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Re: Meteorites from Earth and Venus

Bernd Pauli wrote:
>There is a very interesting abstract in METEORITICS which touches both
>on the current topic of meteorites found in lunar samples, the Venus
>topic we had some days ago

>MELOSH H.J. et al. (1993) Swapping rocks: Ejection and exchange of
>surface material among the terrestrial planets (Meteoritics 28-3, 1993,
>A398, Excerpts):

>Mercury ejecta is nearly all reaccreted by Mercury or erodes in space
>... ( a few percent impact Venus).
>
>Venus ejecta is mostly reaccreted by Venus, but a significant fraction
>(about 30%) falls on the Earth ...
>
>Earth ejecta is also mainly reaccreted by the Earth but about 30% strike
>Venus ...

This was an opportunistic posting.  Joseph Burns from Cornell University
gave a lecture at JPL yesterday which I attended, and the title of his lecture was 
"Swapping Rocks: The Transfer of Eject Between the Planets".   He focused
mainly on the transfer of material from Mars and the Moon to Earth, but did
talk briefly about other bodies in the solar system.  Here are some tibits
from my notes on the lecture.

The impact velocity of a typical meteor is from 10-20 km/second.  If you
look at the escape velocity of some of the planets, it is possible that
some of the kinetic energy from the meteor can be transferred to material
on the surface of the planet:

             Escape Velocity
     Moon    2.3 km/sec
     Mars    5 km/sec
     Earth   11.2 km/sec
  
Also, the areas adjacent to the actual impact craters are the best candidates
for material to be ejected and remain unshocked.

Burns did a number of simulations of ejecta material to see where they
end up, and here are some of the results of the simulation.

If material is ejected from Mars, after 10 million years, the rocks can
literally be anywhere in the solar system.  After 50 million years, here
are the probabilities of where Mars ejecta can be:

  Impact on Venus    - 7%
  Impact on Earth    - 7%
  Impact on Mars     - 9%
  Sun-grazing*       - 32%
  Reaching Jupiter** - 10%
  Survivors          - 35%

*  Sun-grazing means more or less hitting the Sun
** Once an object is in a Jupiter crossing orbit, it can end
   anywhere, including being ejected from the solar system.

These simulations show that the most likely place Mars ejecta will end up
in the Sun.

He also mentioned that a Mars rock can hit the Earth in as little as
16,000 years.  In an extreme case (the odds being 1 in 100 million),
if an impact on Mars hits the planet while it is at aphelion and at
a speed of 5.4 km/sec, and ejects material in the right direction
towards Earth, then a Mars meteorite can arrive at Earth in 6 months.

Burns also gave some numbers for the fate of Mercury ejecta after
30 million years.

  Impact on Mercury - 76%
  Impact on Venus   -  6%
  Impact on Earth   - 0.5%
  Sun-grazing       -  4%
  Jupiter Reaching  - 1.5%
  Survivors         - 12%

The 0.5% chance for Mercury eject hitting Earth doesn't bode too well
for Mercury being the parent body for E chondrites.

Burns also points out that ejecta from Phobos and Deimos are almost as
likely to hit the Earth as from Mars.  However, I should point out that
since Phobos and Deimos are suspected to be C asteroids captured into
Mars orbit, you will be hard pressed to differentiate a meteorite from
Phobos/Deimos from a carbonaceous chondrite meteorite.  

Burn also mentioned briefly about the possibility of ejecta from the
Galilean moons around Jupiter (Io, Europa, Ganymede & Callisto).  Since
Jupiter has such a deep gravity well (escape velocity at about 50 km/sec),
it will be extremely difficult for any ejecta to leave the Jovian system, and
would most likely recollide with the moon from which it was ejected from.

Ron Baalke