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Fraser
2005-May-23, 04:45 PM
SUMMARY: Astronomers have analyzed the dusty discs of newly forming planets around other stars, and have discovered that rocky planets (like our own Earth) form much further from their parent stars than previous theories suggested. Protoplanetary dust that gets to close to its parent star completely evaporates, and never gets the chance to clump together into larger objects. The team made these new, more precise measurements using powerful instruments like the Hubble Space Telescope and the giant Keck Observatory.

View full article (http://www.universetoday.com/am/publish/rocky_planets_further_away.html)

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

om@umr.edu
2005-May-23, 05:09 PM
Interesting.

Rocky planets that formed in the solar system out of Fe, O, Ni, Si, S, etc. definitely formed much closer to the Sun than did the gaseous planets that formed out of light elements like H, He, C, and N.

"Tracer isotopes" of xenon show that heavy elements (Fe, O, Ni, Si, and S) that formed rocky planets close to the Sun never completely mixed with the light elements (H, He, C, and N) that formed the gaseous planets further away from the Sun.

In the inner part of the solar system and in troilite (FeS) inclusions of meteorites the heaviest isotope of xenon, Xe-136, is only about half as abundant as it in xenon from the outer part of the solar system or in xenon trapped in diamond and graphite inclusions of meteorites.

With kind regards,

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

aeolus
2005-May-23, 05:40 PM
I don't quite understand. If Rocky planets don't form as close to the sun as previously thought, how close to the sun did we used to think planets formed? Mercury is pretty close to the sun, it formed. Did we used to think planets formed closer than that?

lswinford
2005-May-23, 05:55 PM
Doesn't the story sort of assume that rocky planets formed as cold lumps, a sort of conglomerate of cold stones? Wouldn't the protostellar cloud have been comparatively cold, consolidating around centers of mass? Then from the pressures of those masses and the energy of matter falling into those centers of mass and compressing those primordial bodies then been the source of the stellar fusion fires? Our planet is more liquid than solid, our cooled and congealed crust floating upon a molten, or definitely more fluidly plastic, mantle about a more solid-seeming core. If, as other discussion states, we had an early atmosphere like that of a certain Saturn moon, perhaps it is the light gasses, the light hydrogen compounds that 'boiled off' from the fires and pressures of our early sun, perhaps swept up by our colder and more distant gaseous giants? I almost get the idea that the rocky material the article describes is more analogous to the rocky "shatter" pieces that make up the astroid belt between Mars and Jupiter, or the Kuyer objects or Oort cloud materials. Those items seem more like splatters from such early shocks and collisions that perhaps broke our moon out of earth. Then maybe our asteroid belt is a marker zone between 'too far' and 'too close' as they were observing? Gases on 'this side' fell into the sun, gasses on 'that side' were swept up by the gas giants--but that is still opposite of what they report. Oh, well, maybe I better go back to my regular job and leave the heavy musings to others. :blink:

om@umr.edu
2005-May-23, 07:01 PM
I agree, Lswinford, that "our asteroid belt is a marker zone."

It marks the boundary between two distinct regions of the protoplanetary disk:

a.) An outer region where light elements (H, He, C, and N) associated with "strange" xenon (with excess Xe-136) were dominant, and

b.) An inner region where heavy elements (Fe, O, Si, Ni, and S) associated with "normal " xenon were dominant.

With kind regards,

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

biknewb
2005-May-23, 08:17 PM
Do I understand correctly that Earth could not have formed if the Sun had been bigger by a mere 50%?

regards

Nereid
2005-May-23, 09:01 PM
Originally posted by aeolus@May 23 2005, 05:40 PM
I don't quite understand. If Rocky planets don't form as close to the sun as previously thought, how close to the sun did we used to think planets formed? Mercury is pretty close to the sun, it formed. Did we used to think planets formed closer than that?
Yes; unfortunately, even the University of Michigan PR doesn't give a link to the relevant papers (you have to dig).

Nereid
2005-May-23, 09:09 PM
Doesn't the story sort of assume that rocky planets formed as cold lumps, a sort of conglomerate of cold stones? Wouldn't the protostellar cloud have been comparatively cold, consolidating around centers of mass?
Unfortunately, the original PR somewhat oversimplies things; Monnier and Millan-Gabet's paper refers only to "dust sublimation" - the accretion processes, their history during the protostar cloud collapse phase, etc are important considerations, as are several other potential mechanisms of possibly great importance in (terrestrial planet formation) are beyond the scope of their paper (apparently).

Bottom line: this is an active area of research, and will likely continue to be for several more decades; expect many interesting details to be filled in - and changed - as theory develops and better data come to hand.

Nereid
2005-May-23, 09:16 PM
Originally posted by biknewb@May 23 2005, 08:17 PM
Do I understand correctly that Earth could not have formed if the Sun had been bigger by a mere 50%?

regards
This is, unfortunately, not an accurate implication from the work being reported in the original PR.

In any case, a tad more precision in the question is indicated - e.g. 'bigger by a mere 50%' (compared to what? in mass/radius/temparature/?); 'Earth' (a planet with comparable composition, size, *and* distance from our Sun? a planet with comparable composition, size, *or* distance from our Sun?) 'formed' (an 'Earth-like planet', by processes hithertofore considered primary? by processes hithertofore considered less important?).

VanderL
2005-May-23, 09:28 PM
Bottom line: this is an active area of research, and will likely continue to be for several more decades; expect many interesting details to be filled in - and changed - as theory develops and better data come to hand.

Bottom bottom line: we know nothing.

Cheers.

Sixbender
2005-May-23, 09:46 PM
I've been curious about something for a time, maybe one of you can explain the conventional thinking. Maybe it's a "dumb" question, if there is such a thing:
It has to do with the proto-planetary disc and the Sun's mass, which has been in constant decline since fusion began, right? While the change is small compared to the whole, it has been going on a long time. This leads me to assume that planetary orbits are, and have been, getting larger in relation to the parent star. Sure, it would be hard to find an astronomical reference, especially if the apparent disc is getting larger over time. Has anything been published that would take this into account so that the orbit, say of Mars for example, is calculated in billion-year increments? Does this have any implications in terrestrial planet formation and evolution?

om@umr.edu
2005-May-23, 11:34 PM
Originally posted by Sixbender@May 23 2005, 09:46 PM
I've been curious about something for a time, . . . It has to do with the proto-planetary disc and the Sun's mass, which has been in constant decline since fusion began, right? While the change is small compared to the whole, it has been going on a long time.
Hi, Sixbender.

