The field, discovered during the 1970's by NASA's Mariner 10 spacecraft, is about 100 times weaker than Earth's magnetic field.

Researchers at the University of Illinois and Case Western Reserve University in Cleveland have produced a model of conditions within Mercury's core that give rise to that weak magnetic field.

Although mostly made of iron, Mercury's core also contains sulfur. The scientists subjected an iron-sulfur mixture in a laboratory to pressures and temperatures similar to those expected to exist within Mercury. When they analyzed the mixture with a scanning electron microscope, they found the iron atoms had condensed into flakes.

Their analyses show the iron flakes fall toward the planet's center as the iron-sulfur mixture in the outer core slowly cools. As the iron flakes sink, they are replaced with a lighter, sulfur-rich liquid. The resulting convection currents produce Mercury's weak magnetic field.

"We can now connect the physical state of our innermost planet with the formation and evolution of terrestrial planets in general," said University of Illinois Professor Jie Li.

The study appears in the journal Geophysical Research Letters.