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FAQ - Radioactive Age-Dating

1. How do scientists find the age of planets (date samples) or planetary time (relative age and absolute age)?

We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars. We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. when the lava cooled and crystallized). We also have meteorites from asteroids and can date them, too. These are the surfaces that we can get absolute ages for. For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface. The biggest assumption is that, to first order, the number of asteroids and comets hitting the Earth and the Moon was the same as for Mercury, Venus, and Mars. There is a lot of evidence that this is true. The bottom line is that the more craters one sees, the older the surface is.

2. Why is it important to establish the age of a planet?

This can be interpreted in two ways: why it is important to know the age of a planet or how is age dating important in determining the age of a planet? Based on our study of meteorites and rocks from the Moon, as well as modeling the formation of planets, it is believed (pretty much well-established) that all of the objects in the Solar System formed very quickly about 4.56 billion years ago. When we age date a planet, we are actually just dating the age of the surface, not the whole planet. We can get absolute ages only if we have rocks from that surface. For others, all we are doing is getting a relative age, using things like the formation of craters and other features on a surface. By studying other planets, we are learning more about our own planet. The effects of impacts and how they might affect us here on Earth, global climate change (Venus vs. Earth) and what could happen to Earth in an extreme case, etc.

3. How do you technically define half-life?

From Wikipedia, radioactive decay is the process in which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. This decay, or loss of energy, results in an atom (element) of one type, called the parent nuclide transforming to an atom of a different type (another element or another isotope of the same element), named the daughter nuclide. For example: a carbon-14 atom (the "parent") emits radiation and transforms to a nitrogen-14 atom (the "daughter"). It is impossible to predict when a given atom will decay, but given a large number of similar atoms, the decay rate on average is predictable. This predictable decay is called the half-life of the parent atom, the time it takes for one half of all of the parent atoms to transform into the daughter.

4. If carbon-14 is so short-lived in comparison to potassium-40 or uranium-235, why is it that in terms of the media, we mostly about carbon-14 and rarely the others?

This may simply have to do with what the media is talking about. When there is a scientific discussion about the age of, say a meteorite or the Earth, the media just talks about the large numbers and not about the dating technique (e.g. "It was a long time ago"). On the other hand, when the media talk about "more recent events," ages that are more comprehendible, such as when early Man built a fire or even how old a painting is (or some ancient parchment), then we bring up the dating technique in order to better validate the findings.

5. Is there a chemical test for carbon?

Carbon is unreactive with a number of common lab substances: sulfuric acid, hydrochloric acid, chlorine, or any of the alkalis. It does burn in oxygen, and if you can pass the combusted gas through limewater, the carbon dioxide will turn the limewater milky by producing calcium carbonate. While not a chemical test, the presence of carbon in a sample (like a meteorite) can be found by vaporizing the sample and passing it through a mass spectrometer. This is also a way to get at the abundance of the various isotopes of carbon.

6. Are carbon isotopes used for age measurement of meteorite samples? We hear a lot of time estimates, X hundred millions, X million years, etc.

We have an activity in one of the PSI workshops "Exploring the Terrestrial Planets," that deals with this topic. So, you can use the radioactive elements to measure the age of rocks and minerals. Below is a list of some common elements. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope-- the parent, to convert into a non-radioactive element/isotope-- the daughter) to 10 times their half-life. For example, Potassium-40 decays to Argon-40. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old). So, Carbon-14 can only measure things up to just over 50,000 years old, great for determining when someone built a wood fire, but not good for determining the age of a meteorite.

Parent Daughter Half-Life (billion years)
Uranium-238 Helium-4, Lead-206 4.47
Uranium-235 Helium-4, Lead-207 0.704
Thorium-232 Helium-4, Lead-208 14
Rhenium-187 Osmium-187 41
Samarium-147 Neodymium-143 106
Rubidium-87 Strontium-87 48
Potassium-40 Argon-40 1.28
Carbon-14 Nitrogen-14 0.0000057 (5730 yrs)

7. How is radioactive decay used to date a surface?

Radioactive decay is a well-known process. It occurs whenever an atom has an unbalanced number of protons and neutrons in its nucleus. The number of protons usually determines the element the atom belongs to and it is fixed for any particular element. On the other hand, the number of neutrons that can be contained in the nucleus can vary. When the number of neutrons is in balance with the number of protons (which does not necessarily means that the number of neutrons has to be exactly the same as the number of protons) then the atoms of a particular element is said to be stable. When the number of neutrons is not in balance with the protons then the atom of that particular element is said to be unstable.

In nature, all elements have atoms with varying numbers of neutrons in their nucleus. These differing atoms are called isotopes and they are represented by the sum of protons and neutrons in the nucleus. Let's look at a simple case, carbon. Carbon has 6 protons in its nucleus, but the number of neutrons its nucleus can host range from 6 to 8. We thus have three different isotopes of carbon: Carbon-12 with 6 protons and 6 neutrons in the nucleus, Carbon-13 with 6 protons and 7 neutrons in the nucleus, Carbon-14 with 6 protons and 8 neutrons in the nucleus. Both carbon-12 and carbon-13 are stable, but carbon-14 is unstable, which means that there are too many neutrons in the nucleus. Carbon-14 is also known as radiocarbon. As a result, carbon-14 decays by changing one proton into a neutron and becoming a different element, nitrogen-14 (with 7 protons and 7 neutrons in the nucleus). The isotope originating from the decay (nitrogen-14 in the case of radiocarbon) is called the daughter, while the original radioactive isotope (like carbon-14) is called the parent. The amount of time it takes for an unstable isotope to decay is determined statistically by looking at how long it takes for a large number of the same radioactive isotopes to decay to half its original amount. This time is known as the half-life of the radioactive isotope.

Once the half life of an isotope and its decay path are known, it is possible to use the radioactive decay for dating the substance (rock) it belongs to, by measuring the amount of parent and daughter contained in the sample. An important point is that we must have an idea of how much of the daughter isotope was in the sample before the decay started.

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