Pluto's icy exterior may conceal an ocean

PLUTO could hide a liquid ocean beneath its icy shell. Indeed, other bodies on the solar system's frigid fringe could also harbour subsurface oceans, and these could provide the conditions to sustain life.

Temperatures on Pluto's surface hover around -230 °C, but researchers have long wondered whether the dwarf planet might boast enough internal heat to sustain a liquid ocean under its icy exterior.

Now Guillaume Robuchon and Francis Nimmo at the University of California, Santa Cruz, say there is a good chance it does. They calculate that an ocean depends on two things: the amount of radioactive potassium in Pluto's rocky core, and the sloshiness of the ice that covers it.

Density measurements suggest a rocky core fills 40 per cent of the dwarf planet's volume. If the core contains potassium at a concentration of 75 parts per billion, its decay could produce enough heat to melt some of the overlying ice, which is made of a mixture of nitrogen and water.

It should have at least that much potassium and probably more, says William McKinnon at Washington University in St Louis, Missouri. He points out that Earth, which probably formed with less of the volatile element due to its closer distance to the sun, has 10 times that concentration in its core.

But merely having a source of heat is not enough to maintain a long-lived ocean. Heat from the core will trigger convection in the surrounding ice, and if the ice churns too quickly, the heat will simply escape into space before it can do much melting.

If it flows substantially more slowly than Antarctic glaciers on Earth, however, then the top 165 kilometres of ice could provide enough insulation for a liquid ocean of the same depth to exist below, the team calculates (Icarus, DOI: 10.1016/j.icarus.2011.08.015).

The viscosity of the ice depends on the size of individual ice particles, with smaller grains flowing more easily. There is no way to measure this from Earth, but Pluto's shape could reveal evidence of an ocean, the team says. 

Pluto's spin is slowing down due to tugs from its large moon Charon. Fast-spinning objects bulge out at their equator, but a soft interior would allow the world to relax into more of a sphere as its spin slows down. NASA's New Horizons probe will image the dwarf planet's shape when it flies past in 2015.

Other distant icy bodies might also have oceans, which could mean that the outer solar system is potentially ripe for life. "It's very exciting to think that these dwarf planets could have astrobiological potential," says New Horizons lead scientist Alan Stern.

NASA Cassini Double Play: Enceladus And Titan

On the left, Saturn's moon Enceladus is backlit by the sun, showing the fountain-like sources of the fine spray of material that towers over the south polar region.

On the right, is a composite image of Titan. Image credit: NASA/JPL/SSI and NASA/JPL/University of Arizona

About a month and a half after its last double flyby, NASA's Cassini spacecraft will be turning another double play this week, visiting the geyser moon Enceladus and the hazy moon Titan.

The alignment of the moons means that Cassini can catch glimpses of these two contrasting worlds within less than 48 hours, with no detours in between.

Cassini will make its closest approach to Enceladus late at night on May 17 Pacific time, which is in the early hours of May 18 UTC. The spacecraft will pass within about 435 kilometers (270 miles) of the moon's surface.

The main scientific goal at Enceladus will be to watch the sun play peekaboo behind the water-rich plume emanating from the moon's south polar region. Scientists using the ultraviolet imaging spectrograph will be able to use the flickering light to measure whether there is molecular nitrogen in the plume.

Ammonia has already been detected in the plume and scientists know heat can decompose ammonia into nitrogen molecules. Determining the amount of molecular nitrogen in the plume will give scientists clues about thermal processing in the moon's interior.

The second of Cassini's two flybys is an encounter with Titan. The closest approach will take place in the late evening May 19 Pacific time, which is in the early hours of May 20 UTC. The spacecraft will fly to within 1,400 kilometers (750 miles) of the surface.

Cassini will primarily be doing radio science during this pass to detect the subtle variations in the gravitational tug on the spacecraft by Titan, which is 25 percent larger in volume than the planet Mercury.

Analyzing the data will help scientists learn whether Titan has a liquid ocean under its surface and get a better picture of its internal structure. The composite infrared spectrometer will also get its southernmost pass for thermal data to fill out its temperature map of the smoggy moon.

Cassini has made four previous double flybys and one more is planned in the years ahead.