The rate of influx of martian meteorites to Earth can be estimated only crudely. Roughly 500 meteorites larger than 0.5 kilogram are thought to fall on Earth every year, but only about four are actually observed because most fall into the ocean, or into sparsely populated areas.16,17,18 Of the 210 meteorites observed to fall between 1815 and 1960, in densely populated areas of Japan, India, Europe, and North America, three were from Mars. Thus, the ratio of martian meteorites to total meteorites is thought to be roughly 1:100. This number is very approximate. So far, about half a dozen martian meteorites have been identified among the 8,000 meteorites recovered from Antarctica. However, considerable analysis is required to identify a martian origin, and most of the antarctic meteorites from Mars have received only cursory examination. If the 1:100 ratio is accepted as being representative, then of the roughly 500 meteorites that fall on Earth every year, perhaps five are from Mars. Because meteorites resemble terrestrial rocks, they are usually recovered only under special circumstances, such as when they have been observed to fall, or by the accumulation of dark-colored meteorites on natural, light-toned surfaces (e.g., accumulation by ablation of the antarctic ice sheet, or aeolian erosion of desert ergs (“sand seas”), like the Sahara, or exposure on playa (dry lake) surfaces of evaporite basins, and so on).
A question of major importance with respect to back contamination of Earth by mechanisms of panspermia is whether putative martian organisms could survive ejection from Mars, transit to Earth, and subsequent passage through Earth’s atmosphere. The Shergottites show evidence for significant shock metamorphism; however, the Nakhlites, Chassigny, and ALH 8400119 show little evidence of shock damage as a result of ejection from Mars.20 Passage through Earth’s atmosphere heats only the outer few millimeters of a meteorite, and survival of organics in ALH 84001 and of thermally labile minerals in several other martian meteorites indicates that, indeed, only minor heating occurred during ejection from Mars and subsequent passage through Earth’s atmosphere.
Transit to Earth may present the greatest hazard to the survival of any microbial hitchhikers. Cosmic-ray-exposure ages of the meteorites in current collections indicate transit times of 350,000 to 16 million years.21 However, theoretical modeling suggests that about 1 percent of the materials ejected from Mars are captured by Earth within 16,000 years and that 0.01 percent reach Earth within 100 years.22 Thus, survival of organisms in meteorites, where they are 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 organisms is possible and has, in all likelihood, already occurred.
Assuming that organisms survive ejection, an important obstacle to long-term viability during transport over interplanetary distances (at low temperatures) is the accumulation of genetic damage from natural background radiation emitted from the radioactive minerals present within the host meteorite. In the absence of active DNA repair, a genome such as that of Deinococcus radiodurans would be degraded and become dysfunctional (i.e., non-repairable) within 200 million years,23 rendering the meteorite sterile with respect to living organisms. A relatively radiation-sensitive bacterium like Escherichia coli present within a meteorite could easily survive for 6 million years.24 Of course, any fossilized remains, or remnant biomaterials, would persist intact, providing a potential record of life. It should be noted that martian materials transported to Earth via a sample return mission will spend a relatively short time (less than a year) in space—all the while protected in containers. (Note that researchers have yet to discover compelling evidence of life in any meteorite, martian or otherwise.) Thus, the potential hazards posed for Earth by viable organisms surviving in samples is significantly greater with a Mars sample return than if the same organisms were brought to Earth via impact-mediated ejection from Mars.
Impact-mediated transfers of terrestrial materials from Earth to Mars, although considerably less probable than such transfers from Mars to Earth, should also have occurred numerous times over the history of the two planets. Thus, it is possible that viable terrestrial organisms were delivered to Mars at some time during the early history