EVOLUTION OF THE INTERIORS OF THE TERRESTRIAL PLANETS

I. FORMATION: step (a)

Around 4.5-4.6 billion years ago, the Terrestrial planets formed through the coalescence of small solid particles into planetesimals and then through the accretion of the planetesimals into protoplanets, planet-sized objects.
The protoplanets were unable to capture and hold gases because of their small masses and the high temperature of the inner Solar Nebula.
The formation process led to an initially homogeneous set of bodies, that is, objects where the chemical elements were mixed together.

II. MELTING AND DIFFRENTIATION: step (b)

A. Melting
Around the time the planets were nearly formed, their interiors started to melt (or at least become soft) through:

accretional heating comes from the energy released when objects collide, happens most often during the early formation years of the planets. Recall how the Moon formed; see a recent simulation of the formation of the Moon. Video made at NASA Ames Research Center.

the decay of radioactive nuclei can release energy over billions of years and so supplies heat to planets over their entire evolution

compression arises when planets shrink and compress (the compression heats the planets), this happened during a planet's early evolution and continues until today in planet's such as Jupiter and Saturn.

B. Chemical differentiation

Chemical differentiation took place in the molten Earth. The reason it occurs is easy to understand; dense material sinks while less dense material floats. For example, in a glass of water, metals sink because they are denser than water. Things like cork and wood float because they are less dense than water.
Chemical differentiation (1) started around 500 million years after the Earth's formation as heating primarily due to accretional heating and radioactive decay, raised the interior temperature of the Earth to around 3,000 F even at depths as small as 500 or so kilometers. At these temperatures, iron melts. (2) The iron and other heavy elements then slowly sank to the Earth's core and lighter materials like silicates and volatile elements rose, separating themselves. The volatiles were eventually released at the surface of the Earth through volcanism contributing to the formation of our atmosphere and oceans. (3) The slowly sinking iron gained energy as the Earth's gravity pulled it downward, the iron releasing the energy through friction as it sank. This release of energy helped to melt the core in addition to the energy release from the decay of radioactive nuclei.

(4) This meant that dense elements sank to the center of the Earth, less dense materials moved closer to the surface where they could crystallize, and volatile materials floated to the surface and were released through volcanism. The above steps led to the three-layered structure of the Earth, layers defined by their chemical compositions

the iron core
the silicate-rich mantle composed of magnesium, silicon, and oxygen
the silicate-rich crust composed of silicon, aluminum, and oxygen

III. INTERIOR STRUCTURES OF THE EARTH AND MOON: step (c)

A. The Earth

The separation of the Earth into the crust, mantle, and core is not the best way to look at the structure of the Earth from a mechanical standpoint. From a mechanical standpoint, it is better to view the crust and solid outer part of the mantle as a single unit. This layer is named the lithosphere. The lithosphere sits upon another layer, which although also made of silicates, can flow. This plastic layer is called the asthenosphere. Below the asthenosphere sits the hot core material.

The current temperature of the Earth increases as you move inward because: (1) there is residual heat left over from the formation of the Earth; and (2) there is still current heat input from the decay of the radioactive nuclei. Based on models of the interior of the Earth, the current temperature of the inner core is 5,200 C or roughly 9,400 F. The pressure is 3.6 million bars, at these high pressures the inner core of the Earth is actually solid.

Because the Earth is hottest at its center and coolest at its surface, heat flows outward from the core to the surface of the Earth. There are three ways for heat to flow:

radiation (important in atmosphere)
conduction (important in solid inner core and solid lithosphere)
convection (important in asthenosphere and in the atmosphere)
That energy is carried by convection in the athenosphere beneath the lithosphere turns out to be very important. These large-scale mass motions produce the plate tectonics found on the Earth.

B. The Moon
The general structure of the Moon is quite similar to that of the Earth, a crust, mantle, and core. This is true even though the heating and melting of the Moon resulted from its violent formation process. There are differences however.
The crust of the Moon is asymmetric. It is thinner on the side which faces the Earth. The thickness of the crust runs from around 60 km to 100 km on the near to far sides, respectively. This interesting property of the Moon is not yet understood.
The core of the Moon is much cooler and smaller than the core of the Earth. The Moon's core is around nearly 3,000 F, close to hot enough to melt its iron core region. However, because of the high core pressure, the inner core appears solid with an outer liquid core, and a partially melted region outside of the liquid outer core not seen in the Earth.
Seismology. Much of the interior information comes from seismometers left by the Apollo missions. What might cause Moonquakes? There are thought are to be 4 types: (1) impacts, (2) tidal forcing by Earth, (3) expansion and contraction of the outer layers of the Moon as it goes from night-to-day: and (4) unknown.

Moonquakes do not happen for the same reasons as do Earthquakes. The Moon does not have active geology as does the Earth. This is due to the fact that the Moon has a fairly cold interior. For planets, in general, this is a determining factor as to whether they show active geologies. The geological activity of a planet is driven by the heat flow from the interior to the surface. It is the size (diameter) of a Terrestrial planet determines how fast it cools and therefore how hot it will be at any given age.

The larger the diameter of a Terrestrial planet, the slower that it will cool and hence, the hotter will be its interior

the smaller the diameter of a Terrestrial planet, the faster it will cool and hence, the cooler will be its interior

IV. INTERIOR STRUCTURES OF THE TERRESTRIAL PLANETS
Compare the Terrestrial planets, Mercury, Venus, the Earth, Moon, and Mars:

Although all Terrestrial planets are divided into the crust, mantle, and core where the crust and mantle are composed of silicates and the core composed of primarily iron, the relative sizes of the cores of the planets changes systematically. The iron core compared to the total mass of the planet is 75% for Mercury, 33% for Venus and Earth, and 20% for the Moon and Mars. This trend naturally reflects the kinds of materials that can remain solid as the temperature increases as one moves toward the center of the Solar Nebula. However, this alone does not explain why Mercury contains so much iron compared to its mass when compared to the other Terrestrial planets. Different workers have suggested that perhaps a large impact early in Mercury's history may have lowered to the mantle mass or perhaps the magnetic field of the young Sun may have affected the early concentrations of iron in the Solar Nebula. This question is more than of esoteric interest because a strong conducting material like iron in the core of a planet is needed for a strong planetary magnetic field. As we find later (for Mars), the existence (or lack) of a magnetic field plays a crucial in the evolution of the atmosphere of a Terrestrial planet.