Meteroite Dating

How do scientists determine the age of meteorites, most of which are around 4.5 billion years old. –Amy


Meteorite Dating

Scientists measure the age of meteorites with the decay of radioactive isotopes. What is an isotope? Isotopes are elements with the same number of protons but a different number of neutrons. For instance, carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Some isotopes are very unstable and tend to decay into lighter elements by alpha or beta particle decay. Scientists use the half-life of certain elements to date objects. First, scientists must determine to isotope to use by examining the elemental composition of the object. For meteorites, scientists generally use Rubidium-87/ Strontium-87 decay, which has a half-life of 49 billion years. Rubidium-87 decays into Strontium-87. So if the object has 50% Rubidium-87 and 50% Strontium-87Β  (only formed by decay process), then the object is 49 million years old. Since some Strontium-87 may have been present originally, scientists use Strontium-86, whose content remains the same, as a reference. Determining the ratio between Rubidium-87/ Strontium-86 and Strontium-87/ Strontium-86 via mass spectrometer (vaporizes a tiny portion of the meteorite to form ions; the ions are then separated by mass in a magnetic field), scientists can then calculate the amount of each isotope present in the meteorite, and thus the age of meteorites. Although radioactive dating is the best method for scientists to date meteorites, many factors, such as the amount of sunshine or heavy rain, can affect measurements.

21 thoughts on “Meteroite Dating

  1. Here is another question that keeps me busy. I know how to calculate half lives for 1st, 2nd, and 3rd order reactions. But how did they come up with the time (half life) first? I mean for many elements including the ones you mentioned, the half lives are millions and billions of years. It can not be found by experiments. In the equations for half lives we need to know at least two terms. K (rate constant), or initial or final concentrations of the matter (depending on the order of the reaction). So, since the concentrations are dependent on the half lives and half life is something that can not be found by experiments for most of the elements, how did they come up with the half lives? Thanks.

    • Scientists determine half-life by measuring the element’s radioactive decay over a short amount of time. For some elements, as you mentioned, their half-lifes are millions and even billions of years. So by measuring the radioactive decay rate over a period of time (and placing the two values into a ratio), scientists can predict mathematically the approximate half-time of the element, using graphs to predict when exactly half of the element has decayed. By various experiments, scientists can then average the results to obtain a final half-life for the element (since the environment of the experiment may alter the result, this eliminates error).

      • OK, but I know that the decay is not constant. An element can lose 1/4 of its initial concentration in 10 seconds, but it might take 10 years to reach half life (another 1/4). What I am trying to say is that measuring the decay over a certain amount of time can’t give reliable data to predict the half life. Hope I could explain what I wanted to say πŸ™‚ Thanks.

      • For exponential decay, use the formula Nt = No x (0.5)^(time/half-life) to calculate half-life of any element if you know the other unknown variables: time elapsed, amount of original substance, and amount of substance after decay.
        Another way to find half-life is by knowing the rate constant, k, the order of the reaction, and the initial concentration. With experimentation, you may find the order of the reaction and the k constant. Depending on the order of reaction, separate formulas will help you achieve the result.

  2. I have read Sam Kean’s Disappearing Spoon. He says that Bismuth has a life of “20 billion billion years”, so this makes it the element which has the longest half life. Is that true? If it is true, why do they use Rubidium isotopes to determine the age of the meteorites? Thanks.

    • That’s true. Bismuth does indeed have the longest half-life. However, a meteorite is only about 4.5 billion years, so using something like Bismuth would be very inaccurate. Even a slight miscalculation could amount to an huge error mathematically. Since the Rubidium-Strontium system is probably one of the closest to the actual ages of meteorites, scientists use rubidium instead. It also has to do with how much of that isotope is in the meteorite. Bismuth is relatively rare compared to Rubidium. Rubidium’s atomic number is 37 while Bismuth’s atomic number is 83! It takes a great deal of energy to fuse nuclei; more energy would be needed to form Bismuth than Rubidium.

  3. Thank you so much, Tina! I feel I’ve learned more from you than I ever learn about Astronomy. Good to know there is a measure to base on… Appreciate you taking time to explain.

  4. I never liked the way our scientists use the radioactive isotopes to determine really old age. Their equation assumes steady state decay and we’ve seen examples where that is not true. Its not perfect, maybe far from perfect, but its the only method we have to measure anything that old 😦

    • Very true, many factors (sunlight and rain) may affect the precision of radioactive dating that makes the process less precise. Unfortunately, it is the only process by which scientists can measure the age of very old objects.

    • πŸ˜€ That would be interesting. There are some small meteors lying around on Earth since the atmosphere usually breaks them into fragments.
      P.S. Would you happen to be a fan of The Simpsons? Referring to the [d’oh], it’s what I usually imagine Homer say.

      • Not really watched any. When I was married, my ex-wife hated it so when I tried to watch it, she would clatter around banging so that I couldn’t hear it. Since the divorce, I haven’t really got back into it

      • Ah well, I used to watch The Simpsons every day in third grade. I just loved the cartoon, especially the way Homer says “d’oh”! I even have a chess set featuring The Simpsons (with Homer as the king, Marge as the queen, etc.)

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