NEARLY 1.4 BILLION kilometres on from Saturn you encounter the first of two more giant planets, Uranus. When I was at school I remember learning that the Solar System was made up of four inner rocky planets, four outer gas giants, and then Pluto. It was only later when my interest in astronomy grew that I discovered the seventh and eighth planets from the Sun are actually ice giants, not gas giants. This does not mean that the planets are simply big balls of ice, but that they differ in terms of their composition. As we saw in the last couple of chapters, Jupiter and Saturn consist almost entirely of hydrogen and helium. In fact well over 90% of their composition is hydrogen. But in the case of both Uranus and Neptune, hydrogen accounts for only about 20% of their makeup. The other 80% comprises heavier volatile elements such as methane and ammonia which have low boiling points.
So the planets we see today are not made of ice, as their collective name suggests, but when they formed over 4 billion years ago they would more than likely have been icy. The chemicals are now in a state described as a ‘supercritical fluid’. In this state, the pressure and temperature of them are well above the levels necessary for phase boundaries to exist, so for example a gas can also behave like a liquid and a liquid can also behave like a solid. Other planets exhibit these conditions too, as we saw with Jupiter earlier. The pressure and temperature at the surface of Venus, too, is well above the critical point of carbon dioxide and nitrogen so the atmosphere at the surface is said to be a supercritical fluid, just like the gas in Uranus and Neptune’s atmospheres.
The discovery of these two planets was a major triumph for science, and it all began on 13 March 1781. In that year, Sir William Herschel was engaged in a series of observations to measure the parallax (apparent shift of position due to the movement of Earth) of fixed stars. While he was observing stars in the constellation Taurus he found an object that he noted as ‘either a nebulous star or perhaps a comet’. He looked for it again on 17 March only to find that it had moved, so he concluded it must be a comet. Following his announcement of a comet discovery, many astronomers across Europe took time to study the new object, but the absence of a nebulous coma or tail soon made it clear that the comet was in fact a planet. Herschel acknowledged this: ‘by the observation of the most eminent astronomers in Europe, it appears that the new star, which I had the honour of pointing out to them in March 1781, is a primary planet in our Solar System’.
Over the following years the movement of Uranus around its orbit was carefully observed and it was found that it seemed to be wandering from its expected path. These perturbations could only be caused by the gravitational influence of another, more distant planet. By applying Newtonian mechanics to the orbit of Uranus, the French mathematician Urbain Le Verrier was able to calculate the possible location of the undiscovered planet, giving astronomers an area of the sky to focus on. The discovery of Neptune in 1846 has since been attributed to both Le Verrier and John Couch Adams, a British mathematician and astronomer who had been working independently of Le Verrier.
For hundreds of years we knew about Mercury, Venus, Mars, Jupiter and Saturn as they were bright and easy to see from Earth. For an equal length of time astrologers have used the movements of the planets to predict the future of people living on Earth. These horoscopes, which are still sadly commonplace, did not of course take account of the impact of Uranus and Neptune – until, that is, they were discovered and all of a sudden they were affecting people’s lives too. Your journey thus far around the Solar System has taught you many things, and one of them is bound to be that human beings are pretty insignificant when it comes to matters of the Universe. Why should all these incredibly beautiful and amazing planets affect us personally? There is no evidence that planets can influence our lives in the way astrologers would have us believe. So next time you read a newspaper and see your horoscope you would do well to avoid it, or at the very least take whatever advice it contains with a big pinch of salt. Certainly at this moment you’d be better off focusing your attention on Uranus as you approach it – it’s what you’ve come all this way for after all.
With an equatorial diameter of 51,120 kilometres, Uranus is the third largest planet in the Solar System, after Jupiter and Saturn. Its average distance from the Sun is 2.8 billion kilometres and it takes a little over eighty-four years to complete one full orbit. What makes this orbit interesting is the manner in which Uranus moves around the Sun. Most planets, the Earth included, orbit the Sun in an almost upright fashion. To quantify that statement, all the planets orbit the Sun roughly in the same plane. You can visualize that by imagining the Sun at the centre of a giant sheet of paper and the planets moving around along that plane. The axis of rotation of most of the planets is almost upright, although as we saw in an earlier chapter, for Earth it is tilted by 23.5 degrees when measured against the plane of the Solar System, or the sheet of paper in the analogy. Most of the planets are near enough upright in the way they rotate, but Uranus is tilted over on to its side so that its axis of rotation is 97.7 degrees. In a sense, it is rolling around the Solar System.
