*sigh*
Do you guys just make shit up?
Internet quote follows.
When we look around our solar system, we notice that there are large amounts of water in the outer regions. This water is mostly in the form of ice, but it is still prevalent. Comets are bodies in the outermost regions of the solar system that contain large amounts of water. They also have highly elliptical orbits that frequently bring them into the inner solar system ? and sometimes on a path that will cause them to collide with Earth. Although we have a general sense of the composition of comets ? they are like dirty snowballs that contain organic matter ? we do not yet know for sure their exact makeup. Astronomers have studied at least three well-known comets ? Halley?s Comet and Comets Hyakutake and Hale-Bopp ? and they have noticed one commonality between the three. In these comets, the percentage of deuterium (a form of hydrogen with double the mass of normal hydrogen) is twice that of what we find in average seawater. It is not yet clear whether or not these comets are representative of the types of comets that would have been colliding with early Earth. Regardless, if Earth had received the majority of its water from comets with similar composition, we would expect the percentage of deuterium to be similar. We could even accept it if the percentage of deuterium in our oceans were greater than that of the comets, due to photodissociation and subsequent loss of deuterium to outer space. Neither of these is the case, so we cannot currently make any conclusions about the contribution of water by comets. Based on the deuterium composition of the oceans, scientists have estimated that no more than 10% of the water on Earth originally came from comet collisions.
Although we have just focused on comets as a possible water bearer for Earth, there are other planetary bodies to consider. Meteoritic material originating in the asteroid belt could also be responsible for bringing water to Earth. Objects in the outer part of the asteroid belt are predicted to contain up to twenty percent water. Jupiter?s large mass and gravitational influence could easily have perturbed the orbits of asteroids in the belt, sending some of them on a one-way trip to Earth. The composition of asteroids within the belt vary depending on their distance from the Sun, so it is difficult to estimate exactly how much water could have been contributed to Earth. However, scientists such as Alessandro Morbidelli have predicted that if asteroids had collided with Earth to bring it the majority of its water, delivery by comets could have contributed some water ? resulting in more appropriate deuterium to hydrogen ratios in Earth?s oceans.
Like so many things in science, the exact origin of water on Earth is still unknown. Yet scientists continue to make observations, collect data, and analyze information to try and develop a more complete understanding of the natural world. One way in which scientists will be gathering more evidence regarding the origin of Earth?s water is the Deep Impact mission, scheduled for July 2005. This mission will send a spacecraft to crash into Comet Tempel 1. Once the spacecraft collides and forms an impact crater, scientists will to try and determine the exact composition of the comet. Discovering a low deuterium concentration in the comet could drastically alter our theories about how water came to Earth.