Planetary Impacts: Friend or Foe?

Planetary impact events have profoundly influenced the origin of life and the habitability of Earth in both constructive and destructive ways. The constructive effects of impacts include building Earth into a habitable world and providing the key ingredients for life, including carbon, hydrogen, nitrogen, oxygen, sulfur, and energy. The destructive effects of impacts include a cascade of transient environmental disruptions that were likely deleterious to life, such as the generation of extreme pressure and temperature conditions at the impact site, ocean vaporization, and ejection of material into the atmosphere. In this review, we retrace the evolving effects of Earth's impact history on Hadean and Archean habitability. We argue that, cumulatively, impacts encourage habitability, whereas, individually, they are more likely to cause significant transient ecological disruptions. Early in Earth's history, when large impacts were frequent, the beneficial cumulative effects likely dominated and resulted in a world primed for the development of life. We discuss novel tools that are being used to trace the origin and nature of these building blocks. From around the Archean onwards, as large impacts waned, they took on the role of occasional disruptors. We consider the ∼66 Ma Chicxulub impact as a case study for these sporadic post-Archean impacts and how they can cause transient environmental disruptions, create new subsurface habitats, and spur evolutionary developments in their wake.