# Do we know anything about the nature of Earth's core that hasn't come from magnetic or seismic measurements?

There is much known about Earth's core from painstaking analysis of seismic data, and from detailed magnetic field maps and trends over time.

Are there any other measurements that have contributed to current understanding of Earth's core besides these two?

• This is a list question, something that's a bit frowned upon across the StackExchange network. I can immediately think of six answers; I'm sure there are more. Those six are very-long-baseline interferometry, satellite radar ranging, inter-satellite ranging, gravity gradient measurements, relative GPS measurements, and precise orbit determination. – David Hammen Mar 28 at 13:31
• @DavidHammen the correct answer will be a Boolean + one example if True, not a list. I'm certainly interested to see how one of those gives meaningful information about the Earth's core that goes beyond seismic and magnetic field measurements. So maybe just pick one you like most? – uhoh Mar 28 at 13:34
• I'm voting to close this question as off-topic because it is a list question that cannot have an objectively correct answer. Questions such as these are better suited to discussion sites as opposed to Q&A sites (e.g., this site). – David Hammen Mar 28 at 13:49
• @DavidHammen The question can be answered with "Yes" or "No". I wrote it that way intentionally! You've already stated the answer is "Yes" and need only to support it with an example. – uhoh Mar 28 at 13:52

Short answer: yes, most of our knowledge is not from seismics and geomagnetism, but from experiment and measurements, deductions from geochemistry and -physics, material science and physics in general (besides the already mentioned measurements of earth's gravity, but the variations are rather connected to mantle convection, water storage, etc. than the core). But all in all those weigh much more than "painstaking" seismics and geomagnetism.

One experiment to replicate pressure/temperature (p/t) conditions is the diamond anvil cell and its cascaded version in combination with laser heating of the probe, to obtain data about mineral and cristalline phases and transitions under conditions in a planet's core.

And some random links on the outcome of the matter:

Laser pulses is a developing technique to reach very high p/t conditions, higher than diamond anvil cells can, but they do not last that long

• Okay this is a wonderful! selection; I'll read through these today. I wasn't really imagining laboratory experiments as examples since they are measurements on concocted mixtures only presumed to be like what's in the core rather than observations directly related to Earth's core directly, but I'll reserve judgement until after I read them. Thanks! – uhoh Mar 28 at 23:33
• The scientific method carries out experiments in a controlled environment and under reproducible conditions. We cannot travel to the earth's core or peel off the bulk. You asked for contributions to our understanding, a "boolean with one example". – a_donda Mar 29 at 9:30
• ...and I up voted and thanked you for it! The seismic and magnetic data are measurements of the Earth's core, the diamond anvil work are measurements on a lab sample hypothesized to be similar to what's in the core. It's possible that the answer to the question is "No", but in the mean time this is really good stuff, thank you again! – uhoh Mar 29 at 9:48
• Just to proove my statement that chemistry and physics contribute much more to our knowledge: seismics lead to the question "Why is the core too light ?". Geochemistry and -physics can answer that, though the exact amount of which elements contribute how much is a discussed item. More lab work will probably one day answer that conundrum. And now you might want to elaborate on what you think is wrong with hypotheses and experiments to disprove or support them. – a_donda Mar 29 at 11:58
• thanks again; I think you're reading too much into my comments, there's nothing up there that matches what your asking for – uhoh Mar 29 at 12:17

The comments to the other question have already alluded to some physical answers that also come out of the lab: One might think that the heaviest elements in primordial Earth should have all sunk into the core by now. But that's not actually the case: For example, uranium with a density 2.5 times that of iron (19.1 vs 7.9 g/cm3) has an abundance of about 4 ppm in the mantle, whereas in the core its abundance is believed to be essentially zero. How do we know this? We can make experiments about how easily uranium dissolves in rocky materials vs in iron melts -- and it turns out that if it has a choice, uranium very much prefers to associate itself with rocks and not with iron.

