Enceladus's geysers spray a hidden ocean into space

Saturn’s moon Enceladus sprays water from a hidden ocean into space: NASA’s Cassini spacecraft found jets of water vapour and ice grains erupting from fractures near its south pole at about 800 mph (400 m/s), building a plume that reaches hundreds of miles out.

The source is a global ocean of salty liquid water beneath the ice of a moon only about 500 km (310 miles) across. Some of the spray falls back as fresh frost, one reason Enceladus is the most reflective body in the solar system; the rest feeds Saturn’s vast E ring.

Cassini pieced this together in stages. Voyager images from the 1980s had shown a bright, oddly smooth little moon. Then Cassini’s magnetometer sensed something, perhaps an atmosphere, pushing against Saturn’s magnetic field near Enceladus, and the team scheduled a closer pass. In 2005 its cameras found a young, almost crater-free south pole, a huge cloud of water vapour, and warm fractures nicknamed the tiger stripes: four cracks each about 135 km (84 miles) long.

In 2008 Cassini flew through the plume and tasted it: water vapour, carbon dioxide, carbon monoxide and organic material, with the organics about 20 times denser than expected. By 2015, measurements of a slight wobble in the moon’s rotation proved too large for an ice shell fixed to the rocky core, so the water must be a global ocean rather than a local polar sea. Cassini made its final flyby that October.

Gravity data point to a sea about 10 km deep under the south pole, beneath 30 to 40 km of ice. Tiny silica grains in the E ring are the hottest clue: they form only where liquid water reacts with rock above about 90 °C, implying hydrothermal vents on the seafloor.

Liquid water, heat and organic chemistry together are why many scientists rank this small moon among the best places to look for life beyond Earth.