LEAVING THE ASTEROID belt behind marks a new phase in your journey. With the exception of the Sun, all the objects you have visited so far have been rocky bodies with a solid crust on which you could move around, and they all belong to the group known as the inner planets. Beyond the asteroid belt is the realm of the gas giants Jupiter, Saturn, Uranus and Neptune, and it is to these planets our attention must now turn.
It was the invention of the telescope that really helped us to understand the secrets of the gas giants. Jupiter, the next destination, was particularly interesting, and not only did telescopic study reveal a little about the nature of its atmosphere and the many moons in orbit around it, it also led to the discovery in 1676 of the speed of light.
The discovery of the speed of light goes back to the hunt for a solution to identify longitude while at sea. To solve that problem, Galileo proposed the idea of taking accurate timings of the eclipses of the moons of Jupiter, which he had discovered earlier in the seventeenth century. The idea stemmed from the fact that all these moons orbit around Jupiter in a very regular fashion and, given that the key to pinpointing your location on Earth relied on accurately telling the time from your location, eclipses of the moons would be a more reliable clock than the man-made clocks of the day. Accurate tables were produced that predicted the numerous eclipses over the years and, in principle at least, by observing an eclipse you could work out the time. A great idea, but a flawed one given that accurate telescopic observation from the deck of a heaving ship was difficult if not impossible.
Several decades later a young Danish astronomer by the name of Ole Rømer was working with colleagues to accurately time some of the eclipses, and in doing so he made a rather startling discovery. He made a series of observations between March 1672 and April 1673 which revealed that the eclipses were taking place later than predicted. By estimating the position of Jupiter and Earth in their orbits as he recorded the timings he was able to deduce that it took light about twenty-two minutes to traverse the diameter of the Earth’s orbit. With more accurate observations in the years that followed, better estimates of the speed of light were achieved. In 1809, Jean Baptiste Delambre calculated that it takes light eight minutes and twelve seconds to travel the distance between the Earth and the Sun, leading to a value for the speed of light of just over 300,000 kilometres per second – quite an impressive calculation given today’s value of 299,910 kilometres per second. It is incredible that the study of a planet that comes no closer than 630 million kilometres to Earth allowed us to deduce the speed of light.
By knowing the parameters of the Earth’s orbit we can time how long it takes Jupiter to complete one orbit of the Sun, and from that we can determine the size of its orbit. Like all planets, the orbit of Jupiter is elliptical, but it has an average distance from the Sun of 778 million kilometres which means it takes sunlight nearly forty-five minutes to get there. Because we are a little nearer at closest approach it takes light from Jupiter just over thirty-seven minutes to get to us. This means that from the surface of the Earth we are looking at Jupiter as it was thirty-seven minutes earlier, so we are essentially looking back in time.
Jupiter is the largest of the planets and you could fit 1,321 Earths inside it, but despite its colossal size the fact that it is a big ball of gas means its gravity at the visible surface is just over twice the pull of gravity on Earth. That may seem pretty weak given its size but its pull is powerful enough for any asteroid or comet wandering towards the inner Solar System to struggle to get past. A great example of the way in which Jupiter is the bodyguard of the inner Solar System was seen in July 1994 when Comet Shoemaker-Levy 9 completed its final dive into the Jovian atmosphere. A look at the data of the comet as it moved revealed that it was actually in orbit around Jupiter and was probably captured from an orbit around the Sun during the later decades of the 1900s. On its last orbit around the Sun it would have wandered just a little too close to Jupiter and was taken, and there it sat for a couple of decades in a highly eccentric orbit that took it even closer to the great planet. With every pass Jupiter’s gravity tugged at it, increasing tidal forces with every pull. Eventually one of its closest approaches, in July 1992, took it to within 40,000 kilometres of the tops of the planet’s clouds and it was here that the comet got ripped apart by the tidal forces into twenty-one fragments. These fragments were then seen to plunge into the atmosphere of Jupiter over six days in July 1994. The fate of Shoemaker-Levy 9 led to Jupiter being dubbed a ‘cosmic hoover’. Its sheer presence attracts many asteroids and comets towards it, keeping us a little safer. Estimates suggest that Jupiter experiences around 5,000 more impacts per year than Earth.
With Jupiter attracting all those lumps of rocks it is no surprise that it is orbited by sixty-seven confirmed moons. Galileo discovered four of them when he turned his telescope to the giant planet over 400 years ago, but since the development of higher-quality telescopes, not to mention the advent of space exploration, many more have been found. The largest of its moons, Io, Europa, Ganymede and Callisto, account for 99.997% of the mass of all the moons put together, so you can see there is a significant difference in their sizes. Europa is the smallest of the four with a diameter of 3,100 kilometres but that dwarfs the next largest, which is known as Amalthea and has an approximate diameter of just 168 kilometres.
