<p>Planets in our solar system have strikingly different chemical compositions - the four rocks closest to the  sun (Mercury, Venus, Earth, Mars) have most of their mass made up of rocky and metallic elements (the centre of the Earth is mainly nickel and iron), whereas the gas giants of Jupiter and Saturn are made up of much lighter and simpler elements (mainly Hydrogen and Helium)</p>
<p>Jupiter has hydrogen all the way to its core, where pressures are so intense that a form of 'metallic hydrogen' is thought to exist there. So yes, there are <em>less</em> step-changes in elemental composition and therefore less step-changes in density - however, density step-changes still do exist where matter (even the same elements) exists in different states under different conditions - e.g. gas, liquid and possibly metallic hydrogen at different pressures and temperatures. That is a feature of chemistry.</p>
<p>The reason for the elemental difference in the planets is partly due to the make-up and distribution within the accretion disk, the swirl of elements that existed when our solar system formed. Also, planets that are as big as Jupiter have such a strong gravitational pull that they are able to hold onto (and pull in more) of the lighter elements like hydrogen and helium, which are the most abundant in the universe. Jupiter is actually like a half-there star, sometimes referred to as a 'failed star'.</p>
<p>However, the Earth is still large enough and has a protective magnetic shield generated by its iron core that allows its atmosphere not to be blasted away by the solar wind. This means we have both a stable gaseous atmosphere as well as a metallic and rocky interior, meaning a step-change in densities between solids and gases.</p>
<p>It is also worth noting though that our gaseous atmosphere is only 60 miles thick, whereas the Earth's radius is about 4,000 miles. The planet is much more of a homogenous solid than you might think, being as we are atmosphere dwellers.</p>

CC BY-SA 4.0
2021-01-12T11:40:14.440Z
2021-01-12T11:56:30.707Z
2021-01-12T11:56:30.707Z
Amphibio
<p>Atoms also interact electromagnetically, and this changes their behaviour.  In particular, iron will form metallic bonds, while silcon will bond covalently with oxygen forming massive solid structures.  On the other hand, Nitrogen will bond to itself forming small molecules that don't link to other molecules. As a result, the iron and silicates will form a solid with much higher density than Nitrogen.  The iron and silicates will then fall together with the nitrogen floating above, and there will be a sharp step change in density as you move from the region where there is Nitrogen (and other gases) to the region where there are silicates (and other minerals)</p>
<p>If Electromagnetic forces didn't do this there would be only a galaxy-sized blob of Hydrogen and Helium nuclei, as we see with the dark matter halo (which interacts gravitationally but not electromagnetically)</p>
<p>It is the electromagnetic interactions that form chemical bonds, and the different types of interaction between different chemical species, that explain why there is a sharp change in density at the boundary between crust and atmosphere, or between ocean and atmosphere.</p>
<p>Similar step changes also exist between ocean and crust, and within the Earth, between the mantle and the core.</p>

CC BY-SA 4.0
2020-12-21T21:56:54.410Z
"If Electromagnetic forces didn't do this there would be only a galaxy-sized blob of Hydrogen and Helium nuclei" - Are you saying there wouldn't be stars?
Keith McClary
CC BY-SA 4.0
2020-12-22T04:58:05.420Z
You wouldn't have a galaxy-sized blob, you'd have stars orbited by "planets" that were blobs of gaseous iron, silicon, &c, just as we have bigger blobs of mostly hydrogen & helium, like Jupiter & Saturn.  Indeed, there seems to be at least one exoplanet that comes close to being an iron gas giant: https://www.scientificamerican.com/article/molten-iron-rain-falls-on-scorching-hot-exoplanet/
jamesqf
CC BY-SA 4.0
2020-12-22T07:21:25.463Z
To forms stars you need to get a gravitational collapse going, That means converting gravitational energy to heat. That means some kind of friction or other diapation, but those processes are electromagnetic.  Without it protons would  behave like dark matter.  On the other hand, they might just fuse via strong interactions. Anyway, you don't get a universe that is anything like ours if you take away electromagnetism
James K
CC BY-SA 4.0
2020-12-21T16:38:59.160Z
2020-12-21T16:38:59.160Z
James K
<p>World is about 4.5 billion years old. At the beginning its density function is linear. Since the beginning world mass loosing its heat. The heat of the earth is high enough to cause convection currents. Convection currents works like an elevator from center to the surface. This convection currents makes the mass inside earth to move upwards, then downwards nonstoppingly, since its formation. This movement center to edge has average 1 mm/year velocity. For 4.5 billion years, thousands of times this tour completed. The action takes place by partial melting. Large ion/charge elements are unwanted in crystal structure and they leave the crystal first during melting. They are transferred to crust via this melt.
Due to this repeatingly mass translation earth has been developing its layered structure with characterised by step function. The process is called differentiation and time is an element of this function.</p>
<p>The differentiation phenomemon is also responsible for plate tectonics; formation of continents, earthquakes, life.</p>

