Planetary bombardment histories provide critical information regarding the formation and evolution of the Solar System and of the planets within it. These records evidence transient instabilities in the Solar System's orbital evolution, giant impacts such as the Moon-forming impact, and material redistribution. Such records provide insight into planetary evolution, including the deposition of energy, delivery of materials, and crustal processing, specifically the modification of porosity.

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... dment histories are traditionally constrained from the surface expression of impacts — these records, however, are degraded by various geologic processes. Here we show that the Moon's porosity contains a more complete record of its bombardment history. We find that the terrestrial planets were subject to double the number of ≥20-km-diameter-crater-forming impacts than are recorded on the lunar highlands, fewer than previously thought to have occurred. We show that crustal porosity doesn't slowly increase as planets evolve, but instead is generated early in a planet's evolution when most basins formed and decreases as planets evolve. We show that porosity constrains the relative ages of basins formed early in a planet's evolution, a timeframe for which little information exists. These findings demonstrate that the Solar System was less violent than previously thought. Fewer volatiles and other materials were delivered to the terrestrial planets, consistent with estimates of the delivery of siderophiles and water to the Moon. High crustal porosity early in the terrestrial planets' evolution slowed their cooling and enhanced their habitability. Several lunar basins formed early than previously considered, casting doubt on the existence of a late heavy bombardment.