<p>We know the density of the core from seismology.<br>
We also know the density of pure iron at given pressure (P) and temperature (T).<br>
From these two, people have long noticed that there is a discrepancy: the actual core is lighter than a theoretical core made of pure iron. This is known as the <a href="https://doi.org/10.1016/S0031-9201(02)00017-1" rel="noreferrer">Core Density Deficit</a> (CDD). To explain this CDD, we need to add light elements in the core composition. There are many candidates, such as sulphur, carbon, silicon, oxygen, etc.</p>

<p>This is actually needed by geochemists, who try to relate the Earth's composition to that of chondrite. They find that the Bulk Silicate Earth (BSE = crust + mantle) is depleted in some elements compared to the chondritic reference, and have to invoke a hidden reservoir to explain this. For some elements, <a href="https://www.nature.com/articles/35051064" rel="noreferrer">this reservoir could be the core</a>.</p>

<p>To further constrain this, petrologists run some high-P/high-T experiments to see the behaviour of elements during the Earth differentiation. Some elements are said to be lithophile, meaning they will stay in the BSE, while other elements are considered siderophile, hence should have joined the metal phase during segregation of the core. The problem is that some experiments disagree with others, as the behaviour of a given element can change depending on the conditions. For instance, niobium and tantalum are considered lithophile, but can be <a href="https://www.nature.com/articles/ngeo2195" rel="noreferrer">siderophile in reduced conditions</a>.</p>

<p>I'm afraid the bottom-line of all this is: there is no simple answer to this question...</p>

CC BY-SA 4.0
2020-02-24T04:10:17.670Z
As an experimental petrologist, I support this answer.
Gimelist
CC BY-SA 4.0
2019-12-02T13:35:37.280Z
2019-12-02T13:35:37.280Z
Jean-Marie Prival
<p>No, simply because we cannot access it and the methods for understanding it are essentially indirect geochemical and geophysical measurements (mostly experimental petrology and wave propagation experiments). We only talk about models, so far. So much so anything deeper than the crust - to be more precise, anything <em>but</em> upper continental crust is either measured indirectly - i.e. we have no access. The mantle? Mostly theoretical with the help of xenoliths (rocks from the mantle that are brought up by ascending magma). </p>

CC BY-SA 4.0
2019-07-04T17:46:25.830Z
You answered "no" to a "how" question...
Spencer
CC BY-SA 4.0
2019-08-02T17:59:13.390Z
@spencer I'm guessing Matheus answered "no" to the "will we ever..." part of the last sentence of the question. I think it's a little arrogant to assume that we'll never figure out a way to get better information, but it does make sense as their opinion.
Semidiurnal Simon
CC BY-SA 4.0
2019-08-02T18:13:27.990Z
"More precisely" is certainly a possibility and the future even, but "precisely" will still get a "no" for me.
Matheus
CC BY-SA 4.0
2019-07-03T01:35:37.753Z
2019-07-03T01:35:37.753Z
Matheus
<p>The consensus view is that that the Earths core is composed mainly of iron alloyed with about 10 percent nickel and traces of other heavy metals, but I have seen at least one account which says it is partly iron sulphide. Has the iron sulphide theory been discredited, or is it a view still held by some geologists? There is a recent theory that about 5 percent of the core is composed of silicon. How valid is this, and will we ever be able to establish more precisely what the core is made of?</p>

CC BY-SA 4.0
2019-06-28T03:54:02.317Z
I see some related questions in the RHS sidebar and by searching ES for [Earths core](https://earthscience.stackexchange.com/search?q=Earths+core).
Keith McClary
CC BY-SA 4.0
2019-06-29T08:55:11.623Z
There is as much chance of someone  travelling  through the Earth's core as there is of me swimming unaided from Land's End to New York in the middle of winter!
Michael Walsby
CC BY-SA 4.0
2019-06-29T16:55:19.293Z
[Mission to Earth’s core — a modest proposal](https://www.nature.com/articles/423239a.epdf?shared_access_token=HfibB4E6q4-bHbBMK22zVtRgN0jAjWel9jnR3ZoTv0PmIWErEShrBqKCWj16RcZHagEq_wdoCLcPYEf8GhxK-uKherfRaw-stybYP7h7iR4tOv2aifqdzt15CzZWNq8rboDAx16WUX0leTvl1Bf9jsCGSxnfPxKoycV6EXkDWTTIkgamDDzE1o53VvnNWFlOGrdtj649jOqJGDzYQHpmFg%3D%3D) , [NASA Frantically Announces Mission To Earth’s Core After Accidentally Launching Rocket Upside Down](https://www.msn.com/en-us/entertainment/humor/nasa-frantically-announces-mission-to-earths-core-after-accidentally-launching-rocket-upside-down/ar-BBUfToo).
Keith McClary
CC BY-SA 4.0
2020-03-30T11:21:34.717Z
@KeithMcClary 2nd link broke but I still laughed out loud, so in that sense NSFW ;-)
uhoh
<geology><geophysics><core>
How Can We Establish Precisely What Earth's Core is Made Of?
Michael Walsby
# How Can We Establish Precisely What Earth's Core is Made Of?

