Role of major terrestrial cratering events in dispersing life in the solar system

The larger and most energetic cratering events from comet and asteroid collisions with the Earth are probably associated with ejection of solid material faster than escape speeds every 100 Myr or so. Metre-sized boulders, we estimate, may have been ejected directly into Venus-crossing and perhaps Mars-crossing orbits from comet impacts at higher speeds and of larger mass, at least on 10 occasions in the last 3.5 Ga. Subsequent close encounters with Earth can also enable slower boulders to reach Mars-crossing orbits. Orbit perturbations from Mars and Jupiter would then have sent a fraction of the boulders to the outer planets and their icy satellite systems. In the so-called late bombardment epoch at 3.9 Ga, when primitive life was developing, ejection-causing impacts were much more frequent, at ∼ 30 per 0.1 Ga, yielding an increased probability of distributing seeds of terrestrial biology to the outer regions of the solar system.