Characteristics and mass distribution of extraterrestrial dust from the Greenland ice cap

Extraterrestrial grains with sizes >~50 µm can now be extracted in large numbers from terrestrial sediments but their origins are still uncertain. At least 99% of these grains have been thought to be spherules^1,2 resulting from the melting of their parent bodies in the atmosphere, which destroyed their mineralogical composition. Here we present analyses of extraterrestrial grains extracted from six samples of cryoconite³ (black dust) collected from the melt zone of the Greenland ice cap. In addition to families of grains never reported before, we have found a surprisingly high abundance of unmelted chondritic fragments, in which the nonvolatile component of the parent bodies has been well preserved. The mass distribution of the grains is very similar to that of the micrometeorite flux at 1 AU (ref. 4), indicating that most of the grains are micrometeorites, and not ablation products of larger meteorites. These results yield new constraints on the ablation of such Greenland micrometeorites, as well as guidelines for using them as glaciological and geological markers.