CC BY-SA 4.0
2021-01-14T00:18:58.857Z
Differentiation alone could not have had the consequences listed at the end of this answer.  We know many differentiated bodies, but not all have the strong orogeny that continents would require; and while some have tectonics, only one example is known to have _plate_ tectonics.  And life is, of course, an open question.
Oscar Lanzi
CC BY-SA 4.0
2021-01-15T16:32:42.927Z
The heat loss of earth, by convection responsible for the differentiation, which have been designing the earth from core to the crust since the beginning. This is the big cycle. The cycle that has been running on lithosphere –which is known as plate tectonics- is the small cycle. The running of the big cycle, make the small cycle to run. When the running of the big cycle will stop (when the fuel of the earth run out), the small cycle will stop. This means the continents will not move any more. This is the case of Mars, where no big cycle running, no plate tectonics taking place and no life.
Muharrem Yavuz
CC BY-SA 4.0
2021-01-15T16:54:29.223Z
**We don't know** whether Mars has no life!
Oscar Lanzi
CC BY-SA 4.0
2021-01-12T17:21:22.837Z
2021-01-13T14:14:38.833Z
2021-01-13T14:14:38.833Z
Muharrem Yavuz
<p>Roughly put, it's the same thing that makes a density step function when you try to combine oil and water:</p>
<p><img src="https://i.sstatic.net/I4hXJ.jpg" alt="enter image description here" /><a href="https://www.thoughtco.com/why-oil-and-water-dont-mix-609193" rel="nofollow noreferrer">Source</a></p>
<p>The components do not mix with or dissolve into each other, so gravity makes the denser material -- water -- settle to the bottom.  The density as a function of height jumps up from the oil density to the water density when you go below the interface.</p>
<p>In the case of Earth, there are several progressively denser phases that do not mix.  The biggest step in the density gradient is when the rock phases (olivine, spinel, perovskites, and ferropericlase which is not shown in the illustration below) give way to the much denser core.  In the illustration below, this boundary is where the bottom part of the rocky mantle, the D&quot; layer, meets the predominantly iron core.</p>
<p><img src="https://i.sstatic.net/LPYC8.jpg" alt="enter image description here" /><a href="http://www.spring8.or.jp/en/news_publications/publications/scientific_results/earth_science/topic21" rel="nofollow noreferrer">Source</a></p>
<p>One major difference between the interior of Earth and the oil-water combination in the first picture is that much of Earth's interior is solid, so it takes a lot of heat and pressure -- both generated by gravity -- to make the materials yield and flow to their equilibrium positions.  The same is true of other predominantly solid celestial bodies, so only relatively large and massive ones have enough gravitational power to enable separation of different density phases.  Planetary scientists call this process <strong>differentiation</strong>.</p>

CC BY-SA 4.0
2021-01-11T18:53:25.957Z
Minor correction: "The biggest steo in the density grad ent is when the rock phases (olivine, spinel, perovskites) give way to the much denser core."

Olivine and spinel are not stable near the earth's core. In addition to perovskite, ferropericlase is thought to be an abundant phase in the lower mantle.
g.z.
CC BY-SA 4.0
2021-01-11T18:58:21.757Z
I knew that, but the illustration unfortunately does not show the ferropericlase, which causes an inconsistency if I were to mention that phase.  Can you offer a solution?
Oscar Lanzi
CC BY-SA 4.0
2020-12-29T22:53:52.300Z
2021-01-11T21:16:53.230Z
2021-01-11T21:16:53.230Z
Oscar Lanzi
<p>Why is the density of particles in Earth's atmosphere above surface-level so much less than ground and below? Why, instead, isn't Earth simply a ball of swirling gasses / liquids, which become progressively denser as you approach the core (since gravitational pull is stronger closer to the core)?</p>

