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.
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. 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. The Earth is the only planet among the terrestrial planets that has this tectonic activity.
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 probably partially molten. 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 |
|---|
Venus is about 95% the size of the Earth and has 82% of the Earth's mass. Like the Earth, Venus has a rocky crust and iron-nickel core. But the similarities stop there. Venus has a thick atmosphere made of 96% carbon dioxide (CO2), 3.5% nitrogen (N2), and 0.5% other gases. Venus' surface air pressure is 91 times the Earth's air pressure. Venus' surface pressure is the same as what you would feel if you were 1 kilometer below the ocean surface on the Earth. A human cannot survive at depths greater than just 70 meters below the ocean surface without special diving suits or a submarine. If you want to send someone to Venus, that person would need to be in something like a diving bell.
The Venus explorer would also need a very powerful cooling system: the surface temperature is 737 K (= 477ƒ C)! This is hot enough to melt lead and is over twice as hot as it would be if Venus did not have an atmosphere. Why does Venus have such a thick atmosphere and why is it so hot on its surface?
Venus is so hot because of a huge greenhouse effect that prevents heat from escaping to space. Plant nurseries use greenhouses to keep their plants warm during the winter months. Energy in the form of visible light from the Sun passes through the glass walls and glass roofs of a greenhouse and heats up the plants and soil inside the greenhouse. The air in contact with the plants and soil gets warmed up. The glass walls and roofs prevent the hot air from escaping to the outside. The same sort of thing happens to the interior of your car when you leave it out in the Sun with the windows rolled up.
On a planet, certain gases like carbon dioxide or water vapor in the atmosphere prevent heat energy in the form of infrared light from leaking out to space. These so-called ``greenhouse gases'' allow visible light from the Sun to pass through and heat up the surface. The surface gets warm enough to emit infrared light. Some of the infrared light is absorbed by the greenhouse gases and radiated back toward the surface, keeping the surface warm. On Venus, the super-abundance of CO2 in its atmosphere is responsible for the huge greenhouse effect. Venus' greenhouse effect probably started from the presence of a lot of water vapor, but Venus is now a very dry place.
Venus' water was always in the gaseous form and could reach high enough in the atmosphere for ultraviolet light from the Sun to hit it. Ultraviolet light is energetic enough to break apart, or dissociate, water molecules into hydrogen and oxygen. The very light hydrogen atoms were able to escape into space and the heavier oxygen atoms combined with other atoms. Venus' water was eventually zapped away. The Earth's ozone layer prevents the same thing from happening to the water here.
Ordinary hydrogen has only one proton in the nucleus, while the isotope deuterium has one proton + one neutron. Therefore, deuterium is about twice as heavy as ordinary hydrogen and will stay closer to the surface on average. Gases higher up in the atmosphere are more likely to escape to space than those close to the surface.
On Earth the ratio of ordinary hydrogen to deuterium (H/D) is 1000 to 1, while on Venus the proportion of deuterium is about ten times greater---the H/D ratio is 100 to 1. The H/D ratio on Venus and Earth are assumed to have been originally the same, so something caused the very light hydrogen isotopes on Venus to preferentially disappear. An easy explanation for it is the ultraviolet dissociation of water.
A summary flowchart of what happened on Venus is given on the Earth-Venus-Mars summary page. Water vapor started the greenhouse heating. Carbon dioxide was baked out of the rocks, further aggravating the greenhouse effect. A runaway greenhouse started. The end result was all of the carbon dioxide in the atmosphere and the water dissociated away. The flowchart on the Earth-Venus-Mars page up to the last arrow occurred several billion years ago. The diamond at the end describes the current state: CO2 maintains the extremely hot temperature.
What makes Mars so
intriguing is that there is evidence for running liquid water in
its past. Some geologic features look very much like the river drainage systems on
Earth and other features points to huge floods. The Mars Pathfinder
studied martian rocks in the summer of 1997 and found some rocks are
conglomerates (rocks made of pebbles cemented together in sand) that
require flowing water to form. Abundant sand also points to widespread
water long ago. Where there is liquid water, there is
the possibility for life to arise.
Tiny structures in a meteorite that was blasted from Mars in a huge impact of an asteroid look like they were formed by ancient simple lifeforms. However, there is still a lot of debate among scientists on that but strong evidence of contamination by terrestrial organic molecules has probably killed the possibility of conclusive proof of martian life in the meteorite. The dissenters are not wanting to be partypoopers. They just want greater certainty that the tiny structures could not be formed by ordinary geologic processes. The great importance of discovering life on another world warrants great skeptism---``extraordinary claims require extraordinary evidence''. The search for martian life will need to be done with a sample-return mission or experiments done right on Mars.