The Sun loses mass from nuclear reactions that convert mass to energy, including fusion.

The Sun loses mass by ejecting H and He from its surface in the solar wind.

These values are given on the upper right side of my web page in a link to Sun (Output).

I am not allowed to post the link to these calculations here. If you cannot find it, send me an e-mail <om@umr.edu> and I&#39;ll send the link to you privately.

With kind regards,

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

Greg
2005-May-24, 02:30 AM
Hmm, this thread seems to be going off on all sorts of tangents.
It is a bit of a shame that the article did not provide specifics to back up the header, so I guess everyone is left to speculate and fill in the blanks. What I think is meant by this header is that the portion of the disc that forms terrestrial planets (the mostly silicon phase which is found closer to the star) may form further out than the authors had anticipated based on previous research articles. OF course this begs the question as to whether this applies only to stars with .5 more solar masses than ours. Also, do not forget that that the composition of the disc itself does largely depend on how old or well processed the starting material is. The first few generatiosns of stars have almost entirely hydrogen and therefore have almost no silicon phase to the ring, and therefore form only gas giants. Later generations of stars will have more and more silicon in the ring and would be expected to form a silicon phase more AUs out from the star.
Once you have your disc, planets form by accretion as silicon tend to be more sticky, it forms solid planets. With little lighter elements there is little in the way of a gasseous atmosphere closer to the star.
One interesting implication of this article, if it is right, is that I think it backs up the theory that it is normal for planets to spiral into their stars. This happens due to friction in the disc causing the orbit of the planets to decay, conceivably right into the parent star and their anihilation. Several planets could have formed in this system, only to wind up crashing into the sun. What may save some solar systems are massive x-ray blasts from some newly formed stars that will help push back planets and keep them from spiraling in. Such blasts may help explain why Mercury has been stripped clean of its outer crust. Alomst all of these insights I have mentioned have only very recently been discovered, but it all seems to fit together and make sense to me. I am eager to learn more and the next 10 or 20 years will provide a horde of data to sate anyone&#39;s appetite.

om@umr.edu
2005-May-24, 03:33 AM
Originally posted by Greg@May 24 2005, 02:30 AM
The first few generatiosns of stars have almost entirely hydrogen and therefore have almost no silicon phase to the ring, and therefore form only gas giants.* Later generations of stars will have more and more silicon in the ring and would be expected to form a silicon phase more AUs out from the star.

That is what I was taught too.

However, evidence of iron-rich material has since been discovered soon after the Big Bang.

So I&#39;m not certain metallicity correlates with age.

There may be other variables, like stellar mixing, that determines the amount of metals at the surface of stars. For example:

1. There was one recent report of a correlation of stellar surface metallicity with the presence of planets. Stars with planets wobble on their journey through space - shaking action that inhibits the star&#39;s ability to segregate and store heavier metallic atoms in the interior of the star.
Stars with Planets Have High [Fe/H] (http://lanl.arXiv.org/pdf/astro-ph/0504154)


2 I have been clear in advising you that I will not allow you to post ideas YOU have about Jason Wright&#39;s findings. You are being deceitful and unethical.

If I had seen this sooner, I would have asked for you to be banned. Consider this your last warning on the subject.

With kind regards,

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

iantresman
2005-May-24, 12:42 PM
Originally posted by fraser@May 23 2005, 05:45 PM
.. have discovered that rocky planets (like our own Earth) form much further from their parent stars than previous theories suggested.
From which we deduce that either: 1. The current theory of planetary formation is wrong.
2. Planets can move around a Solar System after formation.
3. The data/observations are wrong
4. The data has been misinterpretted.
5. My deduction is wrong.
In which case, what is the evidence for the accretion model for the formation of planets?

Regards,
Ian Tresman

om@umr.edu
2005-May-24, 01:07 PM
Originally posted by iantresman+May 24 2005, 12:42 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (iantresman &#064; May 24 2005, 12:42 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-fraser@May 23 2005, 05:45 PM
.... rocky planets (like our own Earth) form much further from their parent stars than previous theories suggested.
From which we deduce that either:
1. The current theory of planetary formation is wrong.

2. Planets can move around a Solar System after formation.

3. The data/observations are wrong

4. The data has been misinterpretted.

5. My deduction is wrong.

In which case, what is the evidence for the heterogeneous accretion model for the formation of planets?

Regards,
Ian Tresman [/b][/quote]
Hi, Iantresman.

I inserted the underlined word, heterogeneous, above.

These measurements tell us planet Earth accreted heterogeneously, beginning with the formation of its iron core in an iron-rich central part of the protoplanetary disk:

1. Decay products of short-lived elements, e.g. I-129 and Pu-244, found inside the Earth. See this paper by Boulos and Manuel, "The xenon record of extinct radioactivities in the Earth," Science 174 (1971) 1334-1336.

2. The relative amounts of radiogenic Xe-129, Xe-136, and Ar-40 in air and still trapped inside the Earth today. See this paper by Manuel and Sabu, "The noble gas record of the terrestrial planets", Geochemical Journal 15 (1981) 247-267.

3. The presence of primordial (non-radiogenic) He-3 trapped inside the Earth today, leaking out in Mid-Ocean Ridge Basalts (MORB) and in CO2 gas inclusions of deep-seated rocks. See above 1981 paper by Manuel and Sabu.

4. Measurements at the University of Tokyo showing that isotopes were never completely mixed, even in massive iron meteorites as big as a building. See the PhD dissertation of Qi-Lu, University of Tokyo (1991).

This last measurement shows that iron meteorites formed directly from material made near the core of a supernova rather than by geochemical extraction of iron from a large interstellar cloud.

