No. 58: Jul-Aug 1988
We begin with the lead paragraph from a recent letter to Nature from H.J. Melosh;
"Recent evidence that the SNC meteorites originated on Mars raises the question of whether large impacts on Earth may eject rocks that could fall on Mars (or other planets in the Solar System) and, if so whether they might contain spores or some sort of viable microorganisms that would have the opportunity to colonize Mars."
After some computations Melosh concludes:
"It seems likely that the impacts that produced craters on Earth that are greater than 100 km in diameter would each have ejected millions of tons of near-surface rocks carrying viable microorganisms into interplanetary space, much in the form of boulders large enough to shield those organisms from ultraviolet radiation, low-energy cosmic rays, and even galactic cosmic rays. Under such circumstances spores might remain viable for long periods of time."
(Melosh, H.J.; "The Rocky Road to Panspermia," Nature, 332:687, 1988.)
Comment. Next we need a reasonable mechanism that spreads life through interstellar space. Light pressure, that's it; and the idea is over a century old! Incidentally, SNC is short for Shergottites, Nakhalites, Chassignites; all rare classes of meteorites.