The misfit moon that spurts liquid into space NASA/JPL-Caltech

Enceladus’ south pole is wounded, bleeding heat and water. Its injury may have come from a huge rock smashing into this frigid moon of Saturn less than 100 million years ago, leaving the area riddled with leaky cracks.

The region near Enceladus’ south pole marks one of the solar system’s most intriguing mysteries. It spews plumes of liquid from an interior ocean, plus an enormous amount of heat. The south pole’s heat emission is about 10 gigawatts higher than expected – equivalent to the power of 4000 wind turbines running at full capacity. The rest of the moon, though, is cold and relatively homogeneous.

“We don’t have a really good explanation for why all this activity is so concentrated,” says John Spencer at the Southwest Research Institute in Colorado.


On most icy moons and ocean worlds, the main heat source is the tide: the worlds are stretched by the gravity of their parent bodies and neighbours, which causes internal heat.

“If it was heated by tides, the north and south should look the same,” says Angela Stickle at Johns Hopkins University in Maryland. “So the fact that Enceladus’s south has these crazy regions with plumes and heat flows is enigmatic.”

Stickle and her colleague James Roberts used computer simulations to see if the enigma could be explained by a giant impact in the past. They found that Enceladus’s strange appearance could be explained by a blow large enough to cause huge cracks in the ice. This kind of collision would leave the south pole warm and weakened, they told the Lunar and Planetary Science Conference in Texas on 21 March.

Scarring a world

“An impact could set up the conditions to form a terrain like what we see now,” says Stickle. To have the desired effect, the blow would have been powerful enough to punch right through the 20-kilometre-thick ice covering the oceans, but we wouldn’t see a crater today because the ice would begin to refreeze immediately.

By an hour after the impact, the exposed liquid could freeze to a depth of 10 centimetres, starting to rebuild the ice shell.“It heals quickly, but it’ll leave a scar,” says Roberts.

Such an impact would deposit energy into Enceladus’ surface, heating and softening the surrounding ice. It would also cause a shock wave and seismic activity that could rip open the shell.

The impact didn’t even have to happen exactly at the south pole. Because the dent would cause a dip in the local gravity, the moon would rotate so that the crater or hole would gradually migrate toward the pole.

“The impact could have happened anywhere and then Enceladus would have rolled over until the impact point ended up at whichever pole happened to be nearest,” says Francis Nimmo at the University of California Santa Cruz.

The patch of weaker ice, riddled with fractures, ends up at the south pole, leaking water and excess heat while it slowly re-freezes, recovering from its ordeal.