Is the earth really 4.6 billion years old?
A few days ago an article with the same title was published here on NEWS24 and can be seen here: http://www.news24.com/MyNews24/Is-the-earth-really-45-billion-years-old-20140123.
In essence the author of the article asserts that there is some sort of debate to be had about the current scientific evidence and calculation of the age of the earth. He asserts this through a series of non-related statements of “evidence”, none of which have any bearing on the methodology used by scientists in calculating the age of the earth and our solar system.
Unfortunately the author of the original article also did not specify precisely which “age-of-the-earth” he found debatable. Did he mean the age of the solar system, or of the earth as a planet within it, or of the earth-moon system, or the time since formation of the earth’s metallic core, or the time since formation of the earliest solid crust? This definition is important for precision but I doubt whether the author of the original article even thought to consider this issue.
In this article I will present a summary of the methodology used by science to in fact provide an accurate (within 2% error) age of the earth.
In getting to the methodology used to calculate the age of the earth we first need to understand some basic nuclear physics.
Isotopes are variants of a particular chemical element such that, while all isotopes of a given element have the same number of protons in each atom, they differ in neutron number. Different isotopes of a single element occupy the same position on the periodic table. The number of protons within the atom's nucleus is called atomic number and is equal to the number of electrons in the neutral atom. Each atomic number identifies a specific element, but not the isotope; an atom of a given element may have a wide range in its number of neutrons. The number of nucleons (both protons and neutrons) in the nucleus is the atom's mass number, and each isotope of a given element has a different mass number.
For example, carbon-12, carbon-13 and carbon-14 are three isotopes of the element carbon with mass numbers 12, 13 and 14 respectively. The atomic number of carbon is 6, which means that every carbon atom has 6 protons, so that the neutron numbers of these isotopes are 6, 7 and 8 respectively.
Some isotopes are radioactive, and are therefore described as radioisotopes or radionuclides, while others have never been observed to undergo radioactive decay and are described as stable isotopes or stable nuclides. For example, 14C is a radioactive form of carbon while 12C and 13C are stable isotopes. There are about 339 naturally occurring nuclides on Earth, of which 288 are primordial nuclides, meaning that they have existed since the solar system's formation. Primordial nuclides include 37 nuclides with very long half-lives (over 80 million years) and 254 that are formally considered as "stable nuclides", since they have not been observed to decay.
A very important concept to grasp, is half-life. Half-life is the time required, probabilistically, for half of the unstable, radioactive atoms in a sample to undergo radioactive decay to form stable isotopes. Radioactive isotopes spontaneously decay into stable isotopes at very predictable rates. The constancy of radioactive decay rates was initially regarded as an independent assumption, until the development of modern quantum mechanics, where it is now understood that these rates were fixed by the fundamental constants of physics.
The mathematical expression that relates radioactive decay to geologic time is:
D = D0 + N(t) (e?t - 1)
t is age of the sample,
D is number of atoms of the daughter isotope in the sample,
D0 is number of atoms of the daughter isotope in the original composition,
N is number of atoms of the parent isotope in the sample at time t (the present), given by N(t) = Noe-?t, and
? is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope times the natural logarithm of 2.
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature. This is well-established for most isotopic systems. However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.
In most cases, if an element has stable isotopes, those isotopes predominate in the elemental abundance found on Earth and in the solar system and the unstable isotopes are referred to as extinct. However, in the cases of 35 unstable element isotopes, we notice something very significant. Every single one of them has a half-life greater than 700 million years, while the short-lived isotopes have a half-life less than 200 million years. Isotopes whose half-life is less than 10% of the age of the earth are already extinct.
This fact is the first indication we have that the earth and solar system have very old ages since all the short lived radioactive isotopes have already decayed to the daughter isotope over long periods of time.
The common methods for radiometric dating used for age determinations of rocks include:
The uranium-lead radiometric dating scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years.
Uranium-lead dating is often performed on the mineral zircon (ZrSiO4), which incorporates uranium atoms into their crystalline structure as substitutes for zirconium, but strongly rejects lead. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert.
One of uranium-lead method great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample.
Now that we understand some basics lets get to how the age of the earth is calculated.
Three basic approaches are used to determine the age of the Earth. The first is to search for and date the oldest rocks exposed on the surface of the Earth. Since the earth has a dynamic surface (plate tectonics) with earlier but now erased histories the ages obtained in this way are minimum ages for the Earth. Because the Earth formed as part of the Solar System, a second approach is to date extraterrestrial objects such as meteorites and samples from the Moon. Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets. The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites. This method represents the time when lead isotopes were last homogeneously distributed throughout the Solar System prior to the time that the planetary bodies were segregated into discrete chemical systems. The results from these methods indicate that the Earth, meteorites, the Moon, and, by inference, the entire Solar System are 4.5 to 4.6 billion years old.
It’s important to realise that the formation of the Solar System and the Earth was not an instantaneous event but occurred over a finite period as a result of processes set in motion when the universe formed. It is, therefore, more correct to talk about formational intervals rather than discrete ages for the Solar System and the Earth. Thus, the ages of the Earth, the Moon, and meteorites as measured by different methods represent slightly different events, although the differences in these ages are generally slight, and so can be treated as a single event for practical purposes.
