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Fraser
2004-Aug-24, 09:03 PM
SUMMARY: Phosphorus is central to life on Earth for many reasons; it forms the backbone of DNA and RNA, and it's an important element in many chemical processes. The questions is, how did the Earth get so much of it? Researchers from the University of Arizona believe that the meteorites that rained down early on during the formation of the Earth could have been the source. They found that many iron-nickel meteorites are rich with minerals that contain phosphorus, and propose that life could have formed around a spot where a meteorite struck the Earth.

What do you think about this story? Post your comments below.

om@umr.edu
2004-Aug-25, 12:17 AM
Meteorites are a reminder that Earth and the other inner planets continue to accrete material. These objects are falling toward the Sun, and just happen to encounter Earth on their journey.

The internal layers of the Earth are a record of changes in the composition of the accreting material.

A few iron meteorites continue to "rain" down in modern times, and an iron meteorite might be one possible source for the Phosphorus found in living organisms.

However, most living organisms contain far more Oxygen, Carbon and Nitrogen than Phosphorus. Further, huge phosphate deposits occur in nature.

With kind regards,

Oliver
http://www.umr.edu/~om

eoleen
2004-Aug-25, 04:20 AM
I think this story is mostly bull. It implies that the Earth was originally formed by the accretion of a group of objects that differed in compostion from "meteorites". This is OBVIOUS nonsense.

The comment preceeding mine, from "om@umr.edu" contains the interesting assumption that the Earth is formed, like an onion, of many layers, and that there has been NO mixing of materials. The application of elementary thermodynamics would reveal that after a period of time (which depends upon the collision rate) the proto-Earth is going to be a molten spheroid: any trace of the original impact sequence will promptly be destroyed. Oliver would seem to be EXTREMELY emeritus.

Fraser
2004-Aug-25, 06:30 AM
I edited your post for language eoleen. Remember the kids... and the rules.

om@umr.edu
2004-Aug-25, 01:05 PM
Eoleen,

Welcome to Universe Today.

As I recall, Professors Turekian of Yale and Vinogradov of the Verdnasky Institute of Geochemistry in Moscow published evidence in reputable journals that Earth accreted heterogeneously, in layers. Those papers were published in the late 1960s and early 1970s.

Isotope analyses of noble gases and a few other elements in the Earth confirmed this finding. Those results were also published in reputable scientific journals in the 1980s.

However, nobody claimed that "there has been NO mixing of materials."

The upper mantle melted and differentiated to form the Earth's crust, atmosphere and oceans. The decay products of short-lived nuclides, I-129 and Pu-244, trapped inside the Earth today constrain this outgassing to the first 200 My of the Earth's history.

The equilibrium position defined by thermodynamics is frequently not reached, unless the temperature is very high.

Witness the "thermodynamically unstable" form of carbon worn as jewlery and commonly believed to "be forever".

With kind regards,

Oliver
http://www.umr.edu/~om

eoleen
2004-Aug-25, 10:59 PM
I'm not quite sure what you mean by "Earth accreted heterogeneously, in layers".
I am certainly not asserting that the various, and varied objects from which the Earth accreted were of uniform composition - just that they were samples from the same large volume of material that made up the early Solar system, which was, in turn, a fairly representative sample of the material in this neck of the galaxy at that time.

The proto-Earth would be hot from released potential and kinetic energy, and would radiate that energy away as a black body. At some point the influx of energy (mostly from collisions, little from the early Sun, which according to my reading, would be expected to be cooler than at present) would exceed the loss due to radiation and the temperature would start rising. As the mass and temerature grew the material would grow molten and differentiate. That it is still molten, or semi-molten, at least, is indicated by several different phenomena, among which is the wandering of the Earth's magnetic poles and the changing of the Earth's magnetic field structure. All of this would seem to indicate that the proto-Earth was thoroughly heated thru and thru, as would the geometry of the situation: the center, surrounded by a large mass of material, would cool the most slowly: the surface, able to radiate into space, the most quickly.

All the models of the Earth that I have seen indicate a core comprised mostly of iron at high temperature and pressure: I find the claim that this came about by iron-rich objects accreting first, and silicacious object last, or some such schema, to be rather far-fetched. For one thing, one would have to explain why the source was so differentiated.

