Salty Ocean Holed Up Inside Saturn’s Moon Enceladus

Spectacular plumes spray water ice from cracks in Enceladus' crust called 'tiger stripes' in this photo by Cassini. Credit: NASA

Enceladus (en-SELL-uh-dus), one of the most awesome moons of the solar system, just doubled in awesomeness. Several years ago, the Cassini probe revealed jets of water vapor and ice shooting from fissures nicknamed ‘tiger stripes’ in the moon’s south polar region, hinting at a water reservoir beneath the 310-mile diameter moon’s crust. But that’s not all that’s being spewed by Enceladus.

The plumes originate in large cracks - seen here in blue - in the moon's south polar region. Credit: NASA

In a paper published in journal Nature this week, Cassini scientists report that the spacecraft found relatively large grains rich with sodium and potassium lower down in the plumes. The salt-rich particles have an “ocean-like” composition and indicate that most, if not all, of the expelled ice and water vapor comes from the evaporation of liquid salt water. When Cassini flies through the plumes, tiny grains hit the craft’s cosmic dust analyzer at 15,000 to 39,000 mph and vaporize. Electric fields inside the device separate out the materials to determine their composition.

Heating from flexure warms the interior of Enceladus, melting ice to form an underlying ocean of salt water. The water erupts at the surface through fissures. Credit: NASA

The salty nature of the water spewed by Enceladus almost certainly originates from a subsurface ocean. The data suggests a layer of water some 50 miles beneath the surface. Water dissolves salts from the overlying rock, similar to how salt accumulates in Earth’s oceans, and then rises through fractures in the crust. When the outer surface cracks and exposes the water reserves to the vacuum of space, the sudden and large drop in pressure propels the salty water out in geyser-like fashion.

In this extreme closeup of the tiger stripe named Baghdad Sulcus, you can make out thousands of individual blocks of ice. The view spans just 7.5 miles side to side. Credit: NASA

How does a moon nearly a billion miles from the sun with a surface temperature of 330 below Fahrenheit get warm enough to host an ocean? Enceladus revolves around Saturn every 1.4 days in an elliptical orbit. Its average distance from Saturn is 148,000 miles, but because the moon’s orbit is slightly “oval” rather than circular, its distance from Saturn varies, causing the pull of the planet to alternatively squeeze and stretch the little moon. Friction from flexing heats up and partially melts Enceladus’ interior. It’s similar to the frictional heat you feel bending a metal paper clip back and forth until it breaks. Radioactive decay of unstable isotopes of elements like uranium also contributes to heating up the orb.

A saltwater ocean suggests a suitable environment for life to arise. Is it too far-fetched to think it could have done so in the midnight-black interior of a remote moon? Maybe, but life continues to surprise us with its insistence on being EVERYWHERE.

Helene photographed from 6,968 miles away on June 18 by Cassini. Numerous flow features are visible. Credit: NASA

Before we leave Saturn, I wanted to share a couple additional photos from Cassini of the tiny but intriguing moon Helene. Measuring just 22 miles across, Helene (HELL-e-nee) was discovered at French observatory in 1980. Recently Cassini made a close flyby and returned some amazing pictures of its unique surface. The moon revolves around Saturn with one hemisphere always facing the planet and the other facing away. Curiously, the Saturn-facing hemisphere shows the usual peppering of craters, but the reverse side is covered in the most beautiful flow features I’ve ever seen on a planetary satellite. Scientists don’t know for sure what causes them, but one hypothesis is that they’re avalanches or flows of dust.

A very evocative image of Helene looking back at the moon from its nightside. The illuminated area is a roughly outlined crescent. Helene has an irregular shape, because it's too small for self-gravity to crush it into a sphere. Credit: NASA