The satellites of Jupiter fall into three main groups: the four largest are collectively referred to as the Galilean moons; Metis, Adrastea, Amalthea and Thebe comprise the inner group; and finally there are the irregular satellites. This latter group gets its name from the shape of the constituent bodies, and all are significantly smaller than the rest with a highly eccentric orbit. It is believed that this group, which totals fifty-nine satellites, are all likely to be captured asteroids that have strayed a little too close to Jupiter, just like Shoemaker-Levy 9. Many of the irregular satellites seem to have similar orbital characteristics with an almost identical orbital period, eccentricity and inclination, so it is quite likely that these asteroids were once one object that was destroyed in a collision, the resultant debris scattered around the orbit.
The inner satellites, by contrast, have nearly circular orbits and, as their name suggests, they orbit closer to the planet than their more eccentric cousins. With the exception of Amalthea, the members of this group are likely to have formed out of a vast rotating disc of material that condensed out as the planet formed; Amalthea is thought to be a captured asteroid like the irregular satellites.
The much larger and more regularly shaped Galilean satellites can be seen easily from Earth, and of course from the Kaldi as you approach the planet. As you coast through the Jovian system and gaze every now and then out of the window you will start to pick out more and more of the smaller and less obvious satellites, too. They all appear as tiny little discs of light with subtly different colours and hues, yet somehow these tiny alien worlds give you a sense of security: you are now among the realm of the gas giants and there are still places where you could set foot on solid ground.
Like the inner satellites, the Galileans formed out of the disc of material that surrounded the planet in its early stages of formation. Unlike all of the other satellites, though, their large sizes dominate the Jovian satellite system. Of the four of them Io is the closest to Jupiter, orbiting at a distance of just 422,000 kilometres, while Callisto is the most distant, 1.8 million kilometres away. At those distances it is thought that they would still have been orbiting within Jupiter’s dust disc in the early stages of the system’s formation. One theory has it that there have actually been a number of generations of Galilean satellites with each one destroyed through the drag exerted on them by the material in the disc. As each generation was destroyed, another was slowly formed from the debris until such a time that the material in the disc dissipated enough that it no longer affected the moons.
The Galilean moons are physically quite different from each other. Io, for example, is home to nearly 100 mountains (some of which are taller than Mount Everest) and an estimated 400 active volcanoes, making it the most geologically active object in the Solar System. The volcanism is caused by tidal forces from Jupiter and the other Galilean satellites constantly pulling on Io from different directions, leading to tidal heating in the interior of the moon. Io completes one orbit of Jupiter in just over forty-two hours; for every two orbits it completes, the next moon Europa completes one orbit, and for every four orbits of Io, Ganymede completes one orbit. These 1:2:4 orbital resonances are one of the key driving forces for the tidal heating and a tidal bulge that measures around 100 kilometres at its maximum. The volcanoes of Io often send plumes of sulphur and sulphur dioxide high into the rarefied atmosphere which then settle back down on the surface, where they can be seen as dark black streaks. The resultant lava flows and deposits are responsible for the colourful appearance of the moon: the sulphur-based compounds scattered over the surface in red, yellow, black and even green make it look like a cosmic pizza.
Quite unusually for a moon in the outer reaches of the Solar System, which are usually high in silicates and water ice, Io is composed of rocky silicates and iron. As with the planets Mercury, Venus and Mars, it has been possible to deduce the internal structure of Io through its gravitational interaction with spacecraft like Voyager and Galileo. The results show a moon that is differentiated with an iron and sulphur core surrounded by a silicate mantle and crust. The presence of a magnetic field suggests that there is a magma ocean under the crust of Io at a depth of around 50 kilometres, which also helps to explain the high volumes of volcanic activity.
