Most people know that the Earth can be divided up into layers, starting with the crust – a skin of hard rock on the surface. Beneath this crust is the mantle; silicate rock which, although solid, moves very slowly over time; convecting and moving heat up to the surface. Beneath this mantle is the core. The Earth’s core is comprised of two parts. The outer core is liquid metal which is spinning away generating the magnetic field for the Earth. The innermost layer is the inner core. A solid lump of iron and nickel.
Mercury, the smallest of the rocky planets (It’s smaller than the moons Titan and Ganymede) but is very dense (denser than both Venus and Mars even though they are bigger). Whilst Mercury is not as dense as the Earth it is much smaller so it doesn’t compress the material inside as much, its high density is best explained by a much bigger iron core and a smaller silicate mantle/crust compared to the Earth. Mercury is also the only other rocky planet other than Earth to have a strong magnetic field, which tells us about the internal composition of the core.
The magnetic and gravity data from Nasa’s MESSANGER probe suggests that it has a crust of about 50 km thick, beneath which there is a mantle, and beneath that we know there is a core at the centre, as shown by the strong magnetic field and the high density.
This core, like Earth, will likely have a solid inner part and then a liquid outer part, made of iron & nickel. As the core cools it solidifies in the middle and iron and nickel is removed from the liquid part, causing a relative increase in the other elements such as sulphur, silica, and oxygen left in the liquid. Depending on the composition this mix of different elements may become like oil and water. A sulphur – iron mix separates out from the rest. This sulphur mix is less dense than the rest and floats up to the core-mantle boundary.
The iron-sulphur mix solidifies and forms a hard layer on the underside of the mantle – this is the anti-crust, a thick layer of iron sulphide separating the liquid outer core from the silica-rich mantle.
Gravity studies of the planet made by looking at small changes in the MESSENGER spacecraft as it orbited suggest that this layer may be present, however, there are a lot of variables to the models used to estimate this. Whilst on Earth seismic studies could be used to verify these, there are no plans to land a probe on Mercury anytime soon to undertake such tests, slight changes in the amounts of Sulpher and other elements could have prevented the formation of the anti-crust.
Whilst the smallest planet in our solar system Mercury has many similarities to that of Earth, the anti-crust is an extra layer and an interesting artefact from cooling worlds.