After its primary accretion had mostly finished, the proto-Earth underwent a catastrophic collision with a stray planetary embryo variably estimated to be at least equivalent to Mars’ mass (Theia). This event effectively reset Earth’s radiometric chronometers such as Pb-isotopes, and gave rise to the Moon. Since then, the Earth-Moon system has been subjected to collisions with smaller solar system objects in a declining flux termed late accretion. Evidence for late accretion mostly comes from the abundance of highly-siderphile elements in the crusts of the Earth and Moon, yet the proportional abundance of these metals between both bodies suggests that the Earth underwent a second large impact with roughly a lunar-sized objected (Moneta) approximately 4.48 billion years ago. Since Earth’s water was most likely acquired during primary accretion and geochemical analyses of Hadean zircons (≤4.4 Ga) suggest clement surface conditions, it is plausible that liquid water could have existed on the early Earth prior to the Moneta impact. Likely impact geometries show that unlike the Theia case, Moneta’s metal core should become shattered with Fe0 becoming oxidized with the water or oxidized mantle to form a dense (<90 bar) hydrogen gas atmosphere with a computed lifetime of ~200 Myr. This startling result means that a highly reducing atmosphere could have provided the conditions for gas-phase synthesis in the atmosphere of the molecular building blocks for life as we know it. Could the RNA world ultimately be a (by)product of a colossal impact?
Here I will discuss: i) the state of the solar system during and after the Moneta event, ii) the rate of decline of the bombardment flux, iii) its effects on the Hadean crust, and iv) propose a temporal window of opportunity for a chemical milieu where life originated. If the Moneta impact indeed provided the atmospheric conditions required to kick-start life on Earth, one could argue that such events on rocky exoplanets are necessary. If so, what can we say about the likelihood of such events, and on what timescales?