We Now Understand Why Enceladus has ‘Tiger Stripe’ Cracks at its Southern Pole

One of the biggest surprises of the 13-year Cassini mission came in Enceladus, a tiny moon with active geysers at its south pole. At only about 504 kilometers (313 miles) in diameter, the bright and ice-covered Enceladus should be too small and too far from the Sun to be active. Instead, this little moon is one of the most geologically dynamic objects in the Solar System.

A new study has modeled how this activity could be taking place, and what mechanism might power the geysers spewing from ‘tiger stripe’ fissures. While previous studies have indicated some type of unknown internal heat source on Enceladus, the new study infers no heat source would be necessary.

Max Rudolph, from the University of California, Davis and colleagues say that cracks in the ice shell caused by changes in Enceladus’ orbit around Saturn would allow water from the subsurface ocean to leak out. And instead of active cryovolcanism, the researchers propose the water spontaneously boils when it hits the vacuum of space.

Cryo-volcanism is a relatively newly found phenomenon, initially discovered by the Voyager missions’ travels to the outer Solar System. Instead of hot, molten lava like volcanoes on Earth, cryo-volcanism spews out water, ice and other materials in environments that can be hundreds of degrees below freezing. For example, temperatures at the surface of Enceladus rarely rise above –200°C (-330 F).

Cryo-volcanism has been observed at Jupiter’s Io and Europa, as well as at Enceladus and other icy moons. While Io appears to be outgassing sulfur dioxide, other moons are erupting with water, methane and ammonia.

Rudolph and colleagues said they modeled the orbital and internal evolution of the ice-covered ocean worlds Enceladus and Europa across 100 million years of time. The eccentricity of the moons’ orbits leads to varying thicknesses of their ice shells. As the ice thickens and thins, the team said, thermal stresses in the ice shell and pressure in the underlying ocean will change, promoting the fracturing of the ice shell, creating the tiger stripe fissures.

This takes place as the ice cools and thickens. The pressure exerted on the ocean below would create stress on the ice, since ice has more volume than water. The pressure and stress could cause cracks, and create pathways for fluid to reach the surface, as much as 20-30 kilometers away. The sublimation of the water as it hit the vacuum of space gives the appearance of “jets” when there aren’t any.

Rudolph said in a press release that this is consistent with the appearance of the surface of Enceladus, which doesn’t show any evidence of cryo-lava flows leaking from the cracks on the surface, which are found on Io. Enceladus appears to be unique in that the tiger stripe cracks are not found anywhere else in our Solar System.  They are parallel and evenly spaced, about 130 kilometers long and 35 kilometers apart, and they appear to be continually erupting with water ice.

But the mechanism identified in this new study of ocean pressure and spontaneous eruption can’t explain the cryo-volcanism that may be happening on Europa, Rudolph said. Further research and observation on that moon is needed to determine the potential causes of those eruptions, since their models don’t show cracks reaching the subsurface ocean on that moon. The upcoming Europa Clipper mission should provide more insights.

This new study, however, does not address previous studies and data which indicate Enceladus’ south pole is warmer than expected just a few feet below its icy surface, and what keeps the interior of this tiny moon warm enough to support a liquid subsurface ocean.

Sources:

AGU press release
Geophysical Research Letters paper: Cooling Crust Creates Concomitant Cryovolcanic Cracks