The Giant Planets

The Giant Planets AST 2010: Chapter 10

Jovian Worlds Saturn Neptune Uranus Jupiter AST 2010: Chapter 10

Missions to Outer Solar System AST 2010: Chapter 10

Early Exploration of Outer Solar System • Pioneers 10 & 11 were launched in 1972 & 1973 • Can they navigate through the asteroid belt? • What are the radiation hazards in Jupiter’s enormous magnetosphere? • Pioneer 10 flew by Jupiter in 1973 and then sped outward toward the edges of the solar system • Pioneer 11 flew by Jupiter in 1974 and was diverted towards Saturn, which it reached in 1979 AST 2010: Chapter 10

Further Exploration of Outer Solar System • Voyagers 1 & 2 were launched in 1977 • They carried 11 scientific instruments, including cameras, spectrometers, and devices for measuring magnetospheres • Their missions were highly productive • Voyager 1 • reached Jupiter in 1979, and used a gravity assist from Jupiter to reach Saturn in 1980 • Voyager 2 • reached Jupiter four month later than Voyager 1 • used gravity assists to reach Saturn in 1981, Uranus in 1986, and Neptune in 1989 • These multiple gravity-assistedflybys were possible thanks to the approximate alignment of the planets • Such an alignment occurs about once every 175 years Voyager AST 2010: Chapter 10

Recent Exploration of Jupiter • For a detailed study of a planet, spacecraft that can go into orbit around the planet are necessary • Flybys of the planet are not adequate • The Galileo spacecraft was launched from the space shuttle Atlantis in 1989 and arrived at Jupiter in 1995 • It began its investigations by deploying a small entry probe for direct studies of Jupiter’s atmosphere AST 2010: Chapter 10

Galileo Spacecraft • In September 2003, the Galileo spacecraft ended its mission by plunging into Jupiter’s atmosphere AST 2010: Chapter 10

Galileo’s Entry Probe • had a mass of 339 kg • plunged into Jupiter’s atmosphere at a shallow angle and a speed of 50 km/s • was slowed down by atmospheric friction • the temperature of its heat shield reaching 15,000°C • as its speed dropped to 2500 km/h, jettisoned the remains of its shield and deployed a parachute for a gentler descent in the atmosphere (animation) • transmitted data to the Galileo orbiter, for retransmission to Earth AST 2010: Chapter 10

Some Results of Galileo Mission • A satellite (Dactyl) of an asteroid (Ida) was discovered • Jovian wind speeds in excess of 600 km/hour (400 mph) were detected • Far less water was detected in Jupiter's atmosphere than estimated from earlier Voyager observations and from models of the Comet Shoemaker-Levy 9 impact • Far less lightning activity was found than anticipated • The individual lightning events, however, are about ten times stronger on Jupiter than on the Earth • The helium abundance in Jupiter is very nearly the same as that in the Sun (24% compared to 25%) • Possible evidence for liquid-water ocean under Europa's ice was found • Website

Recent Exploration of Saturn • Saturn, with its spectacular rings and many moons, is intriguing for many reasons • The Cassini spacecraft was launched 1997 and reached Saturn in 2004 • It deployed the Huygens probe into Saturn’s moon Titan in January 2005 • Titan is of particular interest to scientists because it is one of the few moons in our solar system with its own atmosphere • Animation

Basic Facts about Jovian Planets • They • are at large distances from the Sun • have long orbital periods • rotate (spin) very fast • Jupiter and Saturn have many similarities in in composition and internal structure • Uranus and Neptune • are smaller than the other two • differ from the other two in composition and structure

Appearance of Jovian Planets • Only the atmospheres of the giant planets are visible to us • The atmospheres are composed mainly of hydrogen and helium • The uppermost clouds of Jupiter and Saturn are composed of ammonia (NH3) crystals • The upper clouds on Neptune are made of methane (CH4) • Uranus has no obvious clouds, only deep and featureless haze AST 2010: Chapter 10

Rotation of Jovian Planets • How does one determine the rotation rates of the giants? • For Jupiter • 1st option: use dynamic surface features (storms) • However the cloud rotation may have nothing to do with the rotation of the mantle and core… • 2nd option: look at periodic variations of radio waves associated with the magnetic field produced deep inside the planet • This gave rotation period of 9 h 56 m • The same technique is used to measure the rotation of other giant planets • Saturn has 10 h 40 m • Uranus and Neptune have about 17 hours AST 2010: Chapter 10

Seasons on the Giants • Jupiter’s spin axis is tilted by 3° • It has no seasons to speak of • Saturn’s axis is tilted by 27° and Neptune’s 29° • Both have seasons • Uranus’ axis is tilted by 98° • practically orbiting on its side • its rings and satellites following the same pattern • causing it to have very strange 21-year seasons!!! • Uranus’s odd tilt could have been the result of a giant impact in its past Seasons on Uranus