With kind regards,

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

iantresman
2005-May-24, 01:11 PM
Sorry if this sounds a little flippant, but if the accretion theory if planet formation falsifiable (as good science is supposed to be), can anyone provide any examples of what might constitute such evidence?

In which case, why don&#39;t any of the following falsify the theory? http://www.space.com/scienceastronomy/myst...day_050307.html (http://www.space.com/scienceastronomy/mystery_monday_050307.html) (2005)
&#39;Youngest planet&#39; flouts theory of formation (http://www.newscientist.com/article.ns?id=dn5052) (2004)
Bizarre new planets puzzle astronomers (http://www.theforbiddenknowledge.com/hardtruth/new_planets.htm) (2001)
Planet Formation Remains a Puzzle (http://www.suite101.com/article.cfm/astronomy/15349) (1999)

Regards,
Ian Tresman

iantresman
2005-May-24, 01:25 PM
Originally posted by om@umr.edu@May 24 2005, 02:07 PM
These measurements tell us planet Earth accreted heterogeneously, beginning with the formation of its iron core in an iron-rich central part of the protoplanetary disk:

1. Decay products of short-lived elements, e.g. I-129 and Pu-244, found inside the Earth. See this paper by Boulos and Manuel, "The xenon record of extinct radioactivities in the Earth," Science 174 (1971) 1334-1336.
Surely the decay products suggests when, rather than how planetary formation took place?

Regards,
Ian Tresman

lswinford
2005-May-24, 01:49 PM
Hmm...Jupiter is 11 times as wide as earth and has some 300 times the mass. Saturn is 9 times the earth&#39;s diameter and some 90-odd times the mass. Uranus has something like 4 times the earth&#39;s diameter and over 14 times the mass. Neptune is also about 4 times the earth&#39;s diameter, though smaller than Uranus, and has about 17 times the earth&#39;s mass. I&#39;ll bet they&#39;ve accreted a good share of rocks over the eons, along with the obvious gasses they&#39;ve gathered. Neptune is a case in point, how is it so much denser than Uranus? Not just because of the cold is it? (...as in colder gases occupying a smaller volume) The sun got the obvious lion&#39;s share of the accretion, other centers of gravity gathered theirs. Add up the masses of moons around Jupiter and Saturn and you have a couple of pretty good rocky planet-size masses there too. I suspect if we get too simple in our stories we will simply leave too many possibilities out, suppose? :unsure:

lswinford
2005-May-24, 01:56 PM
Oh, yeah, I meant to ask. Since such an enormous amount of ionized particles are emitted by the sun, and since the sun has this enormous magnetic field, how much might one suppose returns to the sun through its magnetic poles?

antoniseb
2005-May-24, 02:08 PM
Originally posted by iantresman@May 24 2005, 01:11 PM
if the accretion theory if planet formation falsifiable (as good science is supposed to be), can anyone provide any examples of what might constitute such evidence?
Hi Ian,

Planet formation is described with models not theories. These models are being tweaked, and modified based on incoming data, and better simulations. NO ONE say that we have it down exactly right. If we did, we&#39;d stop research. Planet Formation is an interesting and active part of astronomy research today. Saying that there is a thoery so rigid that something observed outside earlier predictions &#39;falsifies&#39; it is demanding more than is there.

If you have some competing model that explains what is observed as well as the current models (note the plural) go ahead and work it out and publish it.

antoniseb
2005-May-24, 02:10 PM
Originally posted by lswinford@May 24 2005, 01:56 PM
how much might one suppose returns to the sun through its magnetic poles?
The results from the Ulysses mission answer that question.
http://ulysses.jpl.nasa.gov/

om@umr.edu
2005-May-24, 02:58 PM
Originally posted by iantresman@May 24 2005, 01:25 PM
Surely the decay products suggests when, rather than how planetary formation took place?

Regards,
Ian Tresman
You are right.

We know little about the formation of planets in general. We know most about the formation of this planet because we can make measurements on it.

This is what the measurements show:

1. The decay products of I-129 and Pu-244 (radiogenic Xe-129 and Xe-136) trapped inside the Earth today mean that this planet was never so hot that it melted so that iron could sink to the Earth&#39;s core.

There is not enough time for that to happen and leave I-129 and Pu-244 still decaying inside the Earth.

2. The decay product of K-40 (radiogenic Ar-40) in air and trapped inside the Earth confirms that there was only partial degassing of Earth.

3. The presence of primordial He-3 trapped inside the Earth further confirms that this planet was never so hot that it melted so that iron could sink to the Earth&#39;s core.

He-3 is much, much more mobile than Fe and would be quickly expelled to the atmosphere if this planet melted so that iron could sink to the Earth&#39;s core.

You are exactly right about the uselessness of models that are not falsifiable.

The model of Earth&#39;s geochemical differentiation has been tested and found wanting.

With kind regards,

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

antoniseb
2005-May-24, 04:24 PM
Note to readers.

Many of the posts in this thread have been made by some members of this forum who are supporters of Alternative Theories, and opponents of mainstream astronomy. As such the content of this discussion may not represent a very balanced look at the subject of what most scientists today think about planet formation.

Nereid
2005-May-25, 12:46 AM
Originally posted by antoniseb+May 24 2005, 02:08 PM--></div><table border='0' align='center' width='95%' cellpadding='3' cellspacing='1'><tr><td>QUOTE (antoniseb @ May 24 2005, 02:08 PM)</td></tr><tr><td id='QUOTE'> <!--QuoteBegin-iantresman@May 24 2005, 01:11 PM
if the accretion theory if planet formation falsifiable (as good science is supposed to be), can anyone provide any examples of what might constitute such evidence?
Hi Ian,

Planet formation is described with models not theories. These models are being tweaked, and modified based on incoming data, and better simulations. NO ONE say that we have it down exactly right. If we did, we&#39;d stop research. Planet Formation is an interesting and active part of astronomy research today. Saying that there is a thoery so rigid that something observed outside earlier predictions &#39;falsifies&#39; it is demanding more than is there.