Mars' surface air pressure is much too low for liquid water to exist now. At very low pressure, water can exist as either frozen ice or as a gas but not in the intermediate liquid phase. If you have ever cooked food at high elevations using boiling water, you know that it takes longer because water boils at a lower temperature than at sea level. That is because the air pressure at high elevations is less. If you were several miles above the Earth's surface, you would find that water would boil (turn into steam) at even room temperature! The fact that Mars had liquid water in the past tells us that the early Martian atmosphere was thicker and the surface was warmer from the greenhouse effect a few billion years ago.
Life may have started there so current explorations of Mars are focussing on finding signs of ancient, long-dead life. Any lifeforms living now would have to be living below the surface to prevent exposure from the harsh ultraviolet light of the Sun. Mars has no protective ozone layer, so all of the ultraviolet light reaching Mars can make it to the surface. The Viking landers that landed in 1976 conducted experiments looking for biological activity, past or present, in the soil but found the soil to be sterile and more chemically reactive than terrestrial soil from the action of the harsh ultraviolet light. What changed Mars into the cold desert of today?
 |
 |
The runaway refrigerator is described in a flowchart on the Earth-Venus-Mars page. The flowchart up to the last dashed arrow occurred several billion years ago. The box at the end describes the current state: frozen water and carbon dioxide below the surface and a very thin atmosphere.
Our home planet, the Earth, is the largest of the terrestrial planets with a diameter of 12,742 kilometers and a mass of 5.977 × 1024 kilograms. It has moderately-thick atmosphere that is 78% nitrogen (N2) and 21% oxygen (O2). It has the right surface temperature and atmospheric temperature for life and liquid water to exist. It is the only place that has either of these things. Some water is also in the form of water vapor and ice. <
Most of the Earth's water is
liquid and some is frozen. The rest that is water vapor
works with carbon dioxide in the atmosphere to create a small greenhouse effect,
raising temperature about 30ƒ C. This natural greenhouse effect makes it warm
enough on the surface for liquid water to exist. Besides making life possible, the
liquid water
also helps to keep the amount of atmospheric carbon dioxide from getting too high.
Carbon dioxide dissolves in liquid water to form ``carbonic acid'' (soda water).
Some of the dissolved carbon dioxide will combine with minerals in the water and settle
to the ocean floor to form limestone. The amount of atmospheric carbon dioxide is
also kept in check by biological processes.
Aquatic plants extract carbon dioxide dissolved in the water to use in their photosynthesis process. Aquatic animals use the carbon dioxide and calcium in the water to make shells of calcium carbonate (CaCO3). When the animals die, their shells settle to the ocean floor where, after years of compacting and cementing, they form limestone, locking up the carbon dioxide. Some of the carbon dioxide is released into the atmosphere via geologic heating processes such as volcanism.
This whole process of the cycling of the carbon dioxide in the water, life, rocks, and air is called the carbon cycle or the carbon dioxide cycle in geology and oceanography. There is the equivalent of 70 atmospheres of CO2 locked up in the Earth's rocks. The contribution of carbon dioxide from the burning of fossil fuels by humans is a new input into this cycle with unknown consequences. Could this artificial input into the carbon cycle upset the natural balance and create a runaway greenhouse effect like that on Venus? Maybe. How much would it take to tip the balance? We don't know. We do know that the amount contributed by human activity has had an effect.
A flowchart of the carbon cycle on the Earth is given on the Earth-Venus-Mars page. While the flowcharts for Venus and Mars show what happened long ago, the flowchart for the Earth shows the cycle as it currently operates.
The temperature of the atmosphere decreases with increasing altitude above the surface up to the ozone layer. The temperature below the ozone layer is below the freezing point of water. If water vapor gets up too high in the atmosphere, it condenses and rains back to the surface. This height is below the ozone layer, so the water vapor does not get high enough to be dissociated---there is a ``cold trap'' below the ozone layer.