This arrangement gives Uranus a rather strange set of seasons, unlike any other planet in the Solar System. During the times of its equinoxes, when the length of day equals the length of night, the Sun is overhead at the equator giving the familiar experience of day and night, but the solstices – on Earth, the longest and shortest days of the year – are very different. At one solstice, one of the poles points towards the Sun, giving that hemisphere almost forty-two years of constant sunlight – although at this distance the light from the Sun is 400 times weaker than it is on Earth. Following a gradual move through the equinox the other pole is presented to the Sun which then enjoys forty-two years’ daylight while the opposing pole is plunged into darkness for the same duration. Interestingly, however, although the poles receive a greater amount of sunlight, the equator has the higher average temperature. The reason for this is not known, but it must have something to do with the redistribution of heat within the atmosphere.
When Uranus is experiencing the summer solstice in either of its hemispheres the movement of the Sun would be quite alien to any visitor to the surface of any of its five major moons (remember, there is no solid surface on Uranus). All these moons share the same axial tilt as Uranus, which is one of the reasons why it is believed they formed out of a rotating accretion disc of material around the young planet. It is the rotation of objects that makes the Sun follow a path across the sky during the period of a day, but from any of the major moons of Uranus, the Sun would just follow a circular path around the celestial pole, never rising or setting, just gradually moving across the sky as the eighty-four-year-long orbit progresses.
We define the tilt of Uranus with reference to the plane of the Solar System, and using the definition from the International Astronomical Union (IAU) of the north pole as the pole which is ‘above’ the plane of the Solar System. Uranus, like Venus, rotates in the opposite direction to the rest of the planets and is said to have retrograde rotation. Uranus, however, rotates prograde. But quite why it is ‘rolling’ around the Solar System is not known. One theory looks to a period of time during the formation of the planets when Jupiter and Saturn experienced an orbital resonance where Saturn completed two orbits for every one orbit of Jupiter. This configuration had a small but cumulative effect on Uranus which may have been sufficient to affect its formation. Perhaps the most popular theory is a more simplistic explanation: it, like Venus, was struck by an Earth-sized object early in the history of the Solar System, knocking it over on to its side. Even the subtle ring system of Uranus and its family of moons orbit the planet around the equator so they spend half their time above the plane of the Solar System and half their time below it, adding some credence to the impact theory. Unfortunately there is little evidence on Uranus of such a significant impact as there would be on a rocky body. It may be that probing the planet’s interior will help us to understand the mystery, but for now that is what it remains.
As you’ve just left behind the stunning ring system of Saturn, the system of rings around Uranus will seem really quite disappointing. They were first suggested by William Herschel in 1789 but his observations seem to lack support: no one else referred to them again for nearly 200 years. That said, he did describe the Epsilon Ring very well, including its colour, appearance and relative angle to the planet. The first confirmed detection came in March 1977, the rings’ presence inferred when observations were made of the occultation of the star SAO 158687. Occultations are events that occur when one object blocks another from view, so in effect a solar eclipse is an occultation where the Sun is blocked from view by the Moon. In the case of the observation of Uranus, the star was predicted to be blocked from view by the planet, but what surprised astronomers is that it faded from view five times before and five times after it passed behind the disc of Uranus. The conclusion was inescapable: Uranus was surrounded by a system of rings that were too faint to be seen directly. When Voyager 2 visited the Uranian system in 1986 it sent back the first direct images of the rings, showing that there were eleven in total. Then data from the Hubble Space Telescope recorded in 2005 showed two more outer rings, bringing the number to thirteen.
The rings of Uranus, which span around 60,000 kilometres of space, differ quite substantially from those around Saturn in that the ring particles are small and dark, usually just a few fractions of a millimetre to up to a metre across. There is a wonderful measure in astronomy known as the bond albedo which describes the total amount of radiation that arrives at an astronomical object which is returned to space through the scattering of light (whereas ‘albedo’, mentioned earlier, refers only to sunlight). The ring particles of Uranus have a bond albedo of just 2%, which suggests they probably consist of ice rather than rock, although their dark nature means they cannot be pure water ice. They are more likely to be a mixture of ice and a dark as yet unidentified material, perhaps even organic in nature. By studying the way the appearance of the rings changes with different angles of light it is possible to determine more about the nature of these ring particles.
During certain lighting orientations the Epsilon Ring, the one Herschel claimed to have observed, appears quite red in colour, suggesting that there may be amounts of dust within the system. Along with being the brightest of the rings, the Epsilon Ring is also among the thinnest with an estimated thickness of just 150 metres. There are nine inner main rings and two dusty rings a little further away from the planet, including the Gamma Ring, which when observed in back scattered light becomes brighter than the Epsilon Ring. Beyond those two dusty rings are the two outer rings that were discovered by Hubble which differ quite significantly from the inner system in many ways, not least because at 17,000 kilometres and 3,800 kilometres across they are quite wide. When the rings were studied in the near infrared with the large multi-mirror Keck telescope on top of Mauna Kea, the outer ring known as the Mu Ring was not detected but the Nu Ring was picked up. This suggests the Mu Ring is bluer in colour and so more than likely composed almost entirely of microscopic dust, unlike the Nu Ring, which is expected to contain more ice.
