10. The Solar System

Transcript: Jupiter's Ganymede is the largest moon in the solar system, just under 5,300 kilometers in diameter. That's 8 percent larger than Mercury and twice the size of tiny Pluto. Ganymede has an old fractured surface covered in groves and fissures. This dark surface is heavily cratered and fairly old. But also, watery material has apparently erupted on the surface of Ganymede at some point in the past to form deposits. In some cases the gray surface soils have been blasted away by meteoric impacts to reveal the whiter ice. The possibility of water and ice on Ganymede is very exciting. Ganymede’s orbit is too circular for tidal heating to have created the heat needed to make water ice erupt. However, its orbit may have had irregularities in the past that could have led to tidal heating.

Transcript: There are many types of interplanetary bodies, and they contain important clues as to the formation and evolution of the solar system. Interplanetary bodies range in size from 1,000 kilometers to chunks of rock the size of a house and smaller. They range in composition from icy to rocky to metallic. They range in distance from 50 to 100,000 astronomical units, or a 100,000 times the distance of the Earth from the Sun, to orbits that plunge inside the Earth's orbit and sometimes into the Sun itself. The largest of these objects are icy comets and rocky and metallic asteroids.

Transcript: Several hundred years ago the astronomer J. Bode noticed a peculiar thing about the spacings and distances of the planets from the Sun. If, for example, you take a sequence of numbers that double, add four to each one and divide by ten you end up almost exactly predicting the distances of the planets from the Sun in units of astronomical units, the Earth-Sun distance. You can see this from the sequence of distances quoted in astronomical units: Mercury at 0.4 AU, Venus at 0.7, Earth at 1.0, Mars at 1.5, then the asteroid belt at 2.8, Jupiter at 5.2, Saturn 9.5, Uranus 19.2, and Pluto at 39.4. This is roughly a sequence of doubling distance. The only planet that doesn’t fit is Neptune, and of course the asteroid belt is not a planet. This peculiar spacing or regularity in the distances of the planets from the Sun is not explained by Newton's law of gravity, but must somehow be to do with the way that the planets formed out of the solar nebula.

Transcript: Astronomers have essentially demoted Pluto from its status as a planet because of its small size and peculiar orbital characteristics. This leaves the solar system with eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There is the possibility of larger objects in the outer solar system, some of which may approach Pluto in size or perhaps even exceed it. Astronomers are continuously hunting for large objects in the outer solar system using the orbital perturbations of Pluto and other bodies to predict where those objects might lie. Some large bodies of hundreds of kilometers have been found, none that exceed the size of Pluto but some that are larger than many of the moons in the solar system.

Transcript: When Pluto was discovered in 1930 it was hailed as the ninth planet, a great discovery of the twentieth century. Its discoverer Clyde Tombaugh was celebrated as the only living person to have discovered a planet. Now astronomers are not so sure whether Pluto is a planet or an interplanetary body for a number of reasons. Pluto is small, less than half the size of Mercury. Its orbit is the most elliptical in the solar system, crossing that of Neptune. There are other interplanetary bodies of similar size, although none quite as large as Pluto. Triton is similar in size, and it is a captured satellite. Pluto does have a moon, but so do interplanetary bodies. A few years ago the astronomers’ official governing body downgraded Pluto causing some consternation among the planetary science community and among the public.

Transcript: Pluto's satellite Charon is half the size of Pluto itself. The ratio between the planet and its moon is the smallest in the solar system. Pluto rotates in 6.4 days, and the orbital period of Charon is the same number, 6.4 days. They are in a synchronous orbit and so always face each other. The density of Pluto is about 2000 kilograms per cubic meter which indicates that Pluto is composed roughly of 70 percent rock and 30 percent ice. The somewhat lower density of Charon indicates that it's more ice rich. The orbit of Charon and Pluto has been beautifully mapped out using the Hubble Space Telescope which can clearly resolve the two objects.

Transcript: When Pluto was discovered in 1930 it was hailed as the ninth planet, a great discovery of the twentieth century. Its discoverer Clyde Tombaugh was celebrated as the only living person to have discovered a planet. Now astronomers are not so sure whether Pluto is a planet or an interplanetary body for a number of reasons. Pluto is small, less than half the size of Mercury. Its orbit is the most elliptical in the solar system, crossing that of Neptune. There are other interplanetary bodies of similar size, although none quite as large as Pluto. Triton is similar in size, and it is a captured satellite. Pluto does have a moon, but so do interplanetary bodies. A few years ago the astronomers’ official governing body downgraded Pluto causing some consternation among the planetary science community and among the public.

