The Late Asteroidal and Cometary Bombardment of Earth as Recorded in Water Deuterium to Protium Ratio
The deuterium to protium (D/H) ratio of the deep mantle may be a remnant of the hydrogen isotopic composition of Earth forming planetesimals, which later evolved as a result of the late accretion of asteroids and comets. If so, the mass of asteroids and comets incident on Earth since the time of its accretion is estimated to be 4×10 20-2×10 22 kg. The combined use of water D/H ratios, the lunar cratering record, and terrestrial mantle siderophiles would favor a rather low mass fraction of comets among impacting bodies (⋦0.01). Asteroids, comets, and the early Earth contributed to 0-0.5, 0-0.1, and 0.5-0.9 of Earth's water inventory, respectively. A two stage model is advocated in which escape to space of terrestrial volatiles predated the late accretion of extraterrestrial gases. We wish to emphasize that our interpretations and conclusions might evolve in the future when additional data on asteroids, comets, and Earth's interior become available.