During the Apollo 8 moon mission, astronaut Bill Anders snapped a photograph of our planet rising over the moon’s surface. It was the first image of its kind ever taken.
It showed our Earth, a spherical geoid, hovering in infinite darkness. The Sun illuminated blue oceans and swirling white clouds on one side, our lonely home planet fading into night on the other.
“We came all this way to explore the moon, and the most important thing is that we discovered the Earth.” – Bill Anders, Apollo 8 astronaut
But what do we really know about Earth?
We’ve ventured into the emptiness of space, and explored the black depths of our deepest oceans. However, the land farthest from our reach does not lie in distant galaxies, but in the very ground beneath our feet.
What We Know About The Inner Earth
Earth is a geoid, not a perfect sphere; it bulges at the equator, and is flattened at the poles. This makes Earth an oblate spheriod — close to a sphere, but not quite.
This equatorial bulge is the result of centrifugal force caused by Earth’s rotation, as well as the gravitational effects on Earth by the Sun and moon. These gravitational effects cause not only the ocean to rise and fall, but also Earth itself, in very tiny amounts that are imperceptible without very precise measurement devices (about 12 cm).
This is known as the land tide.
But that’s the outer Earth. What about the inner?
We know quite a bit about the superficial geology of Earth, its ecosystems and atmosphere. However, it’s important to note that everything we know about Earth’s deep interior has been gathered through secondary observations here on the surface, and that no direct observations have been made.
What follows is a brief overview of what we do know via these secondary observations:
Earth, as a whole, is composed of multiple layers, which grow increasingly dense the deeper they are. The deeper layers are heated by extreme pressure and radioactive elements.
The Earth’s crust has an average thickness of 18 miles (30 km) beneath the continents, and 6 miles (10 km) beneath the ocean. More specifically, the continental crust is anywhere from 16-40 miles thick (25-70 km thick), while the oceanic crust is between 4-7 miles thick (6-11 km thick).
You are, right now, securely planted on this layer of Earth.
All of our deepest drilling efforts have yet to pierce this layer, and while there are oceanic ridges where some of the mantle is exposed, we have not directly observed anything below the crust.
The mantle has an average thickness of 1,800 miles (2,890 km), and comprises roughly 80% of Earth’s volume.
The boundary where the crust and mantle meet is known as the Mohorovicic discontinuity, or Moho, which lies about 3-6 miles (5-10 km) below the ocean floor and 10-60 miles (20-90 km) beneath the continents.
The temperature of the mantle averages about 932-1652 degrees Fahrenheit (500-900 degrees Celsius). Pressure keeps the mantle (mostly) solid despite the immense heat.
However, the mantle isn’t completely solid: It consists of a solid upper area, and a lower, highly viscous layer known as the asthenosphere. The asthenosphere is a ductile, semiliquid plastic moving at a rate of only centimeters a year. The effects of this movement manifest in plate tectonics, earthquakes, and continental drift.
Together with the crust, the upper mantle above the asthenosphere is called the lithosphere. The lithosphere “floats” above the asthenosphere, as it is less dense, and therefore more buoyant, than the layers beneath. This results in the aforementioned plate movements.
A layer below the asthenosphere, called the mesosphere, signifies the boundary where the asthenosphere and the outer core meet.
The core is comprised of two layers: an inner core and an outer core. Most of it is presumed to be iron, or a nickel-iron alloy, containing some traces of sulfur. The outer core consists of turbulent nickel-iron fluid. The inner core is a solid nickel-iron alloy. The core as a whole is about 1,800 miles (2,890 km) beneath the crust.
The temperature of the core is likely close to that of the surface of the Sun.
How Do We Know?
Nothing below the very uppermost portion of the mantle has been observed, and no human has ever passed beneath the crust. But if that’s the case, how do we know so much about Earth’s interior?
The most important method of examining the inner Earth is seismic tomography, in which researchers use P- and S- waves created during earthquakes to plot the various layers. The speed of these waves, as well as the existence of “shadow zones,” show us both the composition and viscosity of the layers.
For example, S-waves cannot travel through liquid. When researchers analyze S-waves traveling through the earth during an earthquake, they see that S-waves do not pass through the core, leaving a “shadow” on the other side. This is called a “shadow zone,” and its existence tells us that at least part of the core is liquid, or acts like a liquid.
- The nature of Earth’s magnetic field implies the existence of a liquid outer core comprised of iron
- Observations of iron meteorites provide clues about the composition of Earth’s core
- Xenoliths from volcanic eruptions show us the composition of the mantle
- Oceanic ridges contain areas of exposed mantle
- Measurements taken from devices on the moon verify the existence of continental drift
- High-pressure experiments allow us to recreate conditions at Earth’s core
Deep-earth drilling experiments
A number of drilling experiments have been undertaken to improve our understanding of the inner earth. To date, the deepest we’ve drilled is approximately 12,262 m (40,230 ft), and though we’ve yet to reach any meaningful depths, research is still ongoing, particularly near oceanic ridges, where the mantle is nearer the surface.
A few of the experiments:
- Project Mohole. An attempt to reach the Moho which was discontinued in 1966. Reached a depth of 601 ft (183 m) below the ocean floor.
- JOIDES Resolution. Reached a depth of 4,646 ft (1,416 m) below the ocean floor in 2005.
- RRS James Cook. In 2007, discovered the “world’s deepest undersea volcanic vents” in the Caribbean, an area of exposed mantle.
- Chikyu Hakken. A scientific drilling ship designed to hopefully pierce the ocean floor and enter the mantle. Currently the project has reached a depth of 1,400 m, and will be completed in 2012. Unfortunately, the ship was damaged in March, 2011 during the Tohoku tsunami.
- Kola Superdeep Borehole. The deepest hole ever drilled, reaching a depth of 40,230 ft (12,262 m). The hole was drilled in the Kola Peninsula by the Soviet Union, and is the very hole at the center of the Well to Hell myth. Despite the fact that it is the deepest hole we’ve ever drilled, it only reached 0.2 percent of the distance to the core.