I don't think that people know exactly where the water on Earth came from. It is believed that pretty much all the material in the Earth (and the whole solar system) was once a swirling cloud of gas and dust, and some ice as well. Gradually gravity made the gas and dust and ice pull together into clumps. The central one was the biggest, and mostly made out of Hydrogen gas. It became the sun. The other clumps became the planets and moons and asteroids of our solar system.

The clump that became the earth probably had a fair amount of ice in it, which became water. Some people also think that over the billions of years that the Earth has existed, comets (which are mostly ice) have crashed into the Earth, bringing water. As far as other planets in the solar system, it is known that some of them have water on them. In fact, it was just discovered that our Moon has water (in the form of ice) in some of its craters.

It is also believed that there are large amounts of water (again in the form of ice) on Mars. The reason that only the Earth has liquid water is that all the other planets and moons are either too hot, and whatever water is there boils away, or they are too cold, and whatever water is there freezes. The one exception might be Jupiter's moon Europa, which I think has a frozen ice surface but a liquid ice core (you could probably get more information about the other planets and moons and their water on the NASA website, or perhaps the Discovery magazine website). In addition, it is worth pointing out that while most of the Earth's surface is covered by water, the water (i.e., the oceans) are only about two miles deep, while the Earth is a several thousand miles in diameter. This means that most of the Earth is actually rock, with a thin layer of water on the surface.

This is an excellent question!!! We believe that VENUS and MARS also initially had the same amount of water that Earth has. On VENUS however, because it was so close to Sun, the water boiled away forming a steamy atmosphere. High in the atmosphere, energy from the SUN broke up the water molecule and the HYDROGEN escaped and the left over oxygen was used to oxidize surface rocks ...hence VENUS has lost most of its water.

On mars, the conditions are so cold that liquid water cannot exist...instead we think that most of the water is present today in polar caps as ice or distributed throughout the soil

Earth is just the right distance from the sun to have liquid water, solid (ice) and also water vapor (clouds and rain)...what a magical planet we live on.

As far as the ORIGINAL SOURCE of water: it is known that the earth formed by a process called COLLISIONAL ACCRETION That is small asteroids banged into one another and STUCK !!Slowly at first by at an accelerating rate the earth accreted by this process. The asteroids that accreted to form the earth are called CARBONACEOUS CHONDRITES. These chunks of asteroids contain about 5% by mass of water....this is the ultimate source of Earth's water. also COMETS which are big snowballs (ice balls ) also fell to earth as it was forming by accretion. the fraction of earths water brought in by comets versus that brought in by asteroids ( as meteorites) is not known, but is probably small.

All of the characteristics of the planets came about during the formation of the solar system. In our solar system, there are two kinds of planets; gas planets like Jupiter and Saturn and rock planets like earth and mars. Notice that all of the rock planets are closer to the sun while the gas planets are further. Lets say that you could take Jupiter and switch places with Earth. Now Jupiter, a gas planet, is closer to the sun. What happens? Well the sun exerts two effects; first it heats Jupiter and second it exerts gravitation. The heat causes the light materials to slowly "boil" off Jupiter leaving behind heavier materials. That means that light gases like Hydrogen and Helium boil off.

Jupiter is now shrinking slowly as its gasses go off into space. The amount of heavy materials like iron and magnesium and water are left behind. These materials sort themselves by density. Heavier materials flow to the center and lighter materials to the outer edges of the planet. Eventually, the planet becomes solid like the earth and exerts it's own forces that keep the remaining part of the atmosphere from boiling off. Now you are left with a planet like Earth--this is no guarantee since there may not be enough iron and other heavy materials and water in Jupiter to form a planet the same size and able to exert enough gravity to keep an atmosphere or even water.

If Jupiter were put closer to the sun like Mars, the heat would be so great that even liquids like water would get boiled off. So you could say that Earth and its water are formed in a "distillation" process where the remaining material depends on what was in the mix in the first place and how close it is from the sun.

When the solar system was formed (see the question from the archives in astronomy "how are stars formed?"), gas, ice, and dust collected in a disk around the sun and within this disk, motion and gravity brought particles of gas, ice, and dust together to form larger and larger objects made of rocky materials as well as trapped gases and ice. Some of these objects grew and tended to collect much of the material around them as they went around the sun in particular orbits. These large objects became the planets while other smaller objects made of collected rocky particles became asteroids and objects made more of gas and ice became comets.

Much of the water on planets probably comes from comets that crashed into the planet early in its history. Because the planets formed from these materials, they all have a lot of rocky material and some gas and ice associated with them. The gravity of planets makes them want to become spherical in shape (see the early question in the astronomy section of answers, "Can you explain why the planets are round?".

To make the best shape, the densest materials in the planet want to go to its core while the less dense materials like ice or gas want to go towards the surface. Gases have a lot of energy so if they escape to a planet's surface, they will want to fly off into space unless the gravity of the planet is strong enough to hold the gas close to the planet as an atmosphere. In space water freezes because it is too cold and so it acts like a solid but once it warms up, ice can turn into vapor or, if there is enough atmosphere and the right temperature, water.

On a planet like Mars, where the atmosphere is thin, there isn't enough pressure for liquid water to form so it either freezes or vaporizes into the atmosphere where the gravity of the planet might not be strong enough to prevent it from flying into space. On Venus, the super dense atmosphere is good for liquid water but it also traps in heat, making the planet so hot that all water becomes vapor. Earth is ideal because it has enough atmospheric pressure that liquid water is stable and the right temperature for water to exist without freezing or vaporizing. In other words, on Earth, water can exist without too much of it freezing or vaporizing so we see a lot of it. Venus might have just as much water as Earth but it has no oceans. Where do you think all of that water is?