As far a "layers" go: that too is something that would need to be explained - both as to exactly what is meant and how it came about. Dropping a rather cool, if fast-moving rock into a hot and viscous if not molten object is going to result in at least some penetration and resultant mixing.

If this is what you posit I would like to see your references.

As far as thermodynamic equilibrium and diamonds go, I certainly make no claim that they are aboriginal: the source seems to comparatively recent, and to be well defined: it should be - there has been enough incentive to explore the mechanism quite thoroughly. There is, I think you must admit, a rather major difference between the cooling of the proto-Earth taking an exceedingly long time (with respect to a human life-time) and the cooling of a plume of HOT rock penetrating the Earth's crust and spewing into the atmosphere at super-sonic speed. (Even then diamonds occasionaly contain inclusions of graphite, or are found as inclusions in a piece of graphite.) The rates of cooling are not at all comparable.

The atmosphere would also seem to be something that has experienced vast changes: the current oxidizing atomosphere can not be aboriginal by any stretch of the imagination. Based upon what I have read about the gas content of the local universe (spectro-analysis of comets, etc.) I would lay money down on it being mostly H2 CO2, H2O, HCN, N2, some NOx, SOx, etc. Kinda hard to breath, and hot light gases would tend to boil off rather rapidly, rather than stick around. In THAT respect, yes, the proto-Earth "formed in layers".

I look forward to your comments.

Ed

om@umr.edu
2004-Aug-26, 02:52 AM
Ed,

Here are three references:

[1] K. K. Turekian and S. P. Clark, Jr., "Inhomogeneous accumulation of the earth from the primitive solar nebula", Earth & Planetary Science Letters 6 (1969) 346-348.

[2] A. P. Vinogradov, "Formation of the metal cores of the planets", Geokhimiya 10 (1975) 1427-1431.

[3] O. K. Manuel and D. D. Sabu, "The noble gas record of the terrestrial planets", Geochemical Journal 15 (1981) 245-267.

With kind regards,

Oliver
http://www.umr.edu/~om

Duane
2004-Aug-26, 02:20 PM
We have been over this in previous topics, and the idea has been explored in the Iron Sun discussion thread in the Alternate Theories forum. To put it shortly, recent p and s-wave studies of the Earth's mantle have revealed the lower mantle to be melted and fully differentiated, with plumes of material rising from the core/mantle boundary and other areas of cooler material subducting.

Put simply, the molten mantle recycles.

The references given above are outdated and do not take into account these more recent studies.

om@umr.edu
2004-Aug-26, 09:28 PM
Seismic studies in no way negate the experimental evidence for heterogeneous accretion of the Earth in the papers cited above.

With kind regards,

Oliver
http://www.umr.edu/~om

Duane
2004-Aug-27, 04:22 AM
No, but they do negate the possibility that the lower mantle remains "unmelted and undifferentiated".

om@umr.edu
2004-Aug-27, 02:37 PM
Ed,

Radiogenic Xe-129 has accumulated inside the Earth from the decay of I-129. This extinct isotope was present at the birth of the solar system, but decayed away with a half-life of 16 million years (0.016 billion years). Radiogenic Xe-129 is commonly seen in meteorites.

Fission produced Xe-136 has accumulated inside the Earth from the decay of Pu-244. This extinct isotope, which is produced only in supernova explosions, was present at the birth of the solar system. Pu-244 decayed away with a half-life of 82 million years. Fission produced Xe-136 from the decay of Pu-244 is common in meteorites, especially in phosphorous-rich minerals.

Radiogenic Ar-40 accounts for almost all the argon in air. Radiogenic Ar-40 has also accumulated inside the Earth from the decay of K-40. This isotope is still around and decaying because its half-life is 1.3 billion years. Radiogenic Ar-40 is common in meteorites and in potassium-rich minerals.

Primordial Helium* is still trapped inside the Earth today. Helium is one of the smallest, fastest moving elements. Helium easily passes through glass at room temperature, for example.

The amounts of primordial and radiogenic noble gases in air and inside the Earth indicate that:

[1.] The outer 17% of the Earth melted and differentiated into the upper mantle, the crust, the oceans, and the atmosphere within the first 200 million years (0.20 billion years) of the Earth's history.

