Most liquid water in the Solar System

The Solar System body with the greatest volume of liquid water is Earth, which has an estimated 1,361,620,510‬ km³ (326,666,904 cubic miles) of the stuff on, in or around it in the atmosphere.Another strong candidate, however, is Jupiter's icy moon Europa, which current models project has a subsurface ocean with a volume estimated at 2.87 billion km3. The existence and size of this ocean is expected to be confirmed (or potentially disproven) by NASA's Europa Clipper and the European Space Agency's Juice (Jupiter Icy Moons Explorer), space probes due to arrive in the Jupiter system in the early 2030s. It is also possible that Jupiter's larger moon Ganymede has an even more extensive subsurface ocean, estimated at 35.4 billion km3 by JPL scientist Steven Vance.

Earth's oceans and seas hold 1,338,000,000 km³ (321,000,000 cubic miles), accounting for 98.2% of all the liquid water on Earth. When combined with the contents of saline aquifers and lakes, the proportion of the planet's liquid water that is saline climbs to 99.2%. Liquid freshwater accounts for only 0.8 percent of all the water on Earth as most of the freshwater (68% of it) is frozen in permafrost, glaciers and ice caps.

If this were all to melt, the balance would shift to 96.5% saline and 3.5% fresh.

Although there are no other planets in the Solar System with liquid water on the surface, there are a few ice worlds where it is thought that an ocean of liquid water exists beneath an icy crust many kilometres thick. The most well-studied of these is Jupiter's moon Europa.

The first detailed images of Europa came from NASA's twin Voyager proves in the 1980s, which took pictures of the icy moon at the relatively high resolution of around 2 km2 per pixel. Researchers analyzing these images quickly noticed a few intriguing details.

The first was that Europa had very few visible craters, suggesting that its frozen surface was being continuously renewed and reformed by some geologic process. The second was that areas of the surface were dotted with large puzzle-piece-like shapes in the ice. The edges of these icebergs matched up with each other, despite often being located some distance apart. This was consistent with a thick sheet of floating ice that had cracked, with liquid water flowing up to fill the gaps between the drifting ice before refreezing. Finally, the ridges and patterns in the ice, formed by gravitational interactions with Jupiter and the other Galilean moons, were not in the orientation scientists expected. This implied that the surface moved independently of Europa's core, freely floating on a world-spanning sea.

Following close observations by the <em>Galileo</em> probe, which made 12 flybys of the moon between Dec 1996 and Jan 2000, and the <em>Juno</em> probe, which made a close flyby on 29 Sep 2022, researchers have a clearer idea of conditions under Europa's surface. By studying data collected on the moon's mass, gravitational field and magnetic field, NASA scientists have calculated an estimated volume of 2.87 billion km3 for the subsurface ocean.

Two missions are currently en-route to Jupiter to specifically study the gas giant's icy moons – NASA's Europa Clipper and the European Space Agency's Juice (Jupiter Icy Moons Explorer).