Infrared Image of Uranus South pole marked with +

Giant Pressures on Giant Planets • Astronomers believe that the interiors of Jupiter and Saturn are composed mainly of hydrogen (H) and helium (He) • Their precise internal structure is difficult to predict • Because the planets are enormous, the H and He in their centers are probably compressed tremendously • Theoretical models predict that Jupiter has • a central pressure of more than 100 million bars • a central density of 31 g/cm3 • The Earth by contrast has 4 million bars and 17 g/cm3 in its center • Giant planets imply giant pressures!! AST 2010: Chapter 10

Consequences of Enormous Pressure • A few thousand km below Jupiter and Saturn’s visible clouds, hydrogen is changed from a gaseous to a liquid state • Still deeper, the liquid hydrogen is further compressed and begins to act like a metal • On Jupiter, most of the interior is probably liquid metallic hydrogen! • Since Saturn is less massive, it has only a relatively small volume of metallic hydrogen • Most of its interior is liquid, but not metallic • Neptune and Uranus are probably too small to liquefy hydrogen AST 2010: Chapter 10

More about Composition • Detailed analyses of their gravitational fields suggest that each of the giant planets has a core composed of heavier materials • The core may have been the original rock-and-ice body that formed before gas was captured from the surrounding nebula • The pressure in the core is in the tens of millions of bars • At such a pressure, rock and ice do not assume their familiar forms

Internal Heat Sources • Because of their large sizes, all the giant planets are believed to be strongly heated during their formation • The contraction due to gravity produced heat • Being the largest, Jupiter was the hottest • Some of this primordial heat still remains inside them • Giant planets may also generate energy internally by slowly contracting • Even a small amount of shrinking can generate significant heat • These internal sources of heat may raise the temperatures in the interiors and atmospheres above the temperatures due to the Sun’s heat • Jupiter has the largest internal source of heat • Most of it is probably primordial heat AST 2010: Chapter 10

Magnetic Fields • All four giant planets have strong magnetic fields • Their associated magnetospheres are large • They extend for millions of km in space • Jupiter’s field was discovered in the late 1950s • Radio waves were detected from Jupiter • They were produced by electrons circulating in its magnetosphere by a process called synchrotron emission • The magnetic fields of Saturn, Uranus, and Neptune were discovered by flyby spacecraft AST 2010: Chapter 10

Atmospheres of Giant Planets • Only their atmospheres are directly observable from space • They show dramatic examples of weather patterns • Storms on these planets can be larger in size than the Earth! AST 2010: Chapter 10

Atmospheric Composition of Jovian Planets • Methane (CH4) and ammonia (NH3) were first believed to be the primary constituents of the atmospheres • We know today that hydrogen and helium are actually the dominant gases • This was first shown by Voyager’s far-infrared measurements by • There is less helium in Saturn’s atmosphere • The most precise measurements of composition were made on Jupiter by Galileo’s entry probe • Since these planets have no solid surfaces, their atmospheres are representative of their general compositions AST 2010: Chapter 10

Clouds on Jovian Planets • Jupiter’s clouds are spectacular • ranging in color from white to orange to red to brown • Movie • The explanation for the colors remains a mystery • Saturn’s clouds are more “subdued” in color • They have nearly uniform butterscotch hue AST 2010: Chapter 10

Atmospheric Structure of Jovian Planets AST 2010: Chapter 10

Winds and Weather • The jovian planets’ atmospheres have many regions of high and low pressure • Air flow between these regions sets up wind patterns distorted by the fast rotation of the planet • The wind speeds are measurable by tracking cloud patterns • The weather patterns are different from Earth’s • Giant planets spin much faster than Earth • Rapid rotation smears out air circulation into horizontal (east-west) patterns parallel to the equator • No solid surface • No friction or loss of energy – this is why tropical storms on Earth eventually die out… • Internal heat contributes as much energy to the atmosphere as sunlight (except for Uranus) AST 2010: Chapter 10

Winds on Giant Planets • Winds on Uranus and Neptune are rather similar to those on Jupiter and Saturn • True on Uranus in spite of the 98O tilt AST 2010: Chapter 10

Giant Storms on Giant Planets • There are many storms on the giant planets • superimposed on the regular circulation patterns • The most prominent storms are large oval-shaped high-pressure regions on both Jupiter and Neptune • The most famous is Jupiter’s Great Red Spot • in the southern hemisphere • 30,000 km long (when Voyager flew by) • present since first seen 300 years ago • changing in size, but never disappearing • Movie From Voyager 1 in 1979

The Giant Planets