Earth, and entry into Earth's atmosphere. The Shergottites1 show significant shock metamorphism, but the Nakhlites, Chassigny, and ALH84001 show little evidence of shock damage as a result of ejection from Mars (McSween, 1994). Passage through Earth's atmosphere would heat only the outer several millimeters, and survival of organics in ALH84001 and thermally labile minerals in several other meteorites indicates that indeed only minor heating occurred during ejection from Mars and passage through Earth's atmosphere. Transit to Earth may present the greatest hazard to survival. Cosmic-ray exposure ages of the meteorites in current collections indicate transit times of 0.35 million to 16 million years (McSween, 1994). However, theoretical modeling suggests that about 1 percent of any material ejected from Mars should be captured by Earth within 16,000 years and that 0.01 percent would reach Earth within 100 years (Gladman et al., 1996). Thus, survival of organisms in a meteorite, where largely protected from radiation, appears plausible. If microorganisms could be shown to survive conditions of ejection and subsequent entry and impact, there would be little reason to doubt that natural interplanetary transfer of biota is possible.
Transport of terrestrial material from Earth to Mars, although considerably less probable than from Mars to Earth, also should have occurred throughout the history of the two planets. It is possible that viable terrestrial organisms have been delivered to Mars and that, if life ever started on Mars, viable martian organisms may have been delivered to Earth. Such exchanges would have been particularly common early in the history of the solar system when impact rates were much higher.
During the present epoch, no effects have been discerned as a consequence of the frequent delivery to Earth of essentially unaltered martian rocks both from the martian surface and from well below. It cannot be inferred, however, that there have been no effects.