Transcript: Pluto is the outer sentinel of the solar system. With a size of only 2,300 kilometers, it is half the size of Mercury and two-thirds the size of Earth's moon. Its mean distance from the sun is 39 astronomical units, but it has a highly eccentric orbit with an eccentricity of 0.25. Pluto is extremely cold, -370 degrees Fahrenheit. Its surface is mostly methane ice, and it has a very thin methane atmosphere.

Transcript: Neptune is a close twin of Uranus with similar size and structure. Its bluish tinge is visible even through a small telescope. At its center is a rocky, silicate core with trace metals surrounded by a mantel of water, ammonia, and methane in both liquid and ice forms. The pressure is insufficient to create hydrogen in its metallic form. The outer atmosphere consists primarily of hydrogen and helium in roughly the solar abundance. Like Jupiter, Uranus and Neptune have large storm systems. Neptune has a great dark spot that has persisted for many years analogous to Jupiter's great red spot.

Transcript: Neptune's Triton is the seventh largest moon in the solar system. With a diameter of 2,700 kilometers it's somewhat larger than Pluto. It has an unusual retrograde or backwards orbit of the planet which probably indicates that it was captured from interplanetary space somewhere during the history of the solar system. Triton has a sparse atmosphere made of nitrogen and methane, similar to the atmosphere on Saturn's Titan. The pressure is very low; it’s a thin atmosphere with barley 0.1 percent of the surface pressure of Mars’ atmosphere. Triton has volcanic vents where smoke plumes rise vertically 8 or 9 kilometers into the atmosphere and then are sheered off by high altitude winds. The surface is young and has probably been resurfaced. The cause of this is almost certainly tidal heating caused by changes in the orbit after its capture from interplanetary space.

Transcript: Neptune is the most distant large gas giant in the outer solar system, about thirty times the distance of the Earth from the Sun. It's a close twin of Uranus with a diameter of about 50,000 kilometers and very similar color and composition to Uranus. Neptune has a nearly circular orbit, and it was discovered using predictions from Newton's law of gravity in 1846. It has one large moon, Triton, with rather unusual properties. Its orbit crosses that of Pluto, and so at some times in the orbit Neptune is more distant than Pluto.

Transcript: Jupiter and Saturn have been known for thousands of years. They've given their names to two of the days of our week. Uranus was discovered in 1781 by the astronomer William Hershel accidentally during a star mapping project. It was a moving target among the pattern of star fields that he was observing. Hershel could not make out any detail on the planet, but he was able to calculate its orbit from its motions measured over several years. Thus Hershel became the first known human to discover a planet. In Greek mythology Uranus is the father of Saturn who is the father of Jupiter. In the entire time since its discovery Uranus has completed less than three orbits of the Sun. Compare this to fleet-footed Mercury, the innermost planet, which completes several orbits of the Sun every year.

Transcript: Orbital eccentricity is the amount by which an orbit deviates from a circle. Mathematically it's defined as the distance between the two foci of an elliptical orbit divided by the major axis. A circle has an ellipticity, denoted by the little symbol "e", of zero. In the solar system most of the planets have small eccentricity and are close to circular. Eccentricity of the Earth’s orbit is 0.017, one and a half percent. The only two planets where the orbital eccentricity is above ten percent are Pluto, with an eccentricity of 0.25 and Mercury with an eccentricity of 0.21. These large numbers probably indicate interactions during the history of the solar system with large bodies or perhaps in the case of Pluto capture from a distant region of space.

Transcript: For most planets in the solar system the orbital inclination is very small. That is, the axis defined by the north and south poles of the planet is almost exactly perpendicular to the plane of the planet’s orbit of the Sun. The only two exceptions to this are the Earth and Uranus. In the case of the Earth, the orbital axis is tilted by twenty-three and a half degrees, giving Earth its seasons. In the case of Uranus the tilt is even more extreme, more than ninety degrees, so Uranus is tilted almost on its side as it orbits the sun. The causes of these tilts for two of the planets in the solar system are almost certainly collisions with large bodies early in the history of the solar system. In the case of the Earth it was probably the collision that lead to the formation of the Moon.

Transcript: In a prograde orbit, or prograde rotation, the planet spins in the same sense as its orbit of the sun. All planets except Venus and Uranus have prograde motion, and so the Earth spins from west to east which is the same sense as its orbit of the Sun. However, in the case of Venus and Uranus the spin is in the opposite sense which astronomers speculate is due to collisions early in the history with giant debris left over from the formation of the solar system.

