Mars Meteorite Transfer: Simulation

The possible detection of ancient life in martian meteorite ALH84001 (D. S. McKay et al ., Research Article, [16 Aug., p. 924][1]) raises questions about the likelihood that microorganisms might have been transported between Earth and Mars ([1][2]). A related question is whether martian samples (to be returned by future spacecraft missions) should be sterilized in order to protect Earth from possible contamination. The survivability of such organisms on their journey between planetary habitats would depend on the flight time, which determines the dose of damaging cosmic rays and ultraviolet radiation, and the sample size, which might protect a meteorite's interior from radiation and from the heat of atmospheric entry. In an attempt to understand the distribution of cosmic-ray exposure ages of the dozen known martian meteorites, we numerically simulated the orbital histories of more than 2000 particles launched from Mars at speeds slightly above escape (B. J. Gladman et al ., Research Article, [8 Mar., p. 1387][3]); these particles were followed as their orbits evolved as a result of close planetary encounters and distant perturbations of all the planets from Venus to Neptune. In our simulation, we found that a few percent of the martian particles struck Earth, having typical transfer times of several million years; ALH84001's measured transfer age of about 15 million years is unusually long. Although terrestrial bacteria are known to have survived in vacuum for several years ([2][4]), transit times of millions of years may preclude safe passage. Our simulations show, however, that a small fraction of martian ejecta launched just marginally faster than the martian escape speed could be found immediately on Earth-crossing orbits. Our simulations show that the rate of Earth deliveries was almost constant for the first 107 years, which implies that on the order of 1 in 107 of the martian meteorites that reached Earth could have spent less than half of an orbital period in space (less than a year). The dozen recovered martian meteorites, the majority of which have been on Earth for much less than 105 years, represent only a tiny fraction of all the pieces of Mars currently on our planet and, when one considers the much higher impact rate early in the solar system's history, an even smaller proportion of those that have arrived throughout Earth's history. Therefore, fast transfers (taking less than a year) from Mars to Earth must have occurred numerous times during Earth's past. We leave to others an appraisal of the likelihood of survival of martian organisms in the rigors of space and their fiery entry through Earth's atmosphere. Because such an entry would hardly affect the center of a meteorite, terrestrial weathering would eventually have exposed the pristine interior. If martian microorganisms can survive a year in space, many may have already arrived. Large impact events could also have liberated some pieces of Earth at high speeds and simultaneously could have cleared an atmospheric channel through which a few particles might have slipped to Mars. Our simulations show that about 0.1% of these rarely liberated terrene meteoroids could have found their way to Mars, but even fewer than 1 in 107 of these would have had rapid transfer times. 1. 1.[↵][5] 1. H. J. Melosh , Nature 332, 687 (1988); [OpenUrl][6][CrossRef][7][PubMed][8] 1. M. Moreno , ibid. 336, 209 (1988); 1. J. Goode 1. P. C. W. Davies in Evolution of Hydrothermal Ecosystems on Earth: Proceedings of Ciba Foundation Symposium No. 202, J. Goode Ed. (Ciba Foundation, London, in press). 2. 2.[↵][9] 1. F. J. Mitchell, 2. W. L. Ellis , Analysis of Surveyor 3 Material and Photographs Returned by Apollo 12 (National Aeronautics and Space Administration Washington, DC 1972 239 248; G. Horneck, Nucl. Tracks Radiat. Meas. 20 , 185 (1992). [1]: /lookup/doi/10.1126/science.273.5277.924 [2]: #ref-1 [3]: /lookup/doi/10.1126/science.271.5254.1387 [4]: #ref-2 [5]: #xref-ref-1-1 "View reference 1. in text" [6]: {openurl}?query=rft.jtitle%253DNature%253B%2BPhysical%2BScience%2B%2528London%2529%26rft.stitle%253DNature%253B%2BPhysical%2BScience%2B%2528London%2529%26rft.aulast%253DMelosh%26rft.auinit1%253DH.%2BJ.%26rft.volume%253D332%26rft.issue%253D6166%26rft.spage%253D687%26rft.epage%253D688%26rft.atitle%253DThe%2Brocky%2Broad%2Bto%2Bpanspermia.%26rft_id%253Dinfo%253Adoi%252F10.1038%252F332687a0%26rft_id%253Dinfo%253Apmid%252F11536601%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [7]: /lookup/external-ref?access_num=10.1038/332687a0&link_type=DOI [8]: /lookup/external-ref?access_num=11536601&link_type=MED&atom=%2Fsci%2F274%2F5285%2F161.3.atom [9]: #xref-ref-2-1 "View reference 2. in text"