In contrast to the fiery world of Io, the smallest of the Galilean moons, Europa, is thought to have a frozen icy crust under which may be an ocean of liquid water. Europa is a moon that at 3,100 kilometres in diameter is a little smaller than our Moon, but it differs quite significantly in composition. As you approach Europa, you’ll see its surface reveals a world markedly different from any other moon in the Solar System. It has a smooth, almost marble-like appearance that seems to lack the craters and mountainous detail of other bodies. What will be evident is its highly reflective surface: Europa has an albedo of 0.64, making it one of the more reflective objects in the Solar System (a perfectly black surface reflecting no light has an albedo of 0; a surface reflecting all light that falls upon it has an albedo of 1). You’ll also see a series of darker lines crisscrossing over the surface. These features are known as lineae, and high-resolution images show them as cracks in the crust where the surface material on either side of the lines has moved relative to each other. Other bands show brighter central regions, suggesting that newer material has risen to the surface as the cracks widen in a process similar to the one seen on Earth along oceanic ridges. The cracks are thought to be the result of a flexing of the moon as tides move around as the planet rotates. Given that Europa is tidally locked with Jupiter so that just one face always points towards the planet, it is reasonable to expect a fairly regular pattern of cracks. With the more recent cracks this is indeed the case, but with the older, less prominent features there seems to be a difference which can only be explained if the crust is rotating at a different speed to the interior. This theory is supported by the idea of some form of global sub-surface ocean upon which the crust ‘floats’.
The surface temperatures on Europa vary from around minus 160 degrees to minus 220 degrees at the equator and poles respectively, and it is these low temperatures that keep the surface frozen solid. Internal tidal heating would melt the sub-surface layers creating an ocean up to 30 kilometres below the thick icy crust and around 100 kilometres deep. Further evidence for such an ocean comes from the few large craters visible on Europa which seem to be surrounded by ripples, as though the heat of impact temporarily caused the surface ice to become partially melted and the crater’s base has been filled with relatively fresh ice.
The idea of a sub-surface ocean gives us another tantalizing possibility of alien life. At the bottom of oceans on Earth there are hydrothermal vents out of which heat is escaping from deep within the planet. Examples of these vents can be seen along the Mid-Atlantic Ridge where two tectonic plates are diverging and new oceanic crust is being formed. No sunlight can penetrate to these depths, yet to scientists’ surprise there are entire ecosystems whose source of energy comes from the vents instead of the Sun. It is just possible that deep under the surface of Europa similar oceanic vents are home to a whole new form of life. This is of course conjecture – as yet there is no evidence – but it is entirely plausible. Even though Europa is smaller than Earth, it is estimated that there is around twice the amount of water locked up in the ice and ocean. This makes it a great place for a stop-off on a voyage around the Solar System, to top up dwindling water tanks.
The other two Galilean moons, Ganymede and Callisto, are respectively the largest and second largest of them all. Like Io and Europa, Ganymede is part of the Jovian resonance system where the three are locked into the orbital pattern of 1:2:4 so it also experiences internal tidal heating and it is likely that it too has a sub-surface ocean, this time nearly 200 kilometres below the surface. The surface of Ganymede seems to be split into two different types of terrain: a darker region that is peppered with impact craters – suggesting an age of nearly 4 billion years, making it one of the oldest surfaces in the Solar System – and a lighter region covered in lines and grooves, thought to be a little younger. One of the unique properties of Ganymede is that it is the only moon in the Solar System to have a magnetic field, thought to be driven by convection in the liquid core. The field is buried within the much larger magnetic field of Jupiter and is only visible as a mere disturbance.
Callisto is different because it is not part of the 1:2:4 orbital resonances that affect the other three moons. This means that Callisto does not experience any internal tidal heating and has a very different internal structure. The core is thought to be silicate and surrounded by the mantle, which is 50% rock and 50% ice. Owing to the lack of heating and the inactive silicate core it is unlikely that Callisto has a sub-surface ocean. The surface is old, perhaps as old as Ganymede’s, and scarred with impact craters. There is also evidence of frost having formed where ice first sublimated before freezing back on the surface. At the distance it orbits Jupiter, 1.8 million kilometres, it does not suffer from as much radiation as the other moons. This makes Callisto a likely place for some kind of future human outpost in the outer Solar System.
Human exploration of the inner Solar System is relatively easy because the distances are considerably smaller so trips out from and back to Earth can happen in a matter of just months. Trips to the outer Solar System are much more difficult, especially in terms of planning a return to the Earth, because of the immense distances and the time it takes for launch opportunities to come back. The setting up of an outpost that could be used for refuelling and resupply would make the possibility much more realistic. Not only are radiation levels low on Callisto, but the moon is geologically stable with high quantities of water reserves. It’s the perfect place. There is even the immense gravity of Jupiter to give any departing spacecraft an immediate swift boost of energy from a planetary fly-by straight after launch. NASA is already looking at using Callisto in this way in their project called HOPE (Human Outer Planets Exploration). It really is only a matter of time before a base like this is scheduled for installation.
