Webbles—and Mercury—wobble but they don’t fall down

By William Atkins
Friday, 04 May 2007 08:57

Science - Space

The time difference in the oscillations of the radio waves when they reached the two Earth telescopes showed that Mercury’s outer core is, at least, in a partially molten state.

Led by Jean-Luc Margot, of Cornell University (Ithaca, New York), a team of U.S. and Russian researchers used measurements from radar speckle patterns to find the spin rate of Mercury. They discovered that Mercury wobbles too much to have a solid core.

In such a molten state, Mercury wobbles as it revolves on its axis to such a degree that would prevent it from having a completely solid core. Consequently, scientists now contend that Mercury has only a 5 to 10% chance of having a solid core.

Their conclusions, under the authorship of Margot, S.J. Peale, R.F. Jurgens, M.A. Slade, and I.V. Holin—titled “Large Longitude Libration of Mercury Reveals a Molten Core”—are found in the Friday, May 4, 2007 issue of Science (volume 316. number 5825, pages 710–714).

Scientists prior to 1974 thought that the closest planet to the Sun and the smallest planet in our solar system had a solid iron core. However, in that year, NASA’s Mariner 10 space probe found that Mercury has a weak magnetic field after measuring Mercury’s atmosphere, surface, and environment. At that time, scientists conjectured that such a weakened magnetic field should be associated with planets with molten cores.

For 33 years, scientists could not prove that Mercury had a molten core. However, the results of the Margot team in 2007 have found that the oscillatory measurements of the radar waves were twice what should be expected for a completely solid body.

The planet Mercury is similar in appearance to the Earth’s Moon—being heavily cratered on its surface. It has only a tenuous atmosphere and a large iron core (possibly as much as 75 to 80% of the radius of the planet), which generates a magnetic field about 1/100th as strong as the magnetic field of the Earth. Still, the planet is the second densest planet in the solar system—a property that makes its evolution still a mystery to scientists.

The conclusions from the Margaot team will help to form the developing theory behind the planet’s history and the origin of its magnetic field. Knowledge about the internal properties and thermal evolution of Mercury and other such planets should help scientists as they research and develop their theories on how habitable worlds, such as the Earth, would form and evolve.