Iron ‘snow’ helps maintain Mercury’s magnetic field

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Iron 'snow' helps maintain Mercury’s magnetic field
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: May 12, 2008

Just like snowflakes form in the Earth’s atmosphere and drift to the ground, new scientific evidence suggests that deep inside the planet Mercury, iron “snow” forms and falls toward the centre of the planet, and could be responsible for Mercury’s magnetic field.

Although scientists know from Earth-based radar measurements of Mercury’s rotation that the planet’s iron-rich core is partially molten, in the absence of seismological data little is known about its physical state, and many models struggle to explain Mercury’s weak magnetic field, which is about 100 times weaker than the Earth’s. Scientists do know, however, that the core contains a proportion of sulphur which lowers the melting point of iron, and is thought to play an important role in producing the planet’s magnetic field.

To try and understand the physical state of Mercury’s core, researchers from the University of Illinois and Case Western Reserve University used high pressure equipment – a multi-anvil press – to study the melting behaviour of an iron-sulphur mixture at high pressure and temperature. After each iron-sulphur sample was compressed to a specific pressure and heated to a specific temperature, the sample was then quenched, cut in two, and analysed. “Rapid quenching preserves the sample’s texture, which reveals the separation of the solid and liquid phases, and the sulphur content in each phase,” says Bin Chen of the University of Illinois, who lead the research. “Based on our experimental results, we can infer what is going on in Mercury’s core.”

Professor Jie "Jackie" Li and graduate student Bin Chen have concluded that iron "snow" forms and falls towards the centre of the planet Mercury and could be responsible for the planet's magnetic field. Image: L. Brian Stauffer.

The results imply that as the molten, iron-sulphur mixture in the outer core slowly cools, iron atoms condense into cubic “flakes” that fall toward the planet’s center. As the iron snow sinks and the lighter, sulphur-rich liquid rises, convection currents are created that power the dynamo and produce the planet’s weak magnetic field. The researchers also speculate that Mercury’s core could be precipitating iron snow in two distinct zones, a unique scenario among the terrestrial planets and terrestrial-like moons in our Solar System.

“Mercury’s snowing core opens up new scenarios where convection may originate and generate global magnetic fields,” said University of Illinois professor Jie "Jackie" Li. “Our findings have direct implications for understanding the nature and evolution of Mercury’s core, and those of other planets and moons.” The findings also provide a new context into which forthcoming observational data from NASA’s MESSENGER spacecraft can be placed.