Mercury is the innermost planet in the Solar System. The bulk composition of Mercury consists of approximately 70 percent iron rich material and 30 percent silicate material. Because most of the planet is comprised of dense iron rich material, Mercury is the second densest planet in the Solar System, with Earth being the densest planet. The Earth’s high density is mainly the result of gravitational compression. In contrast, Mercury is smaller than the Earth and is much less gravitationally compressed. Instead, the reason for Mercury’s high density is due to the planet’s enormous iron rich core. Mercury is basically comprised of an enormous iron rich core with a thin overlying mantle of silicate material. If gravitational compression were factored out, Mercury will be about 20 percent denser than the Earth.
Figure 1: A composite image of Mercury captured by NASA’s MESSENGER spacecraft. Credit: Image compiled by Gordan Ugarkovic.
Figure 2: Most recent models place CoRoT-7b and Kepler-10b on a mass-radius diagram close to a composition similar to Mercury. The black triangle on the vertical axis denotes the Earth. Credit: Wagner et al. 2011.
A number of explanations have been proposed to account for Mercury’s unusually large iron rich core. One explanation involves a giant impact which stripped off most of the planet’s silicate mantle. Another explanation is that high temperatures present during the early Solar System caused part of Mercury’s silicate mantle to evaporate off. Both of these explanations involve intense heating of the planet’s surface. However, data from NASA’s MESSENGER spacecraft show that a giant impact or large-scale evaporation is highly unlikely. This is because the measured abundance of potassium in the crust of Mercury is similar to Venus, Earth and Mars. If intense heating of Mercury’s surface had taken place, the abundance of potassium would have been depleted since the potassium would have simply boiled away. Like Mercury, two exoplanets named CoRoT-7b and Kepler-10b are known to be high density planets that could be similar in composition to Mercury. A paper by Wurm et al. (2013) investigates the process of photophoresis which may explain the formation of such dense iron rich planets.
Photophoresis is a process which takes place when there are solid particles embedded within a low pressure gaseous environment such as in a planet-forming disk of material around a young star. The high temperatures from intense stellar radiation in the inner regions of a planet-forming disk of materials means that almost all solid particles have high melting temperatures, and are mainly comprised of iron rich material or silicate material. A solid particle illuminated by stellar radiation has a warm side and a cold side. Photophoresis occurs in the low pressure gaseous environment because gas molecules bounce off the warm side of the solid particle at a higher velocity than off the cold side. This has the effect of transferring a net momentum to the solid particle.
Figure 3: Photophoresis in the free molecular flow regime. Here, interaction with individual molecules which accommodate to the local surface temperature transfers a net momentum to the particle. Credit: Wurm et al. 2013.
Nevertheless, the effect of photophoresis largely depends on the type of material that makes up the solid particle. Iron has a thermal conductivity of over 50 W/mK while silicates have much lower thermal conductivities that are on the order of 1 W/mK. The high thermal conductivity of metals mean that the temperature difference between the illuminated and non-illuminated sides of a solid metallic particle will be small because heat can quickly conduct from the illuminated side to the non-illuminated side. As a result, photophoresis has a much smaller influence on metals (high thermal conductivity) than on silicate material (lower thermal conductivity). This allows photophoresis to transport silicate material outwards while iron rich material and other metals remain in the inner region of the planet-forming disk of material around a young star. By depleting silicate material and leaving behind iron rich material, photophoresis can account for the formation of high density planets like Mercury, CoRoT-7b and Kepler-10b.
Wurm et al. (2013), “Photophoretic separation of metals and silicates: the formation of Mercury like planets and metal depletion in chondritis”, arXiv:1305.0689 [astro-ph.EP]