Enceladus

Enceladus is a small moon of Saturn, about 504 kilometres across. William Herschel discovered it in 1789. It is the most reflective body in the Solar System: its fresh ice reflects almost all the sunlight that falls on it. Under that bright shell lies an ocean, and unlike any other ocean beyond Earth, this one sprays samples of itself into space.

Cassini’s surprise

In February 2005 the Cassini spacecraft’s magnetometer noticed that Saturn’s magnetic field was bending around Enceladus, as if the moon had a thin atmosphere. That July Cassini flew 175 kilometres above the surface, and images taken over that year showed why: jets of water vapour and ice grains were shooting out of the south pole.

They come from four long cracks, nicknamed the tiger stripes. Each is about 130 kilometres long, 2 kilometres wide and 500 metres deep. Along them the ice is up to 157 K (-116 °C), far warmer than the 68 K (-205 °C) expected so far from the Sun. Altogether the jets throw out about 200 kilograms of material every second, and much of it spreads into a wide, faint ring around Saturn, the E ring, which Enceladus keeps supplied.

Tasting an ocean

Cassini flew through the spray several times and analysed it. On its deepest dive, in October 2015, it found the plume to be about 98% water vapour, with about 1% hydrogen and traces of carbon dioxide, methane and ammonia. The ice grains contain sodium salts.

Each finding added to the picture. Tiny grains of silica, only a few nanometres across, can only form where water hotter than about 90 °C reacts with rock, which points to hydrothermal activity on the seafloor. The hydrogen, reported in 2017, is a possible source of chemical energy for microbes. In 2018 researchers found fragments of large organic molecules made of hundreds of atoms. In 2023 they found phosphates, with phosphorus at least a hundred times more concentrated than in Earth’s oceans, and in a separate study hydrogen cyanide, one of the starting points of chemistry that leads to life.

A global ocean

At first the ocean looked like a regional sea under the south pole. Then in 2015 a careful measurement of how Enceladus wobbles as it orbits Saturn showed that the whole ice shell floats free of the rocky core: the ocean must cover the entire moon. The shell is about 20 kilometres thick on average and under 5 kilometres at the south pole, where the jets break through.

Nobody has seen the ocean itself or the vents on its floor; both are worked out from the spray. Cassini ended its mission on 15 September 2017 by diving into Saturn, partly to avoid ever contaminating Enceladus. In 2024 the European Space Agency named Enceladus the top target for a future large mission with an orbiter and a lander, now under study for a launch around 2042.