Give me a squeeze
On Europa, fractures in the surface come from tidal stresses. As the moon orbits the planet, it is slowly squeezed and released, breaking apart the surface. But Triton’s orbit is nearly circular, one of the roundest of any moon, so Neptune’s kneading today is limited.
In the past, however, Triton probably wasn’t quite as well-rounded. The moon holds the dubious fame of being the largest object to travel around its planet backwards, in what is known as a retrograde orbit. Its backward orbit suggests that Triton didn’t form around Neptune but is instead a captured object, an icy Kuiper Belt Object from the outer solar system. Its original eccentric orbit would have allowed Neptune to squeeze it more frequently, heating the interior and melting the ice.
“We definitely know Triton had an ocean in its past,” Hurford says. “It’s not clear whether or not that ocean stuck around.”
All that squeezing by Neptune would have caused the crust to split, creating fractures. Over time, as Triton’s orbit became more circular, the tidal stress would have decreased. With a circular orbit, tidal heating would decrease and the ocean would eventually freeze, leaving ancient fractures across the surface.
But the old fractions don’t fit well with the otherwise young outer layer suggested by cratering.
“There’s a disconnect between the age of the fractures and the age of the surface,” Herford says.