Earth, with its abundance of liquid water and ability to host and sustain life, is unique among known planets. Let's explore Earth and its layers.
Our species lives primarily on land on Earth's continents, but the planet's surface is primarily covered in water. On Earth, oceans cover about 70 percent of the planet's surface with an average depth of 2.5 miles (4 kilometers). Fresh water exists as a liquid in lakes and rivers and in the form of water vapor in the atmosphere, which causes much of Earth's weather.
Earth has multiple layers, with three main layers: the crust, mantle and core.
What is Earth's crust?
Earth's ocean basins and continents compose the crust, the outermost layer. Earth's crust is between three and 46 miles (five and 75 km) deep. The thickest parts of this crust lie under the continents and the thinnest parts are located under Earth's oceans.
According to "Essentials of Geology" (7th Ed., Prentice Hall, 2000) by Frederick K. Lutgens and Edward J. Tarbuck, Earth's crust is made up of several elements: oxygen, 46.6 percent by weight; silicon, 27.7 percent; aluminum, 8.1 percent; iron, 5 percent; calcium, 3.6 percent; sodium, 2.8 percent, potassium, 2.6 percent, and magnesium, 2.1 percent.
The crust is divided into huge plates that float on Earth's mantle, the next layer under the crust. These plates are constantly in motion; they move at about the same rate as human fingernails grow, according to NASA. Earthquakes occur when these plates grind against each other and mountains form when these plates collide the plates slide under one another, creating deep trenches. Plate tectonics is the scientific theory explaining the motion of these plates.
What is Earth's mantle?
Earth's mantle is our planet's thickest layer and is a mostly-solid layer that lies between the crust and core. It can be found about 1,800 miles (2,890 km) deep and is composed mostly of silicate rocks rich in oxygen, magnesium, aluminum and silicon.
Intense heat from Earth's core causes the mantle rock to move slowly, rising and then cooling off and sinking back down in an ongoing, cyclic process. The upper layer of the mantle, known as the asthenosphere, is below the lithosphere (a region including the crust and upper mantle).
The asthenosphere is where both the pressure and heat are extreme enough that the rock flows like a liquid with a viscous, caramel-like consistency. Below this layer is the lower mantle, also known as the mesosphere, which makes up about 56% of Earth's total volume.
The cyclic convection currents that move heat upwards through the mantle also push this flowing material upwards. This mantle material is known as magma and this movement is thought to be what causes tectonic plates to move.
Volcanic eruptions occur when the mantle material pushes through Earth's crust. When magma from the mantle erupts through the crust it is then known as lava.
What is the core?
The core is made up of two parts and is almost completely composed of metals, namely iron and nickel. Elements that dissolve in iron, known as "siderophiles" and often labeled as "precious metals," can also be found there.
The innermost section of the core is the inner core, which is a hot, solid and dense ball of mostly iron. It has a radius of about 758 miles (about 1,220 km) and is about 9,392 degrees Fahrenheit (5,200 Celsius) with a pressure of about 3.6 million atmosphere (atm). While these temperatures are well above iron's melting point, this intense pressure keeps it solid.
The inner core is surrounded by this layer's second part: a liquid, outer core that is about 1,355 miles (2,180 km) thick. This region is composed mostly of a nickel-iron alloy and its liquid material rotates faster than Earth's surface (but in the same direction).
The increased speed of the inner core, which causes it to rotate an extra time every 1,000 years, approximately, is thought to cause Earth's magnetic field. Magnetism in the outer core is about 50 times more powerful than on Earth's surface.
Earth's magnetic field stretches from our planet's deep interior all the way out to space. There it shields us from charged particles from the sun. These particles routinely flow from the sun, this stream is known as the solar wind. When the solar wind collides with air molecules above Earth's magnetic poles, it causes the air molecules to glow, causing the auroras — the northern and southern lights.
Related: 10 ways Earth revealed its weirdness
How does Earth compare?
To better understand Earth's composition and history, geologists sometimes compare our planet to other rocky planets in our solar system, namely Venus and Mars.
Venus is a similar size to Earth and a little bit closer to the sun, while Mars is only about half the size of our planet. While several spacecraft have been sent to Venus and Mars (though they have only been able to survive on Mars), we are still learning about these planets' interiors.
Venus has an extremely thick atmosphere that blocks visible light from reaching its surface, which means it requires radar to look at the surface. A handful of spacecraft have survived the planet's hellish temperatures and pressures and made it to the surface but they did not last long.
From radar data, we know that Venus' surface appears fresh and young — no more than 500 million years old — due to the amount of volcanic activity on its hellishly hot surface. While Venus likely has a crust, mantle and core similar to Earth, its magnetic field is very weak cby comparison. That may be because the core spins sluggishly to generate the magnetic field, or because there is no core at all.
NASA's InSight mission launched in 2018 to drill into the planet's surface and collect seismic, temperature and other data about the planet's interior. While the craft has had issues with its drill actually making its way into the Martian surface, the mission has collected data that has expanded our understanding of the planet. So far, we have learned that small marsquakes are fairly routine on the planet but inSight has not detected any surface waves; on Earth surface waves, waves as part of marsquakes that travel along the top of the crust, are used to study the planet's interior.
The agency has also landed other craft on Mars that carry drills and are able to study its interior, like the ExoMars rover that is set to launch in 2022.
Mars is a cold planet whose atmosphere is not thick enough to let liquid water flow on its surface (although briny water is a possibility) and almost all the water on Mars today exists as ice or, in very small amounts, as vapor in its minuscule atmosphere.
The planet has a crust covered by dust; it's believed the crust is solid, with no plate tectonics. This allowed Mars to build up huge volcanoes on its surface, such as Olympus Mons. However, the Martian volcanoes appear dormant. The reason why is still not completely understood.
Underneath its surface, Mars likely has a mantle and a core; since Mars has no global magnetic field, its core probably does not spin.
This reference page was created with additional reporting by Elizabeth Howell, Space.com contributor. It was updated by Space.com senior writer Chelsea Gohd in Dec., 2021.