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{\bf Transfer of impact ejecta between the terrestrial planets.}
Brett Gladman, Joseph A.\ Burns. Dept.\ of Astronomy, 
Cornell University, Ithaca, NY, 14853, USA.

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As more and more meteorites from the Moon and Mars are discovered, it is 
increasingly clear that impact fragments can escape from large bodies more 
easily than previously believed.
These escaping fragments are then subject to the gravitational
perturbations of the planets, allowing them to be transferred
to a body other than the one from which they were launched.
The lunar meteorites and SNC meteorites (widely accepted to be
from Mars) prove the plausibility of this process.

We study the transfer dynamics by means of numerical 
simulations, considering the evolutions of ejecta escaping from 
the Moon, Mercury, and Mars.
(The feasibility of impact ejection from Venus and the
Earth, with their massive atmospheres, is an open question).
For the case of the Moon, the escaping material has first
to be followed through the stage of geocentric orbit until
the particles escape to heliocentric space.
In all cases,
we track the material that reaches heliocentric orbit 
using a regularized mixed-variable-symplectic integration package developed 
by Levison and Duncan [1].
The particles are followed for between 2 and 20 Myr, until they impact 
a terrestrial planet, cross the orbit of Jupiter, or evolve to a 
Sun-grazing state.

The lunar ejecta problem is of interest in light of the discovery
of lunar meteorites. 
We find that of the ejecta that just barely escapes the 
Earth/Moon system, slightly more 
than half returns to impact the Earth during 10 million years [2].
Of the Earth impacts that do occur, 
roughly two-thirds happen within the first 50 000 years, in agreement 
with the relatively young cosmic ray exposure ages of most 
recovered lunar meteorites.
The rate of impacts is very high early on because the velocity 
of the escaping particles with respect to the Earth is initially low, 
enhancing the Earth's gravitational cross-section.
Within a few Myr, the material that does not impact the 
Earth scatters throughout the inner solar system.
Over a 10-Myr period the remaining particles continue to collide
with the terrestrial planets.
Many impacts with Venus, and one with Mars, are observed.
We find that after a few Myr, being driven to Sun-grazing (with
perihelia less than 2 solar radii) becomes a very common 
loss mechanism.

Material ejected from Mercury is largely re-accreted by that
planet. 
A small fraction (10\%) is driven to Venus-crossing orbits
on Myr timescales, and collisions with Venus are observed.
The transfer of this material to the Earth would require at
least roughly 10 Myr, and would consist of a very small fraction
of the launched material (in obvious contrast with the lunar
case).

Martian ejecta is perhaps the most interesting in the light of
the SNC meteorites.
The cosmic ray exposure ages of these objects are somewhat
difficult to reconcile with the dynamics.
Secular resonances, which we show to be located throughout
the inner solar system, are very important
for establishing a relatively short timescale for the delivery of 
this material to the Earth. 
These allow fast transfers even if only very small ejection 
speeds off of Mars are typical.

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\begin{footnotesize}
{\bf REFERENCES:}
[1] Levison, H., and M. Duncan. 1994. {\it Icarus}, {\bf 108}, 18-36.
[2] Gladman, B., J.A. Burns, and P. Lee. 1995. {\it LPSC XXVI}, 465-66.
\end{footnotesize}
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