1. A small icy moon was caught spraying water into space
Enceladus (say "en-SELL-uh-dus") is only about 500 km (310 miles) across[1]. The Cassini spacecraft was exploring Saturn, and its observations in 2005 revealed a huge cloud of water vapor above the moon's south polar region[2]. The source turned out to be long cracks that scientists nicknamed "tiger stripes"[2]. Water jets come out of these relatively warm cracks[1].
This is not a stream of liquid water. What flies out is water vapor and tiny grains of ice. The question is where it comes from: ice that is simply warmed and evaporated near the cracks, or a pool of liquid water below? Three kinds of measurement, made at different times, answer it one after another.
2. The ice grains are salty, which points to liquid water below
Cassini flew through the spray and sampled it. A team reported in 2011 that salt-rich ice grains made up more than 99 percent of the mass of the solid material being thrown out[3]. That percentage describes the mass of the ejected solids. It does not say how salty the ocean is[3].
Salt in ice grains is what you would expect if the grains were frozen droplets of salt water. The team concluded that "a salt-water reservoir with a large evaporating surface provides nearly all of the matter in the plume"[3].
That settles that salt water exists somewhere. It does not say whether it is a small pocket under the south pole or a sea around the whole moon. For that, scientists needed to weigh the moon and watch it move.
3. The moon's pull and wobble show how big the ocean is
A tiny tug on the spacecraft: evidence for a sea under the south pole
Cassini passed within 100 km of Enceladus three times in 2010 and 2012. The moon's gravity changed the spacecraft's speed by only about 0.2 to 0.3 millimeters per second (roughly 0.01 inch per second), and that change showed up in its radio signal[4]. Reading the pattern, the researchers found that mass was distributed as if liquid water lay about 30 to 40 km (19 to 25 miles) below the ice surface in the southern hemisphere[4]. That figure is the depth down to the water, not the thickness of the ocean.
In 2014 the best fit was a regional sea reaching from the south pole to about 50 degrees south latitude, although the data could not rule out a global ocean[4].
A wobble that is too big: an ocean around the whole moon
The next clue came from pictures. Researchers measured the positions of craters and other landmarks in Cassini images taken over more than seven years, and worked out how much Enceladus wobbles back and forth as it orbits Saturn. The result was 0.120 degrees, give or take 0.014 degrees (the paper's 2σ uncertainty, roughly 95 percent confidence)[5].
That may sound tiny, but it is too large for a moon that is solid from the surface to the core. A simple way to picture it: try to twist the shell of a hard-boiled egg and the whole egg turns with it, but the shell of a raw egg can slip around the liquid inside. If the ice shell were rigidly attached to the core, the wobble would be much smaller than the one measured[6]. The authors concluded that the wobble "implies the presence of a global ocean rather than a localized polar sea"[5]. NASA announced this in 2015[6].
How thick is the ice? Estimates differ
NASA now describes the ice shell as roughly 1 to 5 km (0.6 to 3 miles) thick at the south pole and about 20 to 25 km (12 to 16 miles) thick on average[1]. The 30 to 40 km figure above is the depth to water in the 2014 gravity study, so the numbers are not directly comparable. Treat any single number for the shell as an estimate that research keeps updating.
One puzzle remains. The wobble paper notes that keeping a global ocean liquid is "problematic according to many thermal models"[5]. NASA's release says what keeps the ocean from freezing is still unknown, and suggests heating caused by Saturn's pull as one possibility[6]. The ocean was found first. The full explanation is still being worked on.
4. Silica and hydrogen hint at hot water meeting rock on the ocean floor
Once there is an ocean, the next question is what its bottom is like. Two clues came from the spray.
The first is silica, the main ingredient of sand and glass. Cassini found extremely small silica grains, 2 to 8 nanometers in radius, in Saturn's E ring, the thin ring of ice fed by Enceladus[7]. A nanometer is one billionth of a meter. The narrow range of sizes pointed to hot-water reactions above 90 °C (194 °F)[7]. A university press release describes the grains forming when hot, mineral-laden water rises from the rocky interior and meets colder water[8]. The 90 °C figure is a lower limit, not a measurement of the water temperature.
The second is hydrogen. In 2015, Cassini made its deepest dive through the plume, and its mass spectrometer found a significant amount of molecular hydrogen[9]. NASA says this supports earlier evidence that warm, mineral-laden water flows out of seafloor vents[9]. Silica and hydrogen are different substances found by different instruments, and both suggest the same thing: hot water reacting with rock on the ocean floor.
5. Ingredients for life may be present, but no one has found life
On Earth, some microbes live on hydrogen and turn it into methane[9]. A similar kind of chemistry could be possible in the ocean of Enceladus. But the lead scientist of the instrument team put it carefully: finding hydrogen "is not demonstration of finding life, but it shows the potential for the existence of life" there[9]. NASA states that Cassini is not able to detect life, and has found no evidence that Enceladus is inhabited[9].
In 2023 a team reported that ice grains from the E ring contain phosphate, a form of phosphorus, which living things on Earth need to build their cells. They estimated the ocean holds more than 100 times as much phosphate as Earth's oceans[10]. The team says there is no evidence that the ocean is actually inhabited[10].
So the honest summary is this: scientists have strong evidence for liquid water beneath the ice, a source of chemical energy, and some of the chemical ingredients that life on Earth uses. Having the ingredients is not the same as having a meal being cooked, and whether anything lives in that ocean is an open question.
How could we find out more, and how can you explore the idea?
- Future missions. The European Space Agency is studying a mission to land on Enceladus, with a launch listed for 2042. It is still under study, so the plan is not fixed. Its goals include sampling the plume and searching for signs of life[11].
- Spin two eggs. With an adult, boil one egg and let it cool completely. Spin it and a raw egg on a large plate or tray, then gently stop each with a fingertip and let go. The cooked egg usually stays stopped. The raw egg may start turning again, because the liquid inside is still moving. Scientists reasoned in a similar way about the ice shell, though the real wobble is far too small to see. Wash your hands after handling raw egg.
- Read the original. NASA's page on Cassini at Enceladus and the 2015 global-ocean announcement are short and readable. Try to spot which clue each one is about.