Tuesday, 14 February 2012

Jupiter – Giant Among Giants

Jupiter Data
Mass: 1.90 x 1027 kg or 317.7 times Earth’s
Diameter: 143 884 km or 11.2 times Earth’s
Surface gravity: 2.6 gees
Axial tilt: 3.1°
Mean surface temperature: -150 Celsius
Rotation period: 9.93 hours
Orbital period: 11.9 years
Inclination of orbit to ecliptic: 1.3°
Orbital eccentricity: 0.048
Distance from the Sun: 4.95–5.45 AU
Sunlight strength: 0.034–0.041 of Earth’s
Satellites: > 28
Largest satellite: Ganymede, diameter 5262 km

Out beyond the rubble of the asteroid belt, like a gargantuan marble in space, we find the fifth planet from the Sun. Giant among giants, Jupiter’s size is breathtaking. It spans fully 11 Earths and contains well over 150 times the mass of all the terrestrial planets put together. But it is nothing at all like these smaller worlds. Jupiter’s composition is very similar to that of the Sun, mainly hydrogen and helium, and it has no solid surface. We see only its atmosphere. There, clouds encircle the planet in distinct bands, stretched around the globe by rapid rotation, and vast cyclonic storms are commonplace. Because Jupiter is a fluid planet, there are no faults, volcanoes or craters to tell us about the planet’s past. But there are clues elsewhere. In attendance around this gas giant is an entourage of at least 16 moons, four of them planet-sized. Their surfaces, at least, have a¤orded astronomers some understanding of the Jovian system’s history.

Physical Overview
At roughly 5 AU from the Sun, Jupiter marks the inner boundary of the giant planets. Because of its composition – mostly hydrogen and helium – Jupiter and its ilk have become known as the gas giants. But the name is a misnomer. Though these elements are gaseous under normal atmospheric conditions on Earth, the crushing pressures found inside Jupiter mean that the materials exist there in a liquid form. In reality, Jupiter is an enormous, spinning blob of fluid. It spins so quickly for its size, in less than ten hours, that it is visibly oblate – bulging at the equator.

Deep within its interior, Jupiter has a rocky and icy core. Recall from Part 2 that this was the original giant planetesimal that, after its early appearance in the Solar Nebula, began to suck in the material that made Jupiter so massive. The core is most likely about 20 times the mass of the Earth, and may be separated into two distinct parts: a rocky centre and an icy exterior. However, it is unlikely, given the core’s extreme conditions, that its constituents look much like their Earthly counterparts. The core pressure is at least 50 million atmospheres, and the temperature is five times that on the photosphere of the Sun. Meanwhile, above the core, the rest of the planet is virtually entirely liquid. Most of it is hydrogen in a form known as liquid metallic hydrogen. It is under so much pressure that the hydrogen is split into its protons and electrons and conducts electricity just like a metal – hence the name. Convective motions within the liquid metallic hydrogen, driven by the hot core, are responsible for Jupiter’s magnetic field. It is ten times stronger than the Earth’s. Above the metallic hydrogen is a smaller shell of ordinary liquid hydrogen. It is under lower pressure and so, though liquid, does not conduct electricity well. It is easy to think that this liquid must have a surface. But this is not the case. There is no sharp boundary separating it from the atmosphere. Instead, the pressure within the liquid hydrogen gradually drops with altitude until, within 1000 kilometres of Jupiter’s outer edge, the interior blends imperceptibly into the planet’s gaseous shroud – the visible face we see.

The atmosphere of Jupiter is totally different from those of the terrestrials: a different composition, different weather patterns – and a radically different appearance. Its brightly coloured clouds encircle the planet in bands, dragged around by the rapid rotation. The dark bands are known as belts, and the lighter ones are called zones. Probe measurements showed that the zones are higher than the belts in the atmosphere. They are highpressure regions where gases are constantly rising. In the low-pressure belts, by contrast, the gases are always sinking. Nobody knows how these patterns are maintained. Meanwhile, superimposed on the zones and belts are circular storms, somewhat like hurricanes on Earth. The most famous is the Great Red Spot. It sits about 8 kilometres above the cloud tops and so is a region of high-pressure gas, swirling around as it rises to the top.

