the outer Planets storm system that is bigger across than Earth Jupiter is one of the brightest objects in the night sky and only the Moon Venus and sometimes Mars are more brilliant Saturn is easily visible to the unaided eye as a nontwinkling point of light It is a planet girded by extraordinary rings composed of dusty chunks of ice and are more complex than the rings of any other planet in our solar system Saturn is arguably the most sublime object in the solar system Uranus is the seventh planet in distance from the Sun and the least massive of the solar system’s Jovian planets It is a dazzling blue-green world with an atmosphere of hydrogen helium and methane and winds that are several times stronger than Earth’s Neptune the eighth planet from the Sun is the smallest in diameter of the four outer planets It is a stormy windswept world with a vivid blue hue Its highly active atmosphere is surprising since it receives so little sunlight to power its weather systems Each of the outer planets is a unique and exotic world As scientists gain the ability to further explore these distant distant neighbours we will have a better understanding of the history and formation of our solar system and of our own planet 8
CHAPTER 1 Giant Jupiter Jupiter is the most massive planet of the solar system When ancient astronomers named the planet Jupiter for the Roman ruler of the gods and heavens also known as Jove they had no idea of the planet’s true dimensions but the name is appropriate for Jupiter is larger than all the other planets combined It takes nearly 12 Earth years to orbit the Sun and it rotates once about every 10 hours more than twice as fast as Earth its colourful cloud bands can be seen with even a small telescope It has a narrow system system of rings and almost 70 known moons one larger than the planet Mercury and three larger than Earth’s Moon Some astronomers speculate that Jupiter’s moon Europa may be hiding an ocean of warm water and possibly even some kind of life beneath an icy crust Given Jupiter’s large proportion of hydrogen and its huge mass it has been traditional to assume that the planet formed by condensation from the primordial solar nebula nebula This hypothesis implies that the elements should all 9
the outer Planets be present on Jupiter in the same proportions that they occur in the Sun However the most recent evidence indicates that the elemental proportions on Jupiter differ from the solar values Current models for Jupiter’s origin suggest instead that a solid core of about 10 Earth masses formed first as a result of the accretion of icy planetesimals planetesimals This core would have developed an atmosphere of its own as the planetesimals planetesimals released gases during accretion As the mass of the core increased it would have become capable of attracting gases from the surrounding solar nebula thus accumulating the huge hydrogen-helium envelope that constitutes Jupiter’s atmosphere and fluid mantle The accumulating envelope would have mixed with the Crescent view of Jupiter a composite composite of three images taken by Voyager 1 on March 24 1979 10
GIant JuPIter outgassed atmosphere from the core Thus the presently presently observed enrichment of the most abundant heavy elements in this envelope compared with solar values reflects the concentration of such elements in the core Jupiter has an internal heat source it emits more energy than it receives from the Sun The pressure in its deep interior is so high that the hydrogen there exists in a fluid metallic state This giant has the strongest magnetic field of any planet with a magnetosphere so large that if it could be seen from Earth its apparent diameter would exceed that of the Moon Jupiter’s system is also the source of intense bursts of radio noise at some frequencies occasionally occasionally radiating more energy than the Sun Despite all its superlatives however Jupiter is made almost entirely of only two elements hydrogen and helium and its mean density is not much more than the density of water Knowledge about the Jovian system grew dramatically dramatically after the mid-1970s as a result of explorations by three spacecraft missions Pioneers 10 and 11 in 1973 74 Voyagers 1 and 2 in 1979 and the Galileo orbiter and probe which arrived at Jupiter in December 1995 The Pioneer spacecraft served as scouts for the Voyagers Voyagers showing that the radiation environment of Jupiter was tolerable and mapping out the main characteristics of the planet and its environment The greater number and increased sophistication of the Voyager instruments 11
