Fig. 4.

Selected model runs for both model 1 and model 2 normalized to the present-day impact rate. Both models suggest a drop in impacts of 2× to 5× over the last 2 Ga, and neither has a spike at the timing of the LHB at ∼3.9 Ga. Also shown is the timing of active crust formation and volcanism on the Moon in red and the LHB in white. During the interval shaded in red, we do not believe ⁴⁰Ar/³⁹Ar to be uniquely interpretable in terms of impacts due to the generally high thermal activity on the Moon.