If you have some competing model that explains what is observed as well as the current models (note the plural) go ahead and work it out and publish it. [/b][/quote]
To amplify just a tad:
a ) &#39;falsifiability&#39; is a concept associated with Karl Popper, a philosopher. Among the professionals who (scientifically) study science, Popper&#39;s &#39;falsifiability&#39; has been thoroughly discredited - it is simply inconsistent with how scientists (and science) actually work. For those interested, here (http://www.eblaforum.org/main/viewforum.php?f=11) is an internet forum devoted to these sorts of issues. Personally, I&#39;m a fan of Imre Lakatos.
b ) what stories such as the ones Ian gives us links to DON&#39;T tell us is the hundreds of &#39;non-stories&#39; - papers written on good observations (or experimental results) which show consistency with the developing models. These are just as much valid contributions to the increasing understanding of planetary formation as the few that make headlines in the popular press.
c ) once an area has become mature (say, QED, or &#39;gardening of the lunar regolith&#39;), when you look back at the papers while it was much younger, you will see hundreds and hundreds proposing models that subsequently were discarded. These discarded models played a role in the development of the field, some (likely) a pivotal role.
d ) in the case of planet formation, I suspect the models are pretty darn complicated&#33; They will aim to incorporate a quite large number of (possibly relevant) physical processes, operating on many different scales; the observational data they have to constrain the input parameters will, in some cases, be very good (e.g. physical properties and chemical reactivity of various compounds); in others not so good.

Greg
2005-May-25, 02:54 AM
One of the benefits of having my statements challenged is that it forces me to actually reread my source material rather than rely on more vague recollections. http://www.universetoday.com/am/publish/ri...ky_planets.html (http://www.universetoday.com/am/publish/right_ingredients_rocky_planets.html)
This is a link to a very interesting article, one of those I was referring to. Within it I think lies the direction in which the current story was going. It suggests that it is processed sillicate grains rather than amorphous grains that are responsible for terrestrial planet formation. Interestingly the extent to which the processed grain extended into the disc varied significantly with each of the stars studied. In one such system the entire disc appeared to be mixed, whereas in the others only the inner portion. Apparently in the larger stars studied in the current article the mixing extends further out, which I would guess could be an intrinsic property of larger stars.
There is a competing model for planet formation with regards to gas giants that I found interesting. It suggests they may form in the same fashion as the parent star rather than by accretion. You may have a better argument debating whether or not gas giants form by accretion than terrestrial planets. There are numerous quality articles supporting the notion of terrestrial planet formation by accretion and the one I linked to above contains observations confirming predictions made based on the models. Circumstantial evidence, yes, as none of us will live long enough to see the proof unfold before our eyes by watching a system evolve. By putting together observations of numerous systems at different stages of development, we can make reasonable inferences about how the process works in general in our lifetimes. The ability to make such observtions is just now coming within our grasp, so stay tuned. I find it fun to try to think it through during the initial stages where the information is still sketchy and the observational evidence is sparse.
To Oliver I say that generally speaking the first few generations of stars are largely devoid of heavier elements. Observations of very old stars in star clusters confirms this. It is possible that there are some stars with high metallicity that formed from the first few supernovas, as all supernovas will produce some heavy elements, but they would be the exceptions and not the rule. Additionally, how would you know that the similar metallicity in the outer portions of stars with planets is not the result of planets spiraling into the star? More and more evidence is piling up that it is normal for planets to spiral into their parent stars. If this is the source of the metallicity then of course the observed metallicity of the outer photosphere would match that of the planets. Your coments regarding the isotopic evidence coming out of volcanoes and mixing on this planet is interesting, however.

om@umr.edu
2005-May-25, 03:21 AM
Thanks, Greg, for the link.

Analyses of samples right here on planet Earth will likely continue to provide important clues on the formation of one rocky planet.

I was taught, for example, that cosmic rays produce He-3 in the upper atmosphere and that alpha-emission from U and Th produces He-4 inside the Earth. We all thought that the He-3/He-4 ratio would therefore decrease with depth into the Earth.

You can&#39;t imagine our surprise when excess He-3 was found leaking out of the Earth in the Mid-Ocean Ridge.

We were also all thrilled and surprised when the decay products of extinct I-129 and Pu-244 were found, first in high-purity CO2 gas wells in New Mexico, then in fluid CO2 inclusions of an olivine xenolith brought to Earth&#39;s surface in an Hawiian volcano, and later in MORBs.

With kind regards,

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

Greg
2005-May-26, 02:51 AM
Thanks, Oliver. Unfortunately I am having a hard time seeing what you are driving at with those comments about radioactive isotopes. A little more detail might be helpful.
Hellium-3 is much more abundant in meteorites than Helium-4 as compared to the ratio found on Earth. It is somewhat surprising that the ration further down deviates from what is predicted by the models. I do not know by how much it deviates, however, and the degree of deviation does matter when consideing how much doubt to cast upon the current model. That said, we know little about the interior of the Earth and its dynamics. Could helium 3 be concentrating in the upper mantle and correspondingly much lower concentrations exist lower down (and higher concentrations of helium-4) as a result of convection over long periods of time? This is certainly a area good for debate and speculation.
I think what you are driving at is that there isn&#39;t as much helium-4 below the crust as expected because heavier elements did not sink into the core of the planet as much as the current model suggests. Some recent information regarding the age of zirconium crystals from the crust may support the notion that the Earth hardened alot faster than had been thought and consequently may not have had as much of a molten phase to allow heavier elements to sink and seperate through the liquid phase when the Earth was hotter. Also, if the liquid phase of a molten Earth had different convection flows at different depths, this may have helped prevent more thorough sorting of heavier elements to lower depths. I had originally thought that the age of the crystals might just imply that the Earth is older (and formed earlier) than previously thought.
I am less certain by what you mean by the fact that Pu-244 and I-129 breakdown products have been found in the lower crust. The I-129 is derived from Uranium and Plutonium is a heavy element also, so I would expect to find evidence of them in even greater abundance the further down you go into the Earth if in fact the current model is right and heavier elements are more abundant as you go further into the interior.

om@umr.edu
2005-May-26, 04:26 AM
Thanks, Greg.