Unlike the rings found elsewhere in the Solar System, the rings around Uranus are thought to be quite young, having formed after the planet. The most popular theory on their formation is that the particles were once part of a moon that was shattered by high-velocity impacts during the earlier history of the Solar System and the resulting debris formed the rings we see today. This is supported by observation of the ring particles which are in orbits that are comparable with the orbits of most of the moons, and, just like the moons of Saturn, they act to keep the ring system in shape.
There are twenty-seven moons in orbit around Uranus and they are broken down into three groups: thirteen inner moons, nine irregular moons and five major moons. Titania and Oberon were the first of the major moons to be discovered, by William Herschel in 1787. Over sixty years passed before the next two were spotted, in 1851. They were named Ariel and Umbriel and were found by William Lassell, a beer brewer and astronomer, using one of his home-made reflecting telescopes. It was almost another century after that, in 1948, when Gerard Kuiper discovered the last of the major moons, Miranda. Voyager 2’s fly-by in 1986 revealed ten more moons, and a further moon was identified later, after closer inspection of the Voyager photographs. The remaining moons were all discovered through the Hubble Space Telescope and Earth-based telescopes in the decades that followed. Unusually, all the moons are named not after Greek or Roman gods but after characters from the plays of William Shakespeare and the works of the poet Alexander Pope.
Miranda is the closest of the major moons, and inside its orbit are the thirteen inner moons. Puck and Mab are the outermost of that group, and the latter is the source of the particles making up the Mu Ring. All the inner moons are closely related in some way to the rings of Uranus, be it providing material for the rings like Mab or acting as shepherd moons to one of the rings, just as Cordelia and Ophelia do for the Epsilon Ring. Although the inner moons are small – only two are over 100 kilometres in diameter – they constantly perturb each other in their orbits, causing them to shift over time. As a result of this, collisions between the moons are not uncommon, which is one of the mechanisms that seeds the rings with fresh material.
Miranda, Ariel, Umbriel, Titania and Oberon, whose sizes range from 470 kilometres to around 1,500 kilometres, dominate the Uranian system of moons. It is unclear exactly when they formed but it is likely they either came out of the accretion disc that surrounded Uranus soon after it formed or they are the result of the impact that is thought to have knocked the planet on to its side. All the moons except Miranda are made up of almost equal amounts of rock and carbon dioxide ice mixed with ammonia. Miranda is composed of significant quantities of water ice with amounts of darker silicate rock and shows a surface with evidence of intense geological activity. It is covered in canyons and grooved features that criss-cross the moon which formed as the crust stretched during tectonic movement. Most of this geological activity has been driven by internal tidal heating caused by orbital resonances with other moons. For example, in the early history of the Solar System, Miranda was in a 3:1 orbital resonance with Umbriel, where it completed three orbits for every one of Umbriel’s. After a few million years Miranda lost the resonance with Umbriel but its orbit changed significantly, causing it to become highly eccentric. With an eccentric orbit the forces exerted on Miranda from Uranus varied significantly over time, so it was constantly being stretched and squeezed in different directions, leading to the internal tidal heating.
The subject of internal heating on Uranus is interesting but for very different reasons. The sheer mass of the giant planets tends to be a contributing factor to the amount of heat they generate internally, so that they radiate more heat than they receive from the Sun. In the case of Uranus, the amount of heat it produces is significantly less than the heat it receives from the Sun. Temperatures recorded in its atmosphere are as low as minus 224 degrees, making it the coldest of all the planets. Neptune is similar in size and composition to Uranus yet its heat output is over two and a half times greater.
The reason for the low temperatures on Uranus still eludes us. One possible theory looks to the impact that dislodged Uranus from its upright orientation and in doing so may have liberated much of the heat from the core. Alternatively it may simply be that there is no vertical transfer of heat through the atmosphere. The transfer of heat around a planetary atmosphere is driven by convection, which relies on the variation of density within the material either as a result of compositional differences or based on thermal properties. These variations or gradients can slowly disappear over time which leads to a reduction in the ability to transfer heat around the planet, unless gradients exist in other regions to continue the transfer. In the case of Uranus, it is quite probable that heat transfer is being limited by a process known as double diffuse convection, where different regions of the atmosphere have different density gradients and hence different convective properties, which may actually inhibit the transfer of heat away from the core.
We can tell a lot about the interior structure of Uranus by studying how it moves around the Sun, how the moons move around it, and how it interacts with passing spacecraft. From this information we can infer a lot about the mass and the distribution of matter, and knowing that it has an equatorial diameter of just over 51,000 kilometres we can deduce that its density is 1.27 grams per cubic centimetre. This makes it slightly more dense than Saturn, which has a density of 0.68 grams per cubic centimetre. This information tells us that almost 80% of the total mass of Uranus is made of ice.
