Comets are generally composed of ice and rocks ((http://www.newton.dep.anl.gov/newton/askasci/1995/astron/AST145.HTM)), and are continually dissipating as their ice evaporates over time. As the ice disappears a commet will tend to break up into smaller pieces, and finally cease to be a comet, as has been observed. Because of their rapid dissipation, they could not have been created very long ago relative to typical evolutionary timescales. ((This is not an acceptance of the theory of evolution))
“In the six hundredth year of Noah’s life, in the second month, on the seventeenth day of the month, on the same day were all the fountains of the great deep broken up, and the windows of heaven were opened. And the rain was upon the earth forty days and forty nights.” Gen 7:11-12 Where could so much water have come from? Today if the water in the atmosphere were to fall as rain all at once, it would only cover the surface to an average depth of approximately one inch (2.54 cm). ((http://ga.water.usgs.gov/edu/watercycleatmosphere.html)) Even if for some unknown reason the pre-flood atmosphere contained 100 times as much water as today (surely impossible), that would still give only about 8.3 feet (2.6 m), hardly enough for a world-wide flood.
On the other hand, it is quite reasonable that such a great amount of water could come from underground, if God originally created the earth so that there was a surface crust sufficiently thick and strong, with large amounts of water under the crust. This would be an unstable system with the heavy rock on top of the much lighter water, and the water beneath would be under extreme pressure. For maximum stability to last about 1500 years before the flood, the part of the crust above water would need to have been very uniform in composition and thickness. That would imply that there would be no significant hills or mountains over that part of the surface, and surface water there would occur only in shallow seas.
Because of the extreme high pressure of the water, any crack in the crust letting enough water escape would start a chain reaction where most of the water underneath would soon end up on top and the rock at the bottom. This would be similar to the failure of a dirt dam when water overflows it. The great force of the ejected water would send it high into the atmosphere or even into outer space, to then fall back in the form of rain and snow. This would explain where enough water would come from for 40 days and nights. It could have ended abruptly as the last crustal rocks pressuring the water settled to the bottom. The water thrown out into the extreme cold of space would freeze, and its falling back could explain the instant freezing of mammoths in Siberia. Some have been found with buttercups still in their mouths and the frozen meat still fresh enough to eat. For an animal that size to have frozen quickly enough for the meat to remain fresh requires extreme cold of at least -150ºF (-101ºC). ((http://www.creationscience.com/onlinebook/FrozenMammoths5.html))
Phaeton is a hypothetical planet (( http://en.wikipedia.org/wiki/Phaeton_(hypothetical_planet))) posited to have existed between the orbits of Mars and Jupiter, whose destruction resulted in the formation of the Asteroid Belt. If two planets come closer to each other than the Roche Limit, ((http://en.wikipedia.org/wiki/Roche_limit)) one or both will break apart partially or completely. It is posited that Mars and Phaeton approached within said limit, and that Phaeton broke apart completely to form the Asteroid Belt, and Mars had a significant piece torn from it. ((Catastrophism and the Old Testament, Donald W. Patten, 1988)) The huge number of asteroids and meteors created by that event seems like the best explanation for what broke the earth’s crust, namely one or more large meteors which hit the earth. Much of the fragments of the break-up would tend to orbit in the Asteroid Belt, but also much would fly off in all directions, explaining the asymmetric orbits of the comets and other space debris. This would explain how some of the debris came towards earth to crack the crust. It also explains the formation of craters on the moon, many of which were formed over a short time span. Some of this water would have fallen onto the earth to contribute to the rain, but could hardly have been significant relative to the water coming up from under the earth’s crust.
- The Grand Canyon of Mars
Space probes have discovered many clues that Mars had a large amount of water, including a water-carved canyon 4 times as deep and 5 times as long as earth’s Grand Canyon. ((http://science.nasa.gov/science-news/science-at-nasa/2001/ast05jan_1/)) The partial break-up of Mars would nicely explain what happened to the water on Mars. Before the break-up, Mars would have been larger with significantly more gravity, and so could have held the water indefinitely and would have had a much thicker atmosphere so that there could have been liquid water. However, once its mass and gravity was so reduced, the remaining surface water and thick atmosphere would have evaporated out into space in a relatively short time.
At the break-up of Mars, the water and solid matter on the side facing Phaeton would have immediately fragmented and flown out into space. But the other side would have remained intact, and furthermore, the extreme tidal forces (caused by the same forces by which the moon produces tides on the earth, but much, much more extreme) would have caused the 
water on the intact side to have formed a large mountain of water. After the break-up Mars would have immediately reshaped roughly into a smaller sphere. This plus the release of the tidal forces would have caused the water, remaining only on one side, to flow rapidly and forcefully, creating the canyon just on one side. Is there a better explanation for how this huge singular canyon could have been formed, and then the water to have disappeared?
The huge blobs of water, ice and debris which flew out into space would have quickly frozen solid in the extreme cold of outer space, creating the comets, meteors and asteroids. ((Conventional theory hypothecates an Oort Cloud to be the source of comets: http://en.wikipedia.org/wiki/Oort_Cloud))
I believe this adequately explains the origin and characteristics of the comets, and is entirely possible within the laws of physics. This also explains the fading problem ((http://star.arm.ac.uk/preprints/380.pdf, http://www.springerlink.com/content/p44058jm6l366777/fulltext.pdf)) (the dying out of the comets), which the Oort Cloud and other hypotheses do not explain.