We’re only here because of Jupiter. That statement is true for a couple of reasons, but it’s no exaggeration.
In spite of appearance and activity, I am of course a herbalist of twenty-three years standing, and you don’t get to be a herbalist without treating the liver. This is absolutely not homeedandherbs, which is effectively moribund, but Jupiter is our liver as a star system. The liver has many functions, but one important one is to detoxify and store toxins. Liver herbs are also associated with Jupiter in the melothesic system. Jupiter the planet performs a similar rôle in absorbing and “detoxifying” asteroids and comets which would otherwise pelt the inner planets, by attracting them to it and often literally absorbing them.
In July 1994 CE, Comet Shoemaker-Levy 9 impacted Jupiter, leaving temporary “bruises” like this one. Jupiter is in any case a big target, making it prima facie more than thirteen hundred times as likely to be hit by débris than Earth even leaving aside its greater gravity and larger Hill Sphere. Considering that, Jupiter becomes more than six thousand times as likely a target.
Jupiter is also responsible for Earth’s formation in the first place. Jupiter’s year lasts 11.86 times as long as ours. This means there’s a potential orbit just within ours whose objects orbit the Sun once every 361 days. This is so close to ours that Earth actually dips into it for a short time each December. Jupiter cleared that orbit of dust and rocks 4 600 million years ago, just marginally to one side because its orbit was exactly twelve times as long and almost every dozen years the protoplanets there were slightly tugged by its gravity. This led to a crowded ring of matter which was to become Earth. Hence in a second sense we are only here because of Jupiter. There isn’t anything special about Earth in that respect either, although the orbits of the other planets don’t work out as exactly. Uranus is close though – its year is close to seven times as long as Jupiter’s. I haven’t checked this out but presume that the ratios are something like 2 in 7 or something less obvious. Earth is the largest terrestrial planet though, and has the most straightforward ratio. It should also be borne in mind that Earth’s gravitational pull may have done the same thing and that the orbits of some of the planets may not be fixed. I haven’t worked all of this out yet.
I may have quoted this too many times, but the Solar System has been described as consisting of “the Sun, Jupiter and assorted débris”. This is a little misleading as Jupiter only has a mass of about a thousandth that of the Sun and all the rest of the matter in the system taken together has a mass of forty percent of Jupiter’s, which is not negligible. In terms of size, there’s a star, a planet about a tenth of the star’s diameter, and the rest. As mentioned yesterday, the Jovian moon system is like a mini-star system in itself and the magnetosphere reaches out almost to Saturn, making it bigger than the entire inner Solar System. From here, Jupiter is usually just slightly too small to make out its disc, but is easy to spot as a very bright star in the night sky, which can sometimes cast visible shadows. Venus is the only other body orbiting the Sun alone which can do that.
Jupiter gives the impression of turbulent and frantic behaviour, like a boiling pot of multicoloured paint. Olaf Stapledon compared it to streaky bacon, although that doesn’t do justice to the colours. Probably in the absence of the opportunity to find out much else about it, Jupiter’s stripes have been meticulously labelled, thus:
The general shape of the planet can be seen in this diagram: Jupiter is notably flattened at the poles and bulges at the edges. This is also true of Earth but in our case the planet is more or less rigid except for the atmosphere and ocean, so it’s only three permille wider at the Equator than at the poles, something I discussed in ‘For The High Jump‘. Jupiter, being largely fluid, is six percent wider at the equator, which is two-thirds the diameter of Earth itself. Saturn is even more squashed. It’s like someone sat on it. Geddit?
Being fluid, the planet doesn’t rotate as a single object but consists of System I, System II and System III, meaning it doesn’t have a single fixed day. In fact no planet has but that’s another story to do with fixed stars versus the Sun. System I is the equatorial rotation up to 9° latitude, System II the polar, actually everything further from the equator than that and is five minutes slower and System III the rotation of the extremely active radio signals from the planet. Additionally there’s the Great Equatorial Current, which is faster at nine hours, fifty minutes and 34.6 seconds, according to an estimate made in 1897. This is over twelve kilometres a second, compared to Earth’s equatorial velocity of 463 metres per second. This is the kind of frenetic and torrid environment Jupiter is. The whole planet takes a bit under ten hours to rotate. It also does so practically upright. There are no seasons. The Jovian year lasts 94 425 days according to the equatorial current rotation, but this is not a definitive figure because Jupiter doesn’t have one definitive day. This differential rotation also means there’s a lot of turbulence in the atmosphere between different latitudes, because they’re rotating at different velocities.
