On August 6, just as the curriculum units in this Teachers Institute volume were going to production, news broke of the first persuasive evidence of life elsewhere in the Solar System, and by extension, elsewhere in the Universe. A team of scientists from NASA’a Johnson Space Center and Stanford University announced that they had detected probable microfossils in a martian meteorite. Meteorite ALH84001 was found in Antarctica in 1984 and was classified at that time as a rock sample most likely from the Main Asteroid Belt. A 1994 publication of data on mineralogical analyses and radioisotopic studies of oxygen in the rock revealed, however, that the most likely origin of the meteorite was the planet Mars. The age of the ALH84001 meteorite is determined to be 4.0 billion years, while the 11 other known martian meteorites have dates of 1.3 billion years old. This unique antiquity of the ALH84001 meteorite led to intensive examination of the meteorite and to the discovery of tubular, bacteria-like forms and further evidence of biological activity (polycyclic aromatic hydrocarbons and calcium carbonate deposits).
The announcement of possible life on Mars was described by President Clinton as “stunning.” In the past days, discussion has focused on the reliability of the description of fossils in the meteorite. Additional information has become available from NASA about the Galileo space probe of Jupiter’s satellite Europa, considered to be the next best bet for past or present extraterrestrial life. That discussion is chronicled in the newspaper files available as part of the classroom materials of this unit. Debate will be carried out in the professional meetings and journal publications to come. Debate will center on the environmental history of the meteorite (its primary and secondary properties), on the microfossil forms and their evidence of cellular structure, cell division, and colonial life, and on the validity of associated organic materials of biological origin. We will arrive at answers to questions generated by study of this and other meteorites through the process of scientific investigation and discourse. The larger debate will be about the possible ubiquity of life, in our Solar System and throughout the Universe. Is panspermia, in the narrow sense the natural innoculation of Earth by extraterrestrial life and in the larger sense the cosmic spread of life, a scientific idea finally to be taken seriously? Regardless of the debate’s eventual outcome, discovery of possible life on Mars will have a profound impact on the American space program in the years ahead, giving far greater impetus for broadening the search for life on Mars and returning rocks from Mars to the Earth.
Thus, the asteroids and comets giveth, and they taketh away. In being identified as having brought to the newly accreted Earth sufficient quantity of water to establish a moist atmosphere and extensive oceans, they are credited with establishing the conditions on Earth which made the evolution of primitive life possible. Through early planetary history and the Late Heavy Bombardment, they delayed this perhaps-inevitable evolution of life. With cessation of steady cataclysmic collisions, life did arise naturally and spontaneously on Earth and perhaps elsewhere in the Solar System. The exchange of life forms among planets and satellite bodies, still very much a matter of conjecture, would have been mediated by the collisions of asteroids or comets with planets or satellites. To borrow the analogy from the ecology of bees and flowering plants, asteroids and comets have acted, in effect, as flying penises. There has not yet developed a scientific literature questioning the supposed common origin of all life on Earth with the suggestion that Earth has received over time one or more new infusions of life from other Solar System refugia of life. This is certain to develop as we learn more about the potential for or reality of life on Mars, on Jupiter’s Europa, or elsewhere in the Solar System. Finally, the asteroids and comets have played a role, whether occasional or repeated, in the major starts and stops of the evolution of life. They have not been associated with all mass extinction events, and their relation to pulsed extinction and background extinction is as yet undetermined. The subject of their potential role in future diversification and attenuation of life should be of interest to those who ask, what is life? where is life found? where does life come from? why is life so diverse? how does life change? why do some living things become extinct?. . . and, what directions will life take in the future?