Swapping Rocks: Ejection and Exchange of Surface Material Among the Terrestrial Planets

The discovery of meteorites originating from both the Moon and Mars has led to the realization that major impacts can eject material from planetary-sized objects. Although there is not yet any direct proof, there appears to be no reason why such impacts cannot eject material from the surfaces of Earth and Venus as well. Because of this possibility, and in view of the implications of such exchange for biological evolution, we examined the orbital evolution and ultimate fate of ejecta from each of the terrestrial planets. This work employed an Opik-type orbital evolution model in which both planets and ejected particles follow elliptical orbits about the Sun, with uniformly precessing arguments of perihelion and ascending nodes. An encounter takes place when the particle passes within the sphere of influence of the planet. When this occurs, the encounter is treated as a two-body scattering event, with a randomly chosen impact parameter within the sphere of influence. If the impact parameter is less than the planet's radius, an impact is scored. Otherwise, the scattered particle either takes up a new Keplerian orbit or is ejected from the solar system. We incorporated several different space erosion models and examined the full matrix of possible outcomes of ejection from each planet in random directions with velocities at great distance from the planet of 0.5, 2.5, and 5.0 km/s. Each run analyzed the evolution of 5000 particles to achieve sufficient statistical resolution. Both the ultimate fate and median transit times of particles was recorded. The results show very little dependence on velocity of ejection. Mercury ejecta is nearly all reaccreted by Mercury or eroded in space--very little ever evolves to cross the orbits of the other planets (a few percent impact Venus). The median time between ejection and reimpact is about 30 m.y. for all erosion models. Venus ejecta is mostly reaccreted by Venus, but a significant fraction (about 30%) falls on the Earth with a median transit time of 12 m.y. Of the remainder, a few percent strike Mars and a larger fraction (about 20%) are ejected from the solar system by Jupiter. Earth ejecta is also mainly reaccreted by the Earth, but about 30% strike Venus within 15 m.y. and 5% strike Mars within 150 m.y. Again, about 20% of Earth ejecta is thrown out of the solar system by Jupiter. Mars ejecta is more equitably distributed: Nearly equal fractions fall on Earth and Venus, slightly more are accreted to Mars, and a few percent strike Mercury. About 20% of Mars ejecta is thrown out of the solar system by Jupiter. The larger terrestrial planets, Venus and Earth, thus readily exchange ejecta. Mars ejecta largely falls on Venus and Earth, but Mars only receives a small fraction of their ejecta. A substantial fraction of ejecta from all the terrestrial planets (except Mercury) is thrown out of the solar system by Jupiter, a fact that may have some implications for the panspermia mechanism of spreading life through the galaxy. From the standpoint of collecting meteorites on Earth, in addition to martian and lunar meteorites, we should expect someday to find meteorites from Earth itself (Earth rocks that have spent a median time of 5 m.y. in space before falling again on the Earth) and from Venus.