Astrobiologists question whether planets in our solar system other than Earth have conditions necessary for life to exist. They study both life on Earth and environments on other planets to answer this question. They have learned that life is adaptable and resilient, capable of thriving in a wide variety of conditions—from the driest deserts to the hottest springs. Missions to study conditions on planets and moons in our solar system are also revealing some unlikely places that may have everything that life would need to exist.

One of the worlds most worthy of further exploration is one of Saturn’s innermost moons, Enceladus (en-SELL-ah-dus). Scientists hypothesized that Enceladus was frozen solid—like a giant ice cube—because it is so distant from the Sun and so small. But in 2005, NASA’s Cassini spacecraft recorded signs of geological activity, implying that Enceladus has its own internal heat source. Cassini also observed jets of ice particles, water vapor, and trace organic molecules—including molecular nitrogen, methane, carbon dioxide, and hydrocarbons—erupting from cracks in the moon’s surface. This suggested that there might be a saltwater sea between its icy shell and rocky core. Because a heat source, liquid water, and organic materials are the three vital requirements for life as we know it on Earth, these findings were highly intriguing.

Enceladus is only one of two bodies in our solar system besides Earth with active volcanoes. The other is Io, a moon of Jupiter. What could be generating the heat necessary for these eruptions? On Earth, the energy that produces volcanic activity comes from heat left over from Earth's formation and from the decay of radioactive materials in its core. But Io and Enceladus are too small and too far away from the Sun to have any residual heat. Further, any radioactive decay that might be occurring inside them could not generate the energy needed to cause these titanic eruptions. The answer lies in the planets that these moons orbit.

Because it is so large, Saturn exerts a tremendous gravitational force on the moons that orbit it. Enceladus is tiny and orbits so close to Saturn that this strong force distorts its shape. But the distortion does not by itself explain the cause of the internal heating within Enceladus. Saturn has many other moons. The gravity exerted by other moons pulls Enceladus in different directions, making its orbit slightly elliptical. And as Enceladus circles Saturn, the amount of gravitational force it experiences at different points in its orbits changes. The resulting squeezing and stretching of its interior causes its surface to rise and fall, generating enormous amounts of heat within its interior. This heating mechanism due to friction and gravitational distortion is called tidal heating.

On Enceladus, tidal heating produces stress that cracks open the surface ice. Through these cracks—the “tiger stripes”—material from beneath the surface is forced up and out. It is estimated that up to 250 kg (550 lbs) of water is being ejected each second.

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