Scientists have finally explained the "tiger stripes" that cover the surface of the mysterious moon Enceladus.
Saturn's icy moon is one of the most interesting places in the solar system for scientists. It is not only one of the prime candidates for being home to extraterrestrial life – because of the ocean that hides beneath its surface – but also because it remains so mysterious.
One of those mysteries are the four stripes that cover its south pole, which are like nothing else known in our solar system. Now researchers have revealed the physics behind the fissures that open up and spill ocean water out of the icy surface, and create those stripes.
Scientists did not even know that Enceladus had the stripes until recently, when Nasa's Cassini mission flew past Saturn and took in Enceladus as it did.
"First seen by the Cassini mission to Saturn, these stripes are like nothing else known in our Solar System," lead author Hemingway explained. "They are parallel and evenly spaced, about 130 kilometers long and 35 kilometers apart.
"What makes them especially interesting is that they are continually erupting with water ice, even as we speak. No other icy planets or moons have anything quite like them."
Astronomers did not know why the stripes appeared to form only on the southern pole, as well as why they were so evenly spaced across that surface.
"We want to know why the eruptions are located at the south pole as opposed to some other place on Enceladus, how these eruptions can be sustained over long periods of time and finally why these eruptions are emanating from regularly spaced cracks," said Max Rudolph, assistant professor of earth and planetary sciences at the University of California, Davis.
One of the breakthroughs was the realisation that there was nothing special about that south pole. The stripes could have formed at either end of the moon, but it was the south's that opened up first.
The other major breakthrough was the discovery that when the cracks formed in the surface, ocean water spewed out of the gap that was left. That allowed three more cracks to form neatly across the surface.
The discovery was made using numerical modelling, which allowed scientists to understand the forces that act on the icy shell that encases Enceladus.
The tiny moon is pulled by Saturn's gravity, which exert tidal forces on Enceladus. That in turn means it is heated and cooled, and the force is especially strong at the poles.
As that happens, water solidifies into ice beneath the outer shell, the researchers think. That in turn means it expands in volume, pushing up against the ie until it cracks.
Those cracks are kept open because the water underneath is continually sloshed around and so can't freeze shut.
As the water spills out of those cracks, it drops back down as ice, which builds up at the edge of the fissure and weighs it down. That means the ice sheet flexes, and so led to the neat cracks about 20 miles away, the researchers say.