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Author Topic: Why is uranium still found on earth  (Read 7383 times)

Offline ampwelder

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Why is uranium still found on earth
« on: 08/03/2006 02:41:43 »
Heavy elements, heavyer than iron, are ALL created in super nova explosions. Uranium (very heavy) has a half life of 760 million years (.76 billion years). We know that the sun formed some 5 billion years ago so the gas that the sun and planets were made from must have been expelled long befor that.
  If I remeber right uranium decays into lead, so shouldn't all the uranium that was formed from the explosion that made the gasses that the solar system was made from all be lead by now?
  Is this a question in physics or is there a logical answer?


 

another_someone

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Re: Why is uranium still found on earth
« Reply #1 on: 08/03/2006 03:28:36 »
quote:
Originally posted by ampwelder

Heavy elements, heavyer than iron, are ALL created in super nova explosions. Uranium (very heavy) has a half life of 760 million years (.76 billion years). We know that the sun formed some 5 billion years ago so the gas that the sun and planets were made from must have been expelled long befor that.
  If I remeber right uranium decays into lead, so shouldn't all the uranium that was formed from the explosion that made the gasses that the solar system was made from all be lead by now?
  Is this a question in physics or is there a logical answer?



It does not fully answer your question, but does modify it somewhat.

http://www.ieer.org/fctsheet/uranium.html
quote:
Code: [Select]
Summary of Uranium Isotopes
Isotope   Percent in        No. of  No. of     Half-Life  
         natural uranium    Protons Neutrons   (in years)    
Uranium-238   99.284           92     146      4.46 billion
Uranium-235    0.711           92     143      704 million
Uranium-234    0.0055          92     142      245,000



But what I would suppose is that the supernova not only created Uranium, but also transuranic elements, some of which then decayed into Uranium.

The other thing to bear in mind is that a half life is not the same as saying that at the end of that period there will be no more of that material left.  What is meant by a half life is that at the end of that period, half of that material will be left.  In other words, if 4.46 billion years ago, there was 1 million tonnes of U-238, today we would have 0.5 million tonnes of U-238 left, and in another 4.46 billion years time, we would have 0.25 million tonnes of U-238 left.



George
« Last Edit: 08/03/2006 03:32:37 by another_someone »
 

Offline daveshorts

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Re: Why is uranium still found on earth
« Reply #2 on: 08/03/2006 08:42:02 »
You are right that in the age of the solar system there have been 4-5 half lives of U235 (the more explosive one) so originally there were 2^5 times more U235 - about 32 times more. This decay is probably why there are no natural nuclear reactors forming today like the one in oklo in gabon:
http://www.ocrwm.doe.gov/factsheets/doeymp0010.shtml
some bacteria concentrated natural uranium and caused it to go critical,,,

This happened about 1.7billion years ago so there was 8-10 times more U235 about so the critical mass of uranium was much smaller and easier to achieve.
 

Offline Ophiolite

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Re: Why is uranium still found on earth
« Reply #3 on: 08/03/2006 12:04:25 »
As a passing note, we often read that carbon is the basis of life - true. Or, that life as we know it is inconceivable without water. So, we add hydrogen and oxygen to our essentials. Then we note that the bulk of organic materila consists of carbon, hydrogen, oxygen and nitrogen. Now we have the ubquitous CHON. [And there is always a pedant who wants to throw phosphorous into the mix.]

I just want to say a word for Uranium. Without the heat generated by radioactive decay of Uranium the temperature of the Earth's mantle  would be much lower than it is. Convection cells would have shut down. Plate tectonics would have ceased. Carbon dioxide would not be effectively removed from the biosphere at subduction zones for sequestration in the mantle. The Earth would have suffered a runaway green house effect and would be a true twin to Venus.

So, for us, on planet Earth, perhaps Uranium is just as crucial to life as CHON.
 

Offline ampwelder

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Re: Why is uranium still found on earth
« Reply #4 on: 08/03/2006 15:21:56 »
Thank you for that answers. They were very helpful

Ophiolite You said "Without the heat generated by radioactive decay of Uranium the temperature of the Earth's mantle would be much lower than it is. Convection cells would have shut down. Plate tectonics would have ceased. Carbon dioxide would not be effectively removed from the biosphere at subduction zones for sequestration in the mantle. The Earth would have suffered a runaway green house effect and would be a true twin to Venus."

I'm sorry I think you're wrong here. Carbon Dioxide is removed from the atmosphear mostly by rain (not subduction) and stored in the oceans and in the ground mostly as limestone. When the limestone is subducted the lighter rocks floats to the surface and the limestone and other rock, along with gasses (water vapor, CO2, and other gasses) are erupted in explosive volcanos like those in the Pacific North West. Without volcanos the CO2 levels would DROP dramaticly and the polar ice caps would extend to the equator. We would see an iceball earth like we had just before the Ophiolite You said "Without the heat generated by radioactive decay of Uranium the temperature of the Earth's mantle would be much lower than it is. Convection cells would have shut down. Plate tectonics would have ceased. Carbon dioxide would not be effectively removed from the biosphere at subduction zones for sequestration in the mantle. The Earth would have suffered a runaway green house effect and would be a true twin to Venus."

