Posted by: SteveInCO | 24 Feb 2012

Radioisotopes and the Old Earth

One of the confirmations that the earth is old–very old–can be found in considering which radioisotopes are present in nature, and which ones are not.  It’s not absolute ironclad proof, but what it is is a strong confirmation of the hypothesis that the earth is billions of years old–and it’s one that was not available to the geologists who first determined that the earth is old.

“If You Can’t Repeat It, It Ain’t Science” — Bull!

Contrary to something many creationists (and other scientific illiterates) claim, you do not need to be able to run experiments in a laboratory in order to do science, as would be the case for chemistry or nuclear physics.  There are sciences where you look at the world, collect information, and use that information to try to understand the world.  The most obvious of these is astronomy.  You cannot experiment on stars (imagine the property tax on a laboratory big enough!) and only rarely on other material in outer space, yet no one seems to doubt that it is a science.  Instead of experimenting to test some hypothesis, you look at more and more things in more and more sophisticated ways, with more and more sophisticated instruments to make sure that everything you see continues to be consistent with the hypothesis.

So that should put paid to Stupid Creationist Argument #537, where they claim that since you can’t run an experiment to reproduce the evolution of dinosaurs, it’s not scientific.

Radioisotopes that appear in nature do so in a way that is exactly as you would expect if all of the material that makes up the earth was created billions of years ago, and that is strong evidence that the earth was indeed created that long ago.

We’ll start with a refresher on the subject, then discuss why what we see is such good evidence against young earth creationism.

Quick Review of Atomic Theory

 


Feel free to skip this if you know what protons and atomic numbers are.

OK, quick recap of science that for me was in the 6th grade, and repeated in high school chemistry and physics.  Your mileage may have varied.  Atoms are basically clouds of electrons “orbiting” a nucleus which contains protons and neutrons.  The nucleus is much smaller than the atom as a whole, something like 1/10,000th the diameter.  It turns out that the electrons, for all the space they take up, individually weigh about 1/1800th as much as individual protons or neutrons.  The protons and neutrons have close to the same weight.

The electrons have a negative electric charge, while the protons have a positive charge.  Neutrons are electrically neutral.   Ideally, an atom has equal numbers of electrons and protons, but under various circumstances that really don’t matter here, electrons may be added or subtracted from the atom, leaving it with a negative or positive charge; atoms like this are known as ions.   Basically, all of chemistry has to do with the way electrons arrange themselves, both in ions and electrically neutral atoms, and the way ions behave around other ions.  But chemistry is not the subject of this post, believe it or not.

An atom’s identity as a chemical element–which chemical element it is an atom of–derives not from the electrons, but from the protons buried deep inside, in the nucleus.  An atom of hydrogen has exactly one proton in it.  No more than that.  Helium has two.  If you find a helium atom with a missing proton, it’s not a helium atom, it’s a hydrogen atom.  A helium atom can no more have one proton, or three or four or 187, than a triangle can have four sides.  Carbon has six protons, nitrogen seven, oxygen eight, aluminum 13, silicon 14, iron 26…  on up through a long list.  Towards the other end, silver has 47, praseodymium has 59, ytterbium has 70, gold has 79, lead has 82, uranium has 92, plutonium has 94.  (OK, I admit I threw praseodymium and ytterbium in there just because the names are really bizarre and roll trippingly only off tongues well used to them.)  The number of protons in an element is its atomic number, which is unintuitively abbreviated Z.

Neutrons and Isotopes

 


OK, but what about the neutrons?  They don’t affect which element an atom is, but they do matter (pun not intended).  A typical hydrogen atom has no neutrons in it, just the one proton.  Sometimes, though, it will have a neutron in it to go with the proton.  And very rarely there will be two neutrons in a hydrogen atom.  Helium, with its two protons, almost always has two neutrons in it, though there might be only one, and could be as many as six.  (Oddly, it will never have five.)  The differing number of neutrons does nothing but alter the weight of the atom, but that can sometime affect chemical reactions–especially if the percentage change is large.  Usually, the number of neutrons is as large or larger than the number of protons; I’ve just covered the two significant exceptions, protium and helium-3.

