Did magnetism make Mercury?

The Sun’s magnetic field may be behind Mercury’s mysterious core

ercury is the smallest planet in our Solar System, as well as being the closest to the Sun and the one with the most elliptical orbit. In some respects it is also the most mysterious. For such a small planet, it’s fabulously dense, so much so that its heavy iron core is estimated to make up around two-thirds of its mass. Proportionally it’s over twice as big as the cores of the other rocky, terrestrial planets, Earth, Venus and Mars.

All terrestrial planets are made up of an iron-rich core with an overlying silicate-rich mantle and crust. But what gave Mercury such a giant heart? Computer simulations of the early Solar System, modelling how small planetesimals collide and aggregate to grow into planets, fail to show how an inner world like Mercury could ever form with such a large iron core.

Several explanations have been put forward. Perhaps Mercury didn’t in fact form with such a swollen centre, but suffered the loss of most of its original silicate-rich mantle early in its history, leaving behind a planet with a disproportionately large core. is an astrobiologist at the University of Westminster and author of

(geni.us/origins) Gerhard Wurm. ZLWK VWURQJ PDJQHWLF HOGV IURP WKH IRUPLQJ VWDU ,Q WKH UVW VWDJH RI SODQHW EXLOGLQJ GXVW SDUWLFOHV QHHG to collide and combine together into bigger and bigger aggregates. But once aggregates get to a certain size they tend to bounce off each other, which hampers the growth process. What the researchers suggest is WKDW D VXUURXQGLQJ PDJQHWLF HOG ZRXOG KHOS LURQ containing particles to stick together, and so planetesimals forming in the innermost region of protoplanetary discs would tend to become more iron-rich and build Mercury-like planets.

They have tested their idea with experiments: RDWLQJ GXVW JUDLQV RI GLIIHUHQW LURQ VLOLFDWH compositions to watch them collide and aggregate with each other, all while surrounded by a magnetic HOG $QG LQGHHG WKH\ IRXQG WKDW WKH FOXVWHUV JUHZ PXFK ODUJHU ZLWK D VWURQJHU PDJQHWLF HOG RU KLJKHU ratio between iron and silicate in the particles.

If they’re right, this could go a long way to neatly explaining why Mercury is so dense with a large core.