The problem of conveying longitude encountered with the Sun, that of attempting to find a fixed point on an essentially fluid surface, is also present here. No less than six systems exist for doing this. They’re significant because of comparing observations made by the various different space probes sent there since 1973. There is a second similar problem with Jupiter: where’s the surface? This and the other issue are characteristic of gas giants. The problem here is that you might say Earth’s surface is the bit we stand on, especially if we’re Jesus, but on Jupiter there just is nothing to stand on and although at some point there is liquid and solid in the interior, conditions there are so extreme that there’s about as much point considering it the surface as the core of the Sun. Most people go for the visible cloud tops, but sometimes you can see further down into the atmosphere than that.
The belts are dark, the zones light. Zone is actually the Greek word for “belt”. There are diverse variations within the belts and zones, but before I get there I should mention the elephant in the room, the Great Red Spot. This is a large oval 22° south of the equator, varying in width and drifts westward, which is a pity as if it didn’t it could be used as a marker for longitude. It also oscillates north and south by around 1 800 kilometres over a cycle of almost ninety days. First observed in 1664 by Robert Hooke, famous for his microscopy, the GRS may or may not be a persistent feature. It actually isn’t permanent. For instance, it disappeared completely in about 1980. It also fluctuates in size somewhat, but has recently been 16 350 kilometres east-west. Its nature and the reason for its colour are still unknown. The earliest idea was that it was a giant active volcano, which was at the time when Jupiter was thought to be largely a solid body. I don’t understand why they thought it was, though, because its density is easy to measure given the movements of the Galilean satellites and it clearly was not a massive lump of rock.
After the volcano theory was rejected, it was suggested that it was a large ovoid object floating in the atmosphere and bobbing up and down, because it appears to change in colour and size. Better resolution and lenses seem to have led to the realisation that it was some kind of anticyclone, being in the southern hemisphere, but this isn’t really an explanation because its persistence, size, location and colour are all puzzling. Two suggestions are that it’s a Taylor Column and a Soliton (‘Star Trek’ fans may have heard of that). A Taylor Column occurs when a rotating fluid meets an obstruction. Drag then forms a cylindrical structure. This would clearly require some kind of body floating in the atmosphere, or possibly in the liquid below it, and moreover an extremely large one considering the enormous strength of the Jovian gravitational field. A soliton is a wave packet which stays bunched together as it moves and is able to collide with other waves without losing its form. It’s hypothetically possible that solitons made of gravity waves (or possibly gravitational waves) could be used to achieve warp drives, but this isn’t relevant to the Great Red Spot, which is a fluid phenomenon, although I imagine that’s why it cropped up in ‘Star Trek’ (TNG – ‘New Ground’). It was first knowingly observed in a Scottish canal in 1834. They’re a bit like sonic booms. Solitons are generated in front of fast moving vessels in canals or rivers because of the horizontal and vertical restriction in the water. They’re like wakes moving ahead of an object instead of behind it because they have nowhere else to go. Once again, the idea of the flow being restricted is a little strange because it suggests the presence of solid obstructions, but maybe it’s more to do with the currents or turbulence being particularly markèd at those points.
There is another fascinating and mysterious aspect to the Great Red Spot which I don’t think has ever been explained. It occurs at the same latitude as several other phenomena on other planets. Olympus Mons on Mars seems to be caused by a hot spot in the planet’s mantle and is 20° north of the equator, and Hawaiʻi, caused by a similar hot spot, is also 20° north of the Equator, although in the latter case this is obscured by the movement of the Pacific Plate. Mars also seems to have drifted because the possible remnants of former moons which impacted its surface are no longer at its equator, which they should be given its current moons’ locations. Also, both of these phenomena are north of the equators rather than south of it. I’ve seen a diagram attempting to explain this by inscribing a tetrahedron one of whose vertices was at a pole, but I don’t know how relevant that is. Neptune has also had a dark spot 23° north of its equator but this may not be the Great Dark Spot as discovered by Voyager. It’s difficult to know if this is cherry-picking.
The other mystery about the GRS is its colour, which varies. Nobody knows what causes this. I find that somewhat surprising because I’d expect its spectrum to reveal its composition, but apparently it doesn’t. One suggestion is that it’s due to tholins generated by the action of solar ultraviolet light on acetylene and ammonium hydrosulphide. Another factor may be the greater altitude of the area. It is of course something like twice the size of Earth’s surface area. I don’t know if anyone has tried to correlate its changes with the activity of the Sun. It’s also colder than its surroundings, which is to be expected considering it’s higher up. It’s also extremely noisy, to the extent that as the sound from it travels up into the upper atmosphere it gets converted to heat and the region above the spot is 1 330°C. This may not be as spectacular as it sounds though, because temperature and heat are different. Low Earth Orbit, for example, technically has a very high temperature but it’s still freezing up there in the shadows.