[2.] This early, catastrophic degassing ceased while I-129 and Pu-244 were still alive.

[3.] The lower mantle did not differentiate and release its volatiles. However, the primordial Helium* trapped in the lower mantle continues to leak into the highly depleted upper mantle today. Primordial Helium is observed in mid-ocean ridge basalts, for example.

Here is a pdf file of the paper.

http://web.umr.edu/~om/archive/NobleGas.pdf

With kind regards,

Oliver
http://www.umr.edu/~om

* See clarification below onEarth's Primordial Helium.

antoniseb
2004-Aug-27, 02:49 PM
Originally posted by om@umr.edu@Aug 27 2004, 02:37 PM
Helium easily passes through glass at room temperature
This caught me by surprise. How rapidly can Helium go through a one millimeter thick glass wall [or bulb] at room temperature? I know that Helium gets trapped pretty well by mylar and rubber balloons. It is hard to picture it just passing through glass as though it wasn't there. [I'm sure this isn't what you meant, but it certainly what it reads like.]

om@umr.edu
2004-Aug-28, 12:18 PM
Clarification:

The Helium trapped in the Earth's mantle is primordial, in the sense it is not radiogenic (from alpha decay of Uranium, Thorium, etc.).

However, this Helium was actually implanted by the early solar wind into the surfaces of silicate and iron particles before they accreted together to form planet Earth.

Measurements reveal that the Helium and Neon in the Earth's mantle have the isotopic composition of solar-wind-implanted gases.

Solar-wind-implanted gases are found in material that formed in the inner part of the solar system:

[1.] Solar-wind-implanted gases are found in the dark regions of many stone meteorites, including Pantar and the Fayetteville meteorite that fell in Fayetteville, Arkansas in 1934.

[2.] Solar-wind-implanted gases have also been reported in at least one iron meteorite: As I recall, Robert Pepin of the University of Minnesota reported solar-wind-implanted gases in the Washington, County iron meteorite.

[3.] Solar-wind-implanted gases are abundant in the surfaces of fine dirt particles from the Moon.

The Helium trapped in the Earth's mantle is not primordial in the sense that it does not have the isotopic composition of the Helium found in carbonaceous chondrites and Jupiter.

With kind regards,

Oliver
http://www.umr.edu/~om

Greg
2004-Aug-30, 02:53 AM
A comment on eoleen's notion that this theory is bunk. I think you should rethink your supposition in regards to time. The timeframe that the authors are referring to includes a period of time after the Earth's crust had cooled as far as I can tell. Any chondrite metorites hitting the Earth before that time would of course have melted and been mixed in with whatever mass had been accreted as you suggest. Looking at nearby objects such as the moon, it becomes quite apparent that significant bombardment by meteroites was taking place after large bodies such as the Earth and Moon had formed a solidified crust. Phosphorous in a metabolically active form could have been quite useful to early organisms trying to gain a foothold in a newly formed ocean at that time.
The notion that different periods of material accumulation existed resulting in an "onion-skin" method of planetoid formation is still quite controversial and I think does not make alot of intuituve sense based on what we currently observe and know (admittedly still very little) about planetary discs and planet formation. If a cloud of dust were to separate (like a centrifuge) into light and heavy elements then I would expect planets made up entirely of iron/nickel in the inner solar system and entirely of silicates in the middle and lithium/sulfur and water in the outer solar system, yet we see all types of elements distributed throughout. I might be willing to entertain the notion of different periods of material being added to planetoids in the context of a series of supernovas in a stellar nursery suddenly adding alot of material of the same type to a disc based on how far away they are when they explode. But even this seems highly speculative to me.

om@umr.edu
2004-Aug-30, 03:59 AM
As I recall, heliocentric sorting has been reported in the different types of asteroids.

Like planet Earth, the bulk planetary system, and the (oninion-skin model of) evolved massive stars, the carbon-rich material is in the outer regions.

I do not recall the reference for this. One of those chaps studying the surfaces of asteroids reported this several years ago, probably in Nature or Science.

With kind regards,

Oliver
http://www.umr.edu/~om