May 7, 2007
CHICAGO, Illinois (Reuters) -- Using a fancy version of a common chef's trick, scientists have discovered that Mercury's core may be partially molten, making it a little more Earth-like than once thought.
Chefs can tell whether an egg is hard-boiled or raw by spinning it on a countertop. A raw egg will wobble.
So apparently does Mercury.
"Essentially, our radar observations have shown that the innermost planet Mercury has a molten core," said Cornell University's Jean-Luc Margot, who led the research published in the journal Science on Thursday.
Using radar and telescopes, Margot's team measured the spin rate of Mercury and discovered that it wobbles too much to have a solid core.
"That is a surprise in the sense that Mercury is so small that most researchers had expected it to have cooled off and solidified by now. The molten core indicates otherwise," Margot said in a telephone interview.
For a long time, scientists had thought Mercury's core was made of solid iron. But NASA's Mariner 10 mission in 1974 discovered that the planet has a weak magnetic field, something that may be associated with a molten core.
That stirred up a decades-long debate about Mercury's core.
Margot said the idea of measuring Mercury's spin to determine the composition of its core was proposed years ago. But it was thought to be too difficult to get the precise measurements needed.
"Nobody expected you could do this from the ground," he said.
"It was thought that the oscillations would be so small they would not be detected until you had an actual lander on the surface to measure that."
Margot and colleagues devised a way to get around that using a strategy he compares to following points of light bouncing off a spinning disco ball.
Instead of light, the team used powerful radio waves and measured the time it took them to bounce off Mercury and travel between two ground-based radio telescopes in the United States -- one in California and one in West Virginia. The time between the two points would be a measure of Mercury's spin.
The strategy involved 21 precisely timed measurements because Earth and Mercury are only in the needed alignment for periods of 20 seconds at a time.
What they found was that the oscillations were twice what they would expect for a completely solid body.
Based on those calculations, the scientists are now 90 to 95 percent sure Mercury's core is at least partially molten.
Figuring out the interior properties and the thermal evolution of Mercury will be helpful in understanding how habitable worlds -- planets like Earth -- form and evolve, Margot said.
"Mercury is an extreme planet because it is made primarily of iron. The core occupies maybe 75 or 80 percent of the radius of the planet," he said.
"Why Mercury is so dense remains one of the most mysterious questions about the terrestrial planets. If we can figure out the answer to that question, we can better understand how the planets are put together."