Formation and differentiation of the planets

Terrestrial planet interiors We can't apply all of the lines of evidence in preceding lectures to the all of the other solid worlds. What we do have:

Planetary interiors
Here is a pictorial synopsis (See also text fig 2.17):


Why are the worlds different?

The density paradox of Earth and other planets seems to be resolved by the presence of dense planetary cores, but this invokes a new paradox: If the planets all formed from the homogeneous material of the Solar Nebula, how have they become inhomogeneous in their structure? To address this we must reconsider the accretion of the planets.

1. Position in the Solar System

Refractory and volatile substances: While the cloud that collapsed to form the Solar Nebula (the protoplanetary nebula giving rise to our solar system) might have been a uniform mix of gas and dust, it did not stay that way for long. Radiation from the protosun would soon have vaporized much of the dust. The composition of substances that condensed from that vapor varied according to the temperature of the region in which they condensed:

Substances like the metals and silicates with high condensation points are refractory. Those with low condensation points are volatile. in the Solar Nebula two processes took place: As a result the bodies of the outer Solar System are compositionally different from those of the inner Solar System.

Bonestell image of Earth during accretion from AntiQuark

2. Heat, Size, and Differentiation

Accretional heat: The process of accretion fundamentally altered the accreting material for the simple reason that in the collision of planetesimals and planetary embryos, kinetic energy is transformed into thermal energy.

E = 1/2 m v2

where m is mass and v is velocity.

The Chesley Bonestell illustration at right shows a mid-20th century idea of what this stage in the Earth and moon's history was like. Earth's outer layer is melted by the heat of a constant barrage of small planetesimals being swept up by the growing planet's gravity. On top of this magma sea ("magma" = molten rock) a thin solid crust of lighter silicates solidifies until the next impact disrupts it.

There is evidence of such a stage in other solar systems. The giant planet imaged circling Fomalhaut is sweeping up the inner margin of a belt of small objects. The results of space age planetary exploration, however, require us to refine this picture substantially.

Accretion and differentiation University of Hawaii - Planetary Science Recent Discoveries
Differentiation: As material accreted, it was heated. Once planetesimals reached the mass at which their own gravity became a significant force, heat and gravity could drive the separation of lighter and heavier substances, provided enough heat was present partially to melt the material. At that point, heavier substances could flow downward toward the center and lighter ones could float upward. In the terrestrial planets, iron primarily moved downward and silicate minerals upward.

Result: a differentiated body. The fact that something as small as Vesta (525 km diameter) seems to be fully differentiated testifies to how uniform this phenomenon was.

Lack of differentiation would result either from:

But we know from the results of the Dawn mission that even an object as small as Vesta (263 km radius) can show full differentiation into rust, mantle, and core. Indeed, iron and achondrite stony meteorites are derived from the cores and mantles, respectively, of planetary embryos (like Vesta) that have been fractured by impacts.

3. The Goldschmidt classification system:

A complicating factor is that liquid silicates and iron are immiscible.

Moreover, various trace elements mix with them to different degrees. These elements were classified by Victor Goldschmidt (1888-1947) according to their chemical affinities:

The big message - Rocky bodies differentiate into: From here on, the words, siderophile and lithophile will be regular parts of our vocabulary.

Giant collision from Mr. Bassrlow's Blog

4. Big Thwacks:

Giant collisions between planetary embryos: As Solar System material coalesced into large planetary embryos during later stages of accretion, large collisions became the common. Compelling evidence of this first came from Project Apollo, although:


The best explanation for these anomalies is that the moon formed from the ejecta of a giant impact between the early Earth and a Mars-sized planetary embryo (Now called "Theia".) This idea was simultaneously proposed by several research teams at the 1984 Kona, HI conference on lunar origins and has robustly resisted falsification ever since. (Even though there is much debate about the fine points.) Such an impact would have resulted in:

The moon is hypothesized to have coalesced from the cloud of vaporized mantle material that condensed in Earth orbit. This scenario explains both lunar anomalies:

Other Impacts: Evidence of planetary embryo collisions has given us a search image for more evidence of possible giant collisions in the histories of other Solar System bodies. Objects with:

all attract researchers' scrutiny.

Mercury: But careful! Mercury, with its oversized core, has long been argued be the remains of the collision that stripped much of the mantle from a larger planetary embryo, leaving only the core and a diminished mantle. Your text embraces this hypothesis. (E.G. Benz et al., 1988.) But note: Analysis of Messenger mission data is now pointing toward a different scenario in which Mercury formed from an iron-rich material called enstatite chondrites that was common in the innermost region of the solar nebula (Vaughan et al., 2013, Namur et al., 2016). Charlier et al., 2016 examined peculiarities in the varying compositions of lava flows of different ages by modeling the evolution its interior. Their result was that lava compositions can only be explained if one assumes that Mercury accreted from enstatite-rich material. Mercury's peculiar history is far from settled.

Native gold in quartz

5. The Late Heavy Bombardment

Too much gold: Despite the affinity of siderophile elements for the iron of Earth's core, there are actually more siderophiles (such as gold and platinum) in Earth's silicate layers than models suggest should be there. Why? Kleine 2011 argues that it was deposited there long after Earth's differentiation and the giant impact, by the late heavy bombardment, between 3.8 and 3.9 ga. Note: That makes two separate lines of evidence for the Late Heavy Bombardment. More to come!

Key concepts and vocabulary:
Additional reading: