Mega-impacts of asteroids/comets were driving the terrestrial habitat in the hadean epoch >3.8 Gyr ago. Rather than sterilising the Earth, these impacts made it suitable for early primitive life, whether de-novo or seeded from space. 13-C depletion in graphitic carbon within 4.1Gyr zircons (zirconium silicate crystals) implies bio-carbon early in or pre-dating the ‘late heavy bombardment’ period. Mineral hot-spring deposits (perhaps older) also show fossil traces of microorganisms, suggesting hydrothermal environments supported hadean life.
Paper presented at RAS NAM2017 National Astronomy Meeting, University of Hull, 4th July 2017: Early Life Traces in the Hadean – compatibility with post-Accretion Bombardment, Max Wallis, Chandra Wickramasinghe + Steve Coulson
ABSTRACT – Evidence from zircons and recently from micron-scale structures in the oldest mineral precipitates from hydrothermal vents implies active micro-biology in the Hadean epoch. The haematite tubes and associated minerals are similar to those associated with micro-organisms in modern hydrothermal vents. The carbonaceous material and carbonate in contact with the supposed microfossil structures are depleted in 13C:12C characteristic of bio-activity. Yet the early Earth is thought to be subject to mega-impacts of comets and asteroids during the Hadean, which cause evaporation of the oceans with global sterilisation due to impact-generated high temperature orbiting debris when dust blocks radiative cooling. Accepting the impact data inferred from the lunar crater record, we challenge the assumption that 25% of the impact energy goes into heat for mega-impacts, where much impact debris is ejected to space. The archaetypal hypervelocity mega-impact with crater size ~1000km (from ~100km impactor) transmits only a few % of its energy, potentially boiling off an early ocean but still leaving water-bearing sub-surface strata that provide ecological niches. These prevent full sterilisation and allow re-population of hydrothermal vents over times short compared with the interval (>10kyr) between mega-impacts. On this picture, early tectonics driving hydrothermal vents play a key role in ensuring continuity of Hadean micro-life.