You are the first space explorer to journey to Callisto, and therefore the first to have the eerie experience of seeing Jupiter sitting motionless in the sky. Callisto is tidally locked with Jupiter and it takes 16.7 days to complete one orbit; but it also takes 16.7 days for Jupiter to rotate once on its axis, so the planet stays in the same part of the sky, hour after hour, day after day and month after month. It would be an amazing sight, with Jupiter appearing nine times as large as the full Moon from Earth. The detail would be incredible … and it would be even more impressive from the surface of Io, appearing thirty-eight times larger than the full Moon and covering an area of the sky around 19 degrees.
Passing by all the Galilean satellites, our trajectory takes us on a close fly-past of the giant planet at a distance of around a quarter of a million kilometres, which puts us just outside the outer ring. Saturn is well known for its splendid ring system but Jupiter also has such a system, although it is nowhere near as impressive. The rings around Jupiter differ from Saturn’s not only in appearance but also in composition, as they are made up almost exclusively of dust rather than ice.
There are four main components: an inner torus-shaped halo, the main ring, and then two outer rings. The halo is the nearest to Jupiter with an inner boundary at a distance of about 30,000 kilometres from the cloud tops; it then extends for a further 30,000 kilometres where it meets the inner boundary of the main ring. It varies in thickness and has a shape reminiscent of a wedge, with the thin end nearest the planet. The appearance of the halo, which varies depending on the direction it is viewed from, suggests it consists of dust particles that are no more than 0.015 millimetres in diameter, although smaller particles have been found some distance away from the ring plane. By studying the optical depth of the halo, which is simply a measure of its transparency, it is possible to deduce that the particles have the same properties as those from the main ring and are likely to have migrated from there and be slowly drifting towards Jupiter.
The main ring is the brightest and thinnest part of the ring system with an inner boundary at the outer edge of the halo and an outer boundary just 6,500 kilometres further out. This distance coincides broadly with the orbit of the inner satellite Adrastea, which clearly makes it a shepherd moon of the ring. Shepherd moons are key to maintaining the sharply defined structure of ring systems as their gravity acts upon the particles, keeping them in their orbit. Any that wander out too far will be tugged on by the shepherd moon, causing them to slow down and drop back into the ring, while others will be accelerated and ejected from the system. If the lighting conditions are right then it is possible to detect a fainter ringlet just beyond the orbit of Adrastea. Another shepherd moon, Metis, orbits within the confines of the main ring and is responsible for the evolution of a gap just 1,000 kilometres inside the outer boundary. The presence of a shepherd moon within a ring will cause ring particles to be ejected from the path along the moon’s orbit.
The appearance of the rings varies with the direction of the light. There are chiefly two ways a ring system can be illuminated, and they are known as back scattering and forward scattering. Back scattering is the reflection of light back in the direction it came from, but it differs from simple reflection because the returning light waves are scattered in different directions rather than obeying the law of reflection, which dictates that the angle of incoming radiation will equal the angle of reflection. This is not the case when light is back scattered. When the rings are illuminated from back scattering, the observer will be roughly between the rings and the source of illumination – in other words, the Sun. Forward scattering occurs when the rings are between the Sun and the observer. Because Jupiter lies further from the Sun than the Earth the rings can only be observed with forward scattered light by visiting spacecraft such as the Kaldi. Forward scattering occurs when light is bent or diffracted around particles and is scattered in the direction it was going before it got to the rings.
Particles within the main ring are thought to last for no more than 1,000 years and are either ejected from the system or drift slowly through the halo and into the upper atmosphere of Jupiter. The cause of the slow but certain demise of ring particles is the radiation that is emitted from the planet, resulting in the so-called Poynting-Robertson drag. The effect is also seen in dust particles in orbit around the Sun and can be understood by considering the process from the point of view of the ring particle itself. Due to their forward motion, the emissions from Jupiter seem to come from a position slightly ahead of them. When the radiation is absorbed by the particles there is a net force acting in the opposite direction to their orbital motion. As a result of the drop in speed, the particles very gradually follow a spiralling path into Jupiter, limiting their time in the rings. If this process takes no more than 1,000 years for each ring particle then there must be some replenishment of particles in the system. One possible source of new dust particles is collisions with the various moons in orbit around Jupiter, either moon-on-moon (rare) or meteoroid-on-moon.