Underneath the hazy blue disc is a world separated into three distinct regions: the atmosphere, a mantle of ice and a rocky silicate core. The core is estimated to be just over 10,200 kilometres across, making it a little smaller than the Earth, but the conditions are very unEarthlike: the temperature is of the order of 5,000 degrees and the pressure equivalent to the surface pressure at Earth multiplied by a factor of 8 million. Under these pressures, the material in the mantle which is described as ice is actually a rather exotic form of hot, dense liquid of ammonia and other volatile elements that in many ways act as an ice.
The atmosphere is rich in methane molecules, but with the immense pressures at these low levels the methane molecules are ripped apart into carbon and hydrogen atoms. The carbon atoms crystallize under the extreme conditions into what scientists at the University of California once described as ‘diamond rain’; there may even be an ocean of liquid diamond underneath the mantle of the planet. If you were to travel from that liquid diamond ocean upward then the liquid would slowly turn into gas as you reached higher levels in the atmosphere. Like all of the giant planets, Uranus has no solid surface, so instead, when discussing altitudes in the atmosphere, a datum is taken at the point where the atmospheric pressure equals the pressure at the surface of the Earth – this is how we calculate the nominal diameter of gas planets, which as we know for Uranus is 51,120 kilometres. Above this level the atmosphere extends out by several thousands of kilometres.
The general composition of the atmosphere is mostly molecular hydrogen and helium and quantities of methane which is responsible for the blue/green colouration of the planet. Unlike Jupiter and Saturn, cloud features in the atmosphere of Uranus are few and far between; indeed when the Voyager 2 probe arrived it detected only ten cloud systems over the entire planet. One of the key driving forces in the production of cloud is heat, which causes parcels of gas to move around the atmosphere. The low levels of heat received from the Sun and the low level of internal heating mean that Uranus is cold so clouds are sparse. When observed visually, the planet lacks any spectacular features, even from this close up as you coast past.
When Voyager 2 visited, the southern hemisphere was experiencing summer so was presented towards the Sun; owing to its trajectory, it did not get to study the northern hemisphere. It found the southern hemisphere to have a few large-scale features, such as a bright cap at the south pole and subtle dark bands that circle the equator. Between the polar cap and the dark equatorial band is a bright belt centred at a latitude of minus 47 degrees. It’s the brightest feature on the planet and has been dubbed ‘the collar’. The collar and cap are now believed to be dense regions of methane clouds which are around 30 kilometres below the zero-kilometre altitude level.
Your arrival in the system coincides with the equinox so you can see a different season and how that has affected the features. Not too surprisingly, the southern collar seems to be disappearing and there is a hint of one forming around the northern pole at a latitude of around 50 degrees, so there is evidence that the south polar region is darkening while the northern pole is brightening. This supports the theory that the features are clouds which form as the hemispheres of the planet slowly warm and cool with the small amounts of solar radiation that arrive. Other clouds seem to be forming in the northern hemisphere too as it turns to face the Sun, but there appear to be differences between them and their southern hemisphere counterparts. Clouds in the southern hemisphere seem to be larger and persist for longer while those in the northern hemisphere seem to be smaller and brighter, perhaps because they lie at a higher altitude and are reflecting more of the incoming radiation.
Studying the clouds in the atmosphere that appear at various different altitudes allows us to determine the wind speeds and directions, and profile how they change with altitude. The wind speeds at the poles are zero but increase to nearly 250 metres per second (equivalent to around 900 kilometres per hour) at a latitude of around 60 degrees where they blow in a prograde direction, the opposite direction as the rotation of the planet. Moving further towards the equator, the winds decrease in speed again to a point at around 20 degrees latitude where once again they are absent because it is at this latitude that the temperatures are at their lowest. The wind direction around the equator is reversed, or retrograde, and blows in the same direction to the rotation of the planet with speeds in excess of 100 metres per second.
The wind speeds on Uranus seem high by Earthly standards but they are nothing compared to those found on the final major planet on the Kaldi’s journey, Neptune. Getting from Uranus to Neptune may in your mind seem like something of a hop, but in reality it will take you another three years to traverse the distance between these two outermost planets.
At its closest approach to Earth, Neptune is 4.3 billion kilometres away. It is incredibly difficult to visualize the true vastness of the Solar System, but one way to do it is to scale the distances down to more manageable numbers. If we say that 1 million kilometres in the Solar System equals 2.8 kilometres on Earth, and place the Sun in central London, then on this scale the nearest planet from the Sun, Mercury, would be about 163 kilometres away in Birmingham and Venus would be in Plymouth. Earth, which is usually about 150 million kilometres from the Sun, would be residing on the Isle of Man and the red planet Mars would be in Prague, no doubt having a highly cultural experience. The distances between the planets then start to become quite extreme. Jupiter is 2,200 kilometres away in Egypt, and the ringed planet Saturn is in Iraq. Uranus, the planet you have just visited, would be over in the Far East, in China, and Neptune, the final planet on your journey, would be on the other side of the world, in New Zealand.
