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:
- Seismic information for Earth and the Moon.
- Samples from Earth and the Moon (although some meteorites seem to come from Mars and Vesta.) For Mars, we have compositional information for a handful of lander sites.
- For any that have been visited by spacecraft, we have bulk density, moment of inertia, and magnetic fields.
- Venus: (Not pictured) Crust and core of similar size to Earth's
- Mars: Core proportional to that of Earth. Crust is much thicker
- Mercury: We have noted Mercury's density anomaly. This suggests that its core proportionally very large compared to that of Earth. Recently, moment of inertia data gathered by Messenger have confirmed and amplified this. Mercury's core is 85% of its radius - even bigger than depicted above.
- The moon: Seems to be of almost uniform density and to have a very small core, if any.
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:
Silicate minerals condense as solids at hundreds to thousands of degrees K. E.G.:
Common metals similarly E.G.:
|Nickel-iron alloy||Ni,Fe||1444 K|
Ices: Substances whose molecules bond through hydrogen bonds in their solid phase tend to have low freezing points. E.G:
|Water||H20||180 K (note, these figures are for condensation in a vacuum!)|
- A condensation sequence: In any given region, more refractory substances condensed before more volatile ones
- Distance from protosun: In warmer regions of the Solar System, volatiles were excluded.
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
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:
- Insufficient gravity: A body lacks the gravitational pull to separate materials with different densities. E.G. very small moons (Hyperion) and asteroids (Gaspra).
- Homogeneity of source materials: The materials from which a body coalesces lack density contrasts. E.G. smaller icy moons, probably Earth's moon (see below.)
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:
- Siderophile elements: Elements that mix freely with iron. These are consequently concentrated in planetary cores, so their chondrite normalized concentrations are:
- depleted in surface rocks
- enriched in core material
- Lithophile elements: Elements that mix freely with silicates because of a tendency to bond with oxygen. These are consequently concentrated in planetary mantles and crusts, so their chondrite normalized concentrations are:
- enriched in surface rocks
- depleted in core material
- Chalcophile elements: Of less importance in the current discussion, are elements with a strong tendency to bond with sulfur. These are not strongly differentiated between the mantle and core.
- Atmophile elements: Goldschmidt identified another class of elements. In the outer Solar System (starting beyond 3 AU), Volatiles like water and CO2 that exist as gasses and liquids in the inner Solar System but condense as solids farther out. Elements that primarily occur as gasses or volatiles (H, He, C, N, etc.) are atmophiles. Of course, on icy outer worlds like Pluto, they can make up a major portion of a word's solid bulk.
- rocky crusts and mantles that are depleted in siderophiles and enriched in lithophiles.
- Metallic cores enriched in siderophiles and depleted in lithophiles.
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:
- evidence for the debris disks resulting from such collisions in young solar systems has been known for decades (See Smith and Terrile, 1984)
- Achondrite stony, iron, and stony iron meteorites have to be the products of the collisions of differentiated bodies.
- Samples returned by the Apollo and Luna programs revealed that Moon and Earth rocks share many geochemical characteristics, indicated that they formed in the same region of the Solar System, however by comparison to Earth rocks, moon rocks are strongly depleted in volatile substances and enriched in refractory ones.
- In contrast to chondritic meteorites, which are radiometrically dated to 4.56 ga, the oldest mineral grain identified on Earth is 4.4 ga., (Valley et al., 2014) and the oldest dated lunar material is 4.5 ga. What happened to older Earth and Moon rocks?
- Contrary to earlier expectation, the moon has, at most, a very small core in proportion to Earth (Weber et al., 2011).
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 coalescence of the impactors' cores
- The vaporizing of significant fractions of their mantles.
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:
- The moon has an undersized core because if the impactors were already differentiated, their merged cores would have become part of Earth. The moon would have formed primarily from previously differentiated mantle material.
- Solar radiation and the solar wind would have driven lighter substances - i.e. volatiles - into the outer Solar System, accounting for the "dryness" of moon rocks. The moon, therefore, assembled from an aerosol of refractory substances. Indeed, a recent Hubble Space Telescope image shows the solar wind's effect on the products of a contemporary collision of small asteroids.
- An enduring enigma has been the uniformity of isotopic geochemistry of Earth and the Moon. Herwartz et al., 2014 now claim to have identified a consistent distinction between the 17O ratios of terrestrial and lunar rocks. Nevertheless, the modelling of scenarios for an actual impact that could yield an Earth and Moon with such similar geochemistries is an ongoing preoccupation of planetary scientists (See Zhang et al., 2012.)
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:
- Bizarre axial tilts
- Retrograde rotation
- Improbable moons
- Physical scars of giant impacts
- Disproportionate cores and mantles
- Uranus' bizarre axial tilt is likely the result of at a giant collision. (But its moons would have had to have formed afterwards because they orbit in the plane of Uranus' current equator.) Morbidelli et al., 2012 suggested that for the moons to have formed in their current orbits, Uranus would have needed to have undergone a series of such collisions.
- The chaotic orbits of Neptune's moons (especially the retrograde orbit of the large moon Triton) could result from a near collision.
- Pluto's oversized moon Charon, along with its four smaller moons seem likely to have formed in a manner similar to Earth's moon.
- Venus' retrograde rotation may be the result of a collision.
- Mars' north polar plains may be the result of an impact by a Pluto-sized planetary embryo. That would make it the largest impact structure in the Solar System. Indeed, Citron et al. argue on chemical evidence that Mars' moon Phobos accreted in orbit around Mars from debris lofted into orbit by a giant impact, giving it an analogous origin to the moon. Rosenblatt et al., 2016 argue on the grounds of numerical simulations that both Phobos and Deimos cannot be captured asteroids but must have accreted from debris lofted by a giant impact.
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:
- Condensation sequence
- Primordial heat
- Siderophile elements
- Lithophile elements
- Chalcophile element
- Atmophile element
- Planetary embryo
- Giant collision
- Late Heavy Bombardment
- Willy Benz, Wayne L. Slattery, and A.G.W. Cameron. 1988. Geoscience: Earth's patchy late veneer. Icarus 74(3) 516-528.
- Bernard Charlier, Timothy L. Grove, and Maria T. Zuber. 2016. Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy. Earth and Planetary Science Letters 363, 50Ð60.
- Origin of the Moon - Papers presented at the Conference on the Origin of the Moon, held in Kona, Hawaii, October 1984. 1986. W.K. Hartmann, R.L. Phillips, and G.J. Taylor, eds. Lunar and Planetary Institute. 781 pp. On line version accessed 6/13/2016 at http://ads.harvard.edu/books/ormo/
- Pascal Rosenblatt, Sebastien Charnoz, Kevin M. Dunseath, Mariko Terao-Dunseath, Antony Trinh, Ryuki Hyodo, Hidenori Genda, and Steven Toupin. 2016. Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons. Nature Geoscience online preprint 04 July 2016.
- John Valley, Aaron Cavosie, Takayuki Ushikubo, David Reinhard, Daniel Lawrence, David Larson, Peter Clifton, Thomas Kelly, Simon Wilde, Desmond Moser, and Michael Spicuzza. 2014. Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience 7, 219-223.
- William M. Vaughan, J. W. Head, S. W. Parman, and , and J. Helbert. 2013. What sulfides exist on Mercury? Mainly CaS and FeS. Abstracts of presentations for 44th Lunar and Planetary Science Conference.