Water is the most abundant substance on Earths surface and also the third most abundant molecule in the universe, after  and . 0.23 ppm of the earths mass is water and 97.39% of the global water volume of 1.38 km is found in the oceans.
Water is far more prevalent in the outer Solar System, beyond a point called the frost line, where the Suns radiation is too weak to vaporize solid and liquid water (as well as other elements and chemical compounds with relatively low melting points, such as methane and ammonia). In the inner Solar System, planets, asteroids, and moons formed almost entirely of metals and silicates. Water has since been delivered to the inner Solar System via an as-yet unknown mechanism, theorized to be the impacts of asteroids or comets carrying water from the outer Solar System, where bodies contain much more water ice. The difference between planetary bodies located inside and outside the frost line can be stark. Earths mass is 0.000023% water, while Tethys, a moon of Saturn, is almost entirely made of water.