Impact bombardments in the first 800 million years of solar system's history fundamentally affected terrestrial planets. Effects of intense impact fluxes include changes in surface morphology, alteration of surface chemical composition via delivery of exogenous materials, melt mixing and differentiation, primordial atmospheric compositions and densities, and perhaps the overall thermal structures of terrestrial planets.

The key to understanding the inner solar system during this seminal epoch may lie in deciphering the lunar bombardment record. To help achieve this goal, we set the following objectives:

1) Greatly expand current numerical models of the thermal effects of impact bombardments. In particular, integrate the existing thermal model with a new class of physical models that quantitatively investigate the evolution of cratered terrains on silicate crusts.

2) Apply the new integrated model to the thermal evolution of Earth's Moon in the effort to "reverse engineer" its bombardment record as revealed by spacecraft observations, lunar samples, and meteorites. Ion microprobe analyses of lunar zircon and apatite grains by collaborator Mojzsis will play a key role in constraining the models.

3) Perform rapid testing of the physical and thermal consequences of bombardments predicted by the Nice-2 dynamical model, currently under development by collaborator Levison.

Once the lunar bombardment history is better established, we will model the effects of impact bombardments to Mars and Mercury. The Mars model will be additionally constrained by known size-frequency distributions of impact craters in the martian highlands, as well as new studies of previously unrecognized impact basins. The Mercury model will be additionally constrained by MDIS imager data from the three MESSENGER fly-bys of Mercury, currently available via the Planetary Data System.

To accomplish these goals, we intend to make use of two exiting numerical modeling tools: (i) a three-dimensional simulation of the thermal effects of impact bombardments based on HEATING, a general-purpose, finite-difference heat transfer program written and maintained by Oak Ridge National Laboratory, and (ii) Cratered Terrain Evolution Model, a computer code developed by Co-I Richardson to model the physical evolution of heavily cratered terrains on terrestrial planets. The thermal model is verified by observations at terrestrial impact craters, as well as hydrocode simulations, and the physical model is verified by observations of heavily cratered terrains on the Moon.

This work will pay particular attention to a putative spike in the number of impact events at ~3.9 Ga dubbed the Late Heavy Bombardment (LHB), but other bombardment scenarios will be examined as well.

This proposal is highly relevant to the Planetary Geology and Geophysics program goal of "synthesis, analysis, and comparative study of data that will improve the understanding of the extent and influence of planetary geological and geophysical processes on the bodies of the Solar System." Replacing the general "geological and geophysical processes" with a more specific "impact processes" in the above sentence succinctly describes the goal of this proposal. More specifically, the proposed work will make a significant contribution to the knowledge of the impact history of the Moon, and, by extension, other terrestrial planets. It will also contribute to the knowledge of thermal histories of terrestrial planets, particularly their crusts. Estimates of the extent of crustal age resetting by impact events have wide-ranging implications to the radiometric ages of planetary surfaces. Additionally, it will supply initial conditions to a new set of models that evaluate effects of large impacts on mantle convection and core dynamos. Finally, this work will weigh in on the LHB vs. uniform decline debate through model testing and comparison with observations.