"Planets are like jawbreaker candy!" exclaimed this one kid while I explained the interiors of planets during a Boy Scout gathering. Very adorable and very true! Interiors of planets have layers, each with its own material composition that would relate to what is going on the surface!
The terrestrial planets: Mercury, Venus, Earth, and Mars, and even our own Moon, all have solid, dense cores. However, we can't immediately image the interior of our planet or others, so we use computer models, earthquake data, and magnetic field analyses for guidelines. Earthquake energy waves behave differently with each layer of the Earth. The magnetic field strength is determined by the density and rate of the spinning inner core. So if a planet has a weak magnetic field, we can guess that the interior layers are slow, and probably cooling more.
Some trends we see with these terrestrial bodies is that "late" worlds cool slower, so they have still-warm interiors and thinner crusts, which would have more variety in geologic formations (the crust could bend or crack easily). This is why we see Earth and Venus to be geologically active. Mercury, on the other hand, has a very thick crust, so we see no volcanoes or mountains. For Mars, we know that there used to be a magnetic field and tectonic activity with its volcano systems, but something caused the core of Mars to stop spinning rapidly and has cooled significantly.
The Gas Giants (Jupiter and Saturn) and Ice Giants (Uranus and Neptune) share commonalities to their interiors. Each has a rocks and metallic hydrogen core. How can hydrogen be metallic if it's a gas? Hydrogen, when under certain conditions, can act as an electrical conductor! This metallic hydrogen core, when squeezed and stretched constantly produces an internal heat. The mantle of these giant planets layers liquid hydrogen to gaseous hydrogen to a "crust" of icy-material clouds with hydrogen, helium, methane, and ammonia.
With the newly imaged Pluto, the questions of what is going on with Pluto's interior is still being poured over with speculations on layer thickness, flowing ice mixtures, and the geology of the crust to reflect what's going on inside.
Thank you for reading and come back next week for a look at the largest impact basin in the Solar System- "The Core Destructor".