Mars , Panspermia , and the Origin of Life : Where Did It All Begin ?

During the nineteenth century, when steadystate cosmological theories were in vogue, Lord Kelvin, Svante Arrhenius, and other eminent scientists believed that the transfer of life from one planet to another was a process made inevitable by the infinite extent and duration of the universe. This hypothesis, known as panspermia, subsequently fell out of favor, partly as a result of the acceptance of the Big Bang theory. Most efforts to understand the origin of life have since been framed by the assumption that life began on Earth. However, in the last decade data have begun to accumulate suggesting that panspermia may in fact be a natural and frequently occurring process. Recent paleomagnetic studies on Martian meteorite ALH84001 have shown that this rock traveled from Mars to Earth without its interior becoming warmer than 40oC (Weiss et al. 2000) (Fig. 1). Experiments aboard the European Space Agency’s Long Duration Exposure Facility indicate that bacterial spores can survive in deep space for more than five years (Horneck et al. 1994; Horneck 1999), and laboratory experiments demonstrate that bacteria can survive the shocks and jerks expected for a rock ejected from Mars (Mastrapa et al. 2001). Finally, dynamical studies indicate that the transfer of rocks from Mars to Earth (and to a limited extent, vice versa) can proceed on a biologically short time scale, making it likely that organic hitchhikers have traveled between these planets many times during the history of the Solar System (Mileikowsky et al. 2000; Weiss and Kirschvink 2000). These studies demand a re-evaluation of the long-held assumption that terrestrial life evolved in isolation on Earth. Figure 1. Magnetic scan of a slice of ALH84001, superimposed on its optical image. Intensity of the perpendicular component of the magnetic field is depicted with a red and blue color scale (corresponding to field lines oriented out-of-the-page and into-the-page, respectively). Note the large magnetic anomaly centered on the fusion crust, and the baked zone that extends less than a few mm inwards from it. The center of the rock has a heterogeneous magnetic pattern, indicating that it has not been heated even to 40oC since before 15 Ma (Weiss et al. 2000). This implies that rocks can travel between Mars and Earth without being heat sterilized.