# Why is Earth's density gradient a step-function, rather than smooth?

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)?

• the stuff Earth is made of doesn't have triple pointss in the right places for such a low mass planet?
– uhoh
Dec 19, 2020 at 7:04
• 1 A small rock is held together by chemical forces (electromagnetism), not gravity. 2 "gravitational pull is stronger closer to the core" Not really. Inside a ball of uniform density, the gravity gets weaker as you approach the centre. See en.wikipedia.org/wiki/Shell_theorem
– PM 2Ring
Dec 19, 2020 at 7:58
• 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, Dec 19, 2020 at 9:49
• @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. Dec 19, 2020 at 9:55

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

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.

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.

• 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.
Jan 11, 2021 at 18:53
• 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? Jan 11, 2021 at 18:58

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.

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.

• "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? Dec 21, 2020 at 21:56
• 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: scientificamerican.com/article/… Dec 22, 2020 at 4:58
• 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 Dec 22, 2020 at 7:21

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.

• 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. Jan 14, 2021 at 0:18
• 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. Jan 15, 2021 at 16:32
• We don't know whether Mars has no life! Jan 15, 2021 at 16:54