Accretion of the outer Planets and its Influence on the Surface Impact Process of the Terrestrial Planets

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Abstract
We study numerically the scattering of residual icy planetesimals towards the terrestrial planets zone during the accumulation of proto-Uranus and proto-Neptune. Our results show that an amount of icy material of severalM⊕(Earth mass) could have reached the terrestrial planets zone in a time scale of a few 108years. An already-formed Earth would have received an amount of about 3 × 1022kg of volatiles (this is about 20 times the amount of water contained in the oceans). Mars would have experienced a similar large volatile injection of about 5 × 1021kg. Yet, it is possible that part of the material injected by the accreting outer planets arrived to the inner planetary region before the terrestrial planets finished to sweep all the massive planetoids of their influence zones. If this was the case, the accreted volatile material would have subsequently been lost in megaimpacts. Moreover, massive icy planetesimals impacting at velocities much greater than the escape velocity might have led to a net erosion of volatile material, rather than accretion. Our numerical results show that the outer planets could have grown to sizes comparable to their current ones in time scales of a few 108years. The tail of residual material injected in the terrestrial planets zone lasted for several 108years, so it might have greatly contributed to the heavy bombardment of the Earth during the period 4.4-3.8 Gyr (before present). The material coming from the outer planetary region might have played a pivotal role in the development of the early atmosphere and life on Earth, though how much volatile material was retained is still an open question whose answer depends on several uncertain parameters.