  New independent constraints on the amount of water delivered to Earth by
comets are derived using the 15N/14N isotopic ratio, measured to be roughly
twice as high in cometary CN and HCN as in the present Earth. Under reasonable
assumptions, we find that no more than a few percent of Earth's water can be
attributed to comets, in agreement with the constraints derived from D/H. Our
results also suggest that a significant part of Earth's atmospheric nitrogen
might come from comets. Since the 15N/14N isotopic ratio is not different in
Oort-cloud and Kuiper-belt comets, our estimates apply to the contribution of
both types of objects.