Scientists need to distinguish between these different types of hydrogen, helium or molybdenum (or whatever) atoms, so what they will do is add the number of protons and neutrons together to get an atomic mass number, A.  So they will talk of helium-3 (two protons, one neutron), helium-4 (two neutrons), helium-5 (three neutrons), or carbon-12 (six protons, six neutrons), or carbon-14 (six protons, eight neutrons).  In the case of hydrogen, there are special names:  Hydrogen-1 is protium, hydrogen-2 is deuterium, and hydrogen-3 is tritium.  (Protium and deuterium differ in mass enough that their chemistry is different.  Heavy water, made from deuterium instead of “regular” hydrogen, is significantly different chemically from regular water.)

For something like uranium, with its 92 protons, what we find most commonly are 142, 143, or 146 neutrons in the nucleus, resulting in U-234, U-235, and U-238.  It turns out that U-235 is what you can make nuclear bombs out of, and it’s quite rare compared to U-238.  In fact, only one percent of uranium atoms are U-235, so very tedious processes have to be gone through to separate it out of the “raw” uranium.  The weight difference is a bit over 1 percent, which doesn’t affect the chemical behavior, but does make it harder to separate them out.  This is what Iran is doing with all those centrifuges you hear about on the news.  Uranium from which large portions of U-238 has been removed, leaving concentrated U-235 in it, is known as “enriched”.  The U-238 which was removed is collected, and is “depleted” uranium, suitable for making very nasty projectiles and also for making tank armor.

Radioactive Isotopes

OK, last step on the refresher.  The number of neutrons matters, because only some combinations of protons and neutrons are stable.  Others are unstable and will try to change themselves into another configuration–and this is what causes radioactivity.  For example, protium and deuterium are stable, but tritium is unstable.  One of the neutrons will split into a proton and electron.  The electron flies out of the nucleus and that one proton, two neutron tritium nucleus becomes a two proton, one neutron helium-3 nucleus.  (This is known as beta decay.)  Note that with beta decay the isotope number (the atomic mass) stays the same, but the atomic number goes up one, because a neutron changes into a proton.

Many elements have only one stable isotope.  Gold, for instance, only has gold-197 as a stable isotope.  Tin, on the other hand, has ten stable isotopes.  But there are elements with no stable isotopes at all; uranium is one of them.  Uranium-238 will undergo alpha decay and spit out an alpha particle, consisting of two protons and two neutrons, and become thorium-234 (90 protons, 144 neutrons).  Alpha decay reduces the isotope number by four, and decreases the atomic number by two–one element becomes another element, the one two steps before it in the periodic table.  Thorium-234 is also unstable and very much so; it goes through beta decay twice and becomes uranium-234–which is still unstable.  (There’s a long chain of unstable intermediaries here, that ends with lead-206.)  There are other ways of decaying but those are the two major ones.

(By the way–what happens to that alpha particle?  Well, it becomes the nucleus of a helium atom!  In fact all of the helium on earth started out as alpha particles, from uranium, thorium, and their decay products.  These decayed deep underground at some time in the past, and the helium has been trapped down there ever since, waiting for us to drill oil wells and bring it to the surface.  That helium party balloon is filled with old, chilled-out nuclear radiation!)

Any unstable isotope is called a radioisotope which is just a contraction of “radioactive isotope.”

Here is another thing to note.  Different radioisotopes have different degrees of instability.  Helium-8, for instance, is so unstable that if you had a gram of it, half of it would decay within an eighth of a second.  Thorium-232, on the other hand, is almost stable.  It takes fourteen billion years for half of it to decay.  (Others are more stable than that; it was recently discovered that bismuth (atomic number 83) which we used to think was completely stable, isn’t.  It takes about 19 quintillion years for half of it to decay, and that’s over a billion times longer than the universe has existed.  Anyhow, the time it takes for half of a radio isotope to decay is known as its half life.  Longer is more stable.  (You may be wondering why not wait for it to all decay?  It doesn’t work that way.   If you start out with a gram of something and half of it decays, you now have a half-gram lump of it.  The atoms in that half gram lump don’t know that they used to be part of a one gram lump, so you have to wait the same amount of time for half of that to decay.  It’s really an expression of the more fact that some certain percentage of atoms of that isotope will decay, at random, in a certain amount of time, and they don’t consult with each other.)