Space travel is not for the impatient.
For the entire duration of this leg of the journey you will be able to see Neptune clearly up ahead as a bright blue disc against the velvety blackness of space. Its blue hue is subtly different to the more aquamarine appearance of Uranus, both, as we have seen, the result of the chemical composition of the planets.
Neptune orbits the Sun at an average distance of 4.5 billion kilometres and takes 164 years to complete one orbit. Despite the fact that it is an ice giant like Uranus and about four times the diameter of Earth there are similarities with our home planet. Neptune rotates once on its axis every sixteen hours and six minutes so, because this is only about eight hours shorter than Earth’s own rotation, a day on Neptune is similar to a day on Earth. The axis of rotation is similar too: Neptune’s axis is tilted with respect to the plane of the ecliptic by 28 degrees while the axis of Earth is just over 23 degrees. This means that Neptune has seasons much like those on Earth, although owing to the much longer orbital period, the seasons last for forty-one years.
As is the case with Uranus, the formation of Neptune is still a subject of scientific debate. One problem with the standard theory, which posits that the planets condensed out of the accretion disc in their current orbits, is the presence of minor bodies in the vicinity of Neptune: the formation of a major planet like Neptune in this location would surely have also swept up these objects. It is also believed that at these distances the disc out of which the planets formed would not have had a sufficient density of matter. Instead it is likely that the two ice giants formed closer to the Sun where the matter density was higher and then migrated out to their existing positions after the accretion disc cleared – a theory known as the Nice (pronounced ‘Nees’) Model.
Neptune does resemble Uranus, but you will notice some prominent features as you draw closer, particularly the Great Dark Spot, which is reminiscent of the Great Red Spot on Jupiter. It too is a raging anticyclonic storm but somewhat smaller, measuring just 13,000 kilometres by 6,600 kilometres – though that’s still big enough for one Earth to fit across its widest axis. The storm was discovered back in 1989 by Voyager 2 when it visited the Neptunian system; but when the Hubble Space Telescope turned its gaze on the planet in 1994, the storm had vanished. Instead, the HST spotted another storm but this time in the planet’s northern hemisphere, so the term ‘Great Dark Spot’ is now more generally used to describe the dark spots seen in the atmosphere of Neptune rather than any specific one. It is around the first spot, known as GDS-89, that the fastest wind speeds in the Solar System have been recorded, at a staggering 2,400 kilometres per hour – a little higher than the maximum cruising speed of Concorde.
The spots seem to lack any features or clouds, suggesting they may be vortex-like structures and that what we are seeing is the top of a hole through the methane clouds that allows us to peer deeper into the atmosphere, perhaps even as far down as the troposphere. White clouds of methane ice crystals similar to Earth’s cirrus clouds seem to form around the periphery of the spots. The methane clouds appear to last longer than the spots themselves, and this persistence means we can pinpoint where previous spots might have existed before the holes either closed up or became obscured. The HST observation showing that GDS-89 had disappeared suggests that in contrast to the Great Red Spot on Jupiter, which has been visible for over 400 years, the spots on Neptune are short-lived, lasting for just a few months or years. It may be that they simply dissipate as they near the equator, or perhaps some other as yet unknown process leads to their demise.
The clouds and wind speeds seen on Neptune are due in part to internal heating. Neptune receives less than half the amount of energy from the Sun that Uranus receives and it is 1.6 billion kilometres further away, yet their temperatures are comparable. At the zero altitude level measured where the atmospheric pressure is 1 bar, the temperature is minus 201 degrees compared to minus 197 degrees at the same level on Uranus. The only explanation for this is that Neptune must be producing and distributing heat internally, with current estimates of 2.6 times the amount of energy received from the Sun. The origin of this internal heat is unknown, particularly as Uranus is so similar to Neptune in many ways yet produces significantly less heat. The most popular theory is that the heat is simply left over from the formation of the planet, but as we have seen this does seem to be contradictory to the observations of Uranus.
All weather systems are driven by heat, be it external (from the Sun) or internal. The source of heat from inside Neptune is generally considered to be constant year on year, although there will be an almost imperceptibly slow decrease as the planet cools. The amount of heat certain areas receive from the Sun, however, will alter with the changing presentation of the planet to the Sun. One of the ways we perceive this is in the changing seasons. Evidence of seasonal changes and large-scale movements of atmospheric gases have been detected on Neptune with concentrations of methane and ethane that are up to 100 times higher around the equator, suggesting there is a general upward movement of gas around the equator and subsidence of gas around the poles. The tropospheric temperatures at the south pole also hint at seasonal changes, with temperatures a few degrees warmer there than elsewhere on the planet. During these southern hemisphere summers the warmer temperatures are sufficient to turn frozen methane into a gas which escapes into space. This causes the southern pole to appear a little brighter than the surrounding regions, but as the Neptunian year progresses and the north pole starts to present itself to the Sun, then it will brighten as it starts to warm. We saw an identical process on Uranus which resulted in darker and lighter polar regions dependent on the seasons.
