While effectively absent for the past year, I've been following threads of interest for some time, unable to participate due to extremely slow dial-up only available to me far out on Colorado's eastern plains and the near constant demands for care-giving. Finally equipped with wireless broadband. Like night and day. Glad to be back.
However, it seems the more I learn, the more I discover I don't know or understand. Help is needed.
I don't know if Earth has an iron core or a hydrogen core, albeit the latter seems more likely to me; given the constant loss of hydrogen into space which appears to be physical evidence that some sort of hydrogen reservoir exists within Earth. It is the unquestioning belief of many that Earth has an iron core that is puzzling to me.
First, I submit the following as a reasonable statement: Currently, temperatures and materials within Earth's core are totally unknown. Exact determination by physical inspection still remains impossible. There are various theories, assumptions and calculations. Some are logical, some illogical, some absurd, but in today's final analysis, it seems to me that no one really knows for sure.
As best I can understand from recent research: Earth's magnetic field convinced early theorists that Earth's core was iron, because, at that time, only iron was known to be magnetic. [Hydrogen's magnetic and metallic properties were unknown until recently.]
In 1906, seismic analysis determined Earth's core was 16% of its volume and this was estimated to contain 32% of Earth's mass; but only if the core was largely iron.
Unfortunately, iron only constitutes 1.1% of all known elements in the cosmos, yet iron is considered to be five times more abundant just within Earth's crust.
To date, two imaginative theories seem to be most popular as explanations for how the theorized enormous excess of iron came to be in Earth's core:
The Grand Bombardment Theory assumes a swarm, consisting of billions and billions (?) of iron-rich planetesimals (micro-planets) bombarded a smaller proto-Earth over a short (?) period of time. These iron-rich planetesimals supposedly added considerable mass and made Earth completely molten through their concentrated impacts within the short (?) period of time. Then, gravity supposedly forced (only) molten iron and nickel into Earth's core, vaporizing the hydrogen originally deposited there when Earth condensed within a spinning mass of dust and gases; largely hydrogen. The vaporized core hydrogen is then assumed to have been blown away by solar winds.
I hope I stated the Bombardment assumption correctly. This is what I was taught in college.
However, it seems to me Earth is a relatively insignificant mass in an immense volume of space, orbiting between Sun and Jupiter; whose individual gravitational attractions greatly exceeds that of proportionately minuscule Earth. Therefore, it seems to me the vast majority of iron-rich planetesimals wandering within our galaxy would much more likely have been attracted to either the Sun or Jupiter and largely ignored Earth.
This appears evident in the fact that neither the Sun or Jupiter contain much iron; apparently even less than the cosmic proportion. This fact alone appears to make it impossible for any swarm of planetesimals to have even existed; iron-rich or stony. It also appears to me that it is more likely for a swarm of planetesimals to be only a figment of someone's imagination; fabricated to support a logical assumption which lacks physical proof.
Then we are left with the problem of where did all these billions of planetesmials come from and how were they formed largely of iron. All of which just seems just like more assumptions to prove an assumption to me.
More recently, we were offered the Collision Theory, which assumes a smaller proto-Earth collided with an even smaller planet containing a relatively enormous iron core. Then, these planets supposedly melted together and the smaller planet's iron core became Earth's core; while a portion was flung into space and ended up condensing into Earth's moon. How the smaller planet developed an iron core is just as unexplained and this scenario appears equally unlikely; albeit very imaginative.
Does anyone know of a more plausible theory of how Earth developed a core containing such a enormous excess of iron?
I found the following interesting, especially since no swarms of planetesimals were observed:
Scientific FrontLine / Hubble Observations Confirm that Planets Form from Disks Around Stars
May 3, 2009: Hubble Observations Confirm that Planets Form from Disks Around Stars
More than 200 years ago, the philosopher Emmanuel Kant first proposed that planets are born from disks of dust and gas that swirl around their home stars. Though astronomers have detected more than 200 extrasolar planets and have seen many debris disks around young stars, they have yet to observe a planet and a debris disk around the same star.
Now, NASA's Hubble Space Telescope, in collaboration with ground-based observatories, has at last confirmed what Kant and scientists have long predicted: that planets form from debris disks around stars.
The Hubble observations by a team of astronomers led by G. Fritz Benedict and Barbara E. McArthur of the University of Texas at Austin show for the first time that a planet is aligned with its star's circumstellar disk of dust and gas. The planet, detected in 2000, orbits the nearby Sun-like star Epsilon Eridani, located 10.5 light-years from Earth in the constellation Eridanus. The planet's orbit is inclined 30 degrees to Earth, the same angle at which the star's disk is tilted. The results will appear in the November issue of the Astronomical Journal.
