New measurements applied kitchen physics to the boiling planet

May 4, 2007 08:40 GMT  ·  By

What does kitchen physics have to do with astronomical measurements? Well...everything.

Pop quiz: You have two eggs on the kitchen table. How do you know which one is boiled and which one is raw? Easy: you spin them and the raw egg will spin slower that the hard-boiled one.

A team of scientists led by Jean-Luc Margot at Cornell University has applied the same reasoning to the smallest planet of the solar system, Mercury and found out that it most definitely must have a liquid core.

To achieve this result, they measured small twists in the planet's rotation by using a new technique that used a radio signal sent from a ground telescope in California that bounced off the planet and was then caught again in West Virginia.

It took them 5 years and 21 repetitions of the radio signals, but it paid off, as they are now firmly convinced that the values they obtained are twice as large as what would be expected if Mercury's core was solid. So, the core of the small planet must be liquid, which would explain its rotation speed, just like in the case of the raw egg.

"The variations in Mercury's spin rate that we measured are best explained by a core that is at least partially molten," Margot said. "We have a 95 percent confidence level in this conclusion."

Mercury is the smallest and innermost planet of our solar system and its yearly trip around the Sun takes 88 days. Although similar to the Moon in physical appearance due to the large number of surface crates, it has no natural satellites, no substantial atmosphere and was previously thought to have a solid iron core that generated a magnetic field 0.1 percent as strong as that of the Earth.

It never stood a chance to form life, as the surface temperatures range from about 90 to 700 K (−180 to 430 C, −292 to 806 F).

The reason why its core has now been calculated as being made of molten iron is thought to be the fact that sulfur or some other light element got mixed with Mercury's iron core when the planet was forming and lowered its melting temperature.

"If you had such a lighter element polluting the iron, it could explain why the core has remained fluid up to the present time," said Margot.

"The surprise," Margot added, "is that you don't expect sulfur to condense out at the distance of Mercury from the Sun."

It is hoped that the mysteries still surrounding Mercury's core will be solved when NASA's Messenger spacecraft will make its first flyby of the planet in 2008.