The Earth is often described as having an iron-nickel alloy core. This is roughly true, about 80% of the Earth’s core is iron and another 15% is nickel. The last 5% is a bit more difficult to work out. When the Earth was a lot more fluid than it is now many of the heavy elements sank as you might expect and so it is logical to believe the core may contain small amounts of mercury, thallium and lead. But despite these heavy elements likely existence the core is still less dense than that of the iron and nickel mix. Methods to examine exactly what is contained in the core that is so light are constantly being developed. This story is particularly interesting because there hasn’t just been a scientific paper published on this topic in the last day, there’s actually been two and they disagree with each other.
It should be clear that there is probably not one sole element whose responsibility it is to keep the core’s weight down. Carbon, oxygen, hydrogen, silicon and sulphur are predicted to each have a part to play. The aim is to find which element contributes the most to the reduced mass and the current options brawling it out are hydrogen and silicon.
Work at the University of Tokyo published in the Nature publications implied that hydrogen was the likely cause. Not only is it the most common element it is also the least dense and so can reduce the core’s density the most for a given volume. Iron, despite not dissolving hydrogen very well at normal pressures, can begin to take on large quantities as pressures rise. By using neutron scattering methods to examine atomic layers it was found that under high temperature and pressure as soon as any local hydrogen containing mineral released it the iron took it up almost instantly. This shows that hydrogen would have been the first element to be absorbed as the Earth formed and so is a likely contender for our weight reducer. Meanwhile a paper released originally at the American Geophysical Union conference last year authored at Tohoku University, also in Japan, and published recently claims that silicon is the likely element. Through the application of Birch’s law (connects the speed of a compressional wave to the atomic weight of any rock it passes through) and experimenting using sound waves in iron compounds the scientists were able to deduce that the most likely mixture was that of iron, nickel and silicon. Of course the brilliant part is that its possible both are wrong and there is in fact a true third option. The brilliance of the scientific method is that these theories will be argued out and the one with superior evidence will win and become the “truth” until an even better idea comes along later.