Continuing my end-of-year research article listicle of favorite research articles from 2014:
- Low Core-Mantle Boundary Temperature Inferred from the Solidus of Pyrolite.
The most abundant element in the universe is hydrogen. Without it, there could be no water, no proteins, and no DNA — so it’s essential for biochemistry too. And yet — amazingly — no one knows how much hydrogen exists on Earth. The major uncertainty is the abundance of hydrogen deep in the Earth, in the lower mantle and the core. Some folks think there’s a lot of it, while some others think there’s very little. If it turns out the Earth’s core contained 1% hydrogen (H) atoms by weight, then the total hydrogen on Earth would be at least 100 times larger than all the H in Earth’s oceans. But if there is no hydrogen in the core and lower mantle, the oceans may well be the largest H reservoir. So we’re talking big error bars here.
Samples of the lower mantle and core would end the debate, of course, but they are presently (and sadly) unavailable. The best geologists can do is to make inferences from seismic and gravitational data. The inferred materials properties of the deep Earth — which depths are solid, which are liquid, what depths which speeds of sound, etc. — are fairly reliable, but the exact materials compositions that give rise to those properties is a tougher nut to crack.
That’s where laboratory experiments and advanced computational chemistry comes in. This paper from a hydrogen-is-probabaly-in-there camp uses advances in high-pressure, high-temperature materials synthesis. What that actually means if you strip away the jargon is that they compressed samples in a diamond-anvil to pressures exceeding one million times the Earth’s atmospheric pressure (169 GPa) while shooting said sample with a laser to heat it to 3900 Kelvins (that’s 6500 °F!). We simply don’t know much about chemistry in those conditions, but thanks to this paper, we know a bit more. The authors determined that pyrolite, the primary component of the mantle, partially melted, forming a molten iron-rich phase, at lower temperatures than previously estimated — a mere 3500 K instead of 4000 ro so. A complex line of geochemical inference leads them to the conclusion that there may well be a lot of hydrogen in the Earth’s core: A large amount of H may have been incorporated into metals from a hydrous magma ocean at the time of core formation.
This is a great example of how science advances: advances in synthesis and measurement technology allow laboratory experiments to test experimental questions (at what temperatures does pyrolite melt?) that are seemingly unconnected to more interesting general questions (how much hydrogen is on Earth?). But accumulated scientific theories often connect these questions in testable ways. New measurements in one area can have profound implications for many others.