CC BY-SA 4.0
2020-12-19T07:04:16.127Z
the stuff Earth is made of doesn't have [triple points](https://en.m.wikipedia.org/wiki/Triple_point)s in the right places for such a low mass planet?
uhoh
CC BY-SA 4.0
2020-12-19T09:49:20.437Z
You seem to be asking "why do liquids and solids exist"  I'm not sure that this is really a question of astronomy.  I think "chemistry" might be a better place or perhaps Physics. The reason is that at low temperatures particles bond electromagnetically,
James K
CC BY-SA 4.0
2020-12-19T09:55:49.943Z
@PM2Ring You're right in your explanation. However often people mean the pressure inside the core when they say gravity; of course the pressure is a result of the gravity pressing the outer layers onto the core.
planetmaker
<atmosphere><geology>
Why is Earth's density gradient a step-function, rather than smooth?
Nikhil Kumar
# Why is Earth's density gradient a step-function, rather than smooth?

(Asked by: username_0 on 2020-12-19)

Why is the density of particles in Earth's atmosphere above surface-level so much less than ground and below? Why, instead, isn't Earth simply a ball of swirling gasses / liquids, which become progressively denser as you approach the core (since gravitational pull is stronger closer to the core)?

**Tags:** atmosphere, geology

> **Comments:**
> **username_1**: the stuff Earth is made of doesn't have [triple points](https://en.m.wikipedia.org/wiki/Triple_point)s in the right places for such a low mass planet?  
> **username_2**: You seem to be asking "why do liquids and solids exist"  I'm not sure that this is really a question of astronomy.  I think "chemistry" might be a better place or perhaps Physics. The reason is that at low temperatures particles bond electromagnetically,  
> **username_3**: @PM2Ring You're right in your explanation. However often people mean the pressure inside the core when they say gravity; of course the pressure is a result of the gravity pressing the outer layers onto the core.  

---

## 4 answers

### Answer by username_8 on 2020-12-29. Score: 13 | Accepted answer

Roughly put, it's the same thing that makes a density step function when you try to combine oil and water:

![enter image description here](https://i.sstatic.net/I4hXJ.jpg)[Source](https://www.thoughtco.com/why-oil-and-water-dont-mix-609193)

The components do not mix with or dissolve into each other, so gravity makes the denser material -- water -- settle to the bottom. The density as a function of height jumps up from the oil density to the water density when you go below the interface.

In the case of Earth, there are several progressively denser phases that do not mix. The biggest step in the density gradient is when the rock phases (olivine, spinel, perovskites, and ferropericlase which is not shown in the illustration below) give way to the much denser core. In the illustration below, this boundary is where the bottom part of the rocky mantle, the D" layer, meets the predominantly iron core.

![enter image description here](https://i.sstatic.net/LPYC8.jpg)[Source](http://www.spring8.or.jp/en/news_publications/publications/scientific_results/earth_science/topic21)

One major difference between the interior of Earth and the oil-water combination in the first picture is that much of Earth's interior is solid, so it takes a lot of heat and pressure -- both generated by gravity -- to make the materials yield and flow to their equilibrium positions. The same is true of other predominantly solid celestial bodies, so only relatively large and massive ones have enough gravitational power to enable separation of different density phases. Planetary scientists call this process **differentiation**.

> **Comments:**
> **username_9**: Minor correction: "The biggest steo in the density grad ent is when the rock phases (olivine, spinel, perovskites) give way to the much denser core."  Olivine and spinel are not stable near the earth's core. In addition to perovskite, ferropericlase is thought to be an abundant phase in the lower mantle.  
> **username_8**: I knew that, but the illustration unfortunately does not show the ferropericlase, which causes an inconsistency if I were to mention that phase.  Can you offer a solution?  

---

### Answer by username_4 on 2021-01-12. Score: 2

Planets in our solar system have strikingly different chemical compositions - the four rocks closest to the sun (Mercury, Venus, Earth, Mars) have most of their mass made up of rocky and metallic elements (the centre of the Earth is mainly nickel and iron), whereas the gas giants of Jupiter and Saturn are made up of much lighter and simpler elements (mainly Hydrogen and Helium)

Jupiter has hydrogen all the way to its core, where pressures are so intense that a form of 'metallic hydrogen' is thought to exist there. So yes, there are *less* step-changes in elemental composition and therefore less step-changes in density - however, density step-changes still do exist where matter (even the same elements) exists in different states under different conditions - e.g. gas, liquid and possibly metallic hydrogen at different pressures and temperatures. That is a feature of chemistry.

