Comet dust as a source of amino acids at the Cretaceous/Tertiary boundary.

Large amounts of apparently extraterrestrial amino acids have been detected recently in rocks at the Cretaceous/Tertiary (K/T) boundary at Stevns Klint, Denmark. The amino acids were found a few tens of centimetres above and below the boundary layer, but were absent in the boundary clay itself. If one supposes that these compounds were carried to the Earth by the giant meteorite thought to have impacted at the end of the Cretaceous, some puzzling questions are raised: why weren't the amino acids incinerated in the impact, and why are they not present in the boundary clay itself? Here we suggest that the amino acids were actually deposited with the dust from a giant comet trapped in the inner Solar System, a fragment of which comprised the K/T impactor. Amino acids or their precursors in the comet dust would have been swept up by the Earth both before and after the impact, but any conveyed by the impactor itself would have been destroyed. The observed amino acid layers would thus have been deposited without an impact.