Rings and Small Satellites

Moving outward from the planet’s cloud tops we next come to its system of rings. But the Jovian rings are nothing like the most famous set, those found around Saturn. Saturn’s glorious rings, as we shall see, are bright and made of ice particles – some of them the size of houses or even larger. But Jupiter’s accoutrements, no more than 30 kilometres thick, are composed of tiny, rocky fragments only about 10 microns across – comparable in size to smoke particles. They are dark and very transparent. And they can only be seen clearly when lit by sunlight from behind.

Because they are so small and lightweight, Jupiter’s ring particles are constantly at the mercy of the planet’s magnetic field, the solar wind and the Sun’s radiation pressure. All exert forces on the fleeting particles and act to disperse them. The fact that the rings are still there means that their stocks of rocky particles are somehow being maintained. How? Micrometeorite impacts on the surfaces of Jupiter’s satellites no doubt chip off fragments that may help populate the rings. Or else the rings could be the remains of entire moons that were shattered in the recent past by tidal forces exerted by Jupiter’s enormous gravity. It is likely that both processes are at work, continually pumping more particles into the rings to replace those swept away. Certainly there are plenty of available satellites. Jupiter has at least 28 of them – and most likely many more remain undiscovered. Most of these satellites are much like asteroids. Like Mars’ Phobos and Deimos, they were captured from the nearby asteroid belt.

The Galilean Satellites
 Not all Jupiter’s satellites are small, though, nor are they all the product of gravitational ensnarement. Four of them are substantial and primordial – that is, they formed alongside Jupiter itself. These are the so-called Galilean satellites, named after the Italian astronomer Galileo Galilei (1564–1642) who discovered them and observed them in the seventeenth century. From Jupiter outwards, they are known as Io, Europa, Ganymede and Callisto.

Io is a rocky world, with little or no ice, a bit larger than our Moon. It is covered in sulphur-rich lava flows and active volcanoes. Around a dozen volcanoes are currently at work on its surface, with the result that Io has the youngest facade in the Solar System. It is only 1 million years old or so, and preserves no trace of the Solar System’s early heavy bombardment. Indeed Io has no impact craters whatsoever. They are covered in lava almost as soon as they appear. The reason for Io’s activity is its proximity to Jupiter. Tidal forces flex the moon’s interior and, like a paperclip bent repeatedly back and forth, it heats up. The heat melts the interior and Io cools down by venting that molten material from its surface in the form of volcanoes. Those volcanoes have furnished Io with a very thin atmosphere. It is little more than a scant shroud of volcanic particles, but it does mean that Io is one of only three or four satellites known to possess a sky. The others are moons of Saturn, Neptune and, possibly, Pluto.

The next Galilean satellite, Europa, is another rocky world, a bit smaller than our Moon. But, in contrast to Io, volcanoes are completely absent on Europa. Being further from Jupiter, Europa no doubt experiences weaker tidal heating. Nevertheless, the moon almost certainly has some internal heat. That this is the case is evident on Europa’s surface. Though the moon is mostly rock, it is covered in a bright veneer of water ice, riddled and heavily scored with long cracks, and with less than a dozen impact craters. The lack of craters means than the surface must be young, so the cracks are geologically recent. Astronomers suspect that tidal heating has melted some of the ice beneath the surface to form an underground ocean of liquid water. This keeps the icy crust pliable and frequently fractures it. As a result, Europa’s surface is a vast patchwork of ill-fitting jigsaw pieces, giant icebergs, constantly jiggling around – and evidently fast enough to obliterate most impact craters very quickly. We shall see in Part 4 that Europa’s ice might well melt in the far future, when the Sun becomes a red giant. But it the meantime it will remain frozen solid.

