How did Mercury get so dense?

I’ve done several radio interviews in the past few weeks on the NASA Messenger mission to Mercury. One question that keeps popping up is – ‘How did Mercury get so dense?”. It’s a great question and one that is sometimes difficult to answer, fully, in a few seconds on air so I thought I would take some time to explore it in a post on the site.

Mercury is a tiny planet, some 4879km across, not much bigger than our own Moon. Its density of 5.3 grams for every cubic centimetre makes it the densest of all the terrestrial planets (those are Venus, Earth, Mercury and Mars) but also one of the most intriguing. The Mariner 10 probe which studied Mercury during the mid seventies mapped not quite half of the planet but it did give us an insight into a peculiar little world with some fascinating geology. Fascination that runs much deeper than the cratered, volcanic and battered surface that we see in the latest Messenger images. Measurements of Mercury’s density infer that much of the planet must be made of a iron-rich core that accounts for 60% of the planet’s mass and an incredible 75% of its radius! But why is it so dense? At the moment scientists just aren’t sure, which is just one reason why Messenger and other missions are going to be scrutinizing Mercury over the next few years. But we do have some ideas.

One theory argues that the bright, extremely hot young Sun played a role in vaporising away rocky material on the surface of Mercury as it was in the process of forming. This would mean that Mercury would be predominantly made of metallic material that had (by then) formed a substantial core; any elements and chemicals that could be easily vaporised and blown away would be notably absent or depleted in the Mercurial geology.

Another suggestion, according to NASA scientists, says that when the Sun and all the planets were forming in the solar nebula (the great cloud of dust and gas around the young Sun), tremendous amounts of gas could have had an important role in deciding what materials Mercury formed out of. To understand this let’s look at a simplified view. Rocky particles (silicates) in the solar nebula, when the planets were forming, were much lighter than their metallic counterparts. As such the gas around our star was able to whisk the lighter particles away and into the Sun, effectively sieving out the lighter silicates in the material from which Mercury went onto form. What was left was a metal rich cloud from which Mercury formed.

Lastly then and perhaps the most exciting idea is that Mercury was smashed into by a large body after its formation. This scenario requires Mercury to have already formed in much the same was as the normal terrestrial planets. The layers in Mercury’s internal structure would be fairly well defined by this stage rather than a mix of all sorts of material. It would have a metallic core with a rocky (silicate) crust and mantle (similar to the Earth’s structure). The theory says that a large body (like a rogue proto-planet) might have collided with the young Mercury blasting away its crust (and much of its mantle) leaving the mostly metallic material at its centre. (Think of it as smashing a boiled egg so that the shell and the white fly away leaving a little yellow core.)

So the answer to the question of ‘How did Mercury get so dense?’ is…we just don’t know yet. Nevertheless Messenger and the handful of other European and Japanese missions to Mercury will be more than equipped with their instruments to try and find out by studying Mercury’s magnetic field, its surface geology and composition.

To find more about the Messenger mission go to the NASA website.

Above: Mercury in colour imaged by Messenger
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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