Why is Ice More Prevalent in Comets than in Early Planetary States?

[skeleton]

I have read a few times that comets are significant transports of ice (perhaps in having supplied water to fill Earth's oceans). That implies that water is more abundant in the few comets (in accumulated mass) that have impacted Earth than that which would have existed within the initial planet when it was forming.

1) WHY is ice more prevalent in comets than in the initial state of a planet?

2) Where did the ice come from when a comet is formed? This question may be equivalent to asking how is a comet formed?

3) Why is H2O, being a light molecule, more prevalent in outer space than other light molecules?

(Thanks in advance for your anticipated replies.)

 

[zhermes]

Good questions skeleton.

1) WHY is ice more prevalent in comets than in the initial state of a planet?

Comets form much farther away from the sun than Earth did. At those larger distances, water accumulates as ice which can stick/clump together into larger bodies (e.g. comets), instead of closer towards the sun where water tends to be gaseous or liquid (if its confined) at best.
See these links: http://en.wikipedia.org/wiki/Ice_line http://en.wikipedia.org/wiki/Habitable_zone

3) Why is H2O, being a light molecule, more prevalent in outer space than other light molecules?

Water is far far less common than Hydrogen or Helium, or simple things like that. But water is also extremely stable (which is a big part of why its so useful/important for biology), and thus it is a little more common that just two hydrogens and an oxygen.

2) Where did the ice come from when a comet is formed? This question may be equivalent to asking how is a comet formed?

I'm not sure about the details, but the overall picture is that 'dust' and 'ice' conglomerate together in the outer edges of the solar system (for the reasons explained in answering 1 and 3). When comets pass near the sun (e.g. haley, etc) they have large tails because the water/ice is being vaporized and blown off. This illustrates that comets would be unstable if they were always in the inner solar system.

Hope this helps.

 

[Chronos]

Hydrogen is the most abundant element in the universe and oxygen is produced in abundance in most stars. Hydrogen bonds readily with oxygen and, once formed, water molecules are extremely stable.