1. University of Colorado, Department of Geological Sciences, 2200 Colorado Avenue, UCB 399, Boulder, Colorado 80309-0399, USA

Correspondence to: Oleg Abramov¹ Correspondence and requests for materials should be addressed to O.A. (Email: oleg.abramov@colorado.edu).

Lunar rocks^{1, 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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