Transcript: Saturn's rings were first seen by Galileo in 1610. However, with the poor optics of his early telescope Galileo only saw fuzzy blobs on either side of the planet, and he actually drew Saturn as triple planet. In 1655, Christian Huygens used a superior telescope to see that the blobs were in fact a ring system encircling the planet along its equator. Even with a backyard telescope the rings are clearly visible, and they present a different appearance in terms of their angle and...

Transcript: The interior of Saturn is almost certainly very similar to the interior of Jupiter, grading down from a pure simple gas in the outer atmosphere region towards molecular gas lower down, when the temperature is still cold but the pressure is high, towards metallic hydrogen at a point where the pressure is high and the temperature is high. Underneath that is likely to be an icy mantel surrounding a rocky core. Scientists do not know for certain if there are rocky cores at the...

Transcript: Saturn, sixth planet out from the Sun, is second only to Jupiter in size and mass. Its atmospheric composition is also similar to the chemical composition of the Sun, 90 percent hydrogen, 5 percent helium, plus methane, ammonia, water vapor, and other gases. The temperature is also similar to Jupiter, and there are winds of three to four hundred miles per hour in the atmospheric bands. Saturn, like Jupiter, has an internal energy source but a much weaker magnetic field than...

Transcript: Saturn is famous for its beautiful ring system, but all four of the gas giant planets have thin and delicate ring systems. The other three were discovered with space probes in the last few decades. Jupiter has a single ring with a sharp outer edge and a diffuse inner edge made of dark, rocky particles. Uranus has a ring that’s not so much a broad sheet but a series of string-like arcs with wide gaps in between. Neptune’s ring system is similar to Uranus’. All the rings...

Transcript: It seems unlikely that life could exist on or in any of the giant planets, yet scientists have speculated over the last few decades, including speculations by the late Carl Sagan, that organic chemicals might combine in the atmospheric layers of Jupiter to produce life forms. These life forms would then be aerial and free floating, analogous to jellyfish in the oceans of the Earth. However, it turns out that there are few organic materials in the upper atmosphere of Jupiter, and...

Transcript: The bulk properties of gases, temperature, volume, density, and pressure, are related to the microscopic motions of atoms and molecules in the gas. This was first realized in the mid-eighteenth century by Daniel Bernoulli, who was the head of an extraordinary family. No less than eight of his sons and grandsons became noted scientists. If you compress a gas, the mean distance between molecules decreases, hence the volume. The number of collisions per second increases, hence...

Transcript: After Boyle established that pressure is inversely related to volume for a gas, P is proportional to one over V, French scientists showed that gas expands when it gets hotter by 0.3 percent for every degree Celsius it's temperature is raised. In the Kelvin temperature scale this relationship turns into the simple proportionality volume is proportional to temperature, V proportional to T. Combining these two relations leads to pressure times volume is proportional to the...

Transcript: The properties of gases were first explored in the mid-seventeenth century by Robert Boyle. Boyle was the youngest of 14 children of the Earl of Cork, one of the richest men in Ireland. As a scientist he carried out an elegant series of experiments of the property of air, the only know gas at the time. By expanding and contracting fixed amounts of air he was able to show that pressure is proportional inversely to volume with the temperature fixed. Pressure is defined as force...

Transcript: Scientists in the seventeenth century realized that we live at the bottom of an ocean of air and that pressure is one of its attributes. Otto von Guericke, the mayor of Magdeburg and a talented amateur scientist, conducted a fascinating series of experiments to illustrate the effects of air pressure. He evacuated the air from a long glass cylinder and showed that feathers and stones fall at the same rate confirming a prediction of Galileo. He evacuated the air from a glass...

Transcript: Each of the giant planets emits more infrared radiation then it receives from the Sun. In the case of Jupiter, Saturn, and Neptune, the factor is 1.7 to 2. In the case of Uranus it’s a slight but detectable amount. This is very different from the situation of the Earth or the other terrestrial planets where the internal heat generated by radioactivity is only five thousandth of a percent of the heat received by the Sun. The answer is that the gas giant planets are still...

Transcript: In the 1960s scientists were surprised to find that Jupiter emits twice as much infrared radiation as it receives from the Sun. We see Jupiter in the reflected light of the Sun, and since no planet is a perfect reflector, like a mirror, we must receive less visible radiation from a planet then is received by that planet from the Sun. The same must be true of heat as well, yet in Jupiter's case the planet is emitting more infrared radiation then it receives from the Sun. The...

Transcript: Astronomers don't know for certain what lies at the center of Jupiter, but current theories suggest that all the gas giant planets have rocky cores about 1.5 to 3 times the size of the Earth. Surrounding the Earth-like rocky core is an icy frozen mantel of water, methane, and ammonia, then, moving outwards, the layers of hydrogen, from slushy near-solid near the icy mantel, to liquid metallic form, to the liquid molecular form, to the simple gas of the outer atmosphere.

Transcript: In the 1960s scientists were surprised to find that Jupiter emits twice as much infrared radiation as it receives from the Sun. We see Jupiter in the reflected light of the Sun, and since no planet is a perfect reflector, like a mirror, we must receive less visible radiation from a planet then is received by that planet from the Sun. The same must be true of heat as well, yet in Jupiter's case the planet is emitting more infrared radiation then it receives from the Sun. The...

