Unbeknownst to most, Mercury, the closest planet to our Sun (and surprisingly, also the closest to planet Earth), has been hiding a large solid inner iron core beyond its crust and mantle. And this core is no conventional one; it is bigger than anything the norm would usually predict, thus taking up a much larger proportion of the planet. Mercury’s core, therefore, has a diameter of about 4100 kilometers (that is, more than the size of the Moon) against a thickness of only 600 km for the inner mantle and 100-200 km for the outer crust, taking a huge proportion of the planet’s full diameter, but why?

Why Does a Planet Like Mercury Have A Core Bigger Than The Moon? 2
A graphic of Mercury’s internal structure. Credit: Antonio Genova

Mercury’s core’s imposing size

Scientists have long debated the reason behind this imposingly large iron core. The idea that had previously prevailed was that various collisions between Mercury and other free objects during the formation of the Solar System had worn off much of Mercury’s outer crust, thus explaining the large ratio of the iron core to the crust.

However, these explanations have recently been disproved in favor of another hypothesis: the influence of the Sun’s magnetic field. In June 2021, two scientists: William McDonough and Takashi Yoshizaki, published a paper in which they presented a model explaining the influence of the Sun’s magnetic field on the distribution of raw materials as the Solar System was forming. In effect, the density and mass of a planet’s core, as well as the amount of iron it contains, were said to depend on its distance to the Sun’s magnetic field, the first step in explaining Mercury’s core as it finds itself the closest to the Sun.

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The complete explanation lies in the behavior of iron as the planets were forming in the early ages of the Solar System. Indeed, when the Sun was younger, it was surrounded by swirling gas storms and clouds of dust, causing the small grains or particles of iron to be drawn closer to the Sun. The concentration of iron, therefore, grew bigger in areas closer to the Sun. The phenomenon was aided by the Sun’s magnetic field, as iron is a metallic element. Then, planets, or bigger lumps, started to form, and therefore, the lumps closer to the Sun started to incorporate more iron into their core.

A colorful version of Mercury as assembled from spectral data taken with the MESSENGER spacecraft.( Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)
A colorful version of Mercury as assembled from spectral data taken with the MESSENGER spacecraft. (Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Explaining trends in the planets’ cores

Thanks to the research conducted on Mercury’s core, we better understand the reason for the core’s size and the correlation with its iron content. We have therefore now been able to quantify these observations into some developing trends which seem to indicate that, within the first four planets of the Solar System (the four “inner planets”), as the planet moves further away from the Sun, the iron content of the core diminishes.

However, this is not only applicable within the Solar System, and scientists are now starting to use the influence of the magnetic field of the central star in a stellar system to explain planetary composition beyond our Solar System. Indeed, it is not only the distance from the Sun’s magnetic field that will influence the iron content but also the strength of the magnetic field at the moment when the planet was forming. The stronger it was, the more concentrated the iron was at the center, leaving less iron available for the outer planets.

This explanation can be considered a breakthrough or a “big deal,” as the study of planetary cores is significant in explaining their overall rocky compositions: the core dictates how the other elements in the crust, the mantle, or the surface behave. This, in turn, maybe the key to explaining whether or not a planet may be habitable – a popular topic nowadays.

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