Scientists Use Earth-based Instruments to Document Mercury's Molten Core

Diagram showing the interior structure of Mercury. The metallic core extends from the center to a large fraction of the planetary radius. Radar observations show that the core is at least partially molten.

[Image courtesy of Nicolle Ragger Fuller, National Science Foundation]

Scientists using Earth-based instruments have discovered that Mercury's core is partially molten and moving independently from the planet's other layers, according to research in the 4 May issue of Science.

Using a technique called radar speckle interferometry, Jean-Luc Margot, professor of astronomy at Cornell University, and colleagues bounced a radar signal off Mercury, looking for small irregularities, or "speckles," in the return signal. These small irregularities showed that Mercury's mantle has small oscillations that are absent in its core.

"Our measurements indicate that an outer shell participates alone in the librations (oscillations)... require[ing] that the core of Mercury be at least partially molten," wrote the authors.

The interior structure of the solar system's tiniest planet has been a puzzle for astronomers since the surprising detection of an internal magnetic field 30 years ago by the NASA Mariner 10 spacecraft. During its flyby, Mariner 10 estimated Mercury's magnetic field, generally a sign of a molten core, at a strength 1 percent of Earth's.

Mercury has a mass about 5 percent of Earth's and was expected to have cooled to the point where it could not sustain a molten core.

The authors believe this research will open new avenues in planetary physics, showing "the capability to monitor the spins of terrestrial planets with Earth-based radar."

Scientists will be able to learn more about the closest plant to the sun beginning in January 2008, when the NASA MESSENGER spacecraft sends back images from its first Mercury flyby.

In collaboration with the NASA mission, AAAS has published Exploring the Inner Solar System: Expecting the Unexpected, a book designed to inspire student interest in space sciences. K-12 teachers and other educators interested in receiving copies of the book should contact Bob Hirshon, senior project director at AAAS.

Kathy Wren and Benjamin Somers

3 May 2007