There are other more transient spots, probably hurricanes, in the atmosphere, but weather systems in general are much longer-lasting in Jupiter’s atmosphere than ours because there’s no friction from a solid surface and also little variation due to the absence of land and liquid regions. Also, because the planet is so much bigger, so are the storms and other winds. Hurricanes often last decades. This raises the question of what weather would be like on a water world. If the figures relating to Jupiter’s axial tilt and surface are fed into a climate model for Earth, the result is a banded arrangement with persistent hurricanes, suggesting that conditions on such a planet, which might otherwise be habitable, could be quite hostile, and the weather conditions at particular latitudes would effectively constitute the climate because they’d be so stable.
Hydrogen and helium make up the bulk of the planet’s atmosphere, and therefore also the bulk of the planet itself, in similar proportions to the Universe in general and also the Sun. It managed to hang on to them because it’s colder on the outside and has such a high escape velocity. In 1939, the South Temperate Zone suffered a disturbance leading to the formation of a single white oval from four merging predecessors in 2000, which started to turn red in 2005. This time scale gives a good indication of how stable the weather is there. It’s also fascinating how Jupiter’s sheer size gives it a known history stretching back into Stuart times, which isn’t true of other planets except for Cynthia and Earth. Features on Mars were well-recognised but the occurrence of storms wasn’t observed until the nineteenth century, and Venus is just blank. This also underlines how dynamic the planet is compared to most others. The Jovian troposphere is somewhat like ours in terms of physical structure, with a falling temperature and pressure with height extending through clouds and leading up to a reversal and gradual increase of temperature marking the lower boundary of the stratosphere, then a mesosphere and thermosphere where the temperature is technically very high, but the chemical composition of the atmosphere is very different. It’s 90% hydrogen, 4.5% helium and has a significant amount of deuterium in it, though well under one percent. This compares to the one in six thousand atoms of hydrogen in Earth’s water. Deuterium also shows up in the compounds replacing the more abundant hydrogen. Methane, ammonia, water vapour, acetylene and phosphine are all present, as is carbon monoxide, but the really surprising constituent, though only present at less than one part per million, is the rather strangely named germane. Germane is like methane but has germanium instead of carbon in its molecules. Like many of the constituents of the Jovian atmosphere, germane would spontaneously ignite in our own. I don’t understand why there’s germane there. Germanium is not a particularly common element and its silicon analogue silane might be expected to be more widespread but it isn’t there. Germane is also denser than methane or silane, so its presence in detectable layers of the atmosphere is peculiar. I don’t think it’s found on any other planet. Incidentally, the presence of phosphine may not be a clue for life existing on Jupiter because the planet’s chemistry is not like that of Venus and conditions are very different. Here, it’s probably formed under high pressure much further down and churned up by convection currents. Methane is no surprise, and the carbon monoxide is probably the result of oxygen being relatively scarce in the original part of the solar nebula from which the planet formed.
You know that bit in ‘Fly Me To The Moon’? “Let me see what spring is like on Jupiter and Mars”? Well, whereas there are seasons on Mars, there are none on Jupiter, so it ain’t gonna happen. This is because Jupiter’s axial tilt is only 3°, so it basically has no seasons, although the butterfly effect might come into play. I suspect this is for two reasons. Firstly, Jupiter is the original planet in this system, so it probably determined the positions of the orbits, and secondly it’s so massive nothing could knock it off-kilter, so it ends up with a tiny tilt. In fact it’s surprising it tilts at all. If it did have seasons, each would last almost three years. Some people draw a link between the traditional Chinese cycle of twelve animals and the Western Zodiac because the planet spents around a year in each sign from our perspective. It should be pointed out that the strict 30° division of the ecliptic used in Western astrology doesn’t correspond to the actual portions of the zodiacal constellations in the ecliptic, and as is practically common knowledge nowadays, Ophiuchus is also in the circle and is ignored for astrological purposes. In the astronomical zodiac, Jupiter is currently in Aquarius but I don’t know how closely this corresponds to the astrological ephemeris, and it’s about to be the Year Of The Tiger. The orbit around the Sun is thrice more eccentric than ours at almost five percent, so there is a little variation in how much radiation and therefore heat Jupiter gets from the Sun.
However, Jupiter actually generates twice as much radiation as it receives, so there’s another reason it has no seasons: it’s actually warm itself, or in fact hot. This is because it’s still hot from the formation of the Solar System, since it has more than a thousand times the volume of Earth but only about 130 times our surface area, and possibly because it’s still contracting, although the contraction may be caused by the cooling rather than the other way round. At the core, the temperature is 20 000 K or higher, more than three times as hot as the Sun’s photosphere and almost as hot as solar flares, with an internal pressure of forty-five million times that of our atmosphere. There are two rival theories about the centre of the planet. One holds that there is no core in the sense of a solid rocky globe, but the planet just gets denser and denser towards the centre, and the other, more popular theory posits the existence of an Earth-sized rocky core. Somewhat away from the centre is a deep layer of liquid metallic hydrogen. Under very high pressures, various gases, such as oxygen and xenon, become metals. This may constitute up to 80% of Jupiter’s radius, and is responsible for generating the enormous magnetic field. The pressure here is a “mere” three million atmospheres and the temperature 11 000 K, so it’s still hotter than the Sun’s surface. Above this layer is molecular liquid hydrogen, twenty-five thousand kilometres below the clouds. The temperature finally drops below that of the Sun three thousand kilometres below the “surface”, where the pressure is ninety kilobars. A thousand kilometres down, the hydrogen becomes gaseous and the temperature is only around 2 000 K, then it falls to -143°C at the cloud tops. The magnetic field generated by the metallic hydrogen is about ten times the strength of Earth’s at this level, but it’s at an angle of almost 11° to the axis of rotation. All of this pressure stuff is exacerbated by the fact that Jupiter’s gravity is over two and a half times ours.
