Jupiter

William Sheehan, Thomas Hockey

• Publisher: Reaktion Books
• Year: 2018
• ISBN:
  9781780239491
• Topic: Biological Sciences
• Subtopic: Science General
• Index: Biological Sciences

Now in paperback, an accessible and engaging introduction to planetary science that will deepen our knowledge both of this magnificent planet and of our own place in the solar system. Majestic and untwinkling, Jupiter is the grandest of all planets. It is the largest planet in our solar system and among the brightest objects in the night sky. It shines with a noble, steady luster, and its calming presence has inspired humans for centuries. Jupiter was the "beloved star" of the first serious observers of the planets, the ancient Sumerians and Babylonians, and has inspired poetic utterances from eminent writers such as William Wordsworth and Walt Whitman. It also continues to inspire contemporary astronomers and stargazers, and this beautifully illustrated volume brings our understanding of Jupiter right up to date.The scientific study of Jupiter is at a watershed: NASA's Juno space probe has entered orbit about Jupiter to investigate the planet, while information gleaned from improved telescopes and other robotic explorers in space continues to improve our understanding of the planet's origin, evolution, and composition. Jupiter provides a concise and expert overview of the history of our observations of this largest of planetary spheres, as well as reports on the much-anticipated initial findings from the Juno space probe.

ONE

THE JOVIAN PLANETS

With many characteristics in common, the outer planets, Jupiter, Saturn, Uranus and Neptune, have sometimes been referred to as the Jovian planets after their grandest member. They are also known as the giant planets or the ‘gas giants’. Worlds of a different order from the Earth and the other ‘terrestrial’ planets that occupy the inner solar system, they are the largest objects that we can study up close, apart from the Sun. Jupiter, in particular, is so gigantic that it has long seemed almost an embryonic star – a reputation it partly merits. Its make-up reflects this: like the Sun itself, it retains all of the gases and even some of the heat of its formation. Its bulk consists of about 90 per cent hydrogen and 10 per cent helium, reflecting the cosmic abundances of the early universe after the creation event of the Big Bang. The other Jovians have similar proportions of these primordial elements. We will devote more time later to a discussion of Jupiter’s composition, and define the precise ‘recipe’ for making a gas giant planet.

An Almost Star is Born

Jupiter and the other planets – including the Earth – formed with the solar system itself, about 4.6 billion years ago, in a process that was long among the most inscrutable and tantalizing of the mysteries of science. After many false starts, though there are still debates about some of the details, we now have a fairly good understanding of just what happened.

All peoples have origin myths. We pass over them here, and take as our starting point the Nebular Hypothesis put forward by Immanuel Kant and Pierre-Simon Laplace near the end of the eighteenth century. Kant was a German philosopher, Laplace a French mathematician, and independently of one another they tried to explain how the planets in our system came to exhibit their configuration of all moving in the same direction (in direct orbits, that is, moving anti-clockwise as seen from north of the plane of the Earth’s orbit, or ecliptic) and in roughly the same plane (marked by the ecliptic). They assumed that the Sun and planets had started out as a swirling nebulous cloud that proceeded to contract gravitationally; the central mass collapsed upon itself and heated up until it could shine by its own light and heat, while the rest of the cloud flattened into a disc. Clumps of material in the disc went on to form the planets and their satellites. Note that, according to the Kant-Laplace scheme, the direction of the planets’ movements was set by the initial rotatory motion of the nebula.

Immanuel Kant (1724–1804), who proposed, along with Pierre-Simon Laplace, what became known as the Kant-Laplace Nebular Hypothesis.

Although the Nebular Hypothesis as set forth by Kant and Laplace was eminently plausible, the Devil is always in the detail, and for a long time there remained an important stumbling block: angular momentum. In a nutshell, although most of the mass of the solar system resides in the slowly rotating Sun, most of the angular momentum lies in the planets. Since it seemed impossible to explain this asymmetry in terms of the Kant-Laplace scheme, a different idea came into vogue, according to which the planets formed as by-products of a rare grazing encounter of another star with the Sun in the early stages of the Sun’s life as a star.

Most astronomers favoured the grazing-encounter theory at the beginning of the twentieth century. For example, in 1909 the American astronomer Percival Lowell, who established his own observatory in Flagstaff, Arizona, for the purpose of studying Mars but soon extended his programme of research to the other planets, went so far as to suggest that the spiral nebulae themselves, which were turning up by the millions in deep-sky photographs, might be budding solar systems in formation, where a dark star and a luminous star were involved in such an encounter. In that case there might be untold millions of solar systems scattered across space. Lowell wrote:

Pierre-Simon Laplace (1749–1827), in old age.

Suppose, now, a stranger to approach a body in space near enough; it will inevitably raise tides in the other’s mass, and if the approach be very close, the tides will be so great as to tear the body in pieces along the line due to their action; that is, parts of the body will be separated from the main mass in two antipodal directions. This is precisely what we see in the spiral nebula[e]. Nor is there any other action that we know of which would thus handle the body . . .

