The study of seismic wave behavior reveals that Earth's core is divided into two parts: a liquid outer core and a solid inner core. Scientists believe the liquid outer core was created when an object roughly the size of Mars collided with proto-Earth. A collision of this magnitude would generate enough heat to melt the outermost part of the planet's core. Scientists believe the transition from liquid to solid occurs in the Earth's core at around a depth of 5,156 kilometers below the surface. The inner core of the Earth is estimated to be around 9,400 degrees Fahrenheit.
Scientists believe Mars had many fewer collisions than Earth and stopped growing in the first few million years of the solar system. Because of the planet's smaller size, less gravitational pull was exerted on loose floating objects. Without Earth's violent history of collisions, Mars' solid inner core is believed to have formed under lower pressures and temperatures.
Venus' density is close to that of Earth's. Because of this, the planet's core is believed to be approximately 3,000 kilometers across and made of metal. The transfer of heat from Venus' core to its surface confirms that the planet has high interior temperatures. Although scientists cannot confirm whether the core is solid or liquid, best guesses indicate that it is solid because of the planet's lack of a magnetic field. The Earth's magnetic field is widely believed to be generated by the convection of its liquid core.
Jupiter's interior is believed to be mostly fluid. Although an iron and silicon core may make up 15 percent of its radius, liquid molecular hydrogen and liquid metallic hydrogen are believed to make up the rest. The hydrogen convects heat from the interior and brings it to the surface. This heat is detectable from Earth using infrared measurements. Jupiter's interior is so hot that it actually radiates twice as much heat from its core as it receives from the sun.
