Astrophile: The outermost ocean in the solar system

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Triton's subsurface ocean
Temperature: About -90 °C

A new day dawns on Triton. It's going to be a cold one, much like the last. And the one before that… and every day since the moon settled into its present orbit around Neptune. Even the volcanoes here spew out cold gases and liquid water rather than hot magma. But below the frigid surface, which registers a temperature of -235 °C, there's something more clement: a liquid ocean.

At first glance, Triton seems to be just another icy moon – a featureless, barren world spinning around Neptune, the outermost planet of our solar system. But Triton is different.

For one thing, it orbits Neptune backwards, moving in the opposite direction to Neptune's rotation. It's the only large moon in the solar system to do so. Satellites can't form in these "retrograde" orbits, so Triton must have begun life elsewhere before being captured by the gas giant. It looks a lot like Pluto, and probably came from the same place – the inner edge of the Kuiper Belt, close to Neptune.

The Voyager 2 spacecraft flew past Triton in 1989, sending back images of the moon's frozen surface. They revealed signs of cryovolcanism – the eruption of subsurface liquids which quickly freeze when exposed to the cold of the outer solar system. As such, Triton joins a short list of worlds in the solar system known to be geologically active.

Its surface ice is unique, too: largely composed of nitrogen, with some cantaloupe-textured terrain, and a polar cap of frozen methane.

But with a name like Triton – the messenger of the big sea in Greek mythology – this moon should really carry one more feature: is there an ocean hiding beneath its icy veneer? A new model suggests there could be. Understanding why requires a quick look at Triton's unique history.

We know that Triton was captured by Neptune. Such captured bodies start in highly elongated orbits, but as they interact with their associated planet, Triton-sized worlds are quickly dragged into more circular orbits. The process releases energy, which heats up the moon. The temperature rise would have melted not just the icy outer layers of Triton, but also its 1900-kilometre-wide core. Then it would have cooled to its current frigid state.

Earlier models had suggested an ocean exists on Triton, but they were quite simplistic. Saswata Hier-Majumder of the University of Maryland in College Park, and his student Jodi Gaeman, have now developed a more detailed model that considers both radioactive decay of core minerals and the orbital interactions that would have heated the moon.

Although heating from radioactive decay is orders of magnitude larger than heating from tidal effects, heat from the core alone could not keep the outer layer from freezing over the 4.5 billion-year life of the solar system, they say.

However, Hier-Majumder and Gaeman have found that even a small amount of heating from orbital forces makes a huge difference because it is applied to the base of the ice covering the subsurface ocean. "It puts a warm blanket on top of the cooling ocean," says Hier-Majumder. As long as the orbit is so circular that its 350,000-kilometre-radius varies by only a few kilometres, Triton should still have a substantial ocean beneath its icy surface.

That watery ocean contains a strong dose of ammonia, which keeps the liquid from freezing unless the temperature drops below about -90 °C. So, while it may be the outermost ocean in the solar system, it is not as cold as the
-180 °C hydrocarbon lakes on Saturn's moon Titan.

Journal reference: Icarus, DOI: 10.1016/j.icarus.2012.05.006