Water allows all known living things on this planet to survive. Without water, earth would be a rock, lifeless, dull, barren even. It is what makes earth unique in our solar system; it allows life to flourish.

Water, or more specifically, H2O, is not a unique phenomenon to our solar system as we know it, however.

The two examples I will use for the purposes of this writeup will be Mars and Jupiter's moons Ganymede and Europa. Mars has polar ice caps. That is to say, water ice at the poles, though humans have never received samples of this substance to scientifically examine all aspects of it to ascertain its full makeup. It's assumed to be water ice, but with what other elements? This is unknown at this time. It's also believe that there is water in Mars' thin atmosphere; clouds appear from time to time, and there is evidence of water at one time being on the surface, in the form of rivers and flood plains, dark grooves on Mars' surface.

Ganymede is believed to be largely composed, at least on the surface, of water-ice, and nickel- or iron-rich dirt. It must be kept in mind, however, that at the estimated surface temperature of Ganymede's surface, about -200 degrees Centigrade, water ice takes on a more steel-like hardness, as compared to the brittle, rubbery ice we have on earth. (As an example, you can take an ice cube from the freezer and crunch it in your mouth; this would be impossible with the ice on Ganymede.

Jupiter's moon Europa is unique in the solar system. It is believed that the entirety of the moon's surface is water-ice, though at temperatures roughly the same as Ganymede. Various tests have been done, and the theory is that the surface of Europa is about 50 miles thick, and it is likely that there are oceans of liquid water underneath. It is impossible to assume that there would be life in these waters; we simply don't know enough.

There are other factors that make water in its liquid form special: as far as we know, our planet is the only body in our solar system where we are certain water exists. This is mainly due to earth's atmospheric pressure and climate. The atmospheric pressure, in combination with the average temperature of earth as a whole (about 15 degrees Celsius), allows water to exist in liquid form. Venus is too hot, the atmospheric pressure too great for water to exist in the form of surface water. Mars' atmosphere is too thin for running water to exist on the surface. The water would not solidify, even at earth's average temperature (which Mars is believed to be able to surpass, on the equator in high Martian summer), to the point where it could run freely on the surface. And on our moon, water could never exist on the surface, because there is no atmosphere; the moon is barren.

Water is what has allowed life to exist on this planet for four billion years. It is unique, and it makes us and our home unique.