A Tiger-Striped Surprise: Ice Volcanoes on Enceladus

I wrote this piece as part of my grad school applications last fall.

At first glance, Saturn’s tiny moon Enceladus appears to be nothing like its namesake, the giant of Greek mythology whose breath caused volcanic eruptions. Enceladus (the moon) is a frozen celestial body smaller than the state of Arizona. No one would have guessed that its name would turn out to be serendipitously appropriate.

When the Cassini spacecraft arrived at Saturn in 2005, scientists didn’t intend to conduct more than a quick flyby of Enceladus. But any expectations of Enceladus as an inert, cratered moon were soon dismantled as Cassini captured images of four large, hot fissures sliced across the south pole, like the jagged claw marks of a tiger. And so it was revealed that Enceladus truly is living up to its mythological namesake: from these so-called “tiger stripes” spewed at least 100 ice volcanoes, jetting particles and water vapor far out into space.

The discovery of these plumes was a shock. One wouldn’t initially expect moons to have active processes like violent geysers of particles — moons are usually too small and cold to produce much energy. Craters on our own Moon indicate that its surface hasn’t changed for three billion years. For tiny Enceladus to have such vigorous volcanoes, something must be happening internally to generate heat and energy.

The most natural explanation for such a small moon to be producing so much power is a process called tidal flexing. Tidal flexing occurs because Enceladus’ orbit around Saturn is not perfectly circular, so the huge planet gravitationally squeezes and stretches its tiny moon as it orbits. This constant exercise of the moon’s rocky interior causes it to heat up. But scientists expect that this tidal flexing can only generate about 10% of the power that the plumes output, so Enceladus must have some sort of other unexpected internal heat storage.

Chemical analyses of the plumes hint at more subsurface secrets. Samples of the plumes reveal “salty” sodium particles, indicating the presence of silicate rocks. Along with the abundance of water vapor particles, this data paints an image of a deep subsurface ocean with a solid rock bottom.

Liquid water, heat, and a water-rock interface are a crucial combination of ingredients for the chemistry of life that has made the once nondescript Enceladus a prime target in our search for extraterrestrial life. The idea that life could potentially arise under a crust of ice on a tiny moon of Saturn has revolutionized concepts of habitable environments. Enceladus is an embodiment of the surprises of our solar system that we have yet to uncover.