Earth looks like the obvious water world. Oceans cover about 71 per cent of its surface, colour the planet blue from space, drive its climate and support every ecosystem we know. Yet if the contest is simply the volume of liquid water, Earth may not even beat a moon one quarter of its diameter.<\/p>\n
NASA\u2019s current Europa comparison<\/a> assigns Earth\u2019s oceans a volume of roughly 1.4 billion cubic kilometres and Europa\u2019s probable subsurface ocean about 3 billion cubic kilometres. Europa achieves that apparent upset not by covering more surface, but by hiding a global sea that may average around 100 kilometres deep beneath ice.<\/p>\n Saturn\u2019s moon Enceladus offers a second correction to our surface-centred intuition. It is only about 500 kilometres wide, yet jets from fractures near its south pole carry water vapour and ice grains from its buried ocean into space, where Cassini was able to fly through and sample them.<\/p>\n This is a synthesis of several mission results and interior models, not one direct measurement. Europa\u2019s ocean has not been visited, the quoted volume has uncertainty, and Enceladus\u2019s plume varies in strength. Neither moon has produced evidence of life. Even with those boundaries, both worlds make Earth\u2019s claim to be the Solar System\u2019s definitive \u201cwater world\u201d much less straightforward.<\/p>\n Europa wins the volume comparison by going deep<\/p>\n Earth\u2019s oceans spread across a vast surface, but their average depth is only about four kilometres. Europa\u2019s proposed ocean is geographically smaller and far deeper. NASA\u2019s illustrative estimate uses an average depth of about 100 kilometres, sufficient to produce roughly 3 billion cubic kilometres of liquid saltwater below the moon\u2019s frozen exterior.<\/p>\n The arithmetic is not mysterious. Volume depends on thickness as well as area. Europa\u2019s diameter is only about 3,120 kilometres compared with Earth\u2019s 12,742 kilometres, but a water layer tens of kilometres deep can compensate for that much smaller circumference. On those estimates, Europa contains a little more than twice the combined volume of all Earth\u2019s oceans.<\/p>\n \u201cMore than twice\u201d should not be mistaken for a laboratory reading. No instrument has drilled through Europa\u2019s shell and sounded the full ocean. Scientists infer its internal layers from surface geology, gravitational behaviour, magnetic measurements and models of how the moon responds to Jupiter\u2019s tides. The 3 billion cubic kilometre figure is a useful scale comparison, not a precisely surveyed inventory.<\/p>\n The evidence for Europa\u2019s ocean comes in layers<\/p>\n Europa\u2019s surface is young by Solar System standards and crossed by ridges, bands and disrupted regions called chaos terrain. The patterns look consistent with an outer ice shell that has shifted, cracked and been resurfaced. Very few large impact craters survive, another sign that the exterior has changed over geological time.<\/p>\n A particularly important clue came from Galileo. The spacecraft measured a magnetic response that changes as Europa moves through Jupiter\u2019s magnetic field. A global electrically conductive layer, with salty liquid water the leading explanation, can produce that induced signal. Tidal flexing supplies a mechanism for keeping the interior warm: Europa\u2019s slightly eccentric orbit repeatedly squeezes and relaxes the moon as Jupiter\u2019s gravity changes in strength.<\/p>\n Current estimates remain broad. NASA describes a possible ocean roughly 60 to 150 kilometres deep and an ice shell commonly modelled at around 15 to 25 kilometres, although the values depend on the assumptions used. Europa Clipper is designed to test this ocean interpretation<\/a> through dozens of close fly-bys, using radar, magnetic, gravity, thermal and compositional measurements.<\/p>\n The mission will assess whether Europa has environments that could support life. NASA explicitly describes it as a habitability mission<\/a>, not a mission that can declare life detected.<\/p>\n Enceladus exposes what Europa keeps hidden<\/p>\n Enceladus is a different kind of ocean world. Cassini observed jets issuing from four long fractures near the south pole, informally called tiger stripes. The jets loft icy particles and water vapour hundreds of kilometres above the surface. Some material falls back as snow; some escapes and helps sustain Saturn\u2019s diffuse E ring.<\/p>\n NASA describes the eruptions as continuous<\/a>, but that does not mean the output is constant. A long series of Cassini observations showed that the plume brightens and fades predictably as Enceladus travels around Saturn. Tidal stresses flex the tiger stripes<\/a>, opening them wider when the moon is farther from Saturn and squeezing them closer to shut at other parts of the orbit.<\/p>\n Gravity measurements and a small rotational wobble indicate that the south-polar activity connects to a global subsurface ocean rather than a local pocket alone. The crucial practical result is access. A future spacecraft need not land, drill or melt through kilometres of ice to collect material from that ocean. It can cross the plume, an advantage I examined more closely in this earlier SpaceDaily explainer on plume sampling<\/a>.<\/p>\n Cassini sampled ocean chemistry, not organisms<\/p>\n Cassini was not built to look for life at Enceladus because the plume had not been discovered when the spacecraft was designed. Even so, its instruments turned fly-throughs and impacts with plume grains into an unexpectedly rich chemical survey.<\/p>\n The spacecraft detected water, salts, silica particles, molecular hydrogen and organic compounds. Silica and hydrogen support the idea that warm water interacts with rock on or within the seafloor. In 2023, researchers analysing Cassini data reported phosphates in salt-rich ice grains<\/a>, adding phosphorus to the collection of elements important for life as we know it.<\/p>\n