Beyond the main ring are the much wider but fainter Gossamer rings which geographically make up the majority of the Jovian ring system. There are two parts, the inner Amalthea Gossamer ring and the outer Thebe Gossamer ring. Both of them get their names from the satellites (Amalthea and Thebe) that orbit at a distance which corresponds broadly with the ring’s outer boundary. The particles within the rings come from Amalthea and Thebe, having been ejected by some kind of high-speed meteoroid impact, and like the particles in the main ring they slowly spiral in towards Jupiter as a result of the Poynting-Robertson effect.
The Jovian ring system is a fascinating structure to study but, as we will see when we travel to Saturn, it is far from being the most impressive system of rings in the Solar System. Also, on a journey through the Jovian system it is hard to maintain focus on the moons and rings when the largest planet in the Solar System dominates the view. The sheer size of Jupiter has been breathtakingly obvious for a while now, and its features are so much more prominent when viewed at close proximity.
The concept of a planet being a giant ball of gas seems a little alien. We are all used to experiencing gas as that stuff that cannot be seen, makes up our atmosphere on Earth and is necessary for our very existence. We can even travel through it seemingly unimpeded. Its appearance in the outer Solar System in the form of a vast spherical ball that looks far from invisible is something that takes a little getting used to. It might be reasonable to think that a spacecraft should be able to fly straight through these so-called gas giants, but the reality is very different.
As with many large-scale phenomena in the Universe, we can look to the force of gravity for the explanation of why it is not possible to fly straight through a gas giant. Like all other normal matter in the Universe, gas molecules are attracted to each other by gravity. When the Solar System formed, the majority of the gas in the protoplanetary disc that had formed around the Sun was forced to the outer reaches. Over millions of years, this gas coalesced into local concentrations which became the outer planets. As time progressed, these concentrations attracted more gas, and with their growth the strength of the gravitational field increased, compressing the gas even more. The force of gravity acts from the centre so it forces the gas to make the most geometrically efficient shape possible – a sphere. If you were to adjust course and attempt to fly straight through Jupiter, ultimately you would fail because of the increasing pressure as you descended through its atmosphere.
Think about your experiences on Earth. Standing on the surface means experiencing pressure from the atmosphere pushing down on you, and in real terms this equates to a pressure exerted on your body of 14.7 pounds per square inch. If you were to descend to the deepest part of the Mariana Trench in the Pacific Ocean then this pressure would increase to around 15,750 pounds per square inch – just over 1,000 times more. You would be crushed. Jupiter is about eleven times the diameter of Earth, so even though gas is less dense than water, descending deep into its atmosphere is clearly going to result in extremely high pressures, perhaps even as high as 500 million pounds per square inch. Gas that is subjected to pressures like that will have a high temperature too, estimated at around 36,000 degrees – hotter even than the surface of the Sun. Gas acts in a very strange way under conditions as extreme as this. As you attempted to fly straight through Jupiter you would first encounter gas in the upper atmosphere which would then turn into liquid with the increasing pressure before becoming solid in the core. For this reason, it is impossible to fly through a gas giant planet. Even without a liquid or solid internal structure, the crushing pressures and roaring heat would mean an end to the mission.
A much more prudent approach is to fly close by the planet and use its gravity to alter the trajectory of your path instead. You will still get a front-seat view of the stunning detail in the upper atmosphere of the planet. Jupiter is often referred to as having an atmosphere, which is a little confusing given that it is a gas planet. The atmosphere is usually considered to start at a point where the atmospheric pressure is about the same as that at the surface of Earth, and we call this 1 bar. This means the atmosphere of Jupiter is about 5,000 kilometres thick. Even this close up, all you can see is the dense ammonia clouds that encompass the planet, embedded in an atmosphere that is broadly the same as the Sun’s with similar proportions of hydrogen and helium.
Along with the moons in orbit around Jupiter, Galileo was the first person to observe the belts of the planet and its beautiful hurricane system. The clouds tend to be separated out into different belts and zones that sit at different latitudes and run parallel to the equator. The belts are the darker features, while the zones are found between them and appear lighter in colour. Spectroscopic studies of the light from the belts and zones reveal that the zones are much cooler than the belts, suggesting that they are upwellings of gas, rising higher into the atmosphere and forming ammonia ice crystals before descending in the dark belts. High-speed winds seem to propagate through the belts and zones with maximum speeds reaching over 300 kilometres per hour. It is not known why there are such well-defined belts and zones, although the high-speed jets driving them are certainly the result of solar and internal heating processes. It may be that the Jovian atmosphere where the clouds exist is quite shallow and overlies a more stable lower layer, or alternatively that it is much deeper than expected and they are the visible effects of deep circulation cells in the lower layers of the planet.
