Science

The Origin of the Earth: Part IV

First of all, check out this recent article on a gigantic stellar explosion. Just think: the stardust released by this giant supernova might evolve into a new solar system, one day. Perhaps this stardust will evolve into a solar system with terrestrial planets not so unlike our own Earth. Also, the shockwave released in the explosion might trigger the gravitational collapse of a different nebula, triggering the birth of a new solar system. Clearly, the universe is still a dynamic, evolving place. Though, I suppose, this giant supernova explosion actually happened some time ago and is only now being observed on Earth. Depending on how far away this supernova explosion occurred (Phil, help here?), I suppose astronomers may have observed a somewhat ancient event in this giant explosion.

Second, here is Part IV of my Origin of the Earth paper. The fifth and final installment of this paper will follow tomorrow. For now, enjoy!

The Origin of the Earth
Part IV: It’s Getting Hot in Here, Differentiation, and Core Formation

Heating of the Early Earth:

Before moving on to a discussion of the differentiation of the accreted Earth, an understanding of the various processes that lead to the heating of the Earth is important. The early Earth was a very hot place and, indeed, the Earth is still fairly hot— plate tectonics and volcanism on Earth are largely driven by convection in Earth’s hot mantle. There are four main sources of Earth’s heat (e.g. Lutgens and Tarbuck, 2003):

1. Potential (gravitational) energy that was converted to thermal energy during the accretion of the Earth.

2. The kinetic energy of the various impactors, which hit and heated the early Earth.

3. Differentiation of planetary layers. Again, gravitational potential energy was converted to thermal energy.

4. Energy released by the decay of both short-lived and long-lived isotopes. In particular, decay of short-lived isotopes such as 26Al may have contributed significantly to the heating of the early Earth and other planetary bodies.

One should also note that the presence of an early proto-atmosphere, perhaps comprised of gases captured from the solar nebula, may have helped blanket the Earth and prevented loss of heat through irradiation to space as the Earth was accreting. Later, atmospheres composed of volatiles released from accreting impactors may similarly have helped blanket Earth and retain heat.

Overall, the various sources of heat in the early Earth suggest that the early Earth was either partially or totally molten as the planet was accreting. Also, heating by impactors and the blanketing affect of a proto-atmosphere suggests that one or more magma oceans may have existed on Earth as the result of especially large impacts. In particular, a large magma ocean is believed to have formed after a Mars-sized impactor hit the proto-Earth in a collision that is believed to have produced Earth’s moon.

The Differentiation of the Earth:

After Earth accreted from the solar nebula, the Earth differentiated into several layers: a solid inner core, a liquid outer core, a solid but plastic mantle, and a thin shell of crust at the surface. The differentiation of the Earth’s core is one of the most significant events in Earth’s history and led to the concentration of iron as well as of siderophile and chalcophile elements at the center of the Earth. Most likely, the formation of the iron core was the first event in Earth’s differentiation. As will be described below, geophysical models and short-lived isotope systems, especially 182H-182W, can be used to place constraints on when and how core formation occurred in the Earth. Very likely, a deep, long-lived magma ocean which formed when a giant impactor hit the Earth and created the moon played a role in Earth’s core formation. After core formation, the upper Earth then differentiated into an enriched upper crust and a depleted mantle, which is the source material for modern oceanic crust. Again, geophysical models as well as isotopic systems and analysis of Rare Earth Elements (REEs) can provide constraints on how and when Earth’s upper mantle differentiated. Additionally, recent evidence from short-lived 142Nd isotopes suggests that there may have been an early protocrust, which was subducted deep in Earth’s mantle and has never been sampled.

Core Formation:

Several models have been put forth to explain the formation of Earth’s iron core (see Figure 10). In early work, the core was modeled to have formed through a one-stage segregation of metal from a solid, fully-accreted Earth (Halliday, 2006). However, this simple model is probably unrealistic. More recent research has modeled Earth’s core forming gradually over millions of years and has examined the possible role of a magma ocean in the differentiation of the core. Additonally, some researchers have developed models of core formation in which Earth accreted (at least partially) from already-differentiated planetesimals (Halliday, 2006). If Earth was formed through giant impacts between differentiated planetesimals, the cores of these planetesimals may have merged to form the Earth’s core (Halliday, 2006).

