Lunar rocks1,2 and impact melts3, lunar4 and asteroidal meteorites5, and an ancient martian meteorite6 record thermal metamorphic events with ages that group around and/or do not exceed 3.9 Gyr. That such a diverse suite of solar system materials share this feature is interpreted to be the result of a post-primary-accretion cataclysmic spike in the number of impacts commonly referred to as the late heavy bombardment (LHB)1,2,3,4,5,6,7. Despite its obvious significance to the preservation of crust and the survivability of an emergent biosphere, the thermal effects of this bombardment on the young Earth remain poorly constrained. Here we report numerical models constructed to probe the degree of thermal metamorphism in the crust in the effort to recreate the effect of the LHB on the Earth as a whole; outputs were used to assess habitable volumes of crust for a possible near-surface and subsurface primordial microbial biosphere. Our analysis shows that there is no plausible situation in which the habitable zone was fully sterilized on Earth, at least since the termination of primary accretion of the planets and the postulated impact origin of the Moon. Our results explain the root location of hyperthermophilic bacteria in the phylogenetic tree for 16S small-subunit ribosomal RNA8, and bode well for the persistence of microbial biospheres even on planetary bodies strongly reworked by impacts.
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This work is funded by the NASA Astrobiology Institute (through the NASA Postdoctoral Program) and the NASA Exobiology program. Office space and computer resources provided by the Department of Space Studies of the Southwest Research Institute in the early stages of this project are greatly appreciated. Reviews by E. Asphaug, as well as comments by D. Trail and T. M. Harrison, are gratefully acknowledged.
This movie shows the thermal evolution of the Earth's lithosphere during the Late Heavy Bombardment in our baseline scenario. Only impactors larger than 10 km in diameter are included in this animation. The upper surface shows temperatures at a depth of 4 km. Dark areas denote crater imprints.
This movie shows the thermal evolution of the Earth's lithosphere during the Late Heavy Bombardment in the extreme scenario: surface temperature of 50∘C, geothermal gradient of 48∘C km-1, and 100X mass delivered. Only impactors larger than 10 km in diameter are included. The upper surface shows temperatures at a depth of 4 km.