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American Association for the Advancement of Science

Mercury's core issues

The inside of the tiny planet Mercury has long been a mystery, but scientists have now begun to solve it. A new study shows that the Mercury's core is at least partially liquid, or "molten," just like Earth's core is.

These findings appear in the 4 May issue of the journal Science.

Mercury's mass is about 5 percent of Earth's. It's so tiny that researchers have generally believed that it would be too cool to have a molten core. (The planets were very hot when they first formed, but the smaller ones would be expected to cool first, just like baked potatoes out of the oven.)

About 30 years ago, researchers were surprised when the Mariner 10 spacecraft flew by Mercury and detected a magnetic field within the planet. A magnetic field is generally the sign of a molten core.

Venus does not have a magnetic field, and ancient magnetic fields of Mars and the Earth's Moon are no longer active. This made Mercury's magnetic field even more mysterious.

In the study, Jean-Luc Margot of Cornell University and his colleagues used a technique called "radar speckle interferometry," which involved bouncing a radar signal off Mercury and analyzing the changes in the signal when it came back. Based on what they learned about the planet's motion, the authors suggest that Mercury's mantle is behaving independently of a core that is at least partially molten.

It's very exciting to researchers that they were able to make this discovery with instruments on the ground. Soon, they'll have a chance to learn more about Mercury, from up close. NASA's MESSENGER spacecraft should be orbiting around the planet by 2011. Soon after that, the BepiColombo mission, sponsored by the European and Japanese space agencies, will also send spacecraft into orbits that cross Mercury's path.