As the stranger passed on, his effect would diminish until his attraction no longer overbalanced that of the body for its disrupted portions.

These might then be controlled and forced to move in elliptic orbits about the mass of which they had originally made part. Thence would come into being a solar system, the knots in the nebula going to form the planets that were to be.¹

Ironically, it would be Lowell’s own assistant, Vesto Melvin Slipher, an Indiana University graduate hired by Lowell in 1901 to take charge of a new spectrograph, initially for the purpose of vindicating Lowell’s contested ideas about Venus’s rotation, who would prove the baselessness of Lowell’s speculations. Instead of confirming Lowell’s (and others’) speculations about the spiral nebulae being solar systems in formation, Slipher made the unexpected discovery that they were (mostly) receding from us at high speeds. This in turn contributed to astronomers’ eventual recognition that the spiral nebulae are something far more consequential even than solar systems in formation – they are galaxies, vast conurbations of stars in their own right, involved in the general expansion of the universe. By the time the expanding universe was being recognized, the close-encounter theory of the planets was also falling by the wayside. The wispy entrails of the Sun pulled away during a close encounter would simply have been too tenuous to stitch together into planets.

Kant and Laplace vindicated: false-colour image of a protoplanetary disc forming about the star Beta Pictoris, which is visible from latitudes south of Hawaii. The disc was discovered in 1984 by Bradford A. Smith (University of Arizona).

The Nebular Hypothesis Makes a Comeback

By the 1940s and ’50s, the Nebular Hypothesis had returned to favour, as astronomers worked out ways of transferring angular momentum from the embryonic Sun to the planets. It is now universally agreed that the planets, satellites and other bodies of the solar system formed from a welter of debris left behind from the eddying protoplanetary disc of gas and dust surrounding the fledgling Sun, which must have looked very much like that now surrounding Beta Pictoris. It follows that planetary systems must be very commonplace throughout the universe – and so they are. Since the first exoplanet (a so-called hot Jupiter, around the star 51 Pegasi) was discovered in 1995, thousands of exoplanet systems – many strikingly different from our own solar system – have been discovered, and there is no end in sight.

Bradford A. Smith, who early in his career was associated with Clyde Tombaugh’s Planetary Patrol, a programme systematically carried out at New Mexico State University to photograph the planets. Smith went on to become Imaging Team Leader of the Voyager spacecraft missions to the outer planets, which included flybys of Jupiter in 1979 and 1980. After the Voyager 2 flyby of Neptune in 1989, he made the observations that showed the existence of the protoplanetary disc around the southern hemisphere star Beta Pictoris. Here he poses in 2016 with the 33-cm Abbott Lawrence Lowell astrograph used by his mentor Clyde Tombaugh to discover Pluto in 1930.

Dark clouds of the Milky Way. One of E. E. Barnard’s classic Milky Way images, of 1913, taken with the 15.25-cm Willard lens of the Lick Observatory on 25 June 1892, exposure 4 hours. This image shows the region north of Theta Ophiuchi, with the ‘Pipe Nebula’, one of Barnard’s dark nebulae, in the lower part of the image.

A great deal is now known about the drama of the origin of the solar system, and after only the Sun itself, Jupiter has always played the leading role. In the beginning, 4.6 billion years ago, the Sun and planets emerged out of cold, dark, interstellar molecular clouds. Examples of these clouds were first recorded in the wide-angle photographs of the Milky Way taken by the great American astronomer Edward Emerson Barnard around the turn of the twentieth century. They were later studied by (and named for) the Dutch-born astronomer Bart Bok, who personally thought they should be called ‘Barnard globules’. We see them as they are silhouetted against the background stars as we look from our position on one of the galaxy’s spiral arms towards the centre of the Milky Way. These clouds are very cold, with typical temperatures of around 10 Kelvin (10° above absolute cold) and with densities of several thousands of molecules per cubic centimetre.

If a dark cloud is dense enough, or if it happens to be suddenly compressed by passage through the dusty arms of the Milky Way or by a supernova blast in its proximity, it begins to collapse in on itself. At first this collapsing tendency is resisted by the presence of magnetic fields, but eventually the magnetic fields ‘leak out’ of the cloud. Once this stage is reached, collapse begins in earnest, and the gravitational energy of collapse is converted into heat. However, because of the cloud’s low temperature and low density, at first it remains transparent to radiation. The radiation simply escapes into cold space. The collapse at this stage is said to be isothermic – that is, it occurs without warming the cloud. During this isothermic collapse phase, the cloud undergoes fragmentation into hundreds of sub-clouds, each massive enough to contract further in its own right. These sub-stars, or protostars, continue to contract until they have become stellar-sized. Stars are what they are destined to become.

Looking along the plane of the Milky Way, the central bulge is visible (located in the direction of the constellation Sagittarius), and the plane is thick with the great dust clouds in which the stars and their systems of planets are formed, 2009.