The features in the atmosphere are sufficiently stable to have been there for hundreds of years, and astronomers have named them. Around the equator is the imaginatively labelled Equatorial Zone, which extends from 7 degrees south to 7 degrees north, and beyond that lie the North and South Equatorial Belts extending to 18 degrees north and south respectively. These are followed by the North and South Tropical Zones to a latitude of around 50 degrees from the equator, then the less well-defined North and South Temperate Belts and Zones, and finally the Polar Zones. Although the belts and zones are considered to be pretty stable, occasionally one or more of the prominent features has faded from view. The last time this happened was in 2009 when the Southern Equatorial Belt disappeared, only to return in early 2010. The reason for the disappearance of the belts is not fully understood, but it is more likely that during these periods they are merely being obscured rather than having vanished. It may be that high-level cirrus clouds of ammonia crystals form in the air above the belts, masking them from view for several months on end until the clouds dissipate.
It is not just the belts that show some degree of change. There are other atmospheric features that genuinely are more transient, like the numerous vortices of varying shapes and sizes. They are just the same as the vortices we find on Earth and can also be categorized as cyclones or anticyclones, different only due to the direction of their rotation. The cyclones tend to form as small dark patches and are often given the descriptive term ‘brown ovals’. Their rotation is in the same direction as that of the planet but their appearance is not restricted to oval shapes: delicate filamentary structures can often be seen in some regions that also show cyclonic motion. But whether oval patches or filaments, they are usually confined to the darker belts. The anticyclones, by contrast, usually appear only in the zones, as white ovals that can last anything from just a few days to over a century. They tend to stay at the latitude within which they formed but do move around the disc of the planet, merging when they meet.
There is one famous anticyclone which appears significantly different to the others, the Great Red Spot (GRS). It was first observed in 1831, having been discovered by the German astronomer Samuel Schwabe, and like all anticyclones on Jupiter it rotates in an anticlockwise direction, taking about six Earth days to complete one revolution. What makes the GRS so impressive is its size: it measures a little over 24,000 kilometres from east to west and about 13,000 kilometres north to south. That may not sound big by astronomical standards but remember, this is a storm, a storm large enough to engulf the Earth … twice.
Interestingly, though, it seems to be shrinking. About a hundred years ago it measured nearly 40,000 kilometres east to west – getting on for twice as wide as it appears now – so at that rate the GRS may eventually become circular in shape and might even disappear. However, a conflicting study at the turn of the twentieth century suggested that it is unlikely to disappear completely because of the way it interacts with the surrounding atmospheric features. The study concentrated on the observation of clouds within the storm, and although the spot showed signs of reduction in size, the velocity of the clouds showed no sign of changing. This suggests the storm was as active at the end of the ten-year study as it was at the beginning. It may be that the environment surrounding the GRS has more to do with its shape and size than its evolution. Quite how it has survived for nearly two centuries is one of Jupiter’s greatest mysteries, but it is likely that it owes its longevity to swallowing up smaller vortex-like disturbances in the Southern Equatorial Belt (SEB) within which it resides, and to energy being fed to it as warmer air is dragged up inside the storm.
Studies of the spot through infrared telescopes have revealed that it is much colder than the majority of other clouds and atmospheric features on the planet, suggesting it stretches to a higher altitude than other visible features in the area, perhaps by as much as 10 kilometres. They also show that a jet stream blowing eastwards sits to its south, and a more powerful westward jet stream blows to the west, confining the GRS to its latitude of around 22 degrees south. Wind speeds around its perimeter vary but have been recorded to peak at just over 600 kilometres per hour, far greater than hurricane force 5 winds on Earth. It may be windy around the outskirts of the GRS but, like similar anticyclones on Earth, there is little or no wind in the centre – the eye of the storm. Observing the storm’s eye in far infrared wavelengths has shown that it is hotter than the surrounding atmosphere because it is dragging warmer air up from lower down in the atmosphere.
The GRS varies in appearance quite substantially from pale pink to a deep salmon colour, although why it is this colour is still not known. There have been occasions when it has disappeared from view, the only clue to its presence a chunk taken out of the SEB. The cause may be the presence of organic compounds like sulphur, but certainly it seems to be somehow related to temperature. The centre of the spot is generally a deeper red than the surrounding regions which ties in with the temperature affecting it in some way. What is known is that it is related to the visibility of the SEB: when the SEB is bright or even white then the GRS is at its darkest, but when the SEB is darker, the spot tends to go light or even disappear.
