Answer to Question #2867 Submitted to "Ask the Experts"

The Earth emits more heat than it absorbs from the sun, and part of the reason for this excess heat is the decay of radioactive elements in the Earth. There are other sources of heat in the Earth, including thermal energy left over from when the Earth first formed, thermal energy from the large collision that formed the moon, and thermal energy remaining from the formation of the Earth's core. However, these other sources of thermal energy are not as long-lasting as that from radioactive decay of natural elements in the Earth.

There are several ways geologists use to try to determine the composition of the Earth's core and mantle (the mantle is the thick layer between the core and the crust). We can sample the mantle directly by looking for places where mantle rocks are brought to the Earth's surface. One place this happens is on the island of Cyprus, another is Iceland, and there are more. By analyzing these samples of mantle rock, we can get a good understanding of how much radioactivity is present today, and by looking at mantle rocks of various ages, we can also see how these concentrations have changed since the Earth first formed.

We also know very well the composition of rocks in the Earth's crust, and we know that, over time, these rocks have become enriched in uranium, thorium, and potassium (the three major sources of radioactivity). This means that these elements must have come from the mantle because the mantle is the source of rocks in the Earth's crust. So we have a fairly good idea of how both mantle and crustal concentrations of radioactive elements have changed over time.

Scientists assume that the entire solar system was fairly similar when it first formed, so they assume that meteorites have the same composition as the Earth as a whole. We have studied a number of meteorites, as well as a number of asteroids, our moon, and moons of other planets (and Mars and Venus, too). From this, we have a fairly good idea as to how much uranium, thorium, and potassium should be in the whole Earth. Whatever we can't account for from our studies of the crust and mantle must be in the Earth's core. From this line of reasoning, we can tell fairly securely that the Earth's core must be almost entirely iron and nickel, perhaps with some sulfur mixed in. This supposition is supported by the fact that the chemistry and geochemistry of the radioactive elements is such that what we infer from the studies mentioned above is very nicely supported by geochemical theory. In other words, there are a few independent lines of thought that give the same answer.

The final line of reasoning relies on math and physics. We know how much energy is given off when an atom decays, and we know how rapidly a given amount of uranium, thorium, or potassium will decay away. So, by adding up all the radioactive elements in the Earth (from sampling and meteorite studies) and multiplying by the amount of heat each should give off, we can calculate how much heat they should generate. These calculations match what we see very nicely, which means that we have a very accurate idea of how much radioactivity the Earth contains.

So, to summarize:

Some of the heat in the Earth comes from physical sources (the Earth's formation, ancient collisions, and the formation of the core), while the rest comes from the decay of radioactive elements.

We can directly sample rocks from the Earth's crust and mantle, and these samples tell us how much radioactivity is in these parts of the Earth.

By knowing the chemical characteristics of each of the radioactive elements, we can confirm that our studies are giving us correct information.

Once we know how much radioactivity is in the Earth, we can easily calculate how much heat should be produced, and these calculations match what we see.