Though most of the material of the cloud falls directly into the incipient star and adds its mass, some leftover remnants are spun into a disc of the kind Kant and Laplace envisaged long ago. The swirling cloud of gas and dust out of whose broken rings planets form derives its symmetry from the same cause as the spiral form of the galaxy itself. It is the symmetry of matter in rotation. The gathering together of matter, realized on a grand scale in the starburst of the galaxy, is writ small in the formation of the solar system.

As the sub-clouds continue to contract, they rotate faster and faster, like a skater speeding up by pulling her extended arms inwards against her chest. In time the swirling material reaches supersonic speeds. The centrifugal force causes material in the cloud to flatten towards the outside, until the cloud’s shape resembles that of a barred spiral galaxy. At this point it has formed a circumstellar disc, like that which has been imaged around Beta Pictoris. The rapid rotation leads to further fragmentation of the cloud, with most of the angular momentum of rotation becoming stored in the relative motion of the largest fragments. These fragments will go on to form the stellar components of binary- or multiple-star systems. Uncompanioned stars such as the Sun are much less common. (Incidentally, there is nothing to prevent binary stars from having their own planetary retinues and as we now know, most of them do.)

As the contracting cloud grows more and more dense, it finally reaches the point where it becomes opaque to radiation. No longer able to escape into space, the gravitational energy that is released as the cloud continues to collapse rapidly warms the womb-like interior of the cloud. As it does so, there is a corresponding build-up of gas pressure resisting further contraction. The opposing forces – gravity inwards, gas pressure outwards – eventually reach a delicately crafted compromise as the gravitational energy of collapse comes into precise balance with the heat energy of expansion. A wonder of nature – in basic structure elegant and simple but intricate in detail – appears. A star is born.

At first the star is a stellar pupa, tucked away inside its gas and dust cocoon, glowing with a softly beating irregular light. It is then known as a T Tauri star (after its prototypical namesake in the constellation Taurus; such a star has strong emission lines in the spectrum and a rapidly varying output of infrared, optical and ultraviolet radiation). Still surrounded by a gas and dust disc, such a star undergoes periodic outbursts, each lasting about a hundred years, in which mass is transferred from the disc onto the young star, increasing its luminosity. The gaseous component of the disc lasts only about 1 to 10 million years against the ravages of these periodic outbursts. However, it takes tens of millions of years – in the case of a one-solar-mass star, about 40 million – for the star to settle into an even-tempered luminary that will burn steadily and predictably for billions of years on the band of stars, graphed according to mass and brightness, that astronomers refer to as the Main Sequence. This means that by the time the star forsakes its turbulent youth and sets out as a stellar debutante on the cosmic stage, it has already begun to form a retinue of planets – or at least of gas-giant planets, such as Jupiter and Saturn. Calculations show that these giant planets must have formed within only the first 1 to 10 million years after the Sun itself, before the gas – hydrogen and helium, formed in the Big Bang and the chief components of the solar nebula – was dispersed into interstellar space.

At present, there are two competing theories regarding the formation of Jupiter and the other giant planets. The first assumes a top-down process, in which Jupiter formed through the very rapid and direct collapse of a cold, dense clump of gas and dust in the outer part of the circumstellar disc. Because the collapse occurs so rapidly, an enormous amount of heat would be trapped deep within the planet. The second theory assumes a bottom-up process, the so-called core-accretion model. Here, planet-sized cores of ice and rock form first and proceed to grow rapidly through an influx of gas and dust. A few objects – by a process that is fundamentally random and unpredictable, hence sometimes referred to as a Monte Carlo process – decisively outgrow the rest, and become the most massive and dominant objects in the solar system after only the Sun itself. This process is more gradual than that involved in a rapid and direct collapse, and the interior of the planet produced would be relatively cooler than in the top-down model, though still red-hot.

In principle, it should be possible to ...

Table of contents

• Front Cover
• Half Title
• Title Page
• Copyright
• CONTENTS
• PROLOGUE
• 1 THE JOVIAN PLANETS
• 2 JUPITER: A PRIMER FOR A GIANT PLANET
• 3 SUPERFICIAL MATTERS
• 4 ATMOSPHERICS
• 5 THE GREAT RED SPOT BECOMES GREAT
• 6 A BEWILDERING PHANTASMAGORIA: JOVIAN METEOROLOGY
• 7 ABOVE JUPITER
• 8 JUPITER IN COLLISION
• 9 JUNO TO JUPITER
• 10 OBSERVING JUPITER
• APPENDIX I: JUPITER BY THE NUMBERS
• APPENDIX II: CONJUNCTIONS OF JUPITER
• APPENDIX III: THE GALILEAN SATELLITES BY THE NUMBERS
• APPENDIX IV: SPACE PROBES TO JUPITER
• GLOSSARY
• REFERENCES
• ACKNOWLEDGEMENTS
• PHOTO ACKNOWLEDGEMENTS
• INDEX