Transcript: The most famous and dramatic feature on any of the giant planets is Jupiter’s great red spot. This huge oval storm has existed for at least 330 years since it was reported by Cassini in 1665. It may of course have existed before the invention of the telescope when it would have been much harder to see, impossible by the naked eye. It's varied in strength throughout the last few hundred years, disappearing for a long period of time and being rediscovered in 1887. It's at least...

Transcript: In 1995 the Galileo space probe plunged into the atmosphere of Jupiter to return readings directly of the chemical composition and atmospheric conditions. There was no camera on board, but instruments were able to show that the chemical composition of the atmosphere matched that of the Sun in its proportion of hydrogen and helium. There were however relatively larger concentrations of carbon, nitrogen, sulfur, and other heavy elements. Few organic compounds were found. The...

Transcript: Jupiter, fifth planet out from the sun, is the largest planet in the solar system, 320 times the mass and 1,000 times the volume of Earth. It may have a solid core, but most of the substance of the planet is gas with the same chemical composition as the Sun, roughly 90 percent hydrogen and 5 percent helium with small amounts of methane, ammonia, water vapor, and other gases. Jupiter has an internal heat source, reflecting the fact that the planet is still contracting slightly. ...

Transcript: We tend to think of the solar system as containing only nine planets, or eight planets plus Pluto, but the satellites, or moons, in the solar system include twelve substantial worlds with interesting properties. Satellites have shown us these properties in detail, and they’re fascinating. Two of the moons in the solar system are bigger than either Mercury or Pluto. There’s a moon with sulfur emitting volcanoes, a moon with sheet ice covering water oceans, a moon with a nitrogen...

Transcript: The chemical composition of the giant planets overall is similar to that of the Sun, roughly three quarters hydrogen by mass and one quarter helium by mass. How do we know this? The density of the gas giant planets gives us a good indication of their chemical composition. For instance, the terrestrial planets have mean densities in the range 4,000 to 5,500 kilograms per meter cubed. This is similar to the density of most rocks. However the gas giant planets have densities in...

Transcript: The gas giant planets are far from the Sun and very cold. Hydrogen and helium of which they are mostly composed remain gaseous down to very low temperatures. However, other compounds which compose trace elements in the atmospheres of the giant planets and on the moons in the giant planets turn into solids at low temperatures. These substances, water vapor, carbon dioxide (CO2), methane (CH4), ammonia (NH3), all turn into ices. These icy materials dominate some of the moons of...

Transcript: The four terrestrial planets are huddled relatively close to the Sun. Beyond the asteroid belt are four giant, or gas giant, planets. Then comes Pluto and a variety of small rocky objects. The four gas giant planets, Jupiter, Saturn, Uranus, and Neptune, are all 4 to 10 times the size of the Earth. Astronomers believe that they all have rocky cores that are similar in size to the terrestrial planets, but in addition they have huge, thick atmospheres of hydrogen and helium, the...

Transcript: Free oxygen in a planetary atmosphere means that something unusual is going on. Oxygen is one of the most reactive gases, and it will cause oxidizing reactions with minerals in a planetary surface on short times scales of only a few million years. In the case of Mars, this has produced the rust kind of minerals that give Mars a reddish color. In the case of the Earth, oxygen is reacted to form carbonates on the Earth's surface. But, if a large amount of free oxygen is seen...

Transcript: In a sense carbon dioxide is a normal constituent of a planetary atmosphere. Early in the history of the solar system all planets in the inner solar system, including Mercury and even Earth’s moon, were hot enough for volcanism to occur. As a result of volcanoes carbon dioxide and water vapor were released. The water was broken down by ultraviolet radiation into hydrogen and oxygen. Hydrogen as a light gas escaped, and oxygen reacts with carbon in the rocks. Thus, carbon...

Transcript: In the inner solar system we can see the tendency for more massive planets to be able to retain an atmosphere and for them to retain an atmosphere with heavier gasses. The secondary effect is temperature which depends on distance from the Sun. At a higher temperature gas is more likely to escape into space. Thus we have the Earth and Venus, the most massive planets in the inner solar system, both with atmospheres containing heavy gases: nitrogen, oxygen, and carbon dioxide. ...

Transcript: The size of a planet affects the way it can retain or hold an atmosphere. Surface gravity increases as the mass of a planet increases, and the escape velocity also increases with mass. And this applies to atoms and molecules as well as spacecraft. Atoms and all planets are slowly leaking into space from their atmospheres, but the kinetic energy of atoms in a gas is proportional to the mass and the velocity squared. In a mixture of gases at same temperature the light molecules...

Transcript: Heat generation by radioactive decay and subsequent heat transfer from the interior to the exterior explains the geological differences among the planets. For example, in the inner solar system, the Earth has a still hot interior, few craters, active volcanism and plate tectonics, and a mean crust age of only four hundred million years. Venus, also still hot in the interior, has few craters, intense volcanism, and a mean surface age of about seven hundred millions years. Mars...