The following video explains in pretty good detail how impact craters can eject material from the surface of a planet into the solar system & find its way onto other planets. In fact, approximately 20% of such material actually gets ejected from the solar system. The probability appears overwhelming that over the 4.5 billion years of the solar system, there has been a relatively significant exchange of material among the planets, including some meteors that fell onto Earth from relatively recent impacts on Mars. If there were any microbial creatures in these rocks, they are highly likely to have survived the original launching (100,000 G's) and the re-entry into the second planet. One Los Alamos scientist placed a steel object on the surface for an underground nuclear explosion in the early 1950's and this object may have achieved priority as the first man-made object to escape the Earth's gravity. This scientist estimates that the Earth gets many tons of material ejected from Mars every year, although much of this material may have been hanging around the solar system for millions of years before finally landing on Earth. He comments that the meteor that caused so much trouble 65 million years ago almost certainly ejected much material into space. Since the Earth was absolutely teeming with life at the time, it would be inconceivable if such life were not carried away with the ejected material. If I understood this video correctly, if an explosion exceeds approximately 250 megatons, then the fireball itself -- i.e., a portion of the atmosphere -- may escape from the Earth. This apparently answers a question I posed here a while back about whether one or more large collisions could have carried away a significant fraction of the Earth's atmosphere -- they answer may very well be "yes". ------- Are We All Martians? The Meteoritic Exchange of Life between Planets Monday, April 20, 2009 10:38 AM Dr. H. J. Melosh is Professor in the University of Arizona Lunar and Planetary Labs. His research interests include theoretical geophysics and planetary surfaces. Presented Feb. 24, 2009. 56 minutes. http://deimos3.apple.com/WebObjects/Core.woa/FeedEnclosure/arizona-public-dz... http://deimos3.apple.com/WebObjects/Core.woa/Feed/arizona-public-dz.57740221...