QUESTION:
What are the three best theories/guesses for the origin of water on mars,
the orgin of water on earth?

ANSWER from Bruce Jakosky on July 16, 1997:
Water is very abundant in the universe, so it is not surprising to find
water on the planets.  It may have been incorporated through two or three
possible mechanisms:

1) It might have been included directly into the materials out of which the
planets formed.  We see asteroids today that have some water in them, for
example.

2) It might have been added through the impact of comets or comet-like
objects early in solar-system history.

3) It might still be in the process of being added through the small comets
that Lou Frank has suggested may be hitting the Earth.  This is the least
likely explanation.

Undoubtedly, 1 and 2 both may have contributed.