Another important factor to consider is that
impacting bodies -- particularly if they were
comet-like -- may have brought a substantial amount
of water with them. In fact, some people have
proposed that most, if not all, of the Earth's
water reservoir was deposited here by incoming
comets between 4.5 and 3.9 billion years ago.
There will be a problem on your next homework
in which I ask you to calculate this impactor
flux and think about the implications.