The similarities with Uranus don’t end there. Neptune’s atmosphere is made up almost entirely of hydrogen and helium with traces of methane which, owing to the way this greenhouse gas absorbs red light, gives the planet its striking blue colour. There are four regions to the Neptunian atmosphere: the troposphere, the stratosphere, the thermosphere and the exosphere. The clouds we see tend to occur at different altitudes within the troposphere and their altitude will determine the nature of the clouds. In the upper regions the pressures are sufficiently low for methane clouds to form, and as the pressure increases at lower levels clouds of hydrogen sulphide and ammonia are found. In the lower levels, the pressure increases to around five times the pressure at the surface of the Earth; the clouds here are made of ammonia sulphide and water. As the pressure increases further in the lowest reaches, there are dense clouds of hydrogen sulphide.
The atmosphere itself accounts for about 10% of the overall mass of the planet; the core and mantle comprise the other 90%. The amount of material in the mantle is equivalent to about fifteen times the material that makes up the Earth, but instead of being a rocky lump it is a hot, super-dense liquid which is made up of ammonia and water. The water is not in a state we are familiar with in our oceans though, as the conditions cause the hydrogen and oxygen to dissociate into hydrogen and oxygen ions, making the liquid highly conductive. At lower levels in the mantle the high pressure causes the methane to separate into hydrogen and carbon, leading to the creation of diamond hail – a process very similar to that found deep in the layers of Uranus. It is also hypothesized that even higher pressures further into the mantle lead to the formation of a diamond ocean, with diamond bergs floating around. There is little direct evidence to support this so for now it remains a wonderfully romantic theory, but if these entities do exist they could well be exactly like the diamonds we know and covet on Earth.
Deep under the mantle is the core of Neptune. It is a silicate rock with a mixture of iron and nickel and a total mass about 1.2 times that of the Earth, and of comparable size. At this depth, buried beneath thousands of kilometres of liquid and gas, the pressure reaches a crushing 7 million times that on the surface of the Earth, and temperatures exceed 5,000 degrees – almost as hot as the visible surface of the Sun.
Also surrounding Neptune is a ring system somewhat reminiscent of the one around Uranus. There are just three main rings in this system: the inner Galle Ring, the Le Verrier Ring and the outermost Adams Ring. They span a 21,000-kilometre-wide region of space around Neptune and are composed of thousands of ice particles that are covered in carbon material, giving them a dull red colour. It was once thought, as a result of observations of stellar occultations, that the rings had gaps in them: the stars flickered in and out of view as the planet approached but not once Neptune had passed. When Voyager 2 visited the system it revealed that instead of there being gaps in the rings, they simply had a rather clumpy structure. The outermost ring, the Adams Ring, is a great example of this with five confirmed arcs of higher density. It is thought that this clumpy nature is a direct result of the gravitational influence of the tiny moons that orbit within the ring system. In the case of the arcs in the Adams Ring, the nearby moon Galatea is responsible. The constant tug and disruption from the moons seems to be causing changes in the rings over geologically short timescales, and it is likely that within a few centuries some of the ring features we can see today may well vanish.
In addition to Galatea there are thirteen other moons in orbit around Neptune, and they fall into two categories. Nearest to the planet are the seven regular moons which orbit in the same direction in which Neptune rotates on its axis, and all are within its equatorial plane. This is in contrast to the irregular moons which tend to orbit further away in a retrograde or backward fashion and with orbits inclined to the equatorial plane with a highly elliptical shape. There is one exception to this broad categorization of the moons, Triton. It should be a member of the outer irregular group because of the direction of its orbit but, unlike all the other irregular moons, it lies close to Neptune.
Triton was discovered just seventeen days after Neptune by British astronomer and brewer William Lassell. It is by far the largest of the Neptunian moons and constitutes 99% of the mass of the material in orbit around the planet, and with a diameter of 2,700 kilometres it is the seventh largest moon in the Solar System. This makes it large enough to have evolved with a broadly spherical shape. It is not unusual for moons to orbit in a retrograde direction around the planet – three other moons orbit Neptune in the same direction. There are other moons around Jupiter, Saturn and Uranus which have retrograde orbits, but they are all much more distant from their planets. Triton is unusual because it orbits at a distance of just 354,000 kilometres.