Briefly, isotope measurements gave us two surprises:

1. This planet formed its current layered structure (crust, mantle, and core) surprisingly soon after the end of element synthesis, while short-lived elements like I-129 and Pu-244 were still alive.

2. Primordial helium seems to be trapped inside the Earth and leaks out at the Mid-Ocean Ridge. [The alternative view is that a reactor at the Earth&#39;s core is producing the He-3.] If the He-3 is primordial helium, as most believe, then its presence inside the Earth today and its high mobility are difficult to reconcile with the idea that iron was extracted and moved to the Earth&#39;s core.

With kind regards,

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

Greg
2005-May-27, 03:06 AM
Thanks, Oliver.

I see what you are driving at regarding the Pu-244. It has a relatively short half-life of 74 myrs, suggesting the crust had formed by then. I think we are in agreement on this topic. The recent evidence (using radioisotopic evidence) regarding the zirconium crystals having been formed in the crust only a few million years after the Earth was thought to have formed suggests that the crust may well have solidified within the 74 myr half life of Pu-244. The evidence you mentioned suggests it may well have solidified down to the current crust-mantle barrier which is even a more radical departure from conventional theory. Clearly the conventional model is in need of revision, especially with regards to crust formation. The helium-3 evidence bears more directly on what is going on below and is troubling for the standard model to explain. I did read about the model about a uranium-plutonium reactor rather than an iron-nickel core being at the center of the Earth. Although I found it thought provoking, I had a hard time buying into it. My own thoughts are that the crust did indeed solidify much earlier than thought and therefore lighter elements that should have risen to the surface never did. The mantle and core are less differentiated and the elements less well separated than had been thought. With so little evidence to go on from the interior of the planet, my specualtion may even be as accurate as anyone&#39;s at this point.

Duane
2005-May-27, 04:05 AM
Greg, I usually find you to be pretty unbiased in your view of things, so I would like you to take a look at this thread:

http://www.universetoday.com/forum/index.p...?showtopic=5495 (http://www.universetoday.com/forum/index.php?showtopic=5495)

Let me know when you&#39;re done, and I&#39;ll engage with you abit about it. (I&#39;ld ask Oliver too, but he has dismissed it without responce previously)

om@umr.edu
2005-May-27, 04:19 AM
Originally posted by Greg@May 27 2005, 03:06 AM
1. I see what you are driving at regarding the Pu-244. It has a relatively short half-life of 74 myrs, suggesting the crust had formed by then.

2. The helium-3 evidence bears more directly on what is going on below and is troubling for the standard model to explain.

3. With so little evidence to go on from the interior of the planet, my specualtion may even be as accurate as anyone&#39;s at this point.
I agree with all three points, Greg. I would add:

1. The half-life of I-129 is only 16 myrs. It too was alive after the crust formed.

2. I agree. But some do not appreciate the problem.

3. Yes, and there is hope in admitting what is.

"To know that you do not know is best."

With kind regards,

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

Greg
2005-May-27, 06:23 PM
Duane,
I appreciate the comments and have begun reading through them, but it is going to take a little while to digest it and process it in my mind. Currently I am busy at work so I will likely get to it by the end of the day. My viewppoint here may only be two-dimensional since it is based on just a few article I have recently read on the forum. I do understand the dicipline but my memory on it is vague going back to Geology 101 which I did take in college, but that is only as deep as my geo knowldege goes. I do try very hard to maintain an objective and unbiased viewpoint. I do think that recent evidence is indicating that the surface of the planet may be older and therefore have solidified earlier than had been thought. My knowledge of the interior is admittedly fuzzy and I think I alluded to that in my last post. Hopefully though, I will be more up to speed by the end of the day. Thanks for the link.

Greg
2005-May-27, 10:42 PM
Ok, I read through the somewhat contentious thread. As usual with deeper and more advanced knowledge I am left with more questions than answers. I will try to explain my thought process based on the wealth of information I have just digested. Maybe now I am up to a Geo 102 or 103 level, but I will leave that for you to judge.
As I understand it, there are numerous variables to consider when trying to determine whether or not the mantle is differentiated or not.
There is time and gradual cooling over time of the Earth&#39;s interior. However due to radioactive isotope decay (especially potassium isotopes), I thought that the interior might be more or less constant in temperature with a tendency to increase over time and therefore release excess heat generated by radioactive decay via volcanism which is what drives plate tectonics. There is also the notion of excess heat trapped in the interior from the Earth&#39;s formation which echos Oliver&#39;s undifferentiated mantle idea, in that if everything degassed and sorted perfectly during a molten Earth state, excess heat should have been able to escape during that phase. So if excess heat did not escape, then maybe this implies that the crust and mantle cooled too quickly to allow adequate sorting and perfect layed distribution of elements to occur during the early liquid phase. I lean towards thinking that there is not much in the way of excess leftover heat that drives volcanism and plate tectonics and in fact the surplus of heat that does this is generated in a constant, gradual fashion by radioactive decay.
The point I am driving at with this notion is that if the interior has been gradually cooling then it is reasonable to expect that the mantle should be expanding to greater depths from its original location. If there has been a balance of heat generated versus expelled, then it is reasonable to expect that the core-mantle boundary has remained roughly where is is over 4 billion years. I think that looking at the mantle composition with respect to changes expected over time due to temperature change (gradual cooling) or a lack thereof is important.
The second factor to consider is pressure. Pressure increases with depth and therefore rock is less likely to melt when pressure is applied. Perhaps I am oversimplifying this, but there would be a tendency to favor a solid phase with greater depth excluding temperature considerations.
The third would be the distribution of elements and minerals. This would incorporate two different time periods. The initial condition would been after the crust and mantle cooled to its current condition from the initial molten phase. I would expect that the mantle would be relatively uniform in the distribution of elements and minerals with depth if the mantle and crust cooled quickly and not separation did not have sufficient time to occur. During intrusions of subducted crust into the mantle, I would expect more uniform sorting of minerals. Minerals with lower melting points would melt first generating volcanoes such as Mount St. Helens, whereas higher melting point minerals would melt deeper down, perhaps not until the CMB is reached. So I think if subduction were primarily recycling mantle, that the mineral composition would be better stratified with depth due to the above process.
The last factor to consider would be the process of convection. What appears to be happening to me is that material is rising from the core and CMB at the CMB via plumes to mid-oceanic ridges and depositing as new formed crust (not the old shield crust.) Subduction zones form at continental boundaries with oceanic crust and take older oceanic crust into the mantle. The assorted minerals melt at different depths depending on their melting point with the majority of the crust reaching the CMB where it melts completely (and due to its less dense composition likely is the first to go up in plumes.) Intrusions into the mantle occur on a vertical basis via the plumes and melting of subducted crust minerals. Recycling occurs on a horizontal level at the crust level at the ridges and along the CMB (due to the liquid nature of the core.) I would argue that the CMB is intruded from both above and below with minerals and elements that might not have originally formed there.
So to answer the question posed regarding mantle recycling, I would ask whether there has been enough vertical intrusions of subducted crust and rising plumes to fully recycle every part of the mantle from its original condition, no matter what we might think that to be? I would tend to think not. Areas of mantle under continental crust (and perhaps the center of large oceanic plates like the Pacific) most likely have not to have been intruded either by rising plumes or descending crust. Since subducting crust does not descend in a perfectly vertical fashion, I would also argue that the higher you go in the mantle under continents (and the the closer to the center of the continent you go) that the more likely it has not differentiated from its originial state after the Earth&#39;s crust and mantle first cooled. So if I were to look for undifferentiated mantle, I would first look directly under the oldest continental crust and nearer to the surface rather than further down.
At least these are my first impressions. Admittedly, I still have alot to read about the topic.

