Core
Iron Meteorites (samples from the cores of planetesimals)
We can’t directly access the cores of planets, but we have an idea of what the cores of planets are made of from iron meteorites. Iron meteorites are composed almost entirely of an iron-nickel alloy (5-25% nickel). Small round blobs of sulfide minerals or graphite (carbon mineral, best known as pencil lead) may also be present. Iron meteorites only account for ~5.7% of observed falls, but usually occur in higher abundances in meteorite collections because they are so much more recognizable than the stony meteorites.Earth
Earth has a solid inner core as well as a liquid outer core. Earth’s outer core is about 2300 km thick and composed of low viscosity molten iron metal that convects turbulently, and creates the Earth’s magnetic field, which extends from the Earth’s interior out into space, where it encounters solar winds.Mercury
The Mariner 10 mission (launched 1973) measured a weak magnetic field during its flyby of Mercury, which suggests it has some internal activity and a molten outer core. This finding was verified by the MESSENGER probe in 2008. Recall that Mercury has a large metallic core for its size and its close proximity to the Sun means that it was subjected to the most extreme heat since its formation.Mars
But not all planets have a liquid outer core. Since their formation, when they were partly or mostly molten, planets have gradually cooled. Mars is smaller than the Earth (diameter is 53% of Earth’s) and relatively far from the sun, so Mars has cooled more rapidly and does not contain a liquid outer core. Today, Mars does not have a single magnetic field like the Earth, although there is evidence that Mars had a magnetic field (and a liquid outer core) in its distant past.And back to Earth
The Earth is also gradually cooling (~100˚C per billion years). At the Earth’s inner-outer core boundary, the temperature is approximately 5700 K (5400˚C). As the Earth cools, its inner core is slowly growing outward as the outer core gradually solidifies. (Don’t worry, we aren’t in danger of losing our outer core or magnetic field any time soon!) Slight changes to the thickness of the outer core may influence the fluid movement and strength or orientation of the magnetic field.Currents within the Earth's outer core are being studied by satellites that look for slight changes in the magnetic field. Recently, a "jet stream" like feature was discovered. You can read about this discovery in the news article below.
