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.

Ganymede

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.

Interplanetary Bodies

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.

Bode's Rule

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.

Planets in the Outer 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.

Structure of Pluto

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.

Charon

Status of Pluto

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.

Pluto

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.

Structure of Neptune

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.

Triton

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.

Neptune

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.

Discovery of Uranus

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.

Orbital Eccentricity

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.

Orbit Inclination

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.

Prograde and Retrograde Rotation

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.

Orbit of Uranus

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.

Miranda

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.

Uranus

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.

Enceladus

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.

Titan

10. The Solar System

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