click for full image and caption

The young Earth appear to have been bombarded by comets for several hundred million years shortly after it was formed. This onslaught, perhaps involving hundreds of millions of comet impacts, is currently the best explantion for the origin of the Earth’s oceans, atmosphere and organic molecules. Although historically a controversial idea, there is now a considerable amount of physical and chemical evidence supporting the theory. Comet scientist Armand Delsemme reviews the evidence and argues that comets from the vicinity of Jupiter contributed the bulk of the constituents found in Earth’s biosphere.