a) Many believe that most of the liquid water on the surface of the Earth came from comets crashing into Earth.

a) Many believe that most of the liquid water on the surface of the Earth came from comets crashing into Earth. Given that there are currently 2 x 1024 grams of water on the Earth, and that the average comet has a mass of 1016 grams, how many comets would have to hit Earth to fill up the oceans?

b) If this "comet rain" occurred exclusively during the first half a billion years of the Earth's history, use your result in part A to determine how many comets, on average, hit the Earth in one year during this period?