> Ron, List - A question, are the number of impacts found on Mars,
> Venus, and Mercury consistent with the number found on the earth?
> Or should I say are they about the same given the different cross
> sectional target area, or is there a variance based on the distance
> form the sun that has a proportion that correlates to something?
> Just curious.... had a few various thoughts running around up there
> I wanted to clear up.... :-) Jamie Platt
Hi Jamie, Ron, and List!
Here is an interesting article from MAPS, the abstract of which I have
scanned:
WETHERILL G.W. (1989) Cratering of the terrestrial planets by Apollo
objects (Meteoritics 24-1, 1989, pp.15-22):
Department of Terrestrial Magnetism, Carnegie Institution of Washington
Washington, D.C. 20015 USA
Abstract:
Absolute and relative cratering rates on the terrestrial planets have
been calculated using the same asteroidal collision model and Monte
Carlo program used for previous studies of the terrestrial meteorite
flux, the steady-state number of Apollo-Amor objects, and the orbital
distribution of both meteorites and Apollo-Amor objects. The most
straightforward result is that projectiles from the asteroid belt appear
to provide about one-third the observed present-day production of
terrestrial craters larger than 10 km in diameter. When uncertainties in
the calculations and observations are included, it cannot be excluded
that the entire terrestrial cratering flux is asteroidal. On the other
hand, assumption of an additional Apollo-Amor source of extinct comets,
in the same quantity permitted by Apollo-Amor observations, provides
better agreement with the observed cratering rate. In addition, a
significant (e.g. about 30%) terrestrial contribution from active long
and short period comets is acceptable within the uncertainties of the
assumptions required.
The ratios of the cratering rates on the different terrestrial planets
are somewhat sensitive to the assumed source. A purely asteroidal source
predicts a martian cratering rate per unit area about four times that on
Earth, whereas the difference is reduced to about a factor of two for
the mixed asteroid-extinct comet source. The opposite effect is found
for Mercury. As discussed by previous authors, the predicted lunar
cratering rate is significantly higher than that observed. It is not
clear whether this is a result of scaling to impacts on a body
considerably smaller than Earth, or if it indicates an increase in the
cratering flux during the Phanerozoic.
Table 7: Summary of cratering rates for
craters larger than 10 km diameter/10^6 yr
Mercury Venus
Earth Mars
Asteroidal Apollos 0.78 3.35 2.63 2.87
Cometary Apollos 3.26 8.08 5.02 1.17
Total calculated craters 4.04 11.43 7.65 4.04
Total/unit area (10^8 km^2) 5.87 2.44 1.50 2.91
Total/unit area normalized 3.91 1.63 = 1.00 1.94
to Earth = 1.0
Best wishes and good night everywhere :-)
Bernd
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