1yioi87 wrote:Not being an Astronomer I find it hard to believe "that enough of those comets (or ice-rich asteroids) crashed onto the Earth, at just the right time,to deliver all the water we have today."
The idea that Earth's oceans came from comets is not highly regarded these days. That's largely because it appears that cometary ice has quite different isotope ratios than we find in water on the Earth. More popular now are models where much of the water was present from the formation of the Earth, with the possibility that more was delivered during early collisions with protoplanets. Nevertheless, the amount of water on Earth is quite small compared with the volume available in comets (especially much earlier in the evolution of the Solar System), so there's nothing inherently implausible about the theory on those grounds.
What was the temp of Earth during this period of a "huge numbers" of comets, and what would you consider a HUGE NUMBER? Is it just a coincidence that the temp was right, and the number of comets was right, at the same time so that the water didn't all boil off, as it apparently has on Mercury and other planets.
The temperature on Earth was probably about 200°C, but that was below the boiling point of water given the dense CO
2 atmosphere that was probably present.
Also the Astronomers believe that the comets also rained down on Mercury and just happen to hit in the right spot to make an ice skating rink in the polar region.
Not at all. The shadowed polar region is simply the only place where water can survive. With high temperatures and no atmosphere, water impacting anywhere else returns to space.
Why isn't there still an occasional comet full of water coming down on earth or the moon or Mercury?
There is. Comet Shoemaker-Levy 9 hit Jupiter less than 20 years ago. We see comets hit the Sun regularly. The likelihood of a comet impact depends in part on the size of the target, which is why it makes sense that we have recently observed them in the cases of Jupiter and the Sun. Other impacts might well go unobserved, being very transient phenomena. On Earth, it is widely speculated that the Tunguska event of 1908 was produced by a small comet or comet fragment. Of course, the density of comets in the inner system is orders of magnitude lower today than it was during the early periods of bombardments in the first few hundred million years of our Solar System.
"Comets, however, formed beyond the snowline, " How do comets form anyway and how long does it take? Do they just wander around out side the snowline like a big snowball rolling down hill and suck up all the H2O that is floating around out there somewhere?
The mechanism by which comets form is not well established. Stellar system formation models suggest that combinations of density and temperature gradients result in regions of the protostellar disc that are suitable for the condensation of icy bodies.
Then when they get big enough to make the trip worthwhile they swoop down on a piece of rock that is just the right temp and in just the right location and deposit their load of water. What are the odds of that.
The odds for any one comet are millions to one. But there are trillions of comets, and they had hundreds of millions of years. So actually delivering the small amount of surface water present on the Earth isn't a statistical reach at all.
What causes them to depart from the birthing place when they are full grown and proceed to a intercept with Earth? If the collisions were just random then there must have been 10 to the 10 numbers of comets that missed the Earth at the same time, where did all of them end up?
Comets are perturbed out of their orbits by interactions with other Solar System bodies, or even interactions with nearby stars. Some are ejected from the Solar System completely, but most end up in the Sun.
As I said I am not an astronomer I am just interested. Seems like when the astronomers can't figure out why things are the way they are they make what they consider to be a probable story and they all agree to it.
Actually, for much of this, the theories have been narrowed down to just one or two that most astronomers consider probable. It isn't the job of scientists to create stories that all can agree upon, but to create theories that can be tested. It is exactly that way of thinking that has led to the loss of popularity in recent years of the theory that comets delivered the oceans- because tests suggest otherwise.
Northern Mercury
how could liquid water exist at -173 deg celcius 
