At-Home STEM Activities: Planetary Structure
We’re spending this week looking at planets, so let’s get to the heart of the matter: planet cores. Although there are common elements in all of the planet cores in our solar system (we think...), there is a lot of variety out there, too. And a planet’s internal structure has a big impact on what happens around the planet, too.
First, we can start by dividing the planets in our solar system into two groups: terrestrial planets and giant planets. Terrestrial planets (Mercury, Venus, Earth, and Mars) are mostly made of silicate rocks and metals- we also refer to them as rocky planets. They are the 4 smallest planets, are closest to the Sun (“interior”), and have a solid surface. The giant planets (Jupiter, Saturn, Uranus, and Neptune) are all bigger than Earth; farther from the Sun; and consist of hydrogen, helium, methane, and other fluids under extreme pressures and temperatures where phases like ‘gas’ and ‘liquid’ don’t really apply anymore.
To get a better sense of how these 8 planets’ cores differ and what that can mean for the planets, you can chart both their sizes and internal layers- because the giant planets are SO GIANT in comparison to the terrestrial planets, it may help to make two charts, one for each group.
I started by folding a piece of paper in half the long way and then making two graphs- for each the y-axis (up and down) was 10 cm with marks every centiment, and the x-axis (side to side) was 26 cm. I labelled each y-axis “Planet’s Radius, km” and left space to write the planets’ names under the x-axis. The upper graph was for the terrestrial planets and the lower graph for the giant planets. For the marks on the y-axis, I used two different scales- for the terrestrial planets, each cm represented 720 km and for the giant planets each cm represented 7200 km (these sound like weird numbers but, since Jupiter, the largest giant planet, is roughly 11 times the size of Earth, the largest terrestrial planet, it means that the top graph shows planets that are actually about 10 times smaller than the planets in the bottom graph (we’ll add one for scale later).
To graph the planet cores, we’ll need to know a few things: the planet’s radius, which layers it has for structure, and how thick those layers are. I found that the information was not all in the same place, so here are links that can help with that (there are many others, as well). *keep in mind we are learning more about each planet all the time, but our knowledge is definitely incomplete- for example, we recently found out that Mercury’s core layers make up much more of the planet’s interior than we had thought. And we still don’t know what the liquid core layer of Venus is like. You can use this information to create a cut-away diagram of ¼ of each planet (go up the y-axis and out the x-axis the same distance and then create and arc connecting the two points so that you are looking at ¼ of a circle), showing the different layers and how thick they are (see the picture below to help you visualize that). Since we are trying to show each planet to scale, remember to divide the radius, thickness, etc., by the value of a mark on the y-axis- that will tell you how many cm big it should be here.
Notice that Venus’s core is a bit undefined because we are still learning about it; the little gray Earth in the lower graph provides a sense of how big the giant planets are.
Here are some useful links:
http://abyss.uoregon.edu/~js/ast121/lectures/lec16.html
https://solarsystem.nasa.gov/planets/venus/in-depth/
http://volcano.oregonstate.edu/earths-layers-lesson-1
https://science.howstuffworks.com/mars4.htm
https://sciencing.com/jupiters-core-vs-earths-core-21848.html
https://solarsystem.nasa.gov/planets/saturn/in-depth/
Why is the structure of a planet important? Partly this information is helpful in understanding both how the planets and the solar system formed- by looking at the distribution of elements, the sizes of the planets, and other aspects, we can better understand what happened in the billions of years after the Big Bang and even get a better sense of what will happen to Earth going forward. It also helps us understand the difference between a star and a planet- for example, right now there is discussion of how a brown star is different from a gas giant like Jupiter or Saturn. And future exploration is aided by understanding the pressures and surfaces (or lack thereof) that probes and astronauts might encounter- Saturn, for instance, doesn’t really have a solid surface to land on, but the pressures experienced there are extreme, so any probe exploring Saturn up close would need to be designed for those conditions. The core of a planet also influences the kind of magnetic field a planet has- a planet’s magnetic field is often generated by convection in an electrically-conducting fluid in the interior of the planet. For Earth, this refers to the liquid outer iron core, while for Jupiter this means the liquid metallic hydrogen around the solid core. But Mars doesn’t have a liquid layer in the core (that we know of) and gets its magnetosphere from the interaction between the atmosphere and solar wind- this means that it doesn’t protect the surface from solar particles the way that our magnetosphere does.
