May 3, 2007 — Tiny Mercury, the innermost planet in the solar system, appears to have a partially liquid core. That new finding may help explain the planet's weak magnetic field, even as it opens more puzzling questions about how the core has maintained a semi-liquid state for billions of years.
Because of its small size — Mercury is 40 percent smaller than Earth — the laws of physics suggest that its core should have cooled and solidified long ago. But an innovative radar technique that measured Mercury's rate of rotation shows distinctive signs that the planet consists of both solid and semi-liquid matter.
"What we've been able to do from the ground is to detect tiny changes in the spin rate of Mercury that indicates the planet has a molten core," Cornell University astronomer Jean-Luc Margot said in an interview with Discovery News.
"The molten core tells us something very important about the planet's body temperature," he added. "A planet the size of Mercury should have cooled off by now. It should have radiated all its heat."
The finding, which is being published in this week's issue of the journal Science, helps explain the weak magnetic field the Mariner 10 probe detected during flybys in the mid-1970s.
One way for a planet to generate a magnetic field is to have a core of liquid or semi-liquid metal, like Earth's.
But the discovery also raises the question of what factors are keeping Mercury's interior hot enough to remain molten.
One possibility is that heat-trapping lighter elements, such as sulphur, are doing the trick. Scientists are stumped, however, as to how those elements could condense in a place so close to the sun.
"The people doing thermal evalution calculations are certainly scratching their heads figuring out what you need to explain a molten core in such a small planet," Margot said.
