I have already discussed several observations and techniques you can use to get initial clues of what a planet's interior is like. I will summarize them and then discuss ways to increase the accuracy of those rough initial models. Methods and observations already mentioned are:
Astronomers have several other tools to probe the interiors of planets. By carefully observing the rotation of a planet, you can detect the precession (wobbling) of its rotation axis (the precession of the Earth's rotation axis is discussed in the coordinates section of the Astronomy Without a Telescope chapter). The rate of precession depends on a parameter called the moment of inertia which tells you how much the mass is concentrated toward the center. The Earth's core is considerably denser than its surface. The jovian planets have even greater concentration of their mass at their cores than the Earth does. Tiny twists in a terrestrial planet's spin can be used to determine if the core is solid or liquid. For example, the tiny twists in Mercury's spin rate are too great to be caused by a completely solid core, so its core (or parts of it) must be liquid.
The mass distribution of a planet can be probed by observing the motions of satellites (moons or spacecraft) in the planet's gravitational field. Mass lumps in the surface layer can be detected, as well as, asymmetries in the overall mass distribution. For example, the center of the Moon's mass is 2 kilometers closer to the Earth than the center of its overall shape (the geometric center). The Moon's crust on the Earth-facing side is several kilometers thinner than the crust on the far side. This is probably a remnant of the Earth's gravity acting on the early Moon's molten interior billions of years ago. Mars' center of mass is north of the geometric center. This is associated with the fact that Mars' southern cratered highlands stand about four kilometers higher than the northern volcanic plains.
The rate of heat loss from the warm interior and the rate at which the temperature increases at greater depths closer to the core are important parameters for determining the interior structure. On the Earth, scientists can drill several kilometers into the crust and measure the temperature difference between the bottom of the hole and near the top. For the jovian planets, infrared telescopes are able to detect their large heat flows.
For the terrestrial planets, the most useful data comes from seismology---the study of the interior from observations of how seismic waves (``planetquake'' waves) travel through the interior. Seismic waves slightly compress rock or cause it to vibrate up and down. They are produced when parts of a planet's crust suddenly shift and can be felt on the surface as a quake. These quakes have been studied extensively on the Earth, so I will focus on the use of earthquakes in what follows but remember that the same principles can be applied on any solid surface where instruments, called seismometers, are placed to study the jolts. Seismometers were left on the Moon by the Apollo astronauts and the Viking 2 lander on Mars had a working seismometer but persistent buffeting by the martian winds prevented it from being able to definitely detect any marsquakes (the seismometer on Viking 1 did not work).
The speed, amplitude, and direction the seismic waves move depend on the particular type of wave and the material they pass through. Just as a physician can use an ultrasound to get a picture of your anatomy or of a fetus, you can use seismic waves to get a picture of the Earth's interior (though it is a bit cruder than the physician's ultrasound). Earthquakes will produce two main types of waves: P (pressure) waves and S (shear) waves.
P waves are like sound waves---matter in one place pushes against adjacent matter compressing it. The result is a series of alternating stretched and compressed rock propagating in the same direction as the compression. It is like what happens when you stretch out a Slinky horizontally on a long table and give one end a sudden horizontal shove. You will see a wave of compressed metal coil move across the length of the Slinky to the other end. P waves can travel through solid and liquid material and move faster than S waves.
S waves are like waves in a jerked rope---matter moves up and down or side to side. Liquid matter prevents S waves from spreading. Timing of the arrival of seismic waves from at least three stations in a triangular array allows the earthquake center to be located. Seismometers on the opposite side of the Earth from the earthquake detect only P waves so there must be liquid material in the Earth's core. The size of the liquid core can be constrained from how far away a seismometer can be and still detect both S and P waves.
Seismic waves refract (bend) inside the Earth because of the change in speed of the waves as they move through material of variable density, composition, and temperature. Abrupt changes in direction occur at the boundary between two different layers. P waves entering the core are bent toward the Earth's center so they only reach the part of the Earth's surface opposite the earthquake. There is a shadow zone between the P waves that pass through the mantle only and those that pass through the mantle and the core. The shadow zone location also puts constraints on the size of the liquid core. It has a radius of about 3500 kilometers and is made of an iron-nickel alloy with a small percentage of sulfur, cobalt, and other minerals and has a density of around 12 (water = 1). Very weak P waves are felt in the shadow zone, indicating that a smaller solid component resides at the very center with a radius of about 1300 kilometers and a density around 14. Even though the temperature of the interior increases toward the center, the high pressures in the inner solid core make it solid while the outer metallic core remains liquid.
The mantle is made of hot but not quite molten iron-rich silicate minerals like olivine and pyroxene and is around 2800 kilometers thick. The density increases from about 3.5 below the crust to over 5 at the core boundary. Even though the mantle is not liquid, it can deform and slowly flow when stressed. Convective motions in the mantle rub on the crust to produce earthquakes and volcanoes.
The crust is broken up into chunks called plates with densities around 3. Oceanic plates are made of basalts (cooled volcanic rock made of silicon, oxygen, iron, aluminum, & magnesium) and are about 6 kilometers thick. The continental plates are around 20 to 70 kilometers thick and are made of another volcanic type of silicates called granite. They are less dense than the oceanic plates. The mantle convection causes the crustal plates to slide next to or under each other, collide against each other, or separate from one another in a process called plate tectonics. Plate tectonics is the scientific theory that describes this process and how it explains the Earth's surface geology. The Earth is the only planet among the terrestrial planets that has this tectonic activity.