Transcript: Uranus has two very unusual aspects to its orbit. The first is the large degree of tilt of the spin axis with respect to the plane of its orbit around the sun. This tilt is ninety-eight degrees, so the pole of Uranus is essentially in the plane of its orbit around the Sun. This is more extreme even then the tilt of the Earth's axis which is only twenty-three and a half degrees. As a result, in its 84 year orbit of the Sun, Uranus experiences extremely long seasons because for 20 or 30 years at a time, when the north pole is pointing directly away from the Sun, the northern hemisphere of the planet is in deep winter, exacerbated by its large distance from the Sun as well. These long and extreme seasons are unique among the planets. In common with only Venus in the solar system, Uranus has a retrograde orbit that is unlike the Earth and the other planets which prograde orbit, that is, they rotate from west to east, and so the spin of the planet is in the same sense as its orbit of the Sun. Uranus is the reverse. Astronomers speculate that the reason for Uranus' strange orbit, as with the Earth, is a collision early in its history. Perhaps a glancing blow left Uranus with a highly tilted orbit.

Transcript: Uranus has several moderate sized moons. Of these, the innermost, Miranda, is the most interesting. Its size is about 470 kilometers. It has an icy, heavily cratered surface. Thus, there is significant indications of young features, the icy regions and also ridges and fractures reminiscent of Ganymede. One cliff rises at a forty five degree slope, 16,000 feet above the surface of this small moon, a unique feature in the solar system. Scientists speculate that tidal heating in the past has caused tectonic activity which has led to the fractured young surface features.

Transcript: Uranus is a large gas giant planet, 51,000 kilometers in diameter, discovered by William Hershel in 1781. Uranus has five major satellites and nine very faint rings. Its atmosphere is composed primarily of methane and hydrogen. The planet is notable for its large and unique tilt of its orbit with respect to the ecliptic. Because of this large degree of tilt of the spin axis of the planet from its orbit of the Sun and its large orbital time of the Sun, Uranus has the most extreme seasons of any planet in the solar system with 42 years of summer followed by 42 years of winter.

Transcript: Enceladus is a modest but important moon of Saturn. Smaller than Titan, Rhea, Iapetus, and Tethys, it is only 500 kilometers in diameter. It has an unusual combination of old crater terrain and smooth icy planes. This moon forms a link between the old cratered surfaces on a moon like Ganymede and the smooth icy surfaces on a moon like Europa. Tidal heating has apparently caused watery eruptions that have resurfaced half of the surface of Enceladus. The icy regions of Enceladus are extremely reflective of light, reflecting 90 percent of the incident light. These bright regions of Enceladus are as white as a field of snow.

Transcript: Saturn's Titan is the second largest moon in the solar system. With a diameter just over 5,100 kilometers it is four times larger than the next largest of Saturn's moons, 40 percent of the diameter of Earth. Yet its pressure at the base of its atmosphere is 60 percent larger than the Earth's pressure, and its atmosphere is composed mostly of nitrogen like the Earth. But unlike the Earth, due to the large distance from the sun, Titan is very cold with a temperature of 95 Kelvin or -290 degrees Fahrenheit. The atmosphere appears as a featureless orange haze, much like a smog situation. In addition to nitrogen there are minor constituents of organic materials such as methane, ethane, acetylene, ethylene, and hydrogen cyanide. The pressure and temperature near the surface are at a place where methane can exist as a gas, a liquid, or a solid, and so it's likely that there's rain or snow of organic compounds and methane itself in the atmosphere. Scientists are virtually certain that there are liquid methane and ethane oceans or lakes on the surface of Titan, making it a very exciting world in the outer solar system for us to explore.

Transcript: Jupiter's Callisto is the third largest moon in the solar system behind Jupiter's Ganymede and Saturn's Titan. It is similar is size to the planet Mercury. Callisto is the outermost of the four Galilean satellites, and it has an orbital period around Jupiter of nearly seventeen days. Galileo used observations of Callisto especially to show that Kepler's laws applied to moons orbiting a planet as well as to planets orbiting the Sun. Callisto has a dark and bright surface, the dark regions representing soils that have been blasted by craters and the bright areas representing geologically younger regions of frozen water ice.

Transcript: What exists under the ice pack on Europa in its liquid oceans? Nobody knows for sure. In his novel 2010 Arthur C. Clark speculated about the possibility of primitive life forms in Europa’s oceans. Scientists do not know if this is possible, but Earth's history is intriguing because on Earth life has evolved in a range of inhospitable environments, including extremely dry and cold situations in the Antarctic ice pack. We may know for sure when in the next decade a fleet of probes head towards Jupiter. One plan is for a probe to land on the ice pack, melt its way thought the surface, and then travel as a bathysphere looking for primitive life forms. Europa is still one of the few possible potential sites for life in the solar system beyond Earth.

