Comparison of the liquid water volume of Earth, Europa, and Titan to scale. It is estimated that Europa has over two times and Titan nearly eleven times more liquid water as subsurface oceans than Earth. Only liquid water is considered in these estimates but water ice is also significantly present in Europa and Titan. The image assumes a mean ocean depth of 4 km, 100 km, and 200 km for Earth, Europa, and Titan, respectively.

Water is not a luxury of Earth, in fact, it is the most abundant compound in the universe because it is composed of the first and third most abundant elements, hydrogen and oxygen, respectively. However, water in liquid form is a special feature of Earth. It is usually present as vapor or ice in the Solar System. After the formation of the Solar System most of the water ended on the farthest planetary bodies locked as ice in their surface or interiors. Earth being closer to the Sun has little water, relatively speaking, mostly in its surface. If Earth is a wet rock the satellites of Jupiter and Saturn are frozen mud balls. Water ice in these satellites is more evenly distributed through their interior than in rocky bodies. We also know that liquid water is not only a luxury of Earth and is also present as deep oceans in Europa and Titan, satellites of Jupiter and Saturn, respectively. The amount of liquid water in Enceladus, a satellite of Saturn, and Mars are probably large too.

CREDIT: PHL @ UPR Arecibo, NASA