[thedoc (OP)]

Mark Harnett  asked the Naked Scientists:
   
A theory about how the water got to the earth is that it came from comets. How many comets of average size would it take to fill the earth oceans with water?

What do you think?

[chiralSPO]

According to a quick google search, the amount of water in Earth's oceans is about 1.4x10²¹ kg. Comets come in all shapes and sizes, and they change. However, another quick search reveals several comets in the 10¹³ to 10¹⁴ kg range. If the comets were 100% water, it would take over ten million (10000000) comets of that size to make up our oceans!