The new findings could help explain why Mercury's magnetic field is so weak. Credit: NASA/JHUAPL/Carnegie Institution of Washington

A storm of falling iron particles may be circulating liquid in Mercury’s core, new research suggests. The results may help explain the origin of the planet's magnetic field, which is much weaker than scientists can explain.

A distinct wobble in Mercury’s rotation suggests that the planet has a molten core1. This liquid layer may create a dynamo, similar to the one that sustains Earth’s magnetic field. But ordinary dynamo models generate magnetic fields that are as much as 10,000 times too strong for what has been seen on Mercury.

It turns out that at the right pressures, a mixture of iron and sulphur might create iron 'snow' close to the outer edge of the core. This falling snow could continuously mix the liquid part of the core and generate the planet’s anomalously faint magnetic field, the new study says.

“Just a little bit of sulphur can make this iron snow effect happen,” says Steven Hauck, a geophysicist at Case Western Reserve University in Cleveland, Ohio, and one of the authors of the study2. “If it’s happening, it suggests there are different mechanisms behind driving dynamos.”

Unusual kinks

Researchers have known for some time that Mercury’s core is rich in iron. But the planet has cooled since its formation, so in order for the core to have a molten part, the iron must be mixed with something, such as sulphur, that remains liquid at lower temperatures.

To investigate the properties of iron–sulphur mixtures under conditions similar to those found on Mercury, team members Bin Chen and Jie Li at the University of Illinois at Urbana-Champaign looked at samples of iron sulphide subjected to the pressures usually found in the planet's core.

The team discovered that as the pressure rose, the temperature at which the iron–sulphur mixture became liquid dropped. This is the opposite of the behaviour seen for iron or for sulphur alone. “It was not expected,” says Li. “There are certain observations for iron-sulphide at high pressure, but within Mercury at very low pressure, nobody has measured anything and nobody expected anything there.”

The implications were not immediately clear, but subsequent modelling showed that a mixture of liquid iron and sulphur in the core could cause iron to condense into a solid and fall toward the core. This 'snow effect' would create a unique kind of circulation and leave behind a liquid with an even higher concentration of sulphur.

Similar types of snow have been predicted for Jupiter’s moon Ganymede and for Mars. The researchers say that iron snow could be a part of the core histories of many small objects in the Solar System.

Messenger and beyond

“It is very interesting, very plausible” that Mercury might have this snow, says Sabine Stanley, a planetary physicist at the University of Toronto in Canada. “The question their results really depend on is how much sulphur is in the core.”

Measuring the sulphur content directly is impossible, but the researchers anticipate that results from NASA’s MESSENGER mission, due to arrive in orbit around Mercury in 2011, will help to pin down details such as the size of the planet’s core. This information will put upper limits on the abundance of sulphur that might be possible.

Even if these unusual snows do exist on Mercury, it is not clear whether they will generate a magnetic field with the right strength. Such validation will await new models that can take this snowing into account, says Hauck.