The figure below shows the boundaries of the major plates on top of a map of the Earth. The arrows show the direction of the plates with respect to each other. The white areas are elevations greater than 2400 meters (7900 feet) above sea level. This figure is an adaption of a map in the "Plate Tectonic Movement Visualizations" website of the Science Education Resource Center at Carleton College and the plate motion data from This Dynamic Earth of the USGS. Select the figure to bring up an enlarged version of it.
The crust and the outermost part of the mantle make up a layer of hard rock called the lithosphere. The lithosphere gradually turns into the softer (and hotter) asthenosphere. Places where molten rock from the asthenosphere rise along weak points in the lithosphere can push apart the lithosphere on both sides (see the figure below). These places are at the midocean ridges (such as the Mid-Atlantic Ridge that bisects the Atlantic Ocean) and continental rift zones (such as the East Africa Rift Zone). Sea-floor spreading caused the Atlantic Ocean to grow from a thin sliver 100 to 200 million years ago to the its present size and now continues at a rate of about 25 kilometers per million years.
This pushing apart of some plates from each other means that others will collide. When oceanic crust runs into oceanic crust or into continental crust, the denser lithosphere material slides under the less dense lithosphere material and melts in the asthenosphere. The region where the lithosphere pieces contact each other is called a subduction zone and a trench is formed there. At the subduction zone, the melted material in the asthenosphere rises up through cracks in the crust to create a range of volcanoes (see the figure below). In another section you will see that this has a profound effect on regulating the climate of the Earth. When two continental pieces bump into each other, they are too light relative to the asthenosphere and too thick for one to be forced under the other. The plates are pushed together and buckle to form a mountain range. It also possible for two plates to slide past each other at what is called a transform fault such as the San Andreas Fault in California.
Examples of ocean-continental plate subduction include the Juan de Fuca plate off the coast of northwestern United States subducting under the North American continental plate to create the Cascade Volcano range, the Nazca plate subducting under the western edge of the South American plate to create the Andes range of volcanic mountains. An example of the ocean-ocean plate subduction are the chains of islands on the Asia side of the Pacific: the Aleutians, Japan, Philippines, Indonesia, and Marianas. An example of continent-continent plate collision is the Indian plate running into the Eurasian plate to create the Himalayas.
Planets exist in a balance between the compression of gravity and the pressure of the liquid and solid. Deeper layers experience more compression from the overlying material so the balancing outward pressure must increase. (This principle can also be applied to the gas of atmospheres to show why the atmosphere is thicker closer to the surface.) The computer model calculates the density in each layer from the equation of state with appropriate values of the temperature at that depth. The computer program starts off with the observed surface conditions and layer-by-layer, works its way toward the center. If the model does not arrive at a value of the total planet mass by the time it reaches the center, it must be revised. The models are checked against the other observables described above (moment of inertia, oblateness, gravity field measurements, heat flow, etc.) and refined further.
What follows is a brief description the other planet interiors found from putting all of the observations and theory together (see also the figure below). Mercury has a very large iron core about 3500 kilometers in diameter that makes up 60% of its total mass) surrounded by a silicate layer only 700 kilometers thick. Its core is partially molten. Its core must also contain some lighter element such as sulfur to lower the melting temperature. Venus's interior is very much like the Earth's except its iron-nickel core probably makes up a smaller percentage of its interior. Mars has a solid iron and/or iron-sulfide core 2600 to 4000 kilometers in diameter, surrounded by a silicate mantle and rocky crust that is probably several hundred kilometers thick.
The jovian planets are made of lighter materials that exist under much higher pressures than can occur anywhere on the Earth. Direct observations of their structure are still limited to the top several hundred kilometers of their atmospheres. Using those observations, computer models are calculated to predict what their interiors are like. Jupiter's hydrogen, helium atmosphere is at least 1000 kilometers thick and merges smoothly with the layer of liquid molecular hydrogen. The liquid hydrogen layer is about 20,000 to 21,000 kilometers thick. The pressure near the center is great enough to squeeze electrons from the hdyrogen atoms to make the liquid metallic hydrogen layer that is around 37,000 to 38,000 kilometers thick. Jupiter probably has a silicate/ice core twice the diameter of the Earth with about 14 times the Earth's mass. Although the core is made of silicates and ices, those materials are much different than the silicates and ices you are familiar with here on the Earth because of the pressures that are many times greater than the pressures at the Earth's core and temperatures in the 20,000 to 30,000 K range. Saturn is a smaller scale version of Jupiter: silicate core 26,000 kilometers in diameter, ice layer (solid methane, ammonia, water, etc.) about 3500 kilometers thick beneath a 12,000-kilometer thick layer of liquid metallic hydrogen, liquid molecular hydrogen layer around 28,000 kilometers thick, and atmosphere about 2000 kilometers thick.
The compression on Uranus and Neptune is probably not enough to liquify the hydrogen. Uranus and Neptune have silicate cores 8000 to 8500 kilometers in diameter surrounded by a slushy mantle of water mixed with ammonia and methane around 7000 to 8000 kilometers thick. This mantle layer is probably responsible for their strange magnetic fields which are not centered on the planet centers and are tipped by large degrees from their rotation axes. At the top is the 9000 to 10,000-kilometer thick atmosphere of hydrogen and helium. Tiny Pluto probably has a rocky core half its size surrounded by an ice mantle/crust.
| equation of state | seismology | plate tectonics |
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last updated: June 1, 2007