Transcript: Jupiter's Europa is one of the Galilean satellites. At 3,100 kilometers in diameter it's similar in size to Io and similar to the Earth's moon. Europa looks completely different from Earth's moon however. It has an uncratered, bright surface of frozen water. In 1996 the Galileo space probe took detailed pictures of the surface of Europa and showed that the surface is covered with a jostling icepack constantly breaking and reforming. The icepack’s thickness ranges from a few hundred meters to a few kilometers, and underneath the icepack is probably several kilometers of depth of liquid oceans. That far from the Sun the water is kept liquid by tidal heating which creates heat in the Europan interior which then melts the water under the icepack. This exciting discovery is the confirmation that the oceans of the Earth are not unique even in the solar system.

Transcript: Jupiter's Io has a highly elliptical orbit. While most satellites in the distant solar system should be cold, icy, and geologically dead, theorists speculated in the 1970s that tidal heating of Io could lead to volcanic activity. In 1979 the Voyager I space craft approached Jupiter after 18 months of travel and photographed a splotchy, yellow-orange surface with no craters, indicating it was geologically young. Over the next few years many active volcanoes were discovered on the surface of Io, and these were seen in motion shooting material up into the weak Io gravity where it would then fall lazily back to the surface. The dark hotspots on the surface were found through infrared techniques to have temperatures of 600 to 700 Kelvin, nearly 800 degrees Fahrenheit. Through its many volcanoes, 10 centimeters of sulfur and sulfur compounds are added to the surface of Io every year.

Transcript: Jupiter's Io is the closest to the planet of the four Galilean satellites. Its diameter is about 3,600 kilometers, and it is the most volcanically active world in the entire solar system. In 1996 a gravitational acceleration experiment on the Galileo space probe was able to measure a large iron core within Io that goes to nearly half the radius of the moon. Io has a larger density than any satellite in the solar system, about 3,500 kilograms per cubic meter, similar density to Mars.

Transcript: Tidal heating is another consequence of gravity acting on moons of planets. The tidal force is the differential force or stretching force on a moon orbiting a planet. Tidal force increases with increasing moon size and decreasing distance from the planet. If the tidal force is too large, the moon will be disrupted, but even a moderate tidal force is enough to cause heating of a moon. Basically the stretching force, in particular in an elliptical orbit where the stretching force changes during the orbit, will cause a flexing of the moon which leads to an increase in its temperature inside. Imagine flexing a tennis ball by squeezing it and letting it go, squeezing it and letting it go. Eventually the tennis ball would warm up. In the same way, tidal forces can cause heating of moons in close orbits around planets, and in extreme cases this tidal heating can lead to volcanic activity.

Transcript: The composition of the atmosphere of any planet depends on three things. The first is the chemical composition of the material available to form an atmosphere. This is the original hydrogen and helium of the solar nebula plus trace elements combined with the amount of gas that outgases from the interior of the planet though it evolution. Second is the mass of the planet which dictates its surface gravity, and third is the temperature of the planet determined by its distance from the Sun which in turn dictates the speed of atmospheric particles. In these terms giant planets have relatively high escape velocities, above 20 kilometers per second, and can retain even the lightest gases, hydrogen and helium. Terrestrial planets like the Earth and Venus have escape velocities around 10 kilometers per second and cannot retain hydrogen and helium but can retain heavier gasses, and the smallest solar system planets and other objects like Mercury and Pluto have escape velocities of only a few kilometers per second and cannot even retain carbon dioxide, nitrogen, or other heavy gasses.

Transcript: How large does a chunk of rock have to be to be called by a moon or a satellite? There is no simple answer to this question; it's largely a matter of definition. However, in general objects that are less than 10 kilometers across are too small to be called moons. Deimos, Mars' smaller of the two moons, is slightly larger than this number. All the moons between 10 and 100 kilometers in the solar system typically have names, and above 1,000 kilometers the moons are distinctive enough to have particular surface features and even atmospheres.

Transcript: How large does a chunk of rock have to be to be called by a moon or a satellite? There is no simple answer to this question; it's largely a matter of definition. However, in general objects that are less than 10 kilometers across are too small to be called moons. Deimos, Mars' smaller of the two moons, is slightly larger than this number. All the moons between 10 and 100 kilometers in the solar system typically have names, and above 1,000 kilometers the moons are distinctive enough to have particular surface features and even atmospheres.

