Action & Reaction - July 1998
by Do-While Jones

Moon Rock Ages

We received this letter from a visitor to our web site.


I believe a scientist whose name is Mitsunobu Tatsumoto ascertained that the earth is 4.5 billion years old after examining moon rocks with mass spectrometers. This was probably early 1980's. It is my understanding that younger scientists now believe the earth is closer to 4.6 billion years.

I believe you missed a link.



The idea that the age of the Earth can be determined from the age of the Moon depends upon a known relationship between the age of the Earth and the age of the Moon. If one knows that the Earth and the Moon are the same age, then knowing the age of one tells the age of the other. If one knows the Moon is younger than the Earth, then knowing the age of the Moon establishes a minimum age of the Earth. If one knows the Moon is older than the Earth, then the age of the Moon establishes a maximum age of the Earth.

Some scientists have made guesses about how the Moon was formed. Some think that lots of rocks were attracted by gravity to form the Earth while fewer rocks were attracted by gravity to form the Moon. They say the Moon formed in orbit around the Earth at the same time as the Earth was formed. Other scientists believe the Moon was belched out of the place where the Pacific Ocean is now by an enormous eruption. That would make the Moon younger than the Earth. Still others believe the Moon is a big rock that had been drifting around the universe for a long time before it got captured by Earth's gravitational field. If that is true, the Moon could be much older than the Earth. All we know for sure is that scientists don't know for sure. Therefore, the age of the Moon has absolutely no bearing on the age of the Earth.

But let's assume, just for the sake of discussion, that the age of the Earth is the same as the age of the Moon. Can we really tell the age of the Moon using a mass spectrometer? Scientists can smash a piece of moon rock down to individual atoms. Each kind of atom (each isotope) has a different atomic weight (mass). A mass spectrometer separates atoms by weight and tells you how much of each isotope is in the rock. What does knowing how much of each isotope is in the rock tell you? It just tells how much of each isotope is in the rock—nothing more.

Certain radioactive isotopes turn into more stable isotopes at a known rate. For example, uranium turns into lead very slowly. Scientists can measure the amount of uranium and lead in a moon rock and figure out how much uranium and lead will be in the rock a billion years from now. (Of course, by that time people will have forgotten about the experiment and lost the rock, so it isn't a very useful experiment to do.)

If we know the amount of uranium and lead in a rock, can we tell how old it is? We can if we know how much uranium and lead were in the rock to begin with. We can make one of two assumptions. (1) There was some lead in the rock when it was formed. (2) There wasn't any lead in the rock when it was formed.

If we make the first assumption, then we have to figure out how much there was. Since scientists don't know what process formed the rock in the first place, we can't possibly know how much uranium and how much lead that process created. Therefore, the accuracy of the computed date depends entirely upon how well we guess the initial concentrations of uranium and lead. There is no more reason to believe that the rock initially contained 20% uranium and 80% lead than there is to believe that the rock initially contained 80% uranium and 20% lead. If you assume an initial concentration of each kind of material, the calculations will yield an age determined entirely by whatever wild guess you make.

If we make the second assumption, the calculation will yield the oldest possible age. This assumption is attractive to people who want to try to justify their belief in an old age of the Earth. The second assumption is a tough one to swallow, though, because one must postulate a natural process that turns hydrogen and helium into iron, oxygen, nickel, carbon, gold, copper and uranium, but not lead. What is there about lead which would make it harder to produce than nickel or copper? Nothing. So the imaginary process that creates uranium must not produce lead for some unexplained reason. This hardly seems like solid, scientific reasoning.

If one uses three different dating techniques on two different rocks from the same rock formation, it is quite possible that one will get six different dates. If one uses Potassium/Argon and Lead/Lead on the same rock, the Potassium/Argon date will probably be millions of years while the Lead/Lead date will probably be billions of years. Geologists know this, so they never bother to do Lead/Lead dating on recent lava flow, nor do they do Potassium/Argon on "ancient" gneiss. Whenever a radioactive date calculation does not agree with the preconceived notion of how old the rock is, that date is declared "discordant" and is ignored.

We'll bet that Mitsunobu Tatsumoto didn't do any Potassium/Argon dating tests on the moon rocks. If he had, he would have come up with ages tens or hundreds of million years old because the Potassium/Argon method simply can't produce dates that are billions of years old. All the original potassium would have decayed into argon in a few billion years, so there isn't anything left to date. If there is any potassium at all, the computed date will be in the tens or hundreds of millions of years.

Given the current prejudice today, few scientists would admit to believing any moon rock age that is expressed in millions of years. Any "young" age calculation would be blamed on potassium contamination during the trip back to Earth, no matter how carefully the rocks were shielded from outside contamination.

But suppose that tomorrow someone comes up with a popular theory that an asteroid struck the Earth 65 million years ago, causing molten rock from the center of the Earth to squirt out into space, where surface tension shaped it into the ball that we call, "the Moon". Then some scientist would certainly do Potassium/Argon tests on moon rocks until he finds one 62 ± 4 million years old, and would offer that as definitive proof of the theory. Mitsunobu Tatsumoto's 4.5 billion year age calculations would be declared invalid for one reason or another.

We hate to sound too cynical, but this sort of thing happens all the time in geology and paleontology. You just have to read the scientific literature about all the controversy of the dating of fossils like skull KNM-ER 1470 and certain Grand Canyon rocks.

They say that someone who has respect for the law and loves to eat sausage has never seen how either one is made. One might also say that anyone who believes in radioactive dates doesn't understand the radioactive dating process.

Ten years after this article was written, the Apollo 11 moon rocks were dated by various menthods. We reported the ages in The Age of the Moon.

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