The Significance of Martian Meteorites
If living microorganisms have existed on Mars, it is possible that such organisms could have been intermittently transported to Earth throughout geological time, carried by meteorites of martian origin. To date, 12 meteorites from Mars have been found on Earth. They are believed to have been ejected from Mars into heliocentric orbits by large impacts and subsequently captured by Earth. The evidence for a martian origin is compelling (McSween, 1994), and there is broad consensus in the scientific community that the meteorites indeed came from Mars.
The rate of influx of martian meteorites onto Earth can be estimated only crudely. Roughly 500 meteorites larger than 0.5 kilograms are thought to fall on Earth every year, but only about 4 are actually observed because most fall in the ocean or sparsely populated areas (Mason, 1962; Brown, 1960, 1961). Of 210 meteorites observed to fall between 1815 and 1960 in densely populated areas of Japan, India, Europe, and North America, 3 were from Mars, and so the ratio of martian meteorites to total meteorites is roughly 1:100. This number is very approximate. So far, 6 martian meteorites have been identified among the 8,000 meteorites recovered from Antarctica. However, considerable analysis is required to identify martian origin, and most of these meteorites have undergone only cursory examination. If we accept the 1:100 ratio as being representative, then of the roughly 500 meteorites that fall on Earth every year, perhaps 5 are from Mars. Because meteorites resemble terrestrial rocks, they generally are recovered only when recovery is favored by special circumstances, such as their having been observed to fall or their landing on the Antarctic ice sheet.
A question of major importance with respect to back contamination is whether putative martian organisms could survive ejection from Mars, transit to
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.