Greg
2005-May-27, 11:17 PM
A few other thoughts to throw in. As I understand plate tectonics, the continents and ocean plates slide over the mantle, which is relatively fixed. So it might not be so easy to determine areas in the mantle that are the least likely to be disturbed by intrusions. My guess would be to look at continents that are more or less trapped in basically the same location over the mantle for long periods like Antarctica or Africa. I would also presume that it would be relatively easy to run a simulation going back in time to determine areas where the mantle has not been disturbed over time by subducting plates or plumes.
The fact that a subducting oceanic plate does not fully melt and is able to reach the CMB I think is very significant. To me it suggests that the subducting crust is not infusing into the mantle but rather infusing into the CMB. There is should form its own layer of distinct minerals, especially if the CMB is dropping lower as the interior cools. Since these minerals are likley less dense than what is normally at the CMB, it is the most likely to be the first stuff extruded in the plumes as it would tend to rise to the top of a liquid phase. Perhaps what is mostly recycling is the oceanic crust itself with some sorted minerals and elements from the core going with it and what is primarily being transported out of the core is heat generated by radioactive decay. Maybe we are fortunate that we have the subducting crust fascilitating the heat transfer process, otherwise the release of heat might be far differnet and catachlysmic. But I do not think much is know about just what is driving the formation of plumes and why they form where they do.

Duane
2005-May-30, 09:23 PM
Whew Greg when you get into it, you really do :)

Ok, a couple of things. The s and p wave studies suggest that the subducting oceanic mantle does not pierce the CMB. The composition of the CMB appears to be difference, and the pressure on the core renders it similar to a solid rotating body.

The oldest rocks found on Earth are ~ 3.6 billion years old. Those rocks identify the period when the continents themselves were forming, or a short time (geologically speaking) thereafter. Yet the Earth apparently formed around the same time as the sun, or roughly 4.6 billion years ago.

The most accepted theory of lunar formation involves the collision between a Mars-sized body and the proto-earth. The suggestiuon is that the core of that body was incorporated into the Earth&#39;s core, suggesting that the entire mantle was melted during the impact at least. Most geologists accept that the Earth&#39;s mantle and core were melted even before that impact.

Continental crust is a differenct composition than oceanic crust. It "floats" on the material making up the mantle, and this is why they are pushed around by plate tectonics. The cores of the continents only reach down a few hundred kilometers, whereas the mantle is some 2800 km thick.

It appears that mantle (and perhaps core) convection is the method this planet uses to allow cooling. (This, of course, precludes a solid core or mantle). The heated magma rises from the CMB to the lithospere, cools, then drops back nto the deep mantle. There is more to it, but this simplified explanation will suffice for now.

There are likely a couple of mechnisms which contribute to the heating of the core/mantle, the most important of which is radioactive decay, especially in the deep core. The pressure there alone is enough to melt rock, and when you add in titdal flexing from the sun and moon and differencial rotation between the core and mantle, you have all of the forces necessary to create heat for a very very long time. The sun will have turned into a white dwarf before the heat generated by these processes will have cooled enough for the mantle to begin solidifying.

The mantle is definately not relatively fixed. It is churning like a pot of boiling water, with large "bubbles" of material rising all the way from the CMB to the surface. Oceanic plates spread as a result of the pressure of the mantle pushing through, and the spreading oceanic crust impacts the continents, pushing them, or carrying them along for the ride.

All of the continents are moving, including Africa and Antartica. This planet will look very different in 100 million years, same as it looked very different 100 million years ago--well at least as it respects the positions of the continents.

At the point between the core and mantle, there is an identified area of "hardened" material, which is mostly accepted as "hard" metals which have precipitated from the mantle.

The very innermost area of the core may be considered "solid" due to the immense pressure.

As for the cause of the plumes--it is known that the areas where the plumes reach the surface are chemically different from the areas where the mantle extrudes to form oceanic ridges. This would seem to suggest they come from a different source. As you say, this is not yet well understood.

om@umr.edu
2005-May-30, 11:45 PM
Hi, Greg.

The oldest rocks may not reside in the Earth&#39;s crust. That material has been extensively re-worked since the crust first formed, well over 4 billion years ago.

The oldest rock known from the crust is probably the one in which Grenville Turner et al. (Manchester U) observed the decay products of Pu-244. That report was published within the past year or so.