Earth’s oldest rocks
All the major continents contain a core of very old rocks. These Precambrian shields are all that remain of the Earth’s oldest crust. Due to the turbulent nature of the earth’s geological systems, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock. In all cases, the measured ages provide only a minimum age for the Earth.
One of the oldest dates for rocks on Earth in fact come from whole-rock samples from the Sand River Gneisses in the Limpopo Valley, South Africa, which have been dated by the Rb-Sr isochron method at 3.79 ± 0.06 billion years.
Studies of the oldest rocks from the Precambrian shields show that the Earth is older than 3.8 billion years. The geology of these oldest rocks also indicates that there was a substantial period of history of the Earth before 3.8 billion years ago for which no datable geological record now exists. If we are to learn more about the Earth’s history before 3.8 billion years ago, we must examine the evidence obtained from other, older sources, particularly meteorites and the Moon.
Ages of meteorites and the moon
The results of radiometric dating on meteorites clearly indicate that these objects formed about 4.6 billion years ago. Because astrophysical considerations require that the formation of the planets and meteorites by condensation from the solar nebula was essentially simultaneous, we can infer with considerable certainty that the age of the most primitive meteorites also is the age of formation of the Earth. Even if we wished to deny this inference, we would still be forced to conclude that meteorites, which must at least post date the formation of the Solar System and the universe, are no less than 4.6 billion years old.
The Apollo missions, for the first time, gave scientists the exciting opportunity to study samples from another body - the moon. The hundreds of radiometric ages on lunar rocks show clearly that the initial formation of the Moon was 4.5 to 4.6 billion years ago.
Model lead age of meteorites and the earth
The generally accepted age of the Earth is based on a simple but elegant model for the evolution of lead isotopes. This model was developed independently by Houtermans and Holmes, and first applied to meteorites and the Earth by Clair Patterson in 1953.
Patterson’s original estimate of the age of the Earth has changed very little over the past three decades. In a recent re-evaluation, the age of the Earth has been fixed at about 4.54 billion years.
What I have attempted to demonstrate above, is that the age of the earth is not some guess or vague set of assumptions but rather an empirically derived calculation. The author of the article I referred to at the start of my article (http://www.news24.com/MyNews24/Is-the-earth-really-45-billion-years-old-20140123.), instead of attempting to question and interrogate the veracity of empirical radiometric data presents several non-related pieces of Psuedo-scientific “evidence” in an attempt to show that the age of the earth cannot possibly be the billions that is calculated by physicists and geologists. Unfortunately he has not taken the time to “debunk” the actual method used to empirically date the earth. I find his article completely and utterly invalid given the subject matter.
Objectivity in science
In the original article the author has the following statement as a conclusion:
“The ideological view of science as totally objective, ignores the fact that the fallible nature of human beings preclude total objectivity. Objectivity in science is created by performing experiments and providing the related experimental data for evaluation (by further experimentation) by other scientists (operational science). Origins science by nature is not repeatable and therefore only partially adheres to the definition of operational science (only limited observations can be made). Many assumptions must be made to arrive at calculated ages (some more plausible than others), which cannot be proven or disproven according to the scientific method.”
The falsehood of this statement should now be abundantly clear. Geological science is based on many related fields of science not least of which physics and chemistry are the most important as demonstrated in my article. The dating of the earth is in fact a classic example of how geological science and physics have come together to resolve a profound question in a relatively simple, elegant and empirical way.
In addition science is the most objective method humans have at establishing the most likely answers to our many questions. It’s amusing to see an individual who already has conclusions in his mind accuse science of not acting objectively.
Radiometric dating methods provide a reliable means of determining the ages of critical points in geologic and planetary history, including the age of the Earth, the Moon, and meteorites. That the age of the Earth is billions of years is now beyond question because it is supported by a wide variety of independently determined scientific evidence which indicates that the Earth is 4.5 to 4.6 billion years old. There is no dispute among scientists about the antiquity of the Earth and her sister planets.
Radiometric dating has independently confirmed and quantified the geologic time scale, which originally was constructed on the basis of stratigraphic and faunal succession, before the development of modern isotopic dating techniques. This is powerful proof that both the dating techniques and the paleaontologic and stratigraphic principles on which the time scale was originally based are sound.
There are more than 100,000 radiometric ages in the scientific literature that date rock formations and geologic events ranging in age from Holocene to earliest Precambrian. These data and all the accumulated knowledge from the science of geology show conclusively that the Earth we now see is the result of natural processes operating over vast periods and not the product of one or two worldwide catastrophic events.
In this article I have not even touched on the evidence for an old earth derived from biological sciences, palaeontology and molecular genetics or any of the mountains of additional evidence from geology. All of these sources of evidence all point to a very old earth and all of the evidence is complimentary.
Sources and further reading:
Geologic Time, 2nd edition, Don l. Eicher
The Story of Earth and Life, A southern Africa Perspective on a 4.6-billion year journey, T. McCarthy and B. Rubidge.
The Greatest Show on Earth, R. Dawkins.