The Modern View of the Origin of the Earth

 


OK, I’ll summarize very briefly modern scientific thinking about how the world came about.

God did it.

OK who let that creationist at my keyboard while I was off in the bathroom?  You’re fired!

Seriously, it turns out that every atom on earth, other than the hydrogen, was created inside a very massive star, perhaps ten times as massive as our sun.  Stars need a source of energy to avoid collapse.  Young stars combine hydrogen nuclei to make helium–releasing a lot of energy; older stars that have begun to run low on hydrogen build atoms of carbon, oxygen, etc. by combining helium nuclei.  Doing this releases some energy (but not as much).  Eventually the star is building iron atoms containing 26 protons and things come to a screeching halt, because making heavier nuclei than that actually consumes energy.  The star no longer is generating energy, and collapses.  Then it goes kaboom! in a supernova.  What happens during the supernova is a sudden explosive creation of heavier elements… up through lead, uranium, and even beyond.  Plutonium is made, and even californium (98 protons) has been detected in the debris of supernovas.  A supernova can outshine the entire galaxy it is in.  This material eventually coalesced into our solar system, in particular, into planets.

Look around you.  Everything you see used to be inside a star–the hydrogen in the water you drink is fuel the star didn’t get around to burning, and everything else is the ash of nuclear fusion.  The earth is made out of the debris of a supernova.  As Carl Sagan put it, we are made of star stuff.

Note that not every atom made in the explosion would be a stable isotope.  Far from it, many radioisotopes would have been made.

Consequences of the Modern Theory

OK.  We’ve made a statement–that some star blew up about 4.6 billion years ago, creating a lot of massive atoms, and those atoms coalesced into the solar system we are in today.  What would be some of the consequences of this?

Well, one would expect that if any really unstable isotopes were made in that supernova, they’d be gone today.  If something has a half life even as long as 40 million years, it has gone through 115 half lives in 4.6 billion years.  If you divide something in half 115 times, you get basically nothing.  One atom in 20,700,000,000,000,000,000,000,000,000,000,000 is left.  Things with somewhat longer half lives should still be around, as traces.  Things with billion year half lives (or more) should still be around, as more than traces.

This is exactly what we see.  We don’t see any aluminum-26 (half life 720,000 years), we don’t see any technetium-99 (half life 213,000 years)–in fact we don’t see any technetium (Z=43) at all, since it has no stable or nearly-stable isotopes.  We do see uranium 235 (700 million years), potassium-40 (1 billion years), uranium-238 (4.5 billion years, and thorium-232 (14 billion years).  And of course, we have bismuth on earth, since virtually none of it has decayed–the earth is less than a third of a billionth of a bismuth-209 half life old.

We know that aluminum-26 used to exist, we see signs of its former existence in meteorites that have signs of great heat–produced by radioactive decay–around their magnesium-26 inclusions.  Magnesium-26 is what aluminum-26 decays into.  Any of this daughter magnesium-26 on earth has long since been mixed together with other magnesium, so we need to look at meteorites to see this.

OK, so there aren’t any short-lived radio isotopes out there?  Well, not quite.  There are two types of short-lived radio isotopes that we do see, but their presence is still consistent with the theory.

One is a short lived radioisotope that is a decay product.  For example, we see uranium-234 out there, in spite of its 245,000 year half life.  Why?  Isn’t it a sign that god created the earth a lot more recently than scientists say?

