The Inner Solar System

There are four rocky, or terrestrial, planets: Mercury, Venus, Earth, and Mars. These planets are called terrestrial planets because they are made up of rocks and metals and have solid surfaces. But even though they're made of the same materials, the four rocky planets in the Solar System aren’t the same. 

In many ways, all the rocky planets are similar. They all have a solid rocky crust, some form of mantle, and a core. But there are important differences, too.

Let’s take a closer look at the rocky planets, inside and out. 

Did you know?

The four rocky planets are the closest planets to the Sun. 

What are the layers of Earth?

Let’s start with the planet we know most about: the Earth. The top layer is called the crust. That’s the layer where we live. The crust can be anywhere from 8 to 50 kilometres thick.  Compare this to the Atlantic Ocean. It is only 8.486 km deep! 

The next layer is called the mantle. It makes up most of Earth. It is made up of rocks and minerals. The mantle is made up of two parts: the upper and the lower portions. Because the inside of the Earth is very hot, the material within the mantle is molten. That means it’s able to move around. It’s kind of like thick molasses.

Under the mantle is the Earth’s core. It is made of nickel and iron. The Earth’s core has two parts: the outer core and the inner core. The Earth’s outer core is so hot that all the rocks and metals there have melted!  They slosh around, creating Earth’s magnetic field. 

At the very centre of Earth is the inner core. Though it is very hot, the metals there are compressed into a solid. That is because of the pressure from all the rock above. 

What are the layers of Mercury?

Next, let’s look at Mercury. Mercury’s surface is rocky and covered in impact craters. It most closely resembles the Moon. Like Earth, Mercury has a crust, mantle, and core. 

Mercury is the smallest of the rocky planets. But it has the largest core! It is mostly made up of liquid iron. Mercury’s core fills over 60% of the planet’s total volume. In comparison, Earth’s core takes up only 15%. 

Scientists aren’t exactly sure why Mercury has such a large core. They think that maybe Mercury was formed very early in the Solar System’s history from water vapour. That water vapour was probably created by objects called planetesimals. These planetesimals probably contained a lot of iron. This would help explain why Mercury’s interior is so rich in iron. 
 

What are the layers of Venus?

Now, let’s look at Earth’s neighbour, Venus. Venus has a very similar size, mass and density to Earth’s. Because of this, scientists generally assume that the inside of the planet is very similar to Earth’s, too. Unfortunately, scientists do not know very much about Venus’ interior. It is difficult to study Venus because it has a very thick, dense atmosphere. Also, Venus is extremely hot!

Did you know?

Geologists who study the geology of the Earth’s Moon, other planets and their moons, comets, asteroids, and meteorites are called astrogeologists.

Unlike Mercury’s surface, Venus’ surface appears quite smooth. This is because there are volcanic plains all over Venus’ surface. Volcanic activity on Venus spreads fresh lava. Lava cools into new rocks. 

Some scientists believe that volcanoes on Venus have recently erupted. They may even still be erupting. Venus may even go through phases where most or all of its volcanoes erupt at the same time. If this happens, then the whole planet is covered in new rock! 

Out of all the planets in the Solar System, Venus has the most volcanic activity by far.
 

What are the layers of Mars?

Finally, let’s look at Mars. Mars has lost much of its internal heat. That means its core is at least partially solid. How do scientists know Mars used to have more internal heat? Because Mars is home to some of the largest volcanoes in the Solar System! Also, some of the planet’s oldest rocks are magnetized. That means that Mars once had a liquid core that created a magnetic field, just like Earth’s. 

Did you know?

The largest volcano in the Solar System is on Mars. It’s called Olympus Mons. 

How do we know what planets look like on the inside?

You might be wondering, how do we know all this? We can’t really take a giant knife and cut open the planets. So how do scientists know what the inside of planets looks like? 

There are actually a few different ways to study the inside of a planet. For example, scientists can study a planet’s vibrations. When rocks move around, they cause vibrations. These vibrations give clues about what is happening inside a planet. Scientists can measure these vibrations with special motion sensors called seismometers . 

There are seismometers located all over Earth. Astronauts even left a few seismometers on the Moon during the Apollo missions. 

As for the other rocky planets, most of our information comes from satellites and other space probes. For example, Mariner 10 flew by Mercury in the 1970s. It provided scientists with a great deal of information about the planet. Venus Express orbited Venus between 2006 and 2014. It sent a lot of information about Venus back to Earth. Also, many different orbiters have been sent to Mars. 

As an orbiter circles a planet, it measures the rocks on the surface. It also measures the entire planet’s mass. These two pieces of information provide a rough idea of what the inside of the planet might be like.
 

Did you know?

The planets farthest from the Sun -- Jupiter, Saturn, Uranus, and Neptune -- are called the gas giants. They are made up of a mixture of gases and do not have solid cores.

Scientists can’t literally look inside a planet, not even our own! But they still have ways of understanding what’s going on beneath its surface. In the case of the four rocky planets, seismometers and spacecraft have revealed many similarities and some important differences.