Local Panspermia

Impacts scatter debris from planet to planet. Some of the debris is remarkably undamaged in the process. The natural speculation is that living organisms might get launched inside these rocks, survive the trip, survive the landing, and once there finds a suitable home, and so innoculate nearby worlds (Melosh, 1989). The lunar and Martian meteorites that hit the Earth indicate the reality of this process and can be used as calibration points (Gladman, 1996; Gladman et al., 1997). Currently about 10 Mars rocks hit the Earth annually (Mileikowsky et al., 2000). The impact rate and hence the rate of rock transfer was 100-1,000 times higher before 3.8 Ga. Roughly a trillion rocks from Mars have hit the Earth over geological time.

Any microbes caught in the ejected rocks faced numerous perils, including high-shock pressures and shock heating from the impact, rapid accelerations, heating again on reentry, and in between a vacuum laced with cosmic rays.

7 Impacts and the Early Evolution of Life 241 Crust age, m.y.

Fig. 7.10. Like Fig. 7.9 but for an impact more directly comparable to an ocean-vaporizing event on the Earth. The range and accessibility of potential refugia are greatly diminished. As with Fig. 9, temperatures at depth rescale with the annual average surface temperature, so that a -60°C surface means that the contour labelled 120°C would correspond to rescaled temperature of 60°C. Although deep survival is promoted by cold surfaces and very low heat flow, one would not expect the corresponding surface environments to be habitable. Climatically warmer or more volcanically active regions that would seem to offer the best ecological options would seem to offer the fewest options for refugia.

Fig. 7.10. Like Fig. 7.9 but for an impact more directly comparable to an ocean-vaporizing event on the Earth. The range and accessibility of potential refugia are greatly diminished. As with Fig. 9, temperatures at depth rescale with the annual average surface temperature, so that a -60°C surface means that the contour labelled 120°C would correspond to rescaled temperature of 60°C. Although deep survival is promoted by cold surfaces and very low heat flow, one would not expect the corresponding surface environments to be habitable. Climatically warmer or more volcanically active regions that would seem to offer the best ecological options would seem to offer the fewest options for refugia.

While none of these things are good for microbes, studies of Mars rocks and extent terrestrial microbes in the laboratory and in space indicate that they need not be fatal (Mileikowsky et al., 2000). For example, the Mars rock ALH-84001 was never heated above 40° C on its trip to the Earth (Weiss et al., 2000). If the typical rock takes of order 10 million years to transit from Mars to the Earth, we expect that billions of rocks made the trip from Mars to the Earth in less than 10 thousand years, and millions in less than 10 years. These are timescales over which many microbes are likely to survive.

Voyage from the Earth to Mars is more problematic as we have no Earth-derived meteorites to calibrate the process. The thicker atmosphere and higher escape velocity inhibit escape. Nonetheless, we might expect K-T-sized impacts to eject rocks from the Earth and that some of these hit Mars. We might also expect some traffic with an early earthlike Venus, had early Venus been earthlike.

Planetary exchange makes it more difficult to sterilize the whole solar system. The relatively modest impacts that scatter suitable rocks between planets are much more common than dangerous large impactors. Spread over many worlds life would be much less vulnerable to a random event.

Seeding the Earth from another planet creates an evolutionary bottleneck (like a pregnant finch landing on an isolated island) and an evolutionary filter (like large mammals not making it to an isolated island). Some DNA-based microbes may have been able to easily make the trip. The situation for the organisms would have been much like free-living microbes on the modern Earth where similar assemblages inhabit simlar environments globally (Finlay, 2003). For example, the same organisms now inhabit, say, a fjord in Norway and one in New Zealand. It would be meaningless to say whether the ancestors of these modern organisms lived in Norway or New Zealand in the previous interglacial period. Similarly, for some organisms, Mars and the Earth might have been one biosphere.

Other organisms would not have found passage on interplanetary rocks to their liking. This obviously includes any large organism, but also may include molecular organisms that cannot tolerate long periods of stasis in space. In particular, the RNA world may have had to stay home on Mars (Joe Kirshvink, personal communication, 2004).

In contrast to interplanetary panspermia - a testable hypothesis and, as argued above, a possible one - interstellar panspermia is a scientific dead-end. It removes the origin of life to an unknown and unknowable location that will be forever unavailable. It is also extremely unlikely to occur naturally. For example, there is only a slight chance that any rock from a terrestrial planet in our solar system has ever hit a terrestrial planet in another solar system (Melosh, 1903).

An origin of life on Mars is more testable than its origin on the Earth. Mars retains a geological record back to at least 4.5 Ga. Some waterlain sediments are inevitable. Biologists will be able to quickly tell whether any extant organisms found on Mars are related to terrestrial organisms. We will probably be able to do this with molecular fossils. Only the logistics pose difficulties. Similar considerations apply to large asteroids like Ceres. In contrast, the record before 3.5 Ga on the Earth is quite sparse and highly metamorphosed. The early terrestrial record may be gone for good, rather than just being hard to access. The only plausible hope is to find a terrene meteorite on the Moon (Armstrong et al., 2002).

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