It is not possible for any moon that orbits a planet in a retrograde motion to have formed out of the same part of the nebula that the planet formed. Taking into account its composition and retrograde orbit, it is thought Triton may be a captured Kuiper Belt object. Since its possible capture from the belt, the orbit of Triton has become almost perfectly circular. One force that is often responsible for the circularization of an elliptical orbit is the tidal force, but in the case of Triton that is unlikely to have provided sufficient drag to affect the orbit that much. It is more likely that drag from the debris disc surrounding Neptune slowed Triton sufficiently to make its orbit circular. Tidal interaction still plays a part in the evolution of Triton’s orbit as it constantly tugs on the moon, slowing it further. Over time, perhaps even within the next 4 billion years – about the same amount of time that has elapsed since the Solar System formed – Triton will get so close to Neptune that it is likely to be destroyed by tidal forces and form a new ring system.
It takes Triton 5.8 days to complete one orbit of Neptune but it also takes 5.8 days to complete one rotation on its axis, so in the same way that our Moon keeps one face pointing towards Earth, so Triton keeps one face pointing towards Neptune. This is known as synchronous rotation and is found in many moon–planet relationships. In fact, take a quick trip down to Triton’s surface in the science-busting RSU, because from this vantage point things are going to appear quite strange. From the surface of Earth we are used to seeing things rise in the east and set in the west, but because of the synchronous rotation, Neptune is hanging motionless in the sky above you. It looks big too, doesn’t it? It spans an area of sky about 8 degrees across, which is sixteen times the apparent size of the full Moon back on Earth. The axis about which Triton rotates is currently at a 40-degree angle to the plane of the orbit of Neptune, so as the pair of them orbit the Sun, the poles of Triton point alternately at the Sun. The changing orientation of the moon to the Sun means that it experiences seasonal changes, and each of those seasons last for about forty years.
Perhaps the most interesting aspects of Triton are its atmosphere and some of the features you can see on the surface. That surface is composed of frozen water, nitrogen and carbon dioxide; measurements of its density suggest the entire body is 45% ice with the remaining 55% comprising rock, giving it a composition somewhat similar to Pluto. The high quantity of ice on Triton’s surface means that it is highly reflective. In fact it reflects 80% more light than our own Moon. This is one of the contributory factors in its discovery: a lower reflectivity would mean it was a lot less visible. On the surface there are many different types of feature visible, from ridges and troughs to icy plains and plateaus. What is noticeable by their absence are craters, which seem to be a common feature in the Solar System. This indicates the surface of Triton is geologically young – anything from 5 million to 50 million years old.
One of the more obvious things you’ll spot are the dark streaks that were first discovered by Voyager. They were quickly identified as being connected to geyser-like eruptions of nitrogen gas; the streaks were sub-surface dust caught up in the eruptions. The geysers tend to be more active at points on the surface where the Sun lies directly overhead, suggesting that solar heating, however weak, has something to do with their origin. One theory suggests that solar radiation penetrates the thin icy crust, warming the rocky surface below. Pressure below the ice builds until it reaches a critical level, leading to the geyser-like eruptions that can reach heights of up to 8 kilometres. Many of the eruptions are thought to last for anything up to one Earth year, throwing up sufficient dust to create streaks that stretch downwind for anything up to 200 kilometres. The icy nature of these events has led to them being dubbed cryovolcanoes, although they differ from other cryovolcanoes which are driven by internal heat rather than heat from the Sun.
The discovery of the streaks also tells us that Triton has an atmosphere, because if there is wind, there must be an atmosphere. It is not dense like the atmospheres on some of the moons in the outer Solar System – its tenuous nature is one of the reasons the cryovolcanic eruptions can reach altitudes of 8 kilometres. We can tell a lot about the atmosphere of moons and indeed planets by observing the way light is extinguished as stars pass behind them. The light from any object that passes behind our own Moon, for example, extinguishes instantly rather than fades and flickers out of view. This tells us that there is no appreciable atmosphere surrounding the Moon. Observations of a star that passed behind Triton showed that its atmosphere was denser than that recorded by Voyager some eight years earlier, and the surface temperatures too were a little warmer, by no more than 5%. These important observations told us at the time that its average temperature was on the increase, perhaps as it heads towards a warmer summer.
Nitrogen is the main component in the atmosphere, with small amounts of methane and carbon monoxide. The majority of the nitrogen has come from the gradual sublimation of surface ice. From the surface level, the temperature slowly decreases from about minus 237 degrees with increasing altitude to the tropopause at a height of around 8 kilometres. Above our own tropopause, and indeed on many other objects in the Solar System, is a stratosphere where temperatures start to increase with height rather than decrease and gases are separated into strata dependent on their temperature. Triton has no stratosphere; instead, the tropopause changes to a thermosphere whose characteristics are like the stratosphere’s, with an increase in temperature with altitude as a result of ionization from the incoming ultraviolet radiation. It differs from the stratosphere due to the way gases are separated into different strata based not on their temperature but on their molecular mass (determined by adding the mass of each of the atoms and multiplying by the number of atoms in the molecule). The thermosphere reaches altitudes of around 950 kilometres, and above that is the exosphere. An exosphere is the outermost layer of any atmosphere, marking the boundary with space. Its molecules are still gravitationally bound to the planet but do not tend to interact with each other in the way most gases do.
