How can we tell what the interiors of planets are like?
I would like to know how you can tell what a planet core is made of.
The answer is part observation, and part intelligent guesswork.
In order to find out what's inside a planet, we can't just drill a hole. Even on Earth we can only penetrate a few miles into the crust! We have to rely on indirect methods.
Gravity: By putting a spacecraft in orbit around a planet, we can observe how the spacecraft's orbit is affected by the planet's gravity. This lets us determine its density profile (how dense the different layers of the planet are.)
Oblateness: No planet is a perfect sphere. They all bulge a bit at the equator due to the centrifugal force caused by their rotation. The amount of the bulge is called the planet's oblateness, and how strong the oblateness is also tells us something about the density profile of the planet.
Seismology: Using seismometers on the surface of the Earth and the Moon, and perhaps one day soon, Mars, we can measure tremors passing through the planet by measuring earthquakes (or moonquakes or marsquakes!) at the surface. Quakes travel through the planet as waves, and these waves travel at different speeds through different materials, and can also be reflected by the boundaries between layers. By tracking the path of seismic waves through a planet, we can tell what layers of a planet are high and low density, and which layers are is solid and which are molten.
Now, here's where the guesswork comes in. Based on the density profile of the planet, we can guess what the planet is made of. Rock has a lower density than metal, for example. And different types of metal have different densities. While making these guesses, we have to keep in mind that the deeper you go into a planet's interior, the higher the temperature and pressure, and this can affect the density of the planet as well. And of course we keep in mind what kind of building blocks there are to make planets from. It would be a little silly to conclude that the center of the Moon is made of gold, for example, because we know from studying the composition of meteorites and the crusts of the planets that gold is fairly rare in the Solar System.
So this is how we can make fairly good guesses at what the interiors or Mercury, Venus, Earth, the Moon, and Mars are like. We can also use these methods to infer the interior structure of some of the better-studied moons in the outer Solar System, Io, Europa, Ganymede, and Callisto.
For gas giants, it's a little tougher because they don't tend to have nice distinct layers like the rocky planets. Also, they are so huge that the pressures and temperatures in their deep interiors are much greater than we can simulate in the laboratory, so we must rely on theoretical predictions rather than actual data to say how the substances they are made of act under those circumstances. Still, we can determine that Jupiter and Saturn are composed almost entirely of hydrogen and helium, and that most of the heavier stuff, including rocky materials and metals, must have sunk down to their cores. But is the core solid, in any real sense of the word? And what exactly is the core's composition? It's very difficult to say, because we have a hard time understanding how materials behave when they are so hot and so dense.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
- What makes the Earth rotate?
- Will the magnetic field of the Earth reverse its direction in the future? Will this cause problems?
- Is the Moon hollow?
- What kind of rock is the moon made of?
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist
This page has been accessed 29934 times since June 21, 2004.
Last modified: June 25, 2004 3:20:29 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)