Above: The internal structure of the four terrestrial planets (NASA)
Earth is covered in rocks. So are Mercury, Venus, Mars and the Moon. But does this mean they’re all essentially the same? Is what’s going on below your feet the same as what’s going on inside the other rocky planets? Even though they're made of the same stuff, the four rocky planets of the Solar System aren’t necessarily similar. Let’s take a closer look at them, inside and out.
Fast fact: There are four rocky planets in the Solar System: Mercury, Venus, Earth and Mars. They are also known as terrestrial planets.For obvious reasons, Earth is the planet we know the most about. If you were to take a giant knife and slice it in half, you would see that it's made up of several different layers, just like an onion:
The top layer, where we live, is called the crust. It’s only between 8 and 50 km thick. By comparison, Lake Ontario is 85 km across at its widest point. The next layer, the mantle, makes up most of the planet. It’s made up of solid dense rock. But because of the higher temperatures deep inside Earth, the rock is able to move around like plasticine. Even closer to the centre is the outer core, which is so hot that all the rocks and metals have melted. They slosh around, creating Earth’s magnetic field. At the very center is the inner core, composed of iron and nickel. Though it's very hot, pressure from all the rock above causes the metal here to solidify.
In many ways, Earth is similar to the Moon and the other rocky planets. They all have a solid rocky crust, some sort of mantle, and a core (see the image at the top of this article). But there are important differences as well.
Mercury has the largest core of all the rocky planets. Like Earth’s core, it's mostly made up of liquid iron. But Mercury’s core fills almost half the planet’s total volume. By comparison, Earth’s core takes up only 17%. Scientists aren’t exactly sure why Mercury has such a large core. It may have been formed in a part of the solar system with a lot of iron and other metals. Or there may have been a giant impact just after Mercury was formed, knocking away all of the lighter surface rocks.
Fast fact: Earth is made up of four main layers, one of which is liquid metal and creates the planet’s magnetic field.When it comes to Venus, scientists generally assume that the inside of the planet is very similar to Earth. And there are even more similarities on the two planets’ surfaces, both of which are relatively young, thanks to erupting volcanoes. Volcanic activity spreads fresh lava, which cools into new rocks. Venus may even go through phases where most or all of its volcanoes erupt at the same time, covering the entire planet with new rocks. Venus has by far the most volcanic activity of any planet in the Solar System.
Mars is the final stop on our tour of the rocky planets. Like Earth, it has a crust, a mantle, and a core. But Mars has lost much of its internal heat, making it likely that its core is nearly or entirely solid. Mars clearly once had more heat because it's home to the largest volcanoes in the solar system. Also, some of the planet’s oldest rocks are magnetized, indicating that Mars once had a liquid core that created a magnetic field, just like on Earth.
You must be wondering: How do we know all this? We can’t really take a giant knife and cut open the planets.
There are actually a few different ways to “look” inside a planet. The best way is to use seismometers. These tools measure the quakes that occur as rocks move around and these vibrations provide clues to what’s going on inside. There are lots of seismometers located all over Earth. There are even a few that were left on the Moon during the Apollo missions.
As for the other rocky planets, most of our information comes from satellites and other spacecraft. For example, Mariner 10 flew by Fast fact: The largest volcano in the Solar System is on Mars. It’s called Olympus Mons.Mercury in the 1970s, providing a great deal of information. Venus Express has been orbiting its namesake since 2006. And several different orbiters have been sent to Mars. As an orbiter circles a planet, it measures the rocks on the surface as well as the entire planet’s mass. This information provides a rough idea of what the inside of the planet is like.
So even if scientists can’t literally look inside a planet, even our own, there are ways of understanding what’s going on underneath the surface. In the case of the Moon and the four rocky planets, seismometers and spacecraft have revealed many similarities and some important differences.
Solar System Exploration (NASA) Views of the Solar System (Calvin J. Hamilton) How Do Planets Evolve? (Scitable, Nature Education) Extraterrestrial Plate Tectonics (Ask A Scientist, US Department of Energy) UCLA scientist discovers plate tectonics on Mars (Stuart Wolpert, UCLA) Plate tectonics on venus (Richard Ghail, Imperial College London)