Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v11e..08a&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V11E-08
Other
0545 Modeling (4255), 0560 Numerical Solutions (4255), 5225 Early Environment Of Earth, 5420 Impact Phenomena, Cratering (6022, 8136), 8424 Hydrothermal Systems (0450, 1034, 3017, 3616, 4832, 8135)
Scientific paper
We model thermal effects of impacts on the terrestrial lithosphere during the period of Late Heavy Bombardment (LHB), a putative ~100 Ma epoch of sharply elevated impactor flux that reached a maximum at ca. 3.9 Ga. The goals of this work include estimating the degree to which the crust was molten or thermally metamorphosed during the LHB and evaluating habitability during this time period. We created a stochastic cratering model which populates all or part of the Earth's surface with craters within a probability field of constraints derived from the lunar cratering record, the size/frequency distribution of the asteroid belt, and dynamical models. For each crater in the model, a temperature field was calculated using analytical expressions for shock-deposited heat and central uplift. The resulting thermal anomaly was then introduced into a 3-dimensional model of the lithosphere, and allowed to cool by conduction in the subsurface and radiation/convection at the atmosphere interface. Parameters tested in the model include the duration, mass flux, and average impact velocity during the LHB, mean lithospheric thickness, lithospheric composition, and the presence or absence of oceans. We also assessed habitability by monitoring habitable volumes for mesophile (~20-50° C), thermophile (~50-80° C), and hyperthermophile (~80-110° C) microbial life in what we term the "geophysical habitable zone"; the volume of inhabited crust within ~4 km of the surface. Results of this work indicate that most of the crust was not melted or thermally metamorphosed to a significant degree under any reasonable scenario evaluated. Smaller impactors (1-10 km) were as important as gigantic basin formers (100+ km) in terms of sterilizing the habitable zone in the near-surface due to their far greater numbers (~170,000 impactors in the 1-10 km diameter range versus ~30 impactors in the 100+ km diameter range). However, large basin-forming craters are nonetheless more thermally and biologically significant because they take a far longer time to cool and drive long-lived hydrothermal systems (~ 106-107 years). Habitable conditions in the near- surface are re-established quickly after crater formation (~ 104-105 years for craters in the 100-1,000 km range), particularly if water is present. Although mesophile habitable volume decreases and hyperthermophile habitable volume increases during the LHB, the total habitable volume does not significantly change in most scenarios. The habitable volume in active hydrothermal systems always increases as the LHB progresses, but hydrothermal environments typically constitute a relatively small fraction (~1%) of the total habitable volume. We find that the LHB was insufficient to extinguish microbial life in the crust under any scenario explored in our model.
Abramov Oleg
Mojzsis Stephen J.
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