The reason for the elemental difference in the planets is partly due to the make-up and distribution within the accretion disk, the swirl of elements that existed when our solar system formed. Also, planets that are as big as Jupiter have such a strong gravitational pull that they are able to hold onto (and pull in more) of the lighter elements like hydrogen and helium, which are the most abundant in the universe. Jupiter is actually like a half-there star, sometimes referred to as a 'failed star'.

However, the Earth is still large enough and has a protective magnetic shield generated by its iron core that allows its atmosphere not to be blasted away by the solar wind. This means we have both a stable gaseous atmosphere as well as a metallic and rocky interior, meaning a step-change in densities between solids and gases.

It is also worth noting though that our gaseous atmosphere is only 60 miles thick, whereas the Earth's radius is about 4,000 miles. The planet is much more of a homogenous solid than you might think, being as we are atmosphere dwellers.


---

### Answer by username_2 on 2020-12-21. Score: 1

Atoms also interact electromagnetically, and this changes their behaviour. In particular, iron will form metallic bonds, while silcon will bond covalently with oxygen forming massive solid structures. On the other hand, Nitrogen will bond to itself forming small molecules that don't link to other molecules. As a result, the iron and silicates will form a solid with much higher density than Nitrogen. The iron and silicates will then fall together with the nitrogen floating above, and there will be a sharp step change in density as you move from the region where there is Nitrogen (and other gases) to the region where there are silicates (and other minerals)

If Electromagnetic forces didn't do this there would be only a galaxy-sized blob of Hydrogen and Helium nuclei, as we see with the dark matter halo (which interacts gravitationally but not electromagnetically)

It is the electromagnetic interactions that form chemical bonds, and the different types of interaction between different chemical species, that explain why there is a sharp change in density at the boundary between crust and atmosphere, or between ocean and atmosphere.

Similar step changes also exist between ocean and crust, and within the Earth, between the mantle and the core.

> **Comments:**
> **username_5**: "If Electromagnetic forces didn't do this there would be only a galaxy-sized blob of Hydrogen and Helium nuclei" - Are you saying there wouldn't be stars?  
> **username_6**: You wouldn't have a galaxy-sized blob, you'd have stars orbited by "planets" that were blobs of gaseous iron, silicon, &c, just as we have bigger blobs of mostly hydrogen & helium, like Jupiter & Saturn.  Indeed, there seems to be at least one exoplanet that comes close to being an iron gas giant: https://www.scientificamerican.com/article/molten-iron-rain-falls-on-scorching-hot-exoplanet/  
> **username_2**: To forms stars you need to get a gravitational collapse going, That means converting gravitational energy to heat. That means some kind of friction or other diapation, but those processes are electromagnetic.  Without it protons would  behave like dark matter.  On the other hand, they might just fuse via strong interactions. Anyway, you don't get a universe that is anything like ours if you take away electromagnetism  

---

### Answer by username_7 on 2021-01-12. Score: 1

World is about 4.5 billion years old. At the beginning its density function is linear. Since the beginning world mass loosing its heat. The heat of the earth is high enough to cause convection currents. Convection currents works like an elevator from center to the surface. This convection currents makes the mass inside earth to move upwards, then downwards nonstoppingly, since its formation. This movement center to edge has average 1 mm/year velocity. For 4.5 billion years, thousands of times this tour completed. The action takes place by partial melting. Large ion/charge elements are unwanted in crystal structure and they leave the crystal first during melting. They are transferred to crust via this melt.
Due to this repeatingly mass translation earth has been developing its layered structure with characterised by step function. The process is called differentiation and time is an element of this function.

The differentiation phenomemon is also responsible for plate tectonics; formation of continents, earthquakes, life.

> **Comments:**
> **username_8**: Differentiation alone could not have had the consequences listed at the end of this answer.  We know many differentiated bodies, but not all have the strong orogeny that continents would require; and while some have tectonics, only one example is known to have _plate_ tectonics.  And life is, of course, an open question.  
> **username_7**: The heat loss of earth, by convection responsible for the differentiation, which have been designing the earth from core to the crust since the beginning. This is the big cycle. The cycle that has been running on lithosphere –which is known as plate tectonics- is the small cycle. The running of the big cycle, make the small cycle to run. When the running of the big cycle will stop (when the fuel of the earth run out), the small cycle will stop. This means the continents will not move any more. This is the case of Mars, where no big cycle running, no plate tectonics taking place and no life.  
> **username_8**: **We don't know** whether Mars has no life!  

---