Ice also covers the surface of the next moon out, Ganymede – in fact this world is half ice and half rock, the latter forming the core. Ganymede is the largest satellite in the Solar System, bigger than the planet Mercury. Parts of its surface are heavily cratered, and in general the terrain is much darker than Europa’s. The dark colour comes from dirt strewn across the surface by aeons of accumulated meteorite impacts – evidence, as well as the craters themselves, that Ganymede’s surface is much older than either Io’s or Europa’s. There are regions that seem to be relatively young, with few craters, but they are still ancient. Covering two-thirds of the surface, this landscape is known as the grooved terrain. It was formed about 1–2 billion years ago, when the ice crust was a bit thinner because of the warmer interior then. Around this time, water from beneath the relatively thin crust flooded the surface – cracked perhaps by tides – and froze. As it did so it filled in the cracks and widened them to create the relatively bright grooves that give this terrain its name. But Ganymede feels Jupiter’s tides even less than Europa does. And so, lacking an appreciable amount of internal flexion, it has now frozen solid to considerable depth. Without the volcanoes of Io or the thin surface of Europa, little has modified Ganymede’s face in a billion years. It remains old and battered.

Lastly, Callisto, also an ice world, has an even older and darker surface. Tidal forces have affected this world least of all, and no cracking seems to have occurred. Evidently Callisto froze very quickly after it formed. It is more heavily cratered than any other planetary body, even Mercury. This is almost certainly because of Jupiter, whose powerful gravity sucks in comets and asteroids, which then collide either with the planet or its moons. Ganymede, being nearer to Jupiter than Callisto, ought to have more craters, but some of them were no doubt wiped out when the grooved terrain was laid down. If Io and Europa had not eradiated their craters, theywould be even more scarred today than Ganymede and Callisto.

History of the Jovian System
Because Jupiter has no surface – and therefore no surface geology – it is very difficult to see how the planet might have changed in the billions of years since it formed. It is probable, however, that the planet has evolved very little. Jupiter’s escape velocity is so high that it has retained virtually all of the material that went into its making – even the lightest and swiftest gas, hydrogen. Thus Jupiter’s composition is essentially primordial, a chunk of the original Solar Nebula. When the planet was first formed, it would have been a lot hotter than now, and even glowing. Gradually it cooled down; molecules began to form from its gases. Thus Jupiter is quite a bit cooler and a little smaller – because of continued gravitational contraction – than when it stopped accreting. Other than that, little else has changed – or if it has, we cannot easily see the evidence.

On the other hand, the Galilean satellites reveal much about the history of the Jovian system. We saw in Part 2 that these large satellites accreted from a disc that surrounded the newly forming Jupiter, in much the same way that the planets themselves grew in the disc around the Sun. Evidence for this scenario is strong. The Galilean moons all orbit in roughly the same plane – that which defined the original disc, long since gone. They also orbit in the same direction. And there is more evidence in the moons’ general compositions and densities. The innermost two, Io and Europa, have high densities. They are rocky, with little ice. Ganymede, further out, has a density between those of ice and rock, and contains equal amounts of each. And Callisto has an even lower density. All of this is to be expected if the moons formed in a disc. Closer in, where it was warm, ices could not condense, and the moons there formed from rock. But further out, where it was cold enough for ices to condense, the moons formed from equal amounts of ice and rock.

Today, the surface of Callisto is our best indicator of the early history of the Solar System near Jupiter. It is much the same story as nearer the Sun, with impact craters betraying the handiwork of the heavy bombardment. Here, though, the bombardment was even more torrential, as more comets and asteroids than usual succumbed to a sticky end, netted by Jupiter’s powerful gravity. Since the end of the heavy bombardment, Callisto has changed very little. Ganymede, as we saw, experienced flooding by water more recently. But Europa is still warmed inside from tidal heating. It has an active surface that betrays little about its past, and volcanic Io is the same.

Source :
Mark A. Garlick. The Story Of The Solar System. University Press: Cambridge. 2002.



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