Exchange of condensed matter among the outer and terrestrial protoplanets and the effect on surface impact and atmospheric accretion

Abstract

One current picture of the accretion process of Uranus and Neptune invokes extensive gravitational scattering of the icy planetesimals by the photoplanets as they grew to significant masses. This phase should last a few 100 million years during which the planetary scattering effect could lead to implantation of comets in the Oort cloud. The injection of the scattered planetesimals into the inner Solar System also leads to a gain of volatile materials by the terrestrial planets. The potential H 2O input to primordial Earth, for instance, could be on the order of 10 25 – 10 26 g even though a large part of it might have been subsequently lost via atmospheric escape process. This early planetesimal impact event was followed by a much more gradual process, i.e., the return of the icy cometary cores from the distant Oort cloud via stellar perturbations. The total amount of volatile material as accreted by the terrestrial planets was quite small as compared with the earlier episode accompanied by the formation of Uranus and Neptune. As an example, the Earth would have recieved about 6 × 10 20g of cometary material in total over the last 4 billion years if the cometary nucleus density is as high as 0.5 g cm −3. Within the context of H 2O inventory (the terrestrial ocean mass is 1.3 × 10 24 g) the cometary influx should have minor effects. On the other hand, because of the paucity of H 2O content in the atmospheres of Mars and Venus, cometary impact could produce strong time variation in their water contents. As for Titan, its nitrogen atmosphere could be derived from cometary impact only if the early phase of comet bombardment was very intense. A comparison of the adopted comet flux model with the crater count statistics indicates a certain discrepancy (a factor of 10–50) in the estimated comet impact rates. Possible sources of errors could come from the size distribution of the long-period comets, their average density, albedo, and other factors. A more complete survey of the population of subkilometer comets in the outer Solar System by Space Telescope and other instruments will be critical importance in clarifying the interrelation among the original mass of the Oort cloud, the volatile accruement by the terrestrial planets, and the impact crater production rates on planetary and satellite surfaces.