Triton is by far the most interesting of the moons, which is why you paid it a short visit, but there are thirteen others in orbit around the planet which deserve a little attention. In the same category as Triton are Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe. Nereid orbits Neptune in a prograde direction but its orbit is highly elliptical, taking it from 1.4 million kilometres to the furthest point in its orbit at 9.7 million kilometres. Its eccentricity is high. We say that 0 is a perfectly circular orbit and 1 is known as a parabolic escape orbit, which in effect means that an object with an eccentricity of 1 would escape the system. Nereid’s eccentricity has a value of 0.75, which leads to the conclusion that it was either an object captured from the Kuiper Belt, like Triton, or was previously a member of the inner regular moons. If it was the latter then it is likely that the appearance and subsequent gravitational interaction of Triton could very easily have disrupted its orbit, turning it into the one we see today.
The regular moons are closer to Neptune than the irregular moons, and in order of distance from the planet they are Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N1 and Proteus. The smallest of them all is S/2004 N1, which is around 15 kilometres in diameter. Contrary to the suggestion in its name, it was discovered in 2013; the ‘2004’ comes from the fact that images from the Hubble Space Telescope taken in 2004 showed the moon, so that was the year in which it was first recorded. All of these moons are in some way related to the ring system. We have already seen that Galatea is responsible for the features in the Adams Ring; Naiad and Thalassa orbit between the Galle and Le Verrier Rings, while Despina is a shepherd moon for the Le Verrier Ring only. All of the inner moons are likely to have formed after a chaotic and disruptive period caused by the capture of Triton. Its immense mass would have sent these inner moons into turmoil, ejecting some and more than likely causing the orbits of the rest to vary wildly, leading to collisions and their eventual destruction. Over time, as Triton settled into its present orbit, the system calmed down and the debris that was left closer to Neptune would have slowly coalesced under the helpful force of gravity, thus forming the small inner system of moons and rings you can now see.
There are a couple of other objects in the Neptunian system which are best described as companions rather than satellites. They are Neptune’s trojans and can be found at the L4 Lagrangian point (we looked at these earlier and saw how they are places where gravitational forces in a three-body system balance). The L4 point lies 60 degrees ahead of Neptune and it is here that six trojans are found, with a further three at the L5 point, 60 degrees behind Neptune. All of them must by definition orbit the Sun over the same period of time as Neptune and follow broadly the same orbit. The discovery of the third trojan, known as 2005 TN53, was significant because its orbit was found to be tilted with respect to Neptune’s at an angle of about 25 degrees. This tells us that there is a high likelihood of a greater number of trojans at this point, almost like a swarm of flies.
Your path through the Neptunian system does not take you near these trojans. Unfortunately, their dim red appearance makes them almost impossible to pick up against the blackness of space without any form of optical aid. The route has taken you close by Triton, however, the largest moon of Neptune, skimming over the top of the ring system, and on a close fly-by of Neptune itself. Neptune then gives you one final boost of acceleration, taking your speed to a little over 17 kilometres per second. At that speed you could fly around the Earth in just thirty-eight minutes.
Not only does your final planetary fly-by increase the Kaldi’s speed, it also adjusts its trajectory and allows you to set the course for the ship’s ultimate destination. Not Pluto, not Proxima Centauri (the nearest star system to our own), but to a star in the constellation of Libra known as Gliese 581 at a distance of twenty light years. It of course takes twenty years for light to reach you from that system, and at the Kaldi’s current speed it will take about 352,000 years to get there. However, it is on this leg of the journey that the ion engine will be beneficial. Firing it over a long period of time will produce a gradual increase in velocity and maybe get the ship up to around 25 kilometres per second, which will reduce the journey time to a mere 239,000 years. Obviously the vast distances involved would make the journey something of a suicide mission for any human inhabitants on board the ship, so fortunately once you reach interstellar space the RSU will allow you to return home once more to the comforting familiarity of planet Earth, leaving the Kaldi to complete the mission unmanned. There are alternative methods that could vastly reduce the journey time, such as nuclear pulse propulsion, where a series of nuclear explosions propel a spacecraft forward at a maximum speed of about 1,000 kilometres per second. This would cut the journey time to Gliese 581 to just six years, but at the moment it is only an idea.
Gliese 581 is a fairly normal red star about a third of the mass of the Sun, but it has been chosen because it’s surrounded by a family of three planets, one of which, Gliese 581c, is believed to be 21 million kilometres from the star itself. It created excitement when it was discovered in 2010 because it orbits the star in the habitable zone, which is the region within which conditions could be just right for liquid water to exist on the surface, and maybe even right for life to evolve.
But before that, and before you become the first human being to enter interstellar space, we must explore the outermost regions of the Solar System.