Of the strange and unexplained terrains in our solar system, the south pole of Saturn’s moon Enceladus is among the most perplexing.
Enceladus is an ocean world, with a vast and briny sea tucked beneath its icy crust; this makes it one of the most tantalizing places in the solar system to look for life beyond Earth. But unlike other frozen moons, Enceladus constantly erupts. The tiny world blasts salty water into space through cracks in its crystalline shell. These fissures, raked across the moon’s southern pole, are roughly parallel and evenly spaced. And ever since scientists first took a good look at this alien moon, they’ve had a tough time explaining those “tiger stripes.”
“What is going on?” said Doug Hemingway of the Carnegie Institution for Science. “In a way, it’s an obvious question — it’s been in the back of everyone’s mind for a long time.”
Now, Dr. Hemingway and his colleagues think they know how the moon got its stripes — and, curiously, why the stripes are found only at the Enceladian south pole. They described their hypothesis Monday in Nature Astronomy. Learning more about how extraterrestrial oceans on worlds such as Enceladus evolve and interact with planetary surfaces is important for understanding how life might exist beyond Earth and how we might find it, Dr. Hemingway said.
In 2005, NASA’s Cassini spacecraft first swooped in and stared at 313-mile-wide Enceladus. The spacecraft saw a stunning array of geysers erupting from the moon’s south pole — eruptions that vent the moon’s ocean into space and sculpt Saturn’s E ring. Later, when Cassini flew through the jets, it tasted an alien soup containing all the ingredients necessary for life as we know it.
The moon’s southern hemisphere is riven by four prominent fractures. Approximately parallel and spaced about 20 miles apart, the massive cracks are, like other features of Enceladus, named after locations in “One Thousand and One Nights.”
Previous ideas about the origins of the cracks — Alexandria, Baghdad, Cairo, Damascus and a smaller crack unpoetically referred to as “E” — included massive impacts, hot spots, strike-slip faulting and a migrating icy shell. Dr. Hemingway and his colleagues modeled the evolution of the moon’s icy shell, accounting for its thickness, elasticity, strength and temperature, and uncovered a simpler, more comprehensive explanation.
Some time after it formed, they think, Enceladus slowly began to cool some time after it formed. Some of its inner ocean froze, expanded — as frozen water does — and strained the moon’s icy crust, which was thinner at the poles.
Eventually, the swelling sea fractured the southern crust.
The first fissure to form was 80-mile-long Baghdad, the largest and most prominent of the tiger stripes. As water began erupting through Baghdad, some of it snowed back to the moon’s surface, piling up near the fracture’s margins. The weight of that accumulating material strained the ice shell, and new cracks — Cairo and Damascus — opened up on either side of Baghdad, each roughly parallel and about 20 miles away.
Then Alexandria and “E” opened up.
“Why doesn’t this cascading sequence just keep going?” Dr. Hemingway said. “It’s not clear. Maybe as you open up more and more of these fractures, the eruption rate per ridge kind of drops, or the overall background ice shell thickness just gets too big.”
The process may have taken between 100,000 and one million years. The tiger stripes’ even spacing is simply a result of the ice’s elasticity and its thickness, which is thinner at the poles and bulkier at the equator.
But why did the stripes only rupture the southern pole?
“It’s kind of a coin toss whether that first fracture happens at the north pole or the south pole,” Dr. Hemingway said. But as soon as the crust breaks open, he added, the swelling ocean’s pressure is relieved “and the other pole will just stay quiet for the rest of time.”
Although this new model answers many questions about the strange moon orbiting Saturn, several remain. Alyssa Rhoden of the Southwest Research Institute called the hypothesis plausible, but she wonders how the model links up with other, less dramatic fractures and features on the moon:
“Now that we have this idea, how do we fit it into the broader picture of how Enceladus evolved over time?”