Planetary Systems by Raymond T. Pierrehumbert

Author:Raymond T. Pierrehumbert [Pierrehumbert, Raymond T.]
Language: eng
Format: epub
ISBN: 9780192577948
Publisher: OUP Oxford
Published: 2021-10-09T00:00:00+00:00

Core segregation: a special role for iron

In the era of exoplanets, astronomers are learning the importance of distinguishing, among the things they have traditionally called ‘metals’, between actual metals and rocks, which are mostly silicates. The importance of the distinction has long been recognized in the Earth science disciplines. Among true metals, iron plays a special role, because of its anomalously high abundance.

When a planetesimal becomes big enough to have significant self-gravity, heavier constituents can sink to the centre, leading to chemical differentiation of the body. This can happen both in the planetesimal stage and when planetesimals aggregate by collision to form planets, and it is generally thought that efficient differentiation requires the body to be in a molten state—i.e. that it has a deep magma ocean. Magma oceans can be formed from the heat of collision, or from the heat released by decay of short-lived radioactive elements. Some iron goes into silicate minerals, but much of it forms dense metallic iron droplets that sink to the centre of the body and form a metallic core. This has happened in all of the rocky bodies of the inner Solar System, including the Moon.

When a metallic iron core segregates near the centre of a planet, it leaves behind a surrounding layer of silicate minerals known as the silicate mantle. The minerals that make up the mantle are partly, although not completely, depleted in iron. Formation of a metallic iron core significantly increases the average density of a planet. Iron has high atomic mass, but is also a small atom so it takes up a lot less space when it is stripped out of a silicate molecule and packed into a volume of pure iron. For example, consider the family of minerals called olivines, which make up most of the Earth’s upper silicate mantle. The family has two end members, Mg2SiO4, called forsterite, which has a density of 3,300kg/m3, and Fe2SiO4, called fayalite, which has a density of 4,400kg/m3. Metallic iron, with a density of 7,800kg/m3, is much denser than either. If a mix of metallic Mg with fayalite is chemically transformed into a mix of metallic Fe and forsterite, the density of the mixture increases from 3,400kg/m3 to 4,400kg/m3. All these densities are given for the solid form at room temperature and low pressure, so in an actual planet they need to be adjusted for density differences in the liquid phase, and due to compression in the planetary interior, but the general trend remains valid.

The chemical effects of core segregation are even more important than its effect on density. In particular, iron loves to react with oxygen, depriving other molecules of their chance to pair up with oxygen. Thus, if there is a lot of iron around in the silicate mantle, non-oxidized species such as H2 and CH4 tend to be outgassed into the atmosphere by volcanism, whereas if metallic iron has segregated into a core the outgassing has a higher proportion of oxidized species, such as CO2. In addition, significant

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