There is another smaller storm that is known as Oval BA – or, more affectionately, Red Spot Jr. It is also found in the southern hemisphere but a little further south in the Southern Temperate Belt. Wind speeds around this storm have reached 618 kilometres per hour, which is comparable to winds around the Great Red Spot. Back in April 1996, Tropical Cyclone Olivia battered Barrow Island in Australia with winds gusting to record speeds of 407 kilometres per hour. Compare that, which was a wind gust, to the sustained wind speeds around the storms of Jupiter and you’ll get an idea of just how violent they are.
When you are travelling on a plane at home, the pilots will do anything they can to avoid flying through giant storms as the experience for you, the passenger, would be pretty unpleasant. Larger aircraft are clearly more stable, but even for you in your spacecraft the GRS presents a serious challenge. If you tried to fly through it at a decent altitude then the first thing you would experience on the approach would be turbulence, starting off as small lumps and bumps but getting more severe as the flight continued. As you got closer to its outer wall the tail winds would pick up to speeds around 600 kilometres per hour with a strong component coming from the left, blowing you to the right. Anyone observing from outside the storm would see you appear to accelerate rapidly at this point as the wind carried you along, and at this stage it would be pretty impossible to abort as the wind speeds would seriously inhibit any means of escape.
As you pressed on into the GRS you would be hit by very strong downdrafts, forcing you down at high speed. This is one danger that aircraft face when flying through storms close to the ground. It’s fine at altitude as there is plenty of time to recover, but experience a downdraft close to the ground and the likely conclusion would be ‘early contact with the ground’ – in other words, a crash.
Assuming you could regain control from the downdrafts, wind-shear would be the next and perhaps most dangerous challenge. Windshear is the variation of wind either vertically or horizontally, and the greater the variation the stronger the windshear. Around the outside of the GRS are the downdrafts just encountered, but dragging warmer air from below are updrafts that pierce through the central column of the storm. Flying from violent downdrafts into equally strong and possibly worse updrafts is the most perilous sector of the flight. At this point there is even a risk of structural failure. The Kaldi may well get ripped to shreds – which is a very good reason for keeping this journey hypothetical.
Pass instead at a safe distance, thousands of kilometres above the GRS, and you will notice that there is a serene beauty about it. Below the almost mesmeric cloud belts and features in the atmosphere of Jupiter is an alien world unlike anything you have encountered so far. Descending through the Jovian atmosphere, there are four distinct regions which bear identical names to the regions in the atmosphere of the Earth: the exosphere, the thermosphere, the stratosphere and finally the troposphere. Like all planetary atmospheres there is no sharply defined boundary where the atmosphere finishes and space begins; instead there is a very gradual transition from the density of gas in the atmosphere and the gas-deficient but not -devoid realms of interplanetary space.
Observations have been made of aurora activity and airglow in the thermosphere of Jupiter. Aurora is a familiar phenomenon – you saw it in the atmosphere of Earth just after you left – but airglow is a new concept, although it too is sometimes visible over the Earth. It is caused by incoming sunlight stripping electrons from molecules in the atmosphere, and then, as they try to reattach to the atomic nuclei, they give off a tiny amount of light and cause the gas to glow.
As the descent into the atmosphere continues, the temperature profile is not what might be expected. Instead of a gentle increase, the opposite is true: it gets cooler the lower you go. This is because the rarefied gas at the top of the atmosphere easily absorbs solar radiation, taking temperatures as high as 1,000 degrees. If you were to hop outside the spacecraft it would still feel cold though because the space between the atoms is so high that heat transfer is non-existent.
At an altitude of about 320 kilometres the top of the stratosphere is encountered, and at this point the temperature decrease stops and a fairly constant 100 degrees is maintained all the way to the tropopause at an altitude of about 50 kilometres. The troposphere, through which the descent now continues, is the region where the majority of the belts, clouds and storms are seen, and now the temperature slowly starts to increase again, to around 400 degrees at the ‘surface’.
The Jovian clouds that can be seen from Earth are probably the most complex seen anywhere in the Solar System. In the upper regions of the troposphere they are made mostly from ammonia ice and ammonium sulphide. Below these are clouds of water, and it is their presence that has had a big influence on the atmospheric conditions. It takes more energy for a given amount of atmospheric pressure to transform water from vapour into a gas compared to ammonia, which, along with the higher quantities of water, causes huge amounts of energy to be transferred to and from the atmosphere.