Within the past couple of decades, the ozone layer has been partially destroyed by some of the chemicals used in modern devices. One class of ozone-destroying chemicals are called chlorofluorcarbons (CFC's) that are used in aerosol sprays, the cooling fluid in older refrigerators and air conditioners and in making styrofoam. Most of the industrialized nations have taken steps to reduce the production of CFC's, but the CFC's already in the atmosphere will take some time to disappear and the ozone destruction will continue for a while. You may have heard that some ozone is produced by engines here on the surface. Unfortunately, that ozone does not make it to the ozone layer high up. Also, ozone can damage our respiratory system.
| dissociation | greenhouse effect | ozone |
|---|---|---|
| photosynthesis | runaway greenhouse | runaway refrigerator |
The Moon also has large, dark smooth areas covering about 17% of the Moon's surface that people originally thought were seas of liquid water so they are called mare (Latin for ``seas''---they are what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out over many hundreds of square miles, covering up many craters that were originally there. The mare material is basaltic like the dark material on the Earth's ocean crust and that coming out many of our volcanoes (e.g., the Hawaiian islands). Mercury also has maria but they are lighter in color because of the different chemical composition and they do not stand from its heavily cratered areas.
Liquid water cannot exist on the Moon because of the lack of an atmosphere---the Moon has only 1/81 the Earth's mass and about 1/6th the Earth's surface gravity. If there is any water to be found on the Moon, it will be in a frozen state in a place of constant shade such as deep craters near the poles. Recent missions have discovered some of those ice blocks near the poles. The ice blocks will be the source of water for any humans that decide to set up bases on the Moon.
Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims. The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts. The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact. The explosion is what carves out the crater so all craters are round (otherwise they would be oblong in shape). The rock on the surface of the planet or moon is bent backward, upward, and outward so the amount of material ejected is much larger than the projectile. Large craters will have a central peak formed by the rock beneath the impact point rebounding upward and they may also have terracing of the inner walls of the crater from the collapsing of the crater rim inward. The size of the craters having central peaks depends on the size (gravity) of the planet or moon: on the Moon craters larger than about 60 kilometers in diameter have central peaks while the crater diameter on the Earth need be larger than just 1 to 3 kilometers.
The number of craters per unit area on a surface can be used to determine an approximate age for the planet or moon surface if there is no erosion. The longer the surface has been exposed to space, the more craters it will have. If you know how frequently craters of a given size are created on a planet or moon, you can just count up the number of craters per unit area. This assumes, of course, that the cratering rate has been fairly constant for the last few billion years. The heavy bombardment of about 3.8 billion years must be taken into account when using the crater age dating technique. For example, the highland regions on the Moon have ten times the number of craters as the maria, but radioactive dating (explained in the next chapter) shows that the highlands are approximately 500 million years older than the maria, not ten times older. Careful studies of how the craters overlap other craters and other features can be used to develop a history or sequence of the bombardment on the moons and planets.
Our knowledge about the Moon took a huge leap forward during the Apollo missions. One main science reason for going to the Moon was to return rock samples to find about their ages and composition. Using their knowledge of geology gained from the study of Earth rocks, scientists were able to put together a history for the Moon. The Apollo astronauts also left seismometers on the Moon to detect moonquakes that can be used to probe the interior using seismology.
The Moon's density is fairly uniform throughout and is only about 3.3 times the density of water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core and slow rotation is why the Moon has no magnetic field.
Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was heated to very high temperatures so that the volatiles escaped to space.
The giant impact theory proposes that a large Mars-sized object hit the Earth and blew mantle material outward which later recoalesced to form the Moon. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. The one ``drawback'' of the theory is that it has a lot of parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result. A complex model can usually be adjusted to fit the data even if it is not the correct one (recall Ptolemy's numerous epicycles). But the giant impact theory is the only one proposed that can explain the compositional and structural characteristics of the Moon.
Even though Io is about the same distance from Jupiter as the Moon is from the Earth, Io experiences much stronger tidal stretching because Jupiter is over 300 times more massive than the Earth. Io's orbit kept from being exactly circular by the gravity of its Galilean neighbor Europa and the more distant Ganymede. Io cannot keep one side exactly facing Jupiter and with the varying strengths of the tides because of its elliptical orbit, Io is stretched and twisted. The tidal flexing heats Io's interior to the melting point just as kneading dough warms it up. The heat escapes through powerful eruptions spewing sulfur compounds in giant umbrella-shaped plumes up to almost 300 kilometers above the surface. The tidal heating from Jupiter has driven away much of the volatile materials like water, carbon dioxide, etc. Io's surface is a splotchy mixture of orange, yellow, black, red, and white. The colors are created by sulfur and sulfur dioxide at various temperatures in liquid and solid states.
