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Mercury is the second most dense planet in the Solar System. Its density can be used to infer many details about the core of Mercury and the planet’s overall composition.
Earth is the only planet that is more dense than Mercury. It is larger, but its density is gained mainly from gravitational compression, especially at the core. Mercury is smaller and does not feature as intense a gravitational compression as Earth, so it must have a core that is much larger in order to compensate.
Scientists wanted to confirm the theory that Mercury had a large core and wanted to know if it was molten or not, so, in 2007, they bounced radio signals off of the Mercurian surface. They were able to confirm that the core was liquid iron. What scientists did was to time how long it took for radio signals to bounce of Mercury. A tell-tale wobble in Mercury’s rotation exactly matched what should be have existed if Mercury had a liquid core.
The core of Mercury accounts for 42% of the planet’s volume. Here on Earth, the core takes up a much smaller percentage. Only accounting for 17% or the planet’s volume. The core is surrounded by about 600 km of mantle(give or take 100 km). Mariner 10 estimated the Mercurian crust covers it all at a thickness that varies between 100 and 300 km.
The high iron content of the core of Mercury is rather unusual and scientists have postulated many explanations. The most widely accepted of these is that Mercury originally had a metal-silicate ratio similar to common meteorites, as is typical of rocky matter in our Solar System. It is thought to have had a mass approximately 2.25 times its current mass, but, early in the Solar System’s history, Mercury was struck by a planetesimal that was about 1/6 its mass and several hundred kilometers in diameter. The impact would have stripped away much of the original crust and mantle, leaving the core as a large percentage of the planet.
The core of Mercury had been a mystery for hundreds of years. The Mariner 10 flyby in 1974 shed very little light on the issue. Radio waves helped somewhat in 2007, but many scientists are hoping for a clearer picture from the MESSENGER spacecraft and others look forward to data from the BepiColumbo mission that will be launched in 2014.
We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed.
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.