Planetary Interiors:

We can not examine the interiors of planets directly (even our own). Thus, we build computer models. A computer model basically takes the known laws of physics (in mathematical form) and calculates what are the conditions at any particular point inside the planet. To perform these calculations, a computer model needs a series of input paramters, such as:

  • density as a function of radius
  • rotation rate
  • magnetic field
  • temperature as function of radius
  • chemical composition

    We can check the computer model by observing boundary conditions, things that we can measure at the surface of the planet, such as:

  • total mass
  • mean density
  • surface temperature
  • strength of magnetic field at surface
  • thickness of crust
  • seismic activity

    Dynamo effect:

    A planet's magnetic field also provides a peek into its interior. For planetary magnetic fields are generated deep in the core of the world. Our understanding of the origin to magnetic fields in planets is very poor. We know that the Earth's magnetic field is not due to the presence of a giant permanent magnet, such as iron, deep in the Earth's core because 1) the core temperature is above the Curie temperature and 2) the field is too variable. It must be a generated field and we know that a conducting fluid in motion generates a dipole magnetic field. The nature of this field and its evolution is governed by the field known as magnetohydrodynamics. The liquid outer core of the Earth is the conducting fluid, free electrons being released from metals, such as Fe (iron) and Ni (nickel), by friction and heat. Variations in the global magnetic field represent changes in fluid flow in the core.

    The field of paleomagnetism examines the behavior of the Earth's magnetic field with time. Whenever a rock crystallizes from molten lava on the surface, the magnetic elements in the rock act like tiny compass needles and are frozen into position aligned with the direction of Earth's field. Fossil evidence for field reversals on timescales of 100,000 years indicates that the process of magnetic field generation is unstable.

    For the planets it is key to know that a magnetic field indicates that:

    1) the planet has a large, liquid core
    2) the planet has a core rich in metals (source of free electrons)
    3) the planet has a high rotation rate

    The strength of the magnetic field is telling you something about the combination of the above factors. For example, Mercury has a weak magnetic field. But, since it has a very low rotation rate we conclude that it has a large liquid core. Mars has a high rotation rate (similar to Earth's), but a magnetic field that is 1/800th the strength of the Earth's. Therefore, we conclude that Mars has a very small core.

    Chemical Fractionation:

    The most important process early in the formation of a planet that influences its structure of its interior is gravity. Gravity causes heavier elements to sink to the core of a planet, this is called chemical fractionation.

    Since this is a slow process, the planet may solidify before chemical fractionation can fully develop. Thus, large, massive planets, like the Earth and Venus, are molten long enough for a Fe and Ni core to form. Whereas, smaller planets, like Mars, cool faster and solidify before the heavier elements sink to the core. Thus, elements like Fe are over abundant in the soil, giving Mars its red color.

    Crust development:

    The thickness of a planet's crust is directly proportional to the rate at which the planet cooled in the distant past. A fast cooling rate (i.e. a small planet) will result in a thick crust. For the major terrestrial worlds, the crust thickness is proportion to the diameter of the planet is:

    Note that the cooling rate is proportional to the total mass of the planet. Large worlds cool slower, have thinner crusts. High cooling rates also determine the interior structure. Slow cooling rates imply planets that still have warm interiors now. Warmer interiors imply more diversified structure (inner core, outer core, semi-solid mantle, etc.)

    Note also that a thicker crust means less tectonic activity.

    Summary of Terrestrial Planet Interiors:

    The make-up of planet interiors is dominated by the physics of materials under high temperatures and pressures. Starting with cold, low pressure regions, rocky materials are straight solids. As one goes deeper into a planet the temperature and pressures go up. Solids become semi-solid, plastic-like materials. With higher temperatures and pressures, semi-solids become liquids. With even higher temperatures and pressures liquid or molten rocky materials undergo a phase change and become solids again. That is why the very inner cores of the Earth and Venus are solid, surrounded by liquid outer cores.

    Just from examination of mean densities, the primary terrestrial worlds fall into two classes. High mean density worlds, Earth, Venus and Mars, with values around 5 gm/cc. And low mean density worlds, Mercury and the Moon, with values around 3 gm/cc. The high density worlds have Fe/Ni cores and clearly defined interior structure. Low density worlds have weaker structure, i.e. no strong cores. Cross sections of the major terrestrial worlds are found below. Areas marked in red are in a liquid or semi-solid state.

    Several trends should be noted:

  • The two largest worlds have the largest liquid outer cores and, therefore, the highest amounts of current tectonic activity.

  • Magnetic fields are generated by planets with molten inner or outer cores. The strength is determined by the size of the core and rotation rate of the planet. For example, the Earth has a strong magnetic field because it has a large core and high rotation rate. Mercury has a weak field since it has a large core, but very slow rotation rate (56 days). Venus has almost no magnetic field because even though it has a large core, its rotation rate is extremely low (243 days).

  • Tectonic activity is caused by heat in the core. But, visible effects (volcanos) is also determined by thickness of crust. Mercury has a large core, but a very thick crust and shows no tectonic activity.

    Galilean Satellites Interiors:

    The primary difference in the formation of the Galilean satellites is the much higher concentration of icy materials in the outer solar system compared to the inner terrestrial worlds. Due to this, the composition of the crusts is dominated by H2O and CO2 ice. Other points to note:

  • The distance to Jupiter determines the amount of tidal friction. The amount of tidal friction determines the amount of heat in the moons core and, therefore, the level of geological activity.

  • Large amounts of outgassing have drained the inner moons, Io and Europa of their icy materials making them rich in rocky materials.

  • The lack of tectonic activity for the outer worlds has left them with warm interiors and large, liquid ice mantles.