Llyd E. Wells

Rummaging through Earth's Attic for Remains of Ancient Life

Abstract We explore the likelihood that early remains of Earth, Mars, and Venus have been preserved on the Moon in high enough concentrations to motivate a search mission. During the Late Heavy Bombardment, the inner planets experienced frequent large impacts. Material ejected by these impacts near the escape velocity would have had the potential to land and be preserved on the surface of the Moon. Such ejecta could yield information on the geochemical and biological state of early Earth, Mars, and Venus. To determine whether the Moon has preserved enough ejecta to justify a search mission, we calculate the amount of terran material incident on the Moon over its history by considering the distribution of ejecta launched from the Earth by large impacts. In addition, we make analogous estimates for Mars and Venus. We find, for a well-mixed regolith, that the median surface abundance of terran material is roughly 7 ppm, corresponding to a mass of approximately 20,000 kg of terran material over a 10×10-square-km area. Over the same area, the amount of material transferred from Venus is 1–30 kg and material from Mars as much as 180 kg. Given that the amount of terran material is substantial, we estimate the fraction of this material surviving impact with intact geochemical and biological tracers.

Reseeding of early earth by impacts of returning ejecta during the late heavy bombardment

Mounting attention has focused on interplanetary transfer of microorganisms (panspermia), particularly in reference to exchange between Mars and Earth. In most cases, however, such exchange requires millions of years, over which time the transported microorganisms must remain viable. During a large impact on Earth, however, previous work (J.C. Armstrong et al., 2002, Icarus 160, 183–196) has shown that substantial amounts of material return to the planet of origin over a much shorter period of time (< 5000 years), considerably mitigating the challenges to the survival of a living organism. Conservatively evaluating experiments performed [by others] on Bacillus subtilis and Deinococcus radiodurans to constrain biological survival under impact conditions, we estimate that if the Earth were hit by a sterilizing impactor ∼ 300 km in diameter, with a relative velocity of 30 km s−1 (such as may have occurred during the Late Heavy Bombardment), an initial cell population in the ejecta of order 103–105 cells kg−1 would in most cases be sufficient for a single modern organism to survive and return to an again-clement planet 3000–5000 years later. Although little can be said about the characteristics or distribution of ancient life, our calculations suggest that impact reseeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.

Cold-Active Viruses

Virus-like particles (VLP) are abundant in the aquatic biosphere, typically ranging between 10 and 10 ml, including in the cold environments of the deep sea, polar oceans, sea ice and high-latitude lakes. In these places, viruses play important ecological roles, e.g., equaling or surpassing grazers as agents of bacterial mortality in some Arctic waters (Steward et al. 1996; Wells and Deming 2006a). Systematic investigations of viral ecology in snow, permafrost or glaciers are only beginning, but viruses are present, preserved or even active there (e.g., Castello et al. 1999; Sawstrom et al. 2007b). Yet, despite the prevalence on Earth of environments ≤ 4°C and the high abundance of viruses in them, little is known about corresponding viral characteristics or ecology. Instead, viral studies have largely been restricted to higher temperatures convenient for experimenters but of little ecological or evolutionary relevance to many viruses. Here, I review information about viruses from cold environments, focusing on bacteriophages when hosts are known. (For a more inclusive review specific to Antarctic systems, see Pearce and Wilson 2003.) Although these viruses are termed “psychrophilic” (e.g., Olsen 1967; Olsen et al. 1968) or “psychrotrophic” (Greer 1982, 1983; Patel and Jackman 1986), such words imply growth characteristics not directly applicable to obligate parasites like viruses. I therefore follow Wells and Deming (2006b) and call viruses capable of infection and production at temperatures ≤ 4°C “cold-active.” The