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Enceladus has fracture lines running across its surface, seen here in blue. 


NASA/JPL-Caltech/Space Science Institute

The giant “tiger stripes” on Saturn’s Enceladus, a moon that hides a liquid ocean under its icy shell, have been something of a mystery to scientists. But now they have a possible explanation. 

In a new study, published Monday in the journal Nature Astronomy, a team of researchers from UC Berkeley, UC Davis and the Carnegie Institution for Science, examined Enceladus’ icy scars using mathematical models to determine the physical forces allowing the unusual stripes to form. The four fissures only appear in Enceladus’ southern hemisphere, and they’re evenly spaced, as if the planet’s been scratched by a giant claw. 

“These stripes are like nothing else known in our solar system,” said Doug Hemingway, astronomer at the Carnegie Institution for Science and lead author on the paper. “No other icy planets or moons have anything quite like them.”

NASA’s Cassini probe, which spent 13 years orbiting Saturn and its moons, discovered that Saturn’s sixth-largest moon is constantly spewing water and organic compounds into space, making it a “tantalizing” site to search for signs of life. The probe also discovered that Enceladus’ south pole contains almost parallel “tiger stripes,” or large fractures, over 80 miles (130 kilometers) long, that run along the icy surface. 

Enceladus’ poles experience the most heating of any area on the moon because of gravitational forces pulling on the moon as it orbits Saturn. The energy keeps the planet from completely freezing over and also means those regions contain the thinnest ice sheet across the entire moon. The researchers suggest that heating and cooling causes pressure to build underneath the shell before, eventually, Enceladus cracks open. 

Notably, this could occur in the north or south pole, according to the modeling, but by chance it occurred in the south. Once the crack appeared, the pressure that had built up was relieved — and no further cracks could be formed in this way.

However, the team hypothesizes that one of the larger fractures, known as the Baghdad Sulcus, was the first to form, which caused further stresses in Enceladus’ icy shell, generating further fracturing in a distinct but related manner. As water ice was ejected from Baghdad, it began to place more pressure on the thin ice sheet, causing it to bend in such a way that it set off another fracture 22 miles away.

“Our modeling of the physical effects experienced by the moon’s icy shell points to a potentially unique sequence of events and processes that could allow for these distinctive stripes to exist,” said Hemingway.   

Originally published Dec. 9, 8:15 a.m. PT.