The ocean of liquid water below Europa's icy surface may extend down serveral tens of kilometers (or more). Could life forms have developed in the warm waters below the icy surface? Recent discoveries of fish, albino crabs, and 10-foot-long tube worms huddled around active volcanic vents on the Earth's ocean floor far below where the sunlight energy can penetrate has bolstered the view that Europa could harbor life below its icy surface away from sunlight. Before the discovery of life around the geothermal vents, scientists thought that all life depended on sunlight. More recently, bacteria have been found to exist in rock a few kilometers below the sea floor and land surface. Clearly, life is more versatile than originally thought.
The second largest of the Galilean satellites and the farthest from Jupiter is the heavily-cratered moon called Callisto. It has a density of 1.8 times that of water, so it has proportionally more frozen water surrounding a smaller rocky core than Ganymede. Callisto's surface does not appear to have undergone any sort of geological activity. Callisto has a huge impact site called Valhalla that was produced about 4 billion years ago. When the asteroid hit Callisto, it exploded on impact. The explosion heated the ice to above the melting point and the shock waves produced a ripple pattern away from the impact site. The ripples later froze so Valhalla now looks like a big ``bull's eye''.
Titan's surface probably has frozen methane ice fields with organic material accumulated on the surface. Titan is probably like the early Earth's chemistry. Its very cold temperatures may then have preserved a record of what the early Earth was like before life formed. The Cassini spacecraft now on its way to Saturn (it will arrive in 2004), will launch a probe into Titan's atmosphere and will hopefully send back data of what lies below the hazy layer.
Triton has many black streaks on its surface that may be from volcanic venting of nitrogen heated to a gaseous state despite the very low temperatures by high internal pressures. The nitrogen fountains are about 8 kilometers high and then move off parallel to the surface by winds in the upper part of its thin atmosphere. Another unusual thing about Triton is its highly inclined orbit (with respect to Neptune's equator). Its circular orbit is retrograde (backward) which means the orbit is decaying---Triton is spiralling into Neptune. Triton's strange orbit and the very elliptical orbit of Neptune's other major moon, Nereid, leads to the proposal that Triton was captured by Neptune when Triton passed too close to it. If it was not captured, Triton was certainly affected by something passing close to the Neptune system.
All of the jovian planets have a system of rings. Jupiter has four faint rings: a flattened main ring, a puffier inner ring, and two wispy outer rings that are inside the orbit of Io. The rings are made of very small, dark particles the size of smoke particles. They are produced by dust kicked up from the tiny innermost moons of Jupiter by impacts on the moons.
Saturn's rings were discovered by Christian Huygens in 1659. Galileo's telescope was too small to make them look like more than just a couple of bumps on either side of the planet. In 1675 Giovanni Cassini discovered a gap between the two large (A & B) rings, now called the Cassini division in his honor. With improved telescopes, astronomers were able to see that one of the large rings was in fact, two rings (B & C) and there is a gap in the A ring (the Encke division). There is also a hint of another ring closer to the planet than the C ring (the D ring). When the Pioneer and Voyager spacecraft flew by, astronomers found more rings and complex structure in the rings.
The rings that are visible in even low-power telescopes on the Earth (A, B, and C) extend from about 74,000 kilometers to about 137,000 kilometers from Saturn's center (or 1.23 to 2.28 Saturn radii). The rings are very thin, less than a few hundred meters thick. A scale model of the rings with the width equal to a single piece of regular paper would be 70 to 100 meters across! In 1859 James C. Maxwell (of electromagnetism fame), showed that the rings could not be solid, but, rather a swarm of particles. A solid ring the width of Saturn's ring system would become unstable and break up. James Keeler proved Maxwell correct in 1895 when he measured the doppler shifts of different parts of the rings and found that the outer parts of the ring system orbited at a slower speed than the inner parts. The rings obeyed Kepler's third law and, therefore, must be made of millions of tiny bodies each orbiting Saturn as a tiny mini-moon.
More recently, astronomers bouncing radar off the rings and analyzing the reflected signal found that ring particles must be from a few centimeters to a few meters across. When the Voyager spacecraft went behind the rings with respect to the Earth, astronomers could measure the particles sizes from how Voyager's radio signal scattered off the particles and from how sunlight scattered through the rings. The ring particles range in size from the size of a small grain of sand to the size of a large house, but on average, they are about the size of your clenched fist. Spectroscopy of the rings shows that the particles are made of frozen water. Collisions between the ring particles keeps the ring system very flat and all of the particle orbits circular.
