New simulation gives Jupiter double-sized core
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 28 November, 2008

New computer simulations, conducted at the scale of individual atoms, say Jupiter has a rocky core surrounded by ice that is more than twice as large as previously thought.

“We performed computer simulations of hydrogen-helium mixtures at high pressure and temperature conditions that occur inside Jupiter. Laboratory experiment cannot reach those extreme pressures yet,” says Professor Burkhard Militzer of the University of California, Berkeley, who calculated the properties of hydrogen and helium for temperature, density and pressure at the surface all the way to the planet's centre. Combined with known data for the planet’s mass, radius, surface temperature, gravity and equatorial bulge, co-author William Hubbard of the University of Arizona's Lunar and Planetary Laboratory used the theoretical data to build a new model for Jupiter's interior.

According to new simulations, Jupiter's core is twice as massive as originally believed. Image: NASA/R.J.Hall.

The new model suggests that Jupiter's core is an Earthlike rock 14 to 18 times the mass of Earth, equivalent to about one-twentieth of Jupiter's total mass, and with a metallic ball or iron and nickel at the centre. Previous models predicted a much smaller core of only seven Earth masses, or no core at all. The simulations also suggest that the core is made of layers of metals, rocks and ices of methane, ammonia and water, while above it is an atmosphere of mostly hydrogen and helium.

"Our simulations show there is a big rocky object in the centre
surrounded by an ice layer and hardly any ice elsewhere in the
planet,"says Militzer. "This is a very different result for the
interior structure of Jupiter than other recent models, which predict a relatively small or hardly any core and a mixture of ices throughout the atmosphere."

Militzer explains that hydrogen gradually changes from a molecular fluid in the outer layers to a metallic fluid in the deeper interior, which offers good electrical conductivity and gives rise to Jupiter's magnetic field. The homogeneous mantle is the key difference compared with older models that assume a different composition in the molecular and the metallic layers, giving rise to a smaller core.

"Our simulations show no evidence of any sharp phase transition, which led us to conclude that Jupiter's mantle is homogeneous in composition," he tells Astronomy Now. "The uncertainties in the previous models are why and where is there sharp transition and how does this change the chemical composition. No satisfactory explanation has been given, however, this is subject to further research. Our model is simpler because we assume the mantle is homogeneous since it does not make an assumption about a phase change."

Juno will reach Jupiter in 2016 and will make measurements of Jupiter's core. Image: NASA/Juno.

The results are bringing Jupiter's interior in line with that of Saturn, with a Neptune or Uranus at the centre. Neptune and Uranus are known as ice giants because they also appear to have a rocky core surrounded by icy hydrogen and helium, but without the giant gas envelopes of Jupiter and Saturn. The new Jupiter has ices that are concentrated in the outer layer of the core, while only a small amount, around one percent, is mixed in the hydrogen-helium gas envelope that contains 95 percent of the planet's mass.

The new model strongly supports the idea that Jupiter and other gas planets formed through the collision of small rocks that accreted to make a core, capturing a huge atmosphere of hydrogen and helium through its new-found gravitational attraction. "According to the core accretion model, as the original planetary nebula cooled, planetesimals collided and stuck together in a runaway effect that formed planet cores," says Militzer. "If true, this implies that the planets have large cores, which is what the simulation predicts. It is more difficult to make a planet with a small core."

In order to match the observed gravity of Jupiter, Militzer's
simulation also predicts that different parts of Jupiter's interior rotate at different rates. Jupiter can be thought of as a series of concentric cylinders rotating around the planet's spin axis, with the outer cylinders - the equatorial regions - rotating faster than the inner cylinders, in a similar fashion to how the Sun rotates. Future data from NASA's Juno mission, to be launched in 2011 and reaching Jupiter by 2016, will measure the planet's magnetic field and gravity, and provide a check on Militzer and Hubbard’s predictions.

The team also plan to use the new model to simulate other planets' interiors, and to investigate the implications for the formation of planets outside our Solar System.