Well, no.  Remember when I stated that uranium-238 decayed into thorium-234 (half life 24 days), which decays into uranium-234?  That’s where the uranium-234 comes from.  All of the uranium-234 out there used to be uranium-238, in fact it was uranium-238 which decayed fairly recently.  Any primordial (original) uranium-234 is long gone.  It’s fair to say that uranium-234 would not exist on earth today, if there wasn’t a bunch of uranium-238 around.  In fact many of the elements high up in the periodic table are only still around because there is still some original uranium-238, uranium-235, and thorium-232 around.  Were it not for them, there would be no radon and no radium.  There would be no polonium to kill ex-KGB agents with, either.

So a short lived isotope can be found, if it is a “daughter isotope” of something much longer lived.

The other type of short-lived isotope is most famously exemplified by carbon-14.  Carbon-14 has a 5,730 year half life, and decays into nitrogen-14.  It should be long, long gone with almost a million half lives having elapsed!  But we see it out there all the time, in fact we use it to date wooden artifacts.  It’s a not a daughter isotope of anything, either.  (One could imagine it being a daughter of oxygen-18 (if by alpha decay) or boron-14 (if by beta decay), but oxygen-18 is stable and boron-14 doesn’t exist at all.  Surely carbon-14 is a sign of recent creation.  Though why god would want to create carbon-14 that scientists will only use to date things to be older than the earth, is beyond me.

So where does it really come from?  (I mean, the god suggestion is clearly bullshit.)  Well, it is being made continuously, even today.  Cosmic rays hit nitrogen-14 in the upper atmosphere and essentially run the beta decay backwards, creating carbon-14.

So a short lived isotope can also be found, if it is being created today by some process other than the supernova going kaboom!

[By the way, there are an inordinate number of people out there today (not just creationists, from whom I would expect such a mistake) who seem to think that carbon-14 dating specifically is what is used to date dinosaur fossils and old rocks.  This is not the case; carbon-14 is only useful for very recent organic matter; it’s generally considered good to about 50,000 years.  Rather, other methods using other radioisotopes are used, most famously uranium-lead, where the ages gotten from uranium-238 and uranium-235 can be used to cross check each other.  But that’s a topic for a different post.]

So once you account for decay products, and radioisotopes being created today, everything ties in to an older earth.  The absence of other short lived radioisotopes suggests the earth is old, the presence of longer-lived ones puts an upper limit on how old.  In all particulars, the radioisotopes we find today here on earth are consistent with a 4.6 billion year lifetime.

The Icing On The Cake

I’ll share with you a borderline case–one where we just barely detect some remaining atoms of a radio-isotope, and that’s what we would expect–if we saw too many, the earth would be significantly younger than 4.6 billion years old, if we saw none at all, the earth would be quite a bit older.  An 82 million year half life is about right.  That’s 56 half lives, and one atom in 77 quadrillion should still be around.  So 77 kilograms of that isotope originally present leave you with a billionth of a milligram of the stuff today.  Those are amounts we can barely detect, and we have to use cyclotrons converted into mass spectrometers to do it.

There is such an isotope.  It’s plutonium-244.  Plutonium, one of the scariest words in today’s world.  But this isn’t your grandfather’s plutonium.   The plutonium used in bombs is plutonium-239, half life 24,400 years, and it has to be artificially made by adding a neutron to uranium-238 and letting the uranium-239 go through a couple of beta decays.  It was believed until recently that no natural plutonium existed on earth, except maybe where the reactions we use to manufacture it accidentally happen in nature.  But now we know that isn’t true, for plutonium-244 has been detected in minute traces–just a few atoms–still hanging around, survivors of 56 half lives of decay.  And by the way, one of its daughter isotopes is thorium-232.  But that remnant plutonium is also star stuff.  It’s interesting to think that a couple of billion years ago, it could have been mined, purified, and maybe put to some use we cannot imagine, if any intelligent life had been here to do it.

The world as it actually is, is so much more fascinating than anything imagined by the writers of the Bible.

Well this is yet another walk through an interesting bit of science.  At least I found it interesting, and if you don’t agree, you can take Richard Dawkins’ advice.

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Responses

  1. […] day my last post got published (I had written it a couple of days earlier and queued it) I read an article in Analog […]

  2. […] glossed over this a bit in this post in the section The Modern View of the Origin of the Earth.  But maybe someone wants to know more, […]


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