To celebrate Earth Day, we are tackling the age old question: How old is the earth. You have probably already learned that the earth is about 4.5 billion years old, but you probably haven’t heard of how specifically we got that answer. So, we’ll be stepping through it and showing just how scientists go about measuring it.
To understand how we measure the age of profoundly old things we must first understand about what elements are made out of, a bit about radioactive materials and how they break down. Let’s start with elements and have a look at the periodic table:
The entire universe is on this table. Every, planet, every chemical, every creature is made with a combination of those elements. Crazy, right?! Let’s go one step further. All those elements are made up from a combination of Protons, Neutrons and Electrons. Protons have a positive charge, and mass. Neutrons have mass but no charge, and electrons have a negative charge but have virtually no mass. Let’s zoom in an element:
We will just look at Carbon because it’s one of my favorite elements, and we’re just looking to understand how the periodic table works. The number above the big C in the center, is how many protons it has. The number of protons is what makes an element. Every element that has 6 protons is Carbon. The most stable form of carbon has 6 protons and 6 neutrons. We call this carbon 12 (6protons+6 neutrons). But, carbon can have 7 neutrons and 6 protons, or 8 neutrons and 6 protons and still be carbon, and in fact it does, fairly often.
We call these other two forms of Carbon Isotopes, and they will often be labeled Carbon 13 (7neutrons and 6 protons), and Carbon 14 (8 neutrons + 6 protons). These other isotopes aren’t very stable. And this instability is the key factor for understanding radioactive materials.
Radioactive materials have a combination of Protons, Neutrons and electrons that are unstable. And like all things in nature, they are trying to move to a more stable state. The rate at which radioactive materials decay into their more stable elements is easily measurable, and scientists have determined the rate of decay to be exponential. This makes radioactive materials a perfect stopwatch to measure how old things are. This is known as radiometric dating.
Let’s look at some isotopes of Uranium. Uranium can exist in nature as Uranium 238, Uranium 235 or Uranium 234. They are all trying to get to more stable states. For all three isotopes of Uranium, they break down through a series of steps that eventually changes them to lead. There are several different types of radioactive break downs and the exact steps that each of the isotopes go through to get to lead are different. I won’t go into those here because this article is already starting to get a bit lengthy. Let’s just simplify here and say that the unstable uranium atoms break down to the much more stable lead atoms.
The rate at which the uranium isotopes convert to lead happens in a very predictable amount of time. For any given isotope of a material there is a measurable probability that a molecule will break down in a given amount of time. So a scientist can measure the rate of change for a year or so, and because we know that the decay is exponential, we can plot this and determine a material’s half life. The half life of a material is the amount of time that a half of a material will break down. No matter how much uranium you start with, half of it will break down in a certain period of time. For uranium 238, we know that half of Uranium will convert to lead 208 in about 4.46 billion years. For uranium 235 to break down to lead 207, the half-life is about 704 million years.
This is where scientists get smart and creative. Scientists looked for minerals called Zircons. Zircons contain small amounts of Uranium, but because of the structure of their atoms when forming, they reject lead, so they are perfect clocks for measuring time using radiometric dating because they start with no lead, but have uranium.
They are strong minerals and resist weathering, erosion, or even high temperatures. They are found almost everywhere, and samples have been found on both the Moon and mars. Zircons are formed in high heat and pressure.
If you want to read more about zircons and how they were likely formed here is a great article from business insider:
How Zircons Crystals Formed During Early Earth
Because zircons don’t have any lead in them when they are formed, scientists can measure the amount of lead, and uranium in the sample. Since the uranium breaks down into lead at a predictable rate we can get this curve:
Based on the relationship between the amount of lead 207 and uranium left in the zircon, the can get an age of the material. Let’s do a simple example. Let’s say that you measured a zircon and it had 160 micrograms of lead 207, and 20 micrograms of Uranium 235. We take 160/ (160+20) to get about 89% lead 207, and 20/(160+20) and about 11% Uranium 235. On our curve from before:
That zircon was formed about 2.1 Billion years ago.
This test can be repeated with radioactive materials other than the uranium 235 to confirm the age as well.
The oldest zircon ever found on our planet was found in Australia, and was measured to be about 4.375 billion years old, so we know that our earth is at least that old.
We have also measured rocks from the moon and dated them to be between 4.4 and 4.5 billion years old, and we know meteorites are some of the oldest materials in our solar system, and have been dated to be between 4.4 and 4.6 billion years old.
Most of the samples taken were measured with multiple radiometric methods to confirm the age of the samples.
Based on all these data, scientists have approximated that the earth is 4.54 +/- 0.5 billion years old.