Transcript: Each planet in the solar system is surrounded by an imaginary spherical region called the sphere of gravitational influence. For any object, such as a moon or ring particle situated within this sphere, the motions are dictated by the planet. Beyond this sphere the motions are dictated by the Sun. Thus, a moon placed just outside the sphere of gravitational influence of Jupiter would gradually drift apart from Jupiter and would move into a different orbit of the Sun. In a similar way the Sun has a sphere of gravitational influence that includes all the planets and moons of the solar system.

Transcript: Is there something that sets the maximum size of the moon of a planet? The answer to this question complex, but one part of the answer is Roche's limit. The tidal force on a moon increases both with the size of the moon and it's proximity to the planet. Thus, a moon that is large and close to a planet will have a large tidal force, and at some point that tidal force will disrupt the solid material and break apart the moon leading perhaps to a ring system. Thus, moons that are too close and too large to their planets will be disrupted, destroyed, and eventually end up as ring systems. Thus, we should not be surprised that most moons are substantially smaller then their parent planets and relatively far away. Earth's moon in this regard is relatively large being only a quarter the size of the Earth and about one-eightieth of the mass. We should also remember that not all moons and rings are related in an evolutionary sense; some moons have been captured gravitationally from other parts of the solar system.

Transcript: All ring systems in the solar system have outer edges that are somewhere between 1.8 and 2.5 times the planet radius from the center of the planet. What is particular about this ratio and how does it arise? The answer was derived in the mid-nineteenth century by the French mathematician Edward Roche. He calculated that the edge of planetary rings is defined by tidal forces. If we consider the gravity force between particles composing a planetary ring, the Roche limit, as it's called, is defined by the distance in which the tidal force caused by the planet on the particles equals the gravity force between them. Therefore beyond the Roche limit rings cannot exist because the tidal force is too large.

Transcript: Gravity keeps planets in their orbit of the Sun and keeps moons in their orbit of the planets. There is a second type of force that is important, however, in the solar system, tidal force. A tidal force is caused by the difference between the gravity force on one side of an object and the other side. It's essentially a stretching force. The size of a tidal force depends on the ratio between the front to back distance, or diameter, of an object like a moon or a planet and its distance from the object causing the gravity. If you work this out for the Earth, Sun, and the Moon you find out that although the Sun’s gravity pull on the Earth is much larger than the Earth’s gravity pull, the tidal force of the Moon exceeds that of the Sun by a factor of two. Thus, tides are caused primarily by the Moon but with a secondary contribution from the Sun. This explains why tides on the Earth are larger at new and full moons when the Sun and the Moon and the Earth all line up.

Transcript: Resonance is a phenomenon that’s common to all vibrations and waves. It’s a situation where a vibration, a wave, or an oscillation caused by one object induces a vibration, a wave, or an oscillation in another object. Obvious example of resonance is the fact that soldiers must break step when they walk across a bridge in case the pattern of their marching motion corresponds to a natural frequency of the bridge, in which case the marching adds energy to the bridge motion which amplifies it into a wave that could destroy the bridge. A better example is pushing a child on a swing. Assume you walk up to a swing where a child is swinging. If you then push the swing with random intervals you will on average loose energy from the swing and the swing will stop. If, however you wait until the swing is just moving forward or every second forward motion or every third or forth or fifth forward motion you will add energy to the swing. This is a resonance. At a fixed period interval, adding energy to the system will increase the original motion. You can see this also occurring with a bell or a tuning fork. If the bell is struck or the tuning fork is struck a similar bell or tuning fork some distance away will resonate.

Transcript: The rings of Saturn are spectacular and highly complex. The Voyager space probes showed the existence of thousands of individual ringlets, with the widest gap being Cassini's division, which was discovered in the seventeenth century. The particle sizes within the rings range from golf ball size up to about the size of a house. Larger blocks of material are broken down by collisions. The material is mostly made of frozen ices rather than dark, rocky material, so the rings are relatively pale in color. The most spectacular thing about the rings is their aspect ratio. The rings are 270,000 kilometers from edge to edge but only 100 meters thick. They are millions of times thinner than they are wide. It's as if you had a pizza that was less than the thickness of a human hair.

Transcript: Where did the rings of the giant planets come from? Interplanetary debris has rained down upon the giant planets and the moons of the giant planets since the formation of the solar system four and a half billion years ago. Some of the giant planets undoubtedly accreted a ring of debris material early in their history, yet more of the material must have come from impacts of interplanetary debris on the inner moons of the planets themselves. Some of these impacts sandblasted the moons, leading to small particles that get ejected from the moons and spread out into a ring, yet more material that forms rings must have come from the disruption of moons entirely by a shattering impact. Thus, its possible that many of the moon systems of the planets have lead directly to the ring systems and that those ring systems have not been the same over the history of the solar system.