¿ìè¶ÌÊÓƵ

TWO teams are racing to reach the core of Jupiter. Not by actually boring into the giant planet, but by squeezing hydrogen to incredibly high pressures to simulate what’s it’s like there. The results could tell us the maximum possible size of a planet like Jupiter—if only the two teams could agree on what they mean.

Jupiter is made mainly of hydrogen, and in the centre of the planet it is crushed by pressures millions of times the air pressure on Earth. Under these conditions, physicists reckon hydrogen should behave like a liquid metal. But no one has been able to investigate what this material will be like because researchers had only managed to compress hydrogen up to a few hundred thousand atmospheres until recently.

Now a team at the Lawrence Livermore National Laboratory in California, and a rival group at Sandia National Laboratories in Albuquerque, New Mexico, have squashed deuterium to millions of atmospheres. Deuterium is chemically identical to hydrogen but denser, making it easier to generate high-pressure shock waves in. At Sandia, Marcus Knudsen’s team used an intense magnetic field to accelerate metal plates and smash them into a deuterium target at 20 kilometres per second. At Livermore, Robert Cauble and his colleagues compressed their deuterium using a shock wave generated by the super-powerful Nova laser.

Both groups reported their results this week at the American Physical Society meeting in Albuquerque. They found that the deuterium began to turn shiny, a sign it was becoming metallic. To their surprise they found that this happened only gradually, suggesting that there is no hard boundary to Jupiter’s metallic core. Many theorists had predicted a sudden phase change into the metallic form.

But the two teams disagree about what happens within the core. Researchers at Livermore say that at the pressures they reached, the metallic deuterium’s density was twice that claimed by the Sandia team. Cauble says this leads to a 15 per cent disagreement about how large and dense Jupiter’s metallic core is.

It would also affect how big a planet can possibly be. The denser hydrogen becomes when compressed, the easier it is for nuclear fusion to start, turning a planet into a star.

Deciding who is right may have to wait for future space missions, which could reveal the size of Jupiter’s metallic core via precise measurements of its gravitational field.

It’s possible that two groups are measuring slightly different things. The Sandia method compresses deuterium for about five times as long as the Livermore team. This should mean their measurements more closely reflect what is happening in Jupiter’s core, where the gas is in equilibrium.

But Cauble thinks the Livermore results could still be relevant, because planetary cores aren’t always in equilibrium. In 1994, Comet Shoemaker-Levy crashed into Jupiter, causing shock waves that reverberated around the whole planet. They could have disturbed the core, he thinks, perhaps briefly increasing its size.