Currently, in the year 2024, we have a space station orbiting the Earth, and we have seen twelve astronauts standing on the Moon, with more soon to follow. There are space probes which have flown by all the planets in the solar system and on Mars there are several rovers making their way slowly across the surface and one helicopter. We also know the likely composition of asteroids, some planets and their moons. And so, it has been said that we know more about the composition of the Moon and Mars than we do about our home planet, the Earth. So, what do we know?
There are a number of ways to try to understand the structure of our planet. For example, we can use equipment which measures the P and S waves produced by earthquakes. These seismic waves change as they pass through different densities of matter under the surface and can be analysed by geologists as readouts on seismographs. The other method is to analyse meteorites and asteroids. They give us clues as to what lies beneath our feet.
Some may ask if we could drill down and withdraw samples in a similar way as taking ice core samples from Antarctica. The answer is yes we can and we have, but there is one big problem. And that is because of the distances involved. The diameter of the Earth is 7,926 miles. However, even with these challenges, we have a pretty good idea of the structure of the Earth.
The outer layer is called the crust and is made of solid rock. It consists mainly of the lighter elements of the periodic table, such as oxygen, silicon and aluminium. The next layer is the mantle. But between the two is a boundary layer called Mohorovic Discontinuity known as Moho for short. Attempts to reach this boundary layer have been made for decades. The mantle also consists of oxygen and silicon but has some heavier elements like magnesium. The mantle has a very important role as it is responsible for plate tectonics which literally moves continents, very slowly and builds mountains, also very slowly. The mantle is also the thickest layer, being 2,890 kilometres.
We next come to the Earth’s core which is split into two layers. There is the outer core which consists of a fluid form of iron and nickel. The temperature increases as we go deeper and so the outer core has a temperature of roughly 5,000 degrees Celsius. This is approximately the same as that found on the surface of the Sun (at approximately 6,000 degrees Celsius). The outer core is also responsible for the Earth’s magnetic field. It is generated by eddy currents in the molten material.
The inner core is in the same temperature range (around 5,700 degrees Celsius), as the outer core but it is not a fluid. The inner core is solid and consists, we believe of mostly iron and nickel.
Finally, let us return to the subject of drilling into our planet. How far have we actually gone? In the 1950s and 1960s, the USA made an attempt with Project Mohole. But it ran out of funds and was abandoned. The Soviet Union also attempted it and reached a depth of 12,260 metres before giving up in 1969. It took fifteen years for them to get that far. Therefore, books and movies along the lines of travelling to the centre of the Earth will remain just that…fiction.
John Pullen 