I'm sorry I think you're wrong here. Carbon Dioxide is removed from the atmosphear mostly by rain (not subduction) and stored in the oceans and in the ground mostly as limestone. When the limestone is subducted the lighter rocks floats to the surface and the limestone and other rock, along with gasses (water vapor, CO2, and other gasses) are erupted in explosive volcanos like those in the Pacific North West. Without volcanos the CO2 levels would drop dramaticly and the polar ice caps would extend to the equator. We would see an iceball earth!!Ophiolite You said "Without the heat generated by radioactive decay of Uranium the temperature of the Earth's mantle would be much lower than it is. Convection cells would have shut down. Plate tectonics would have ceased. Carbon dioxide would not be effectively removed from the biosphere at subduction zones for sequestration in the mantle. The Earth would have suffered a runaway green house effect and would be a true twin to Venus."

I'm sorry I think you're wrong here. Carbon Dioxide is removed from the atmosphear mostly by rain (not subduction) and stored in the oceans and in the ground mostly as limestone. When the limestone is subducted the lighter rocks floats to the surface and the limestone and other rock, along with gasses (water vapor, CO2, and other gasses) are erupted in explosive volcanos like those in the Pacific North West. Without volcanos the CO2 levels would drop dramaticly and the polar ice caps would extend to the equator. We would see an iceball earth!! Just like what exsited just before the Cambrian Period.
 

Offline daveshorts

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Re: Why is uranium still found on earth
« Reply #5 on: 08/03/2006 16:33:52 »
Actually if it wasn't for plate tectonics I think we would have the opposite problem to venus, all the carbon would besequestered at the bottom of the ocean and there would be no way of getting it out. The greenhouse effect would stop, the planet would freeze, and there would be no way out of the problem.
 

another_someone

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Re: Why is uranium still found on earth
« Reply #6 on: 08/03/2006 16:53:56 »
I'm sorry, but I think you are both wrong.

Volcanism does pump CO2 into the atmosphere, but it is removed from the atmosphere by photosynthesis (which converts it to sugars and free oxygen – without photosynthesis, there would be no free oxygen in the atmosphere, and without CO2 there would be no photosynthesis), not subduction (subduction will more likely cause it to pop out of a volcano somewhere else).

You are right that without uranium there would be no life, because without CO2 there would be no life, but it also requires water, but that too comes from volcanism.

Incidentally, the Martian atmosphere is also predominantly CO2, and yet is too cold for substantial life to exist (there may be some single cell life forms, but that is yet to be ascertained with certainty).  The problem is that because Mars lacks volcanoes (although it may well probably have had them in the past), most of its CO2 has escaped into space, and has not been replenished (also, the lighter gravity has not helped it to retain its atmosphere).



George
« Last Edit: 08/03/2006 17:07:03 by another_someone »
 

Offline daveshorts

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Re: Why is uranium still found on earth
« Reply #7 on: 08/03/2006 22:43:30 »
However most of the carbon removed by photosynthesis is quite rapidly recycled in the carbon cycle - since some of the carbon cycle is as CO2 gas, more carbon in the cycle probably means more CO2 in the atmosphere. If this wasn't the case there would be no problem with burning coal.

Essentially without the plate tectonics all the fossilised carbon (in the form of fossil fuels and limestone) wouldn't get released => not enough CO2 => the world would get too cold.

There are in fact 'snowball earth' theories that say this has happened in the past when green plants were a new idea.
 

Offline DoctorBeaver

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Re: Why is uranium still found on earth
« Reply #8 on: 09/03/2006 00:14:40 »
Well I've learned something else. I never knew that about subduction & plate tectonics.
I was aware of the half-life thing, so I'm not completely dumb :)

Brand new forum
http://beaverland.forumup.us/
More than just science
 

another_someone

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Re: Why is uranium still found on earth
« Reply #9 on: 09/03/2006 00:19:31 »
quote:
Originally posted by daveshorts

However most of the carbon removed by photosynthesis is quite rapidly recycled in the carbon cycle - since some of the carbon cycle is as CO2 gas, more carbon in the cycle probably means more CO2 in the atmosphere. If this wasn't the case there would be no problem with burning coal.

Essentially without the plate tectonics all the fossilised carbon (in the form of fossil fuels and limestone) wouldn't get released => not enough CO2 => the world would get too cold.

There are in fact 'snowball earth' theories that say this has happened in the past when green plants were a new idea.



What do you mean by “quite rapidly”.

Clearly, some of it is recycled in a matter of hours, some in a matter of months, some a matter of thousands of years; but at the extreme, it is clear that coal that was laid down during the carboniferous period has taken hundreds of millions of years to be recycled (that coal itself being as a result of the capture of carbon through photosynthesis).  Similarly, other sedimentary rocks bearing carbon (such as limestone and chalk) can lock up carbon for hundreds of millions of years.

As for whether the burning of coal is a problem, that is still a matter of conjecture.

As I understand it, the snowball earth was not a consequence of too much photosynthesis (in fact, it seems that CO2 was probably 350 times what it was today (and at times virtually free of oxygen, as unoxidized deposits of iron have been found from that period), as the ice may have killed off all the photosynthesis).  The snowball earth appears to be presumed to have happened because of the nature of the distribution of land, the land at the time being predominantly around the equator, and this allowing extreme fluctuations in climate, because of the lack of land in the temperate regions that would slow down the climate fluctuations.  The main driving force seems to have been the feedback effect of changes in albedo as the ice progressed across the sea, rather than atmospheric greenhouse gases.  That having been said, ofcourse, even the existence of the snowball Earth, albeit substantiated with some evidence, is still conjecture; and the mechanisms by which it was triggered or terminated are even more conjectural.





George
 

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Re: Why is uranium still found on earth
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