May 8, 2008 -- Scientists have long wondered about how a planet as small as Mercury could have kept its iron core fluid enough to account for the planet's magnetic field. Physics would seem to dictate that the core cooled and solidified long ago.
Now researchers are chewing on another theory: It's snowing iron flakes inside Mercury.
The scenario opens new possibilities for explaining the planet's global magnetic field. Mercury is the only other terrestrial, or rocky-bodied, planet in the solar system besides Earth to have one.
Recent findings from the MESSENGER probe, which made its first pass by Mercury in January, verified that the planet's magnetic field is dipolar, meaning it has north and south polar orientations, like Earth's. Such fields typically are caused by what is known as the dynamo effect, which happens when a conductive molten core rotates.
Most of the dynamo models, however, predict fields thousands of times stronger than those existing on Mercury today. Researchers at the University of Illinois and Case Western Reserve University in Ohio may have found an explanation for the anomaly: A mix of iron and sulfur could be creating flakes of iron snow near the core's outer edge.
The scientists experimented with an iron-sulfur blend to simulate Mercury's core. The mix was compressed, heated, cooled and analyzed. The results were then applied to how the process would work on Mercury.
The researchers determined that as Mercury's outer core cools, iron atoms condense into cubic flakes that sink toward the planet's center. As the iron sinks, the lighter, sulfur-rich liquid deep in the core rises upward, creating a convection current that powers the process and produces the planet's weak magnetic field.
"Our findings provide a new context into which forthcoming observational data from MESSENGER can be placed," said University of Illinois geology professor Jie Li, who co-authored on a paper about the research appearing in the April issue of Geophysical Research Letters.
Ralph McNutt, MESSENGER project scientist, said he finds the theory interesting.
"I think these guys may be on to something," McNutt told Discovery News. "There is certainly something going on that is very different than what is going on at Earth."
Sulfur was fingered long ago as an element that could be keeping Mercury's iron core partly molten, much like salt on a snowy driveway lowers the melting point of ice, turning it liquid even in temperatures below 32 degrees Fahrenheit.
MESSENGER is due back at Mercury in October. After a third flyby in 2009, the probe is scheduled to settle into orbit in March 2011 for a year-long, detailed study.
With regard to whether Mercury's core is snowing iron, McNutt said the probe should be able to provide detailed maps of Mercury's magnetic field, as well as chemical studies to identify sulfur and sulfur-rich deposits on the planet's surface.
"Trying to understand globally how much sulfur we're seeing might be important," McNutt said.
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