Even though we’ve ventured far into the Solar System and our telescopes tell us a lot about distant stars and galaxies, the inner workings of our own planet have remained shrouded in mystery. Much of our understanding of what it’s like underground comes from what are basically educated guesses, because we can’t really send probes down to the center of the Earth. There are some inconsistencies with these models, like why are there certain elements in certain proportions in the planet’s crust and below. Another great problem is the fact that we don’t understand how the Earth’s magnetic field, one of our planet’s most remarkable features, works. Recently, however, scientists from Oxford University have proposed a possible explanation – and it involves a massive collision between our planet in its youth and a smaller cosmic object with similar properties to those of Mercury.
Like all the objects in our Solar System, the Earth formed from a huge molecular cloud which spanned this area of space about 4.6 billion years ago. The early Solar System comprised a young Sun and probably hundreds of planets, which either merged with one another or were destroyed by the gas giants like Jupiter. A lot of what we know about the composition of these celestial objects comes from the study of meteors called chondrites, which have underwent little change since the formation of the Solar System billions of years ago. It is believed these kinds of rocks clumped together under the effect of gravity to create the rocky planets, such as our own.
There is, however, recent data which isn’t entirely consistent with this model, namely there’s a shortage of neodymium relative to samarium (two elements with magnetic properties) in the Earth’s crust and mantle compared to what is found in chondrites. This seems like a rather small and innocuous anomaly, nevertheless it’s something which puzzles scientists, who haven’t managed to come up with a definitive explanation for it. The key, according to Oxford University geochemists Bernard J. Wood and Anke Wohlers, who have recently published a study in the journal Nature, might be sulfur, which made its way into our planet’s crust after it “consumed” a Mercury-like planet the size of Mars early in its lifetime.
The closest planet to the Sun, Mercury is a hellish world which is, quite appropriately, rich in sulfur. If an object with a similar composition had smashed into the Earth billions of years ago, the resulting iron sulfide would have mixed with the neodymium, and subsequently sink together to the core of the planet, leaving relatively more samarium (which is more attracted to the silicate rock found in the crust and mantle and thus less likely to sink) behind in the upper layers. To test this hypothesis, scientists added sulfur to a mixture of elements similar in composition to the primitive Earth, then subjected the sample to the conditions thought to have occurred at that time: temperatures between 1,400 and 1,640 °C (2,550 and 3,000 °F) and pressures of 1.5 gigapascals (150,000 times greater than atmospheric pressure at sea level today!). After adding sufficient quantities of sulfur, neodymium sunk to the core of the model planet, which is consistent with the theory.
This could also help explain the origin of the Earth’s magnetic field. The planet’s inner core is mainly made up of iron which is extremely hot (about 5,700 °C or 10,300 °F), however due to the huge pressure at that depth it remains solid. Higher up, where the pressure isn’t as high, there’s an outer core of molten iron. Convection currents, as well as the Coriolis force cause the flow of torrents of liquid iron, generating electric currents, which then produce magnetic fields. How the core has remained molten for so long is another mystery, which could be explained by uranium (which generates heat through nuclear decay) also sinking together with the iron sulfide.
If a catastrophic collision with a Mercury-like, Mars-sized object sounds a bit far-fetched, it shouldn’t. Remember there were perhaps hundreds of objects of various sizes and compositions in the young Solar System, which were constantly jostling for position. It’s not inconceivable that many of them collided, getting obliterated and forming new celestial bodies or merging together. In fact, one of the most widely accepted theories regarding the Moon’s formation holds that the Earth collided with a planet similar in size to Mars we now call Theia, and the debris resulting from the crash clumped together to form our sole natural satellite.
Our magnetic field is extremely important for life on Earth. It protects us from potentially devastating solar winds and cosmic rays, which would otherwise blow away the upper parts of the atmosphere, and it also helps birds, turtles, and even humans navigate. You could argue we wouldn’t be here if it didn’t exist. Although the collision with a Mercury-like object hypothesis is still far from being confirmed, it’s still interesting to think life on Earth was made possible by a cataclysmic event early in our planet’s lifetime.