At the bottom of the troposphere, the pressures and temperatures are so extreme that they are above the point where hydrogen and helium blend seamlessly from a gas to a liquid without a solid phase in between. They are said to be supercritical fluids in this state as they just get denser and hotter the lower you go. This is thought to continue all the way through to the core of Jupiter, where, as we know, it may become so dense as to be a solid metallic core. It is also likely that convection currents within the supercritical liquid layers may have mixed with a metallic hydrogen core, redistributing it within the interior of the planet. For now, the exact nature of the interior of Jupiter remains a little bit of a mystery.
Some interesting stories have been written about the Jovian atmosphere that suggest life may have evolved among the clouds. Any creatures could only survive high up in the atmosphere because of the higher pressures lower down. That means they would need to float constantly, so the idea seems a little far-fetched. Even if the organisms were able somehow just to float they would need to be resistant to extreme levels of solar radiation. And all this, of course, assumes a nice, stable, almost quiescent atmosphere that allows them to float, as if on a gentle breeze. As we have already seen, the conditions in the Jovian atmosphere are very different; jet streams, vortices and violent vertical winds could easily suck any life form into the crushing pressures down below. Even if they could survive the extreme pressures, the temperatures would make the environment completely inhospitable.
There is, however, one hardy little micro-organism that gives some hope for finding life elsewhere in the Solar System. It is known as a tardigrade and measures just half a millimetre when fully grown. Research has shown that these tough little critters can survive pressures of around 150,000 pounds per square inch, which is about ten times the pressure found at the bottom of the Mariana Trench so they would certainly be capable of surviving at lower depths in the Jovian atmosphere, but not strong enough to survive the million-pounds-per-square-inch pressures down in the liquid metallic hydrogen levels. They can withstand temperatures in excess of a few hundred degrees, and also significantly more radiation than us lowly humans. They can survive in the vacuum of space and are able to do so without any food or water for about ten years. These creatures are tough. But still it seems even they cannot survive in the atmosphere of Jupiter. It looks like the only place where life may be able to evolve around here is in the sub-surface oceans on some of the moons.
As you swing past the planet on the closest approach you get a boost, trading some of Jupiter’s orbital velocity for spacecraft speed. It is amazing to think that the Kaldi is not the first spacecraft to be here and to do this. Pioneer 10 was the first of a flotilla that have visited the outer Solar System, having been followed by Pioneer 11 and Voyagers 1 and 2. Pioneer 10 was launched from Cape Canaveral in Florida on 3 March 1972. In February the following year it became the first spacecraft to fly through the asteroid belt, which it did suffering no damage at all. It first encountered the magnetic field of Jupiter on 16 November 1973, when it made the startling discovery that the magnetic field was actually inverted compared to the Earth’s.
Planetary magnetic fields like Jupiter’s and Earth’s have two poles, a north and a south, just like the bar magnets used in schools. Earth is actually the peculiar one rather than Jupiter because its north pole sits under the geographic south pole while the south pole of the magnetic field is in the northern hemisphere. That is why the needle marked north on a compass points north, because it is attracted to the south magnetic pole. When the fields reverse, the north needle will point in the other direction and all compasses will have to be remade. Jupiter has its magnetic poles under the geographic poles where you would expect them, but it is not unusual for planetary magnetic fields to reverse. It is even possible to study past magnetic reversals by studying the magnetic properties of metallic rocks. There have been a number of reversal events on Earth over its history. The last major one happened 780,000 years ago when the field strength dipped by just over 5%.
Having been in the neighbourhood of the mighty Jupiter for a total of sixty days and having studied the magnetic field of the planet and made numerous discoveries about new moons and atmospheric features, Pioneer 10 finally went on its way. Unlike yours, the path it took then sent it off and out of the Solar System, away from the other outer planets. At the speed it is flying at – around 12 kilometres per second (most commercial jets travel at about 0.2 kilometres per second) – it could travel the diameter of the Earth in just over seventeen and a half minutes. That sounds pretty impressive, but the distances between the stars where Pioneer 10 is heading are mind-bogglingly vast. It is now flying towards the bright red star in Taurus known as Aldebaran, sixty-five light years away (which of course means that when we observe Aldebaran from Earth, we are seeing it as it was sixty-five years ago). Travelling at its current speed it will take 2 million years to get there.
For you, though, the outer planets are very much on the agenda. Your encounter with Jupiter is over and it is on with the journey and another long haul of empty interplanetary space for around 500 million kilometres. At your current speed, that will take a little over a year. Clearly interplanetary space travel is not for the fainthearted, but to be able to see the beautiful and enigmatic Saturn, the jewel of the Solar System, at first hand is going to make it all worthwhile.