Inside the Earth, Ozima and co-workers (University of Tokyo) found some Zaire cubic diamonds with very old K/Ar ages (5-6 billion years). The question is whether some of the Ar-40 was incorporated from surrounding rock.

The insides of the Earth is one of the most exciting frontiers&#33;

I&#39;m at home and do not have access to all my reprints, etc.

With kind regards,

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

Greg
2005-May-31, 03:00 AM
I have done some more reading on the topic since my last post. Forgive me if it seems a bit rambling, but I have a tendency to put my thought processes into words at times. From what I have gathered so far it would seem that the Mantle is only partially melted. Therefore I still believe the comment I said above taht some areas of the mantle (especially in the upper half) may still be pristine and unintruded. Trying to determine exactly where those areas are might not be so easy. The evidence I am relying on now are comparisons of basalt flow from Archaean times to those of more recent times. There is variability in the flows, from place to place (along the same oceanic ridge) suggesting there are pockets of melted and less melted mantle. More importantly, the composition on average of the minerals in the basalt flow is little if at all changed over 4 billion years. If melting were to be a significant and ongoing process, I would expect some change in the composition of the basalt. (ie if there is convection and melting within the mantle then the magma composition at oceanic ridges should be changing over time and it hasn&#39;t) An alternative explanation might be that the whole mantle was homogeneous from the start and that convection and melting is irrlevant.
One other thing, if indeed subducted oceani crust reaches the CMB, what is to stop it from going a few hundred km further? My guess is that it melts completely soon after reaching there, implying that at least some crust infuses into the core or at least the CMB with the implications I outlined above.
I agree wit hthe notion of the mantle being melted by a huge impact, but would this apply to the entie mantle or just the half impacted?

Greg
2005-May-31, 03:19 AM
One or two other thoughts on the subject. Another possibility as to why the balsalt flow at oceanic ridges has changed little over time could be that the material is always emanating from the same part of the mantle.
The fact that compositions of basalt vary at plume sites like Hawaii to me suggests that these plumes emanate from different areas within the mantle. I think the difference suggests a heterogeneous mantle if both streams of magma emanate from the mantle. My own take on the plumes is in part derived from my understanding of how Mars volcanism works. Since radioactive decay and heat generation occurs everywhere, those areas of the mantle (or core if they emanate from there) not blessed with a conviently nearby ridge to release pent up heat will suffer from a buildup of heat. Eventually this buildup will melt surrounding rock and form a magma chamber. (chambers most likely form in areas of less dense rock) Eventually the liquid magma will find a fault line going up and follow the path of least pressure to the surface. As the magma chamber evacuates, pressure drops within allowing more hot rock to liquefy and a feedback cycle ensues. So maybe the shield vlocanoes here are similar to those on Mars, but on a Mars with an absence of plate tectonics, this is the only way for radioactive decay generated hat to reach the surface.

Greg
2005-May-31, 03:34 AM
One last thought, if indeed a huge impact occured that disrupted the mantle, should there not be evidence of a disruption in the mantle in the seismologic data?
If there is none, then I would agree that there must be an insidious process reshaping the mantle. Of course, this assumes such an impact did occur.

om@umr.edu
2005-May-31, 04:12 AM
Hi, Greg.

I agree with you.

Several isotope geochemists have concluded that parts of the mantle have been extensively re-worked and that other parts are undifferentiated.

I suspect that the upper mantle differentiated to produce the crust and atmosphere, and the lower mantle is more pristine. Professor Allegre (Paris) and his co-workers agreed with that conclusion.

With kind regards,

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

Svemir
2005-May-31, 05:09 AM
Plate tectonics?
Isn&#39;t it obsolete?
http://www.ias.ac.in/currsci/mar25/articles9.htm
http://www.newgeology.us/presentation20.html

Tinaa
2005-May-31, 01:03 PM
This first article doedn&#39;t really throw out plate tectonics, it adds to it.

In the second paper the author of this paper is a theosopher and he&#39;s an Arp fan. From some of the stuff on his home page, it is plain that he doesn&#39;t believe mathematics have a place in physical science. He doesn&#39;t even seem to believe in science. This stuff belongs in AT. No discussion of it here&#33;

lswinford
2005-May-31, 03:07 PM
Here is something more in line with what I learned a while back, that the mantle has a fluid convection flow, http://www.lpi.usra.edu/science/kiefer/Res...convection.html (http://www.lpi.usra.edu/science/kiefer/Research/convection.html).

Another summary is found at http://www.npaci.edu/successes/1999_mantle.html.

Still another, http://www.faculty.iu-bremen.de/course/fal.../mantlebody.htm (http://www.faculty.iu-bremen.de/course/fall02/c210101/students/PlateTectonics/mantlebody.htm), shows it as a more substantiated idea than the vague theory one of my geology profs gave me a few decades ago.

There seems to be quite an opportunity for material mixing in the mantle, albeit slow, still over 4-ish billion years it ought to be a well-mixed cake batter that pushes or plumes through the crust from time to time.

Duane
2005-May-31, 10:54 PM
Greg, just some random thoughts back at ya&#33; :)

The mantle is definately layered, with at least three distinct boundaries--the upper mantle, the hydrated zone at ~ 660 kms deep, then the lower mantle.

Subducting oceanic lithosphere is seen to extend well through the hydrated zone, and reach a depth of at least 2500 km, or just short of the core-mantle boundary. It would seem then, that part of the explanation of the convection of the mantle is a direct result of the subducting oceanic plates.

Without the subduction zones, it would seem that there would be 2 or possible 3 different areas of convection, at each of the layers mentioned above. Because of the deep penetration of the slabs however, upper mantle material is being introduced onto the lower mantle. In fact, it seems that the hydrated layer may be a direct result of this intrusion, as that is the depth at which the temperature is high enough to de-gas the subducting slab.

There is no question anymore that the plumes and super plumes of material are rising from a different source than the material forming the ocean crust at mid-ocean ridges. They are chemically distinct from each other. While it was once thought by Allegre and others that this may suggest a reservoir of material that is supplying the plumes, studies done since his time (he went into politics in the early 70&#39;s and hasn&#39;t really done any study of geology since then) have clearly and unequivicolly confirmed the plumes rise from the CMB and drift in an eastward pattern as would be expected from a "hot" plume rising through a "cool" liquid magma in a rotating body.