I think the answer is 'no', in the sense that 'seismology (i.e., the study of waves propagating within the Earth) tells us the details in a far superior way'. See this paper for an informative treatise: Brush, S. G. (1980). Discovery of the Earth’s core. American Journal of Physics, 48, 705-724, and these lecture notes http://www.geo.uu.nl/~berg/geodynamics/lecturenotes.pdf on Geodynamics.

Interesting additional tools that you'll find in these works are:

1. Figuring out the mass of the Earth (using Newton's laws). People found that the average density of the Earth had to be $$5500$$ kg/m$$^3$$, while rocks at the surface only had a density of about $$2700$$ kg/m$$^3$$, so the interior of the Earth had to be of higher density. Under the assumption that pressure alone cannot be responsible for the increase in density, the high densities had to come from chemical changes such as the presence of metals, particularly because of the Earth's magnetic field. Under the assumption that the pressure was responsible for the high density, the Earth's interior could be gaseous, for example, plain air is more compressible than just rocks, so the Earth's interior could be air (as proposed by Benjamin Franklin in the US!).

2. A second clue that the Earth had to have a radially varying density came from the Earth's moment of inertia (which was a known quantity, based on astronomical measurements such as the Earth's axial precession), which makes it possible to create models that relate density and radius. Of course, with neither quantity well-known, you couldn't do too much with this. But assuming an iron core (with known density, and responsible for the Earth's magnetic field) overlaying a mantle (with density close to that of the Earth's surface), the first radial profiles could be made.

3. Models could be made that concerned themselves with the rigidity/deformation of the Earth under forces, such as the response to ocean tides, terrestrial tides and the Chandler wobble. These could be used to even construct some radial profiles of some approximate elastic properties. Only in the 1960s, after seismology was a firmly established technique, people additionally found additional non-seismic evidence of the Earth's eigenvibrations/free oscillations where the Earth (as a whole) rings like a bell, which was a very useful independent tool to verify layered profiles of the Earth's elastic properties, and which were important in the discovery of, e.g., the solid inner core.

4. The rigidity (from the previous point, and mentioned in the first point) can be linked to densities using (thermodynamical) equations of state. Hence, figuring out the appropriate relations between density/pressure/rigidity was of large importance.

5. Similarly, with the assumption that the Earth was made with the rest of the solar system, it was possible from the spectral lines of the sun and the chemical makeup of meteorites to figure out the likely bulk chemical composition of the Earth (https://en.wikipedia.org/wiki/Chondritic_uniform_reservoir). If you additionally know the pressure, density and rigidity information, you can make relatively informed guesses about candidate materials that must be present. That is, similar to the answer by a_donda, one must still test how materials really behave in these strange pressure and temperature regimes, which is mostly laboratory work using, e.g., a Diamond Anvil Cell.

6. Similarly, these lab experiments can tell us that for a given pressure on a given material, what the expected temperatures can be.

But if you want very solid answers about the 'nature' of the Earth's core, all the most direct answers come straight from seismology. That is the best method to actually constrain the location of the (elastic) properties within the Earth, at a much higher accuracy than any of the other methods can. As secondary consequences, we can use these measurements to make chemical hypotheses about the makeup of the core etc., but those are entirely based on the assumption that the seismic data is correct....so they're not an 'independent' kind of measurement/model!

• Heavily needs citations. "Weight of the earth" is a physical misconception. What have the "spectral lines of the sun" to do with "chemical makeup of meteorites". Are you talking about an absorption spectrum of the sun ? Or spectral classes of asteroids ? Where is the link between the two ? The earth "rings like a bell" ? And the precession can be used to determine density ? How so ? Citation pls ! There seems to be a lot of confusion in this answer. – a_donda Apr 5 at 13:43
• @a_donda All the information comes from the two linked documents which review the material... I must refer you to those if you care about the complete story, these are merely some hints about the full story. You're of course right about the "weight of the Earth"! :-) – Erik Apr 5 at 14:50
• While my question turned out to be much more nuanced than I'd expected, this answer best addresses what I was hoping would be explored, so while I can't yet accept any answer as "the right" answer I'll definitely award the bounty. Now I'll go read up on these. Thanks! – uhoh Apr 10 at 3:59