• By measuring the relative amounts of certain radioactive elements or their derivatives in meteorites, astronomers have been able to construct the timeline for the evolution of our solar system. Indeed, the chronology of events is probably what we know best about the solar nebula.
• Naturally occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay.
• The rate of the transformation is constant for a given element, but different elements decay at different rates. Once this rate is known, we can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.
• The rate is usually described in terms of half-life, defined as the time for half of the parent to decay to daughter.
• Radiometric dating is often compared to the measurement of time by an hourglass. For an hourglass, the relative amounts of sand on top and bottom give a pretty good estimate of how much time has passed. Similar logic is used with radiometric dating. An unstable parent element (top sand) is transformed into a daughter element (bottom sand).
• Many radioactive elements can be used as geologic clocks (see below).
• Radiodating of Moon rocks and meteorites suggests that our solar system is around 4.6 billion years old.
• Are the results of radiometry inconsistent with the Bible? Click here for an excellent site on radiometric techniques, along with some commentary on religious biases toward the subject.

Brief history of radiometric dating

• Radioactivity was discovered in 1896 by Henri Becquerel.
• In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium).
• In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.
• The invention of the mass spectrometer after World War I (post-1918) led to the discovery of more than 200 isotopes.
   

Ages from vanished isotopes

• Unstable isotopes with relatively long half-lives can be used to estimate the ages of the various rocks in our solar system. However, these isotopes tell us nothing about the history of the rocks before they solidified.
• The timeline of the early (molten) solar system can be reconstructed by studying short-lived isotopes. Although these isotopes may no longer exist in our solar system, their presence in early rocks and molten matter can be deduced from the presence of their decay products.
• The ages of the oldest lunar rocks and meteorites indicate that the formation of the major bodies in the solar system was complete by about 4.6 billion years ago. Most of the meteorites formed within 20 million years of each other, while the Earth (and probably the other planets) formed within the next 100 million years.
• At the beginning of the solar system there were several relatively short-lived radionuclides like ²⁶Al, ⁶⁰Fe, ⁵³Mn, and ¹²⁹I present within the solar nebula. These radionuclides—possibly produced by the explosion of a supernova—are extinct today but their decay products can be detected in very old material such as meteorites.
• It has been found, for instance, that meteorites contain the magnesium isotope ²⁶Mg in minerals that normally are made from aluminum. The ²⁶Mg isotope was formed by the decay of the radioactive aluminum isotope ²⁶Al. This is remarkable because the half-life of ²⁶Al is only 700,000 years and there is no known way in which ²⁶Al could have been made within the solar system. This means that ²⁶Al must have come from somewhere else. The most likely origin for the ²⁶Al is a supernova explosion of a massive star within the molecular cloud from which both the star and the Sun were formed. The supernova explosion could not have occurred more than a few million years before the formation of the meteorites; otherwise, all of the ²⁶Al would have decayed to ²⁶Mg before it could have been incorporated into meteorites. This tells us that only a few million years elapsed between the start of the collapse of the cloud core and the time that the parent bodies of the meteorites formed.
• Another radioactive isotope present in the solar nebula was I-129 (¹²⁹I) , which decays with a half-life of 15.7 million years to produce ¹²⁹Xe, an isotope of xenon. Excesses of Xe-129 (¹²⁹Xe) in meteorites have been shown to result from decay of I-129. While Iodine-127 (and its unstable isotope I-129) readily forms bonds with other elements, Xenon is an inert gas that normally does not. Consequently, the presence of Xenon in a rock mineral implies that it was produced from its parent, ¹²⁹I, after the rock solidified. Otherwise, it would have escaped. A body that solidified early must have originally contained a relatively large quantity of ¹²⁹I and should now contain a relatively large quantity of the daughter, ¹²⁹Xe. On the other hand, a body that formed millions of years later, after much of the original ¹²⁹I had already decayed, should now contain very little ¹²⁹Xe. The fact that Earth rocks contain much less Xenon than do meteorites implies that the Earth was still molten millions of years after the solid meteorites formed.

How does Carbon-14 dating work?

• Carbon is familiar to us in various substances, such as the black part in charred wood, diamonds, and the graphite in ‘lead’ pencils, But more importantly, in the current context of radiodating, carbon is an essential component of living matter.
• All elements, including carbon, occur in several elemental forms, called isotopes. All isotopes of an element have the same number of protons but differ in the number of neutrons. Two important isotopes are carbon-12 (¹²C) and its rarer cousin carbon-14 (¹⁴C). They both have 6 protons, but carbon-14 (¹⁴C) has 8 neutrons vs 6 in the more common carbon-12 (¹²C).
• Carbon-14 (¹⁴C) is produced when cosmic rays knock neutrons out of atomic nuclei in the upper atmosphere. These displaced neutrons, now moving fast, hit ordinary nitrogen (¹⁴N) at lower altitudes, converting it into ¹⁴C. Unlike the common carbon (¹²C), ¹⁴C is unstable and slowly decays, changing back into nitrogen and releasing energy. This instability is the reason it is considered radioactive.
• Living things are in equilibrium with the atmosphere and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and into the carbon cycle.
• All living things contain a constant ratio of Carbon-14 to Carbon-12 (roughly 1 in a trillion) based on the proportions in the environment. At death, Carbon-14 exchange ceases and any Carbon-14 in the tissues of the organism begins to decay to Nitrogen-14 and is not replenished by new C-14. The change in the Carbon-14 to Carbon-12 ratio is the basis for dating.
• This method assumes that the rate of Carbon-14 production (and hence the amount of cosmic rays striking the Earth) has been constant in recent history.
• Carbon-14 has a half-life of about 5730 years. After many half-lives, the parent daughter-ratio becomes too small for accurate measurement. This element should therefore not be used to age things that are older than about 50,000 years (i.e., 10 half-lives).
• The following table shows the approximate amount of carbon-14 that will remain years after a plant or animal has died.
   

  Half-Life	Years	C14 Remaining
  1	5,730	50.0%
  2	11,460	25.0%
  3	17,190	12.5%
  4	22,920	6.3%
  5	28,650	3.1%
  6	34,380	1.6%
  7	45,840	0.8%

Limits to carbon-14 dating

• After about 10 half-lives, very little ¹⁴C remains, and it is therefore barely measurable.
• The limit of radiocarbon dating depends on the ability to measure very low activity in old samples.
• 40,000-50,000 years is the current limit of age-dating using ¹⁴C.

Some naturally occurring radioactive isotopes and their half-lives