Recent studies (including those linked here by Iswinford) still using seismic waves have identified areas of convection and eddying in the mantle, essentically at all levels. While there is little mixing between the layers, it is clear that the mantle is melted throughout. As the plumes rise from the lower mantle, this likely explains the distinct chemical differences between basalt rising at the mid-ocean ridges and basalt reaching the surface at the plume zones.

AS for the giant impact, well it occured well before the last HBE some 700 million years after the formation of the solar system, as evidenced by the recent story in this forum regarding the moon basin formation. I would suggest the proto-earth was still a big ball of melted rock at that point, as was the impactor, such that the mantles of both objects merged. It shouldn&#39;t really be that surprising that there is little or no difference beteen the two objects, considering they probably coalesced in the same general area.

Greg
2005-Jun-01, 05:48 AM
Thanks, Iswinford for the links. I reached several important conclusions from them that have helped me resynthesize my understanding of heat transfer and its implications. The first is that mid oceanic plumes origniate from the core and not the mantle. The second that the vast majority of the heat needed to be transferred resides at the CMB. The third is that the mantle can be differentiated into at least two distinct layers.
My understanding now is that the core and cmb will release heat anywhere it can, at plumes and into mid ocean ridge systems. The mantle itself is also going to generate heat that also must be released. Since it does not release in discrete pockets of volcanoes, it must be released at mid ocean ridges in bulk via convection. So I agree that the upper mantle, especially being the most free of pressure most likely has a liquid phase and convects. I think that the lower mantle most likely does this as well, similarly dumping heat as magma into a mid-ocean ridge system, but the flow most likely has a different speed and composition of elements, resulting in the heterogenity we see in different flows at different locations within the ridge itself. I doubt that the different density of the magmas would result in mixing very easily at the ridge locations. I also think it likely that some core-composed magma would be present in these ridge locations. I see the rising core heat through a ridge system driving the mantle convections layers which facilitates removal of trapped heat within the mantle as well. Alternatively it may be mantle convection itself that drives ridge formation and since mantle convection will reach the cmb, cmb heat gets taken along secondarily. The limit on mantle convection then would be the location of the subducting plates, which will in turn affect the extent of convection cycles and secondarily where mid-ocean ridges appear (where mantle stored heat will be released.)
If convienent ridge systems are not available for the core to dump heat into, then it may drive plume formation in central areas in oceanic plates where magma follows the path of least resistance within less dense, more faulted, more separated rock. The limit of the mantle convection would be location where subducting plates block lateral convection. CMB plumes would be least likely to form beneath subducted plates, which convienently extend under continents which is why we do not see many shield volcanoes on continents. I would suspect that areas where continents are locked and there is little subduction would be areas where cmb plumes can and will penetrate through continental crust. I would be curious to see whether basalt outflows over continental crust such as in siberia and the southwest US bear more similarity to outflows from mid-ocean ridges or shield volcanoes in order to explore this point better.
Another idea I had for what might be going on in the western N. American plate involves a little thought experiment I had regarding what happens when a spreading ridge gets subducted? This is an ongoing phenomenon along the N. American plate. My guess is that upwelling mantle flow is temporarily blocked by dense rock, penetrates the subducted crust further flowward (where the subducted plate is more melted and thinner), bisects the continental mass and forms a new spreading center there. An area of a recently subducted spreading center would appear locked until the new spreading center emerged within the continent a bit further away but in the same direction the ridge was being moved in (ie new spreading center forming between mexico and baja california a bit further south and east of where the plates are currently "locked" might be an example of the same process having occured earlier further south.) Thus the location of regional mantle flow upwellings (ridges)are preserved in location for long periods of time.

Svemir
2005-Jun-01, 06:01 AM
In the second paper the author of this paper is a theosopher and he&#39;s an Arp fan. From some of the stuff on his home page, it is plain that he doesn&#39;t believe mathematics have a place in physical science. He doesn&#39;t even seem to believe in science. This stuff belongs in AT. No discussion of it here&#33;

He might well be homosexual and Knicks fan. Who cares if his arguments about plate tectonics are OK?
I didn&#39;t refer to his home page (I didn&#39;t read all that stuff about theosophy or whatever), just to this particular page.
I don&#39;t believe in science eather. Belief in science would make science just another religion.

This first article doedn&#39;t really throw out plate tectonics, it adds to it.

How can this add to plate tectonicists case:
"All the ocean floor bathymetry maps that plate tectonicists have published and used, right from Bruce Hezeen and Marie Thrap’s work to the much publicized and used GEBCO 1975–1982 and GEBCO 1978–1992 and National Geographic Society Atlas 1992, are far from the actual ocean floor bathymetry and geomorphology"

So, if someone builds his case on the wrong data, what shall we believe in ;) ?

Let&#39;s see what happens when mathematics takes a place in science:

The cores of the continents only reach down a few hundred kilometers, whereas the mantle is some 2800 km thick.
...
The heated magma rises from the CMB to the lithospere, cools, then drops back into the deep mantle.
Wow&#33;

Are mathematical calculations suppported by observation?
How long it takes for low density material of litosphere to reach 2800 km into high density layer?

And how does it explain formation of rocky planets further away?

No discussion of it here&#33;

Greg
2005-Jun-01, 06:11 AM
One other though experiment regarding the possible effect of an impact. I do believe recent evidence is pointing towards faster cooling of the crust than expected. This may well have applied to the mantle as well by 700 million years later. My guess would be that the impact would have had an effect on the mantle. perhaps having a lasting effect on the formation of the initial plate boundaries and locations of the first continents. I would expect to see a difference with regards to one half of the globe to the other regarding plate boundaries and subsequent continent formation. (more dense material on the side of the impact with lighter material/minerals on the other side if at least part of the mantle was solid on impact.) I would guess that mantle recycling would have removed any "scarring" evidence within the mantle itself of the initial impact.