In planetary science, planetary differentiation
is the process of separating out different
constituents of a planetary body as a consequence
of their physical or chemical behavior, where
the body develops into compositionally distinct
layers; the denser materials of a planet sink
to the center, while less dense materials
rise to the surface, generally in a magma
ocean.
Such a process tends to create a core and
mantle.
Sometimes a chemically distinct crust forms
on top of the mantle.
The process of planetary differentiation has
occurred on planets, dwarf planets, the asteroid
4 Vesta, and natural satellites (such as the
Moon).
== Heating ==
When the Sun ignited in the solar nebula,
hydrogen, helium and other volatile materials
were evaporated in the region around it.
The solar wind and radiation pressure forced
these low-density materials away from the
Sun.
Rocks, and the elements comprising them, were
stripped of their early atmospheres, but themselves
remained, to accumulate into protoplanets.
Protoplanets had higher concentrations of
radioactive elements early in their history,
the quantity of which has reduced over time
due to radioactive decay.
Heating due to radioactivity, impacts, and
gravitational pressure melted parts of protoplanets
as they grew toward being planets.
In melted zones, it was possible for denser
materials to sink towards the center, while
lighter materials rose to the surface.
The compositions of some meteorites (achondrites)
show that differentiation also took place
in some asteroids (e.g. Vesta), that are parental
bodies for meteoroids.
The short-lived radioactive isotope 26Al was
probably the main source of heat.When protoplanets
accrete more material, the energy of impact
causes local heating.
In addition to this temporary heating, the
gravitational force in a sufficiently large
body creates pressures and temperatures which
are sufficient to melt some of the materials.
This allows chemical reactions and density
differences to mix and separate materials,
and soft materials to spread out over the
surface.
On Earth, a large piece of molten iron is
sufficiently denser than continental crust
material to force its way down through the
crust to the mantle.
In the outer Solar System a similar process
may take place but with lighter materials:
they may be hydrocarbons such as methane,
water as liquid or ice, or frozen carbon dioxide.
== Chemical differentiation ==
Although bulk materials differentiate outward
or inward according to their density, the
elements that are chemically bound in them
fractionate according to their chemical affinities,
"carried along" by more abundant materials
with which they are associated.
For instance, although the rare element uranium
is very dense as a pure element, it is chemically
more compatible as a trace element in the
Earth's light, silicate-rich crust than in
the dense metallic core.
== Physical differentiation ==
=== Gravitational separation ===
High-density materials tend to sink through
lighter materials.
This tendency is affected by the relative
structural strengths, but such strength is
reduced at temperatures where both materials
are plastic or molten.
Iron, the most common element that is likely
to form a very dense molten metal phase, tends
to congregate towards planetary interiors.
With it, many siderophile elements (i.e. materials
that readily alloy with iron) also travel
downward.
However, not all heavy elements make this
transition as some chalcophilic heavy elements
bind into low-density silicate and oxide compounds,
which differentiate in the opposite direction.
The main compositionally differentiated zones
in the solid Earth are the very dense iron-rich
metallic core, the less dense magnesium-silicate-rich
mantle and the relatively thin, light crust
composed mainly of silicates of aluminium,
sodium, calcium and potassium.
Even lighter still are the watery liquid hydrosphere
and the gaseous, nitrogen-rich atmosphere.
Lighter materials tend to rise through material
with a higher density.
They may take on dome-shaped forms called
diapirs when doing so.
On Earth, salt domes are salt diapirs in the
crust which rise through surrounding rock.
Diapirs of molten low-density silicate rocks
such as granite are abundant in the Earth's
upper crust.
The hydrated, low-density serpentinite formed
by alteration of mantle material at subduction
zones can also rise to the surface as diapirs.
Other materials do likewise: a low-temperature,
near-surface example is provided by mud volcanos.
=== Moon's KREEP ===
On the Moon, a distinctive basaltic material
has been found that is high in "incompatible
elements" such as potassium, rare earth elements,
and phosphorus and is often referred to by
the abbreviation KREEP.
It is also high in uranium and thorium.
These elements are excluded from the major
minerals of the lunar crust which crystallized
out from its primeval magma ocean, and the
KREEP basalt may have been trapped as a chemical
differentiate between the crust and the mantle,
with occasional eruptions to the surface.
=== Fractional melting and crystallization
===
Magma in the Earth is produced by partial
melting of a source rock, ultimately in the
mantle.
The melt extracts a large portion of the "incompatible
elements" from its source that are not stable
in the major minerals.
When magma rises above a certain depth the
dissolved minerals start to crystallize at
particular pressures and temperatures.
The resulting solids remove various elements
from the melt, and melt is thus depleted of
those elements.
Study of trace elements in igneous rocks thus
gives us information about what source melted
by how much to produce a magma, and which
minerals have been lost from the melt.
=== Thermal diffusion ===
When material is unevenly heated, lighter
material migrates toward hotter zones and
heavier material migrates towards colder areas,
which is known as thermophoresis, thermomigration,
or the Soret effect.
This process can affect differentiation in
magma chambers.
=== Differentiation through collision ===
Earth's Moon probably formed out of material
splashed into orbit by the impact of a large
body into the early Earth.
Differentiation on Earth had probably already
separated many lighter materials toward the
surface, so that the impact removed a disproportionate
amount of silicate material from Earth, and
left the majority of the dense metal behind.
The Moon's density is substantially less than
that of Earth, due to its lack of a large
iron core.
== Density differences on Earth ==
On Earth, physical and chemical differentiation
processes led to a crustal density of approximately
2700 kg/m3 compared to the 3400 kg/m3 density
of the compositionally different mantle just
below, and the average density of the planet
as a whole is 5515 kg/m3.
== Theories of core formation ==
Iron catastrophe
Rain-out model
== 
See also ==
Core-mantle differentiation
== Notes ==