Based on constraints from short-lived isotopic systems, Earth’s core is believed to have formed very early in Earth’s history. Most of Earth’s core was probably formed by about 10 million years after the formation of the solar system, and the final stage of core formation likely occurred in a magma ocean which resulted from a giant, moon-forming impact about 30 million years after the formation of the solar system (Jacobsen, 2005). Short-lived isotope systems have come into use recently as mass spectrometry has become sensitive enough to detect very small variations in these systems and are helping geochemists constrain the timing of core formation and other events in Earth’s history. The short-lived isotope systems 182Hf-182W (1/2-life: 9 million years) and 129I-129Xe (1/2-life: 16 million years) as well as the longer-lived U-Th-Pb system are all used to try to constrain when core formation occurred.

The 182Hf-182W system is the most straightforward system and has shed the most light on the formation of the core. Again, 182Hf has a ½-life of 9 million years, which means that variations created by the decay of this element cannot be detected after ~60 million years after the formation of the solar system (Lee and Halliday, 1995). Importantly, both Hf and W are refractory, which means that their ratio should not have been significantly altered during the accretion of the Earth from the solar nebula (Lee and Halliday, 1995). However, Hf is strongly lithophile while W is strongly chalcophile. This means that the Hf/W ratio would be greatly affected by core formation as W would be incorporated into the core while Hf would have remained concentrated in the upper, silicate Earth. The 182W/184W ratio of the Earth is slightly above chondritic (Jacobsen, 2005). This means that Earth’s core formation must have occurred before ~60 million years, after which time it would not be possible to significantly alter the 182W/184W ratio. By modeling the development of the observed 182W/184W ratios on Earth and the moon, scientists think that Earth’s core formed very early in Earth’s history and that the moon-forming impact occurred around 30 million years after the formation of the solar system (Jacobsen, 2005).

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Figure 10: Different Models of Core Formation on Earth (Halliday, 2006)

Evelyn

Evelyn is a geologist, writer, traveler, and skeptic residing in Cape Town, South Africa with frequent trips back to the US for work. She has two adorable cats; enjoys hiking, rock climbing, and kayaking; and has a very large rock collection. You can follow her on twitter @GeoEvelyn. She also writes a geology blog called Georneys.

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9 Comments

  1. The giant supernova was 240 million light years away, so unless some Jurassic animal had a good 'scope, I think they would have missed it.

    In other news, though, Fe60 has been found in sediments in the ocean floor, and it clearly left over from a nearby supernova a few million years ago. I'm reading up on it now for my book; it looks like the star was less than 300 light years way when it blew, and may have been as close as 100. That's surprisingly close!

  2. In the case of short-lived isotopes being partly responsible for early heating, by the time the Earth formed, it must have been some time after the isotopes were generated.

    Do we have any idea how long that time would have been – is it just luck that the isotopes hadn't already decayed away to a negligible level?

  3. Bad Astronomer:

    The giant supernova was 240 million light years away, so unless some Jurassic animal had a good ’scope, I think they would have missed it.

    By "good 'scope," do you mean one which uses tachyons?

  4. Good introduction into planetary evolution. I had taken a isotop geology class as an elective in my undergrad and I still find those methodes facinating in their ability to probe the earliest ages of the Earth.

    I even cited some of the same authors some turm papers back in the day.

  5. Well, Blake, crap. What on Earth (or anywhere else, for that matter) was I thinking? I remember trying to make some joke, erasing it, and then writing that. Maybe I got something mixed up in editing?

    Or more likely I had a Total Brain Failure. Crap. This one is going to take some polish off my smug.

    Oh, who am I kidding? I'm still smug.

  6. Similar to Bjørnar's comment: I'm not familiar with the notation but does the result of 182H-182W allow you to make some really really heavy water which might be handy to keep your free energy nuclear device under control?

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