Earth's Core

Question 1

What are the elements that occur in the core along with Iron?

Answer 1

Nickel (Ni), Cobalt (Co), Rhenium (Re), Osmium (Os), Phosphorous (P), Sulfure (S), etc

Question 2

The external core is assumed to have how much Sulfur?

Answer 2

9-12%

Question 3

Why do we know that there must be Iron in the Core?

Answer 3

Because Iron is the only element that satisfies the geophysical calculations of inertia, and also it is very abundant in the solar system.

Question 4

How much of the Earth’s mass is the inner core?

Answer 4

1.7%

Question 5

How much of the Earth’s mass is the outer core?

Answer 5

30%

Question 6

Why can’t the inner core be only made of Iron?

Answer 6

Seismic tests have shown that the inner core has too high of a density to be pure Iron, so it is believed to have 10-20% Nickel.

Question 7

Why can’t the external core be made of only iron?

Answer 7

Seismic tests have shown that the external core has too low of a density to be solid Iron, so it is believed to have either Si, O, Mg, or S in order to reduce the density.

Question 8

What is the argument for silicon to be in the external core?

Answer 8

For: A small amount will reduce the density of Iron (10% Si), there are also observations of some silicon in metal asteroids.

Against: High melting temp. Tends to form ions that mixes with oxygen.

Question 9

What is the argument for Oxygen to be in the external core?

Answer 9

For: Iron-oxide behaves like a metal at high temps, and oxygen is very abundant.

Against: Very little is known about oxides at those pressures, and it does not reduce the melting temperature of iron.

Question 10

What is the argument for Magnesium to be in the external core?

Answer 10

For: Very abundant
Against: Does not appear too much in rocks

Question 11

What is the argument for Sulfur to be in the external core?

Answer 11

For: A strong affinity for Iron. There is the presence of FeS in meteorites. (9-12%) would justify observed densities.

Against: Sulfure is a volatile element (which makes it difficult to stay in the external core)

Question 12

What is some research that tries to show the composition of the external core?

Answer 12

Poirier (1994) and Sherman (1997) stated that there are 4 possibilities…

1) Fe(Ni) - S
2) Fe(Ni) - O and Fe(Ni) - S
3) Fe(Ni) - O
4) Fe(Ni) - S and Fe(Ni) - O

Question 13

Why is the external core liquid?

Answer 13

The presence of Sulfur lowers the melting temperature for Iron. 1,539 C to 1,230 C at 1 atm for Iron.

Question 14

What are possible sources of energy for convection of the external core, which drives the magnetic field?

Answer 14

1) Radioactive Decay (U, Th, K)
2) Release of heat from crystallization of the Inner Core
3) Release of heat produced by electrical currents
4) Effects of tides and moon’s gravity
5) Gravitational Convection

Question 15

What are two models to explain the core?

Answer 15

Jacobs (1953)

• Chemically Homogenous
• Melting Temp. increases with depth
• Pressure changes quicker than heat
• Silicates are in the mantle but not core due to the pressure and temperature.

Higgins & Kennedy (1971)

• Decided Jacobs model was not sufficient to explain convection
• The core has an internal and external part

Question 16

How long have conditions stayed similar in the core?

Answer 16

for 3.5 billion years

Question 17

What is the density of the core?

Answer 17

Mantel - External Interface (9.9-12.2 g/cm3)
Internal Mantel (12.6-13.5 g/cm3)

Question 18

What is the radius of the core?

Answer 18

Whole - 2,500 km

Inner - 1,200 km

Question 19

What is the pressure and temp of the core?

Answer 19

External - 1.3 million atm
Internal - 3.5 million atm
Max temp - 4000 to 5000 C

Question 20

How much FeS is in the outer core according to density data?

Answer 20

33%

Question 21

What is layer “D”?

Answer 21

A layer at the boundary of the mantle and outer core which is a result of the reaction between silicates and metallic material. Probably reactions between Iron, Oxygen, Silicon, and Magnesium.

Question 22

What did the primordial Core consist of?

Answer 22

A mixture of Fe(86%), S(11%), and Ni(3%); Half of S was lost during accretion. Fe and S may have also been incorporated then.