(Asked by: username_0 on 2019-06-26)

The consensus view is that that the Earths core is composed mainly of iron alloyed with about 10 percent nickel and traces of other heavy metals, but I have seen at least one account which says it is partly iron sulphide. Has the iron sulphide theory been discredited, or is it a view still held by some geologists? There is a recent theory that about 5 percent of the core is composed of silicon. How valid is this, and will we ever be able to establish more precisely what the core is made of?

**Tags:** geology, geophysics, core

> **Comments:**
> **username_1**: I see some related questions in the RHS sidebar and by searching ES for [Earths core](https://earthscience.stackexchange.com/search?q=Earths+core).  
> **username_0**: There is as much chance of someone  travelling  through the Earth's core as there is of me swimming unaided from Land's End to New York in the middle of winter!  
> **username_1**: [Mission to Earth’s core — a modest proposal](https://www.nature.com/articles/423239a.epdf?shared_access_token=HfibB4E6q4-bHbBMK22zVtRgN0jAjWel9jnR3ZoTv0PmIWErEShrBqKCWj16RcZHagEq_wdoCLcPYEf8GhxK-uKherfRaw-stybYP7h7iR4tOv2aifqdzt15CzZWNq8rboDAx16WUX0leTvl1Bf9jsCGSxnfPxKoycV6EXkDWTTIkgamDDzE1o53VvnNWFlOGrdtj649jOqJGDzYQHpmFg%3D%3D) , [NASA Frantically Announces Mission To Earth’s Core After Accidentally Launching Rocket Upside Down](https://www.msn.com/en-us/entertainment/humor/nasa-frantically-announces-mission-to-earths-core-after-accidentally-launching-rocket-upside-down/ar-BBUfToo).  
> **username_2**: @username_1 2nd link broke but I still laughed out loud, so in that sense NSFW ;-)  

---

## 2 answers

### Answer by username_3 on 2019-12-02. Score: 6

We know the density of the core from seismology.  
We also know the density of pure iron at given pressure (P) and temperature (T).  
From these two, people have long noticed that there is a discrepancy: the actual core is lighter than a theoretical core made of pure iron. This is known as the [Core Density Deficit](https://doi.org/10.1016/S0031-9201(02)00017-1) (CDD). To explain this CDD, we need to add light elements in the core composition. There are many candidates, such as sulphur, carbon, silicon, oxygen, etc.

This is actually needed by geochemists, who try to relate the Earth's composition to that of chondrite. They find that the Bulk Silicate Earth (BSE = crust + mantle) is depleted in some elements compared to the chondritic reference, and have to invoke a hidden reservoir to explain this. For some elements, [this reservoir could be the core](https://www.nature.com/articles/35051064).

To further constrain this, petrologists run some high-P/high-T experiments to see the behaviour of elements during the Earth differentiation. Some elements are said to be lithophile, meaning they will stay in the BSE, while other elements are considered siderophile, hence should have joined the metal phase during segregation of the core. The problem is that some experiments disagree with others, as the behaviour of a given element can change depending on the conditions. For instance, niobium and tantalum are considered lithophile, but can be [siderophile in reduced conditions](https://www.nature.com/articles/ngeo2195).

I'm afraid the bottom-line of all this is: there is no simple answer to this question...

> **Comments:**
> **username_4**: As an experimental petrologist, I support this answer.  

---

### Answer by username_5 on 2019-07-03. Score: 3

No, simply because we cannot access it and the methods for understanding it are essentially indirect geochemical and geophysical measurements (mostly experimental petrology and wave propagation experiments). We only talk about models, so far. So much so anything deeper than the crust - to be more precise, anything *but* upper continental crust is either measured indirectly - i.e. we have no access. The mantle? Mostly theoretical with the help of xenoliths (rocks from the mantle that are brought up by ascending magma).

> **Comments:**
> **username_6**: You answered "no" to a "how" question...  
> **username_7**: @username_6 I'm guessing username_5 answered "no" to the "will we ever..." part of the last sentence of the question. I think it's a little arrogant to assume that we'll never figure out a way to get better information, but it does make sense as their opinion.  
> **username_5**: "More precisely" is certainly a possibility and the future even, but "precisely" will still get a "no" for me.  

---

