Here is another quote from Science this time.
Science 28 April 2006:
Vol. 312. no. 5773, pp. 535 - 536
DOI: 10.1126/science.1126896
Perspectives
PLANETARY SCIENCE:
Ice Among the Rocks
Alan Fitzsimmons*
At the same time, Hsieh and Jewitt [2006] note that the outer asteroid
belt has been proposed as a source of the water deposited on Earth
after the end of the planet-building phase. This work should spur a
closer assessment of recent dynamical models predicting delivery of
large numbers of objects from this region into near-Earth space
[Gomesl et al. 2005]. It is interesting that many astronomers have
pursued comets to greater and greater distances in their pursuit of
understanding the evolution of comets and the early history of the
solar system. All this time, it would have also been worthwhile to
look a little closer to home.
> > R. Gomes1, H. F. Levison, K. Tsiganis & A. Morbidelli
> > Nature 435, 466-469 (26 May 2005) | doi:10.1038/nature03676; Received
> > 6 December 2004; Accepted 18 April 2005
>
> > Origin of the cataclysmic Late Heavy Bombardment period of the
> > terrestrial planets
> There are currently a good many asteroids known to have Venus crossing
> obits:
Yes, but they are probably dead comets from beyond Jupiter and the
results of asteroid collisions.
> Planets and asteroids don't all orbit in the same plane. The planets
> obital planes vary by 1-7 degrees. Obits are elliptical and not in
> the same plane, so the probability of Earth colliding with a Earth-
> crossing asteroid can can be small or even zero:
In general, any existing asteroid with and Earth-crossing orbit is not
going to hit the earth now, since it has been missing us for 4.600
billion years! In other words, Earth crossing asteroids are the the
exceptions that prove the rule.
The orbital inclination of Venus is 3 23', Earth 0 0', Mars 1 52', and
Jupiter 1 18'. From that it appears that Venus and Earth are equally
likely to receive collisions from an asteroid with an inclination
roughly equal to that of Jupiter/Mars.
I've run http://www.orbitsimulator.com/ (Thanks Phil :-) with one
asteroid that had an orbit greater than any at present but less than
that of Jupiter. Its orbits slowly filled the space between Mars and
Jupiter, and probably would have continued to fill the space as far as
Earth. Left longer yet it could have reached Venus, but if it had
already collided with Mars or Earth that would not happen.
What is being suggested is that the outer Main Belt asteroids are
carbonaceous chondrites, containing 30% water with a similar D/H ratio
to that of Earth's water. Beyond that are the the newly discovered
Main Belt comets with a dirty exterior and an icy interior, say 50%
water perhaps with the same D/H ratio. Beyond that were the icy
asteroids with say 75% water with the same D/H ratio. Because they
were too close to Jupiter, their orbits became more elliptical until
they collided with Earth. In fact, it was the resonance of Jupiter
with Saturn that disturbed these icy asteroids, and their orbits would
have changed from circles into strange attractors, not ellipses.
This is still speculation, but it does mean that Venus need not have
had oceans like Earth.
Cheers, Alastair.
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