When somebody loses weight, where does the fat go?
This is a simple question as while as the title of an interesting explanatory piece in the British Medical Journal’s Christmas issue. Scientists have known the answer for a about a century. But, in a survey of 50 doctors, 50 dieticians, and 50 personal trainers, not a single doctor or personal trainer got the answer right. And only 6% of the dieticians did. The right answer is elusive because most of the weight is lost as an invisible gas that passes unnoticed from your body with every breath. That’s right, when you manage to keep your new year’s reslution and lose those five pounds, you exhale most of it as CO2, about 84% of it to be precise. People are similarly stumped by where the mass of a tree comes from. This paper (and that tree video I linked) are neat reminders seeing is *not* believing — relying too much on what we can easily see or otherwise sense can lead our intuitions to some spectacularly wrong conclusions.
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.
“Best of last year” listicles are a time honored tradition, and I want in. So in the next few blog posts I want to highlight some some of my favorite research articles of 2014. Here’s the first one: Continue reading →
Enzymes are nature’s best catalysts. The how and why of it are the central questions of enzymology. Over the decades, enzymologists have come a long way in understanding and even designing enzyme function. But three new results are highlighting previously unappreciated complexities of enzyme function, complexities that are often neglected by enzyme designers. Continue reading →
All scientists have heroes. Some of our heroes are internationally renowned, like Galileo, Darwin, or Curie. Other heroes are personal, whether they be an inspiring high school teacher, an always-helpful senior colleague, or a wise, generous professor or boss. But what about villains? Do scientists have villains too? Continue reading →