The Earth-moon system during the late heavy bombardment period; Geochemical support for impacts dominated by comets

Jorgrnsen, U.*; Appel, P. W. U.*; Hatsukawa, Yuichi; Frei, R.*; Oshima, Masumi; Toh, Yosuke   ; Kimura, Atsushi   

The solid planets assembled 4.57 Gyr ago during a period of less than 100 Myr, but the bulk of the impact craters we see on the inner planets formed much later, in a narrow time interval between 3.8 and 3.9 Gyr ago, during the so-called Late Heavy Bombardment (LHB). It is not certain what caused the LHB, and it has not been well known whether the impactors were comets or asteroids, but our present study lend support to the idea that it was comets. During a field expedition to Isua, Greenland, we sampled three types of sedimentary river load from larger areas of surrounding land surface and of contemporaneous seawater. Our samples show evidence of the LHB impacts that took place on Earth, by an average of a seven times enrichment (150 ppt) in iridium compared to present day ocean crust (20 ppt).