2002 Annual Science Report
University of Washington Reporting | JUL 2001 – JUN 2002
Giant Impacts in Earth's Early History: Reseeding the Planet and the Search for Earth Rocks on the Moon.
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. In order 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
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 (Armstrong et al., submitted) 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 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 10-4 – 10-2 cells kg-1 would in most cases be sufficient for a single 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 re-seeding is a possible means by which life, if present, could have survived the Late Heavy Bombardment.
PROJECT MEMBERS:Conway Leovy
RELATED OBJECTIVES:Objective 5.0
Describe the sequences of causes and effects associated with the development of Earth's early biosphere and the global environment.
Identify the environmental limits for life by examining biological adaptations to extremes in environmental conditions.
Understand the natural processes by which life can migrate from one world to another. Are we alone in the Universe?
Define climatological and geological effects upon the limits of habitable zones around the Sun and other stars to help define the frequency of habitable planets in the universe.
Determine the resilience of local and global ecosystems through their response to natural and human-induced disturbances.