The Voyager and Pioneer spacecraft confirmed the presence of the inner D ring and discovered two other rings beyond the outer A ring: a narrow F ring just outside the A ring and a broad, but faint E ring. The broad rings we see from the Earth are actually systems of thousands of tiny ringlets each just a few kilometers wide, so the rings look like grooves in a phonograph record (youngsters only familiar with CD's will have to ask their parents or grandparents about them). Voyager also found some unusual things in Saturn's ring system: rings that change shape, eccentric ring shapes (some even twist around each other to make a braid), and dark features that look like spokes extending radially outward across the rings.
The grooved pattern of Saturn's rings are probably the result of spiral density waves forming from the mutual gravitational attraction of the ring particles. Narrow gaps in the rings are most likely swept clean of particles by small moonlets embedded in the rings. The tiny moons can also act as shepherd satellites. Two shepherd satellites with one orbiting slightly outside the other satellite can constrain or shepherd the ring particles to stay between the moonlet orbits. The narrow F ring is the result of two shepherd satellites about 50 kilometers across with orbits about 1000 kilometers on either side of the F ring. The shepherd satellites are probably also responsible for the braids and kinks in the F ring, but how they do that is unclear.
Bigger gaps in the rings (such as the Cassini division) are the result of gravitational resonances with the moons of Saturn. A resonance happens when one object has an orbital period that is a small-integer fraction of another body's orbital period, e.g., 1/2, 2/3, etc. The object gets a periodic gravitational tug at the same point in its orbit. Just as you can get a swing to increase the size of its oscillation by pushing it at the same point in its swing arc, a resonance can ``pump up'' the orbital motion of an object. Particles at the inner edge of the Cassini division are in a one-two resonance with the moon called Mimas---they orbit twice for every one orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap. Other resonances with Mimas are responsible for other features in Saturn's rings: the boundary between the C and B ring is at the 1:3 resonance and the outer edge of the A ring is at the 2:3 resonance. It is amazing what a simple inverse square law force can do!
The dark spokes in Saturn's B ring were a surprise. The different orbital speeds of the ring particles should quickly shear apart any radial structure in the rings, but the spokes clearly survived the shearing! The spokes are probably caused by very tiny dust particles hovering just above the rings by their interaction with Saturn's magnetic field or by electrostatic forces created from ring particle collisions.
Where did the rings of Saturn come from? Studies of the various forces on the ring particles show that the rings are transient---they did not form with Saturn as part of the formation of the main planet, nor will they always be there. The rings of Saturn are within the distance at which a moon would experience extreme enough tidal stretching to be torn apart. This distance is called the Roche limit, after M.E. Roche who developed the theory of tidal break up in the 1849. The exact distance of the Roche limit depends on the densities of the planet and close-approaching moon and how strongly the material of the moon is held together. The classical Roche limit considers a moon held together only by its internal gravity. Such a moon would break up at a distance of about 2.44 planetary radii from the center of the planet.
Saturn's rings lie within Saturn's Roche limit so it is likely that they were either formed by the breakup of a moon that came too close or particles too close to Saturn to ever form a large moon. Another possibility is that large collisions on the large moons outside the Roche limit spewed material into the region inside the Roche limit. Saturn's E ring lies outside Saturn's Roche limit and is most concentrated at the orbit of the icy moon, Enceladus. Eruptions of water vapor from Enceladus probably are the source of the E ring material.
The rings of Uranus and Neptune are much darker than Saturn's rings, reflecting only a few percent of what little sunlight reaches them (they are darker than pieces of black, burned wood) while Saturn's rings reflect over 70% of the Sun's light. The rings are also much narrower than Saturn's rings. Uranus' outermost and most massive ring, called the Epsilon Ring, is only about 100 kilometers wide and probably less than 100 meters thick. The other ten dark and narrow rings have a combined mass less than the Epsilon Ring. The six rings of Neptune are less significant than Uranus' and the ring particles are not uniformily distributed in the rings. Like Saturn's F ring, the rings of Uranus and Neptune are kept narrow by shepherd satellites. The narrowness and even clumpiness of the rings means that the rings can last for only a short time---a million years or so, unless the rings are replenished by material ejected off the moons in large collisions.
| giant impact theory | mare | resonance |
|---|---|---|
| Roche limit |
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last update: 26 April 1999
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