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 tilt as seen from the Earth throughout Saturn's 29 year orbit of the Sun.

Saturn's Rings

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 center of any of the gas giant planets because those cores are very small compared to the overall size of the planets, but its almost certain that the way the planets grew was by a nucleus of rocky material which attracted and accreted by gravity the large gaseous atmosphere during the formation of the solar system.

Interior of Saturn

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 Jupiter.  It's less dense than water on average, and Saturn, in addition to its spectacular ring system, has over two dozen moons.

Saturn

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 systems have been followed in detail using space probes that have approached within a short distance.

Gas Giant Ring Systems

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 the circulation patterns would move material from very hot to very cold extremes making it unlikely that life could survive.  However, the truth is that we don't know what the true and full range of conditions under which biochemical life is possible are, and so the jury must be out on the possibility of life on the giant planets, however unlikely it may seem.

Life on Jupiter

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 the pressure increases, and the average energy per collision increases, hence the temperature goes up.  The microscopic form of the ideal gas law is pressure times volume equals nKT, n: the number of molecules, k: the Boltzmann constant, and t: temperature in the Kelvin scale.

Gas Properties and Atomic Motion

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 temperature.  It's called the ideal gas law, ideal because it involves some assumptions: that the number of molecules is very large, that the molecules occupy a negligible fraction of the gas, that they are in constant random motion, that there are no forces between the molecules, and that in molecule collisions no energy is lost.

Ideal Gas Law

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 per unit area.  When you press down on a plunger of gas, such as in a bicycle pump, the volume decreases; it’s linear and inversely proportional.

Boyle's Law

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 hemisphere and showed that life depended on the presence of air.  In his most spectacular demonstration he took two metal hemispheres, joined them together, and evacuated the air from between them with a pump.  In this situation, two teams of eight horses could not pull apart the hemispheres, yet when he opened the valve and let the air back in the hemispheres fell apart.

Air Pressure

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 contracting slightly, and in an illustration of the law of conservation of energy the gravitational potential energy from the contraction is being slowly but steadily converted into heat energy which moves to the planetary atmosphere and is radiated into space as longs waves of electromagnetic radiation.  Despite this, none of the giant planets are anything close to being stars.  The heat source internally is mild and is not nearly enough to cause the fusion process.

Gas Giant Heat

9. Outer Planets and Planetary Atmospheres

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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 three times the size of the Earth.  This hurricane-like system occurs at the junction between difference jet streams.  Simulations in the laboratory have shown that for a planet of Jupiter's size it is not surprising that a persistent atmospheric system could survive for hundreds of years as opposed to the several weeks that is the maximum duration of storm systems on Earth.

Great Red Spot

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 atmosphere thicker than the Earth’s and possible ocean on its surface.  All this diversity of properties makes the outer solar system a spectacular place to study

A Variety of Moons

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 the outer planets, and they are also found in objects in the outer solar systems, such as comets.

Planetary Ices

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 same composition as the sun itself.

Giant Planets

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 something must be replenishing it.  On Earth that something is life.  Elsewhere in the universe we expect that free oxygen, when it’s seen in a planetary atmosphere, will represent microbial metabolisms at work.

Oxygen Atmosphere

Transcript: How good is the evidence for ancient life on Mars based on the Allen Hills Meteorite?  The evidence comes in two forms, first evidence for organic material.  There are indeed hydrocarbons within the meteorite, but they need not represent decayed organisms.  There are also magnetite and iron sulfite compounds that are often found in juxtaposition to living organisms but do not directly represent those organisms.  Also notable in the photographs of slices of the meteorite were the slender forms that were very reminiscent of microfossils, ten thousand times smaller than a human hair, some 200 nanometers long.  These are very evocative fossils, but they do not necessarily represent living organisms.  No cell walls are seen, and there is no direct evidence of amino acids.  The jury is definitely still out on whether the Allen Hills Meteorite indicated ancient life on Mars, and in fact, most planetary scientists consider the evidence less than compelling.

Evidence for Life on Mars

Transcript: Centuries of speculation came to a head in 1996 when a team of scientists announced very publicly that they had discovered evidence for life on ancient Mars.  The evidence concerned material found in the middle of the Allen Hills meteorite, ALH84001, one of the most famous rocks in astronomy.  The history of the meteorite is extraordinary itself because it formed billions of years ago, was blasted off the surface of Mars by a meteoric impact, floated through space for tens of millions of years, and landed on the Earth perhaps 13,000 years ago and was discovered in 1984 by a team of scientists.  This discovery, the idea that microbial life might exist somewhere beyond the Earth, galvanized the scientific community into scrutinizing the evidence more carefully.

Life on Mars

Transcript: Mars is a relatively small planet, and since the mass of a planet drives the likelihood of geological activity, we would not expect a lot of volcanism.  Mars is relatively quiet at the present time, but there's evidence for lava planes covering much of the surface of Mars.  They date back about 1.5 billion years ago, so major volcanism has been important in the last third of Mars’ history.  Individual volcanic features are spectacular.  The Olympus Mons volcano dwarfs any volcano on Earth, including Mona Loa and Mona Kea in the Hawaiian Islands.

Volcanoes on Mars

Transcript: Mars is cold and dry, a frozen desert.  The temperature ranges from about -120 degrees Fahrenheit, or 190 Kelvin, up to about -20 degrees Fahrenheit, or 240 Kelvin.  On a summer afternoon in Mars the soil can be warmer, even a little above the freezing point of water, because the soil directly absorbs the Sun’s radiation.  The atmosphere is very thin, made primarily of carbon dioxide, 95 percent, with a surface pressure of about 0.7 percent of the pressure of the Earth’s atmosphere at sea level.  The poles of mars are more complicated than the poles on the Earth.  They consist of a small permanent deposit of frozen water ice, permafrost, with a larger amount of frozen carbon dioxide that changes with the seasons.  In the winter on Mars, carbon dioxide goes directly out of the atmosphere into the solid form.  Carbon dioxide has no liquid phase.

Atmosphere of Mars

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