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...

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...

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,...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...

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...