Jupiter has jet streams like Earth’s, but because of the coloured clouds, the white ones being mainly frozen ammonia, they’re more vividly colour-coded than ours. A jet stream is a relatively narrow, fast, horizontally undulating air current moving east to west and drifting north and south assuming the planet spins prograde. They’re formed by the Sun heating the atmosphere. There are four such streams on Earth, two subtropical and two polar. On Jupiter they’re driven by internal heating. Moving through the latitudes, there are alternating regions of faster and slower east-west winds, each of which is a jet stream, even though models show fewer jet streams on larger planets. Each stream is also “rolling”, in that it is a kind of horizontal whirlwind separated from its neighbours north and south.
Zones have more ammonia than belts, hence their paler appearance – they’re cloudier. The belt clouds are lower and thinner, and belts are warmer than zones. This makes sense if you think of ammonia condensing or freezing out of the atmosphere. I get the impression on looking at pictures of Jupiter that the belts look lower and possibly have shadows cast upon them by the clouds in the zones. Air seems to be warmed and rises in the zones, causing clouds to form as it expands and cools. In the belts, it sinks, becoming warmer and losing its clouds. The air flow generally tends to “stay in lane”. It doesn’t deviate in latitude much except within its belt or zone. At the poles, there are large circles in which not much seems to be happening. These caps can extend further towards the equator or less so, and the northernmost two bands after the north polar region can become incorporated into them temporarily. This extends to the NNTB (North North Temperate Belt), which can fade entirely, as it did in 1924. Consequently the NTZ varies in width. South of that, the NTB often has dark spots on its southern edge. The North Tropical Zone, NTrZ, is where the System I movement of the atmosphere comes uncoupled from the more polar System II. This leads me to ponder whether the planet consists of a series of nested hollow cylinders, such that the temperate regions north and south are in fact continuous but hidden under the more equatorial regions. They wouldn’t be homogenous in properties of course because the conditions deeper in the atmosphere are bound to be very different. Also, the liquid hydrogen ocean is not that far beneath the cloud tops.
The largest region on Jupiter is the EZ, or Equatorial Zone, with an area about an eighth that of the whole planet. That’s eight thousand million square kilometres, making it the largest visible feature in the entire Solar System. It’s something like six or seven times the entire surface area of all the inner planets taken together. While I’m at it, Earth mapped onto Jupiter would be the size of India on a map of Earth. There are many features in the EZ compared to most of the rest of the planet. For instance, it often shows plumes from its northern edge projecting southwest. A narrow belt appears occasionally at the equator itself. The southern side includes a “dent” where the Great Red Spot begins. The GRS itself is a feature dominating the South Tropical Zone, and this raises the question of why it’s in the southern hemisphere without any corresponding feature in the north. then again, the bands are not symmetrical either side of the equator either. The only thing I can think of right now is the very slight tilt of the planet combined with its greater orbital eccentricity creates slightly different conditions in the northern and southern hemispheres.
The planet emits decametric radio waves. This is of the order of thirty megahertz but they peak at seven to eight megahertz, so it’s close to the analogue VHF band used for FM radio on Earth, though the frequency is slightly lower. There are amateur radio projects monitoring Jupiter’s radio transmissions, which were discovered in 1955. Since they’re stronger in some parts of the planet than others, they provide fixed points enabling longitude and a “true” rotation period to be determined, but they aren’t associated with any visible features. They’re also polarised, like visible light passing through the plastic in front of a flatscreen monitor – they vibrate only at a fixed angle. This is due to Jupiter’s magnetic field and the charged particles moving within it. One of the moons, Io, influences the radio transmissions but I’ll talk about that more when I get to it. The decametric transmissions occur in short bursts sporadically. They last between a few minutes and several hours.
There are also decimetric waves, and these are continual and don’t have peaks at particular frequencies within that wave band, which is in the UHF range now used by mobile phones and previously by analogue PAL TV. They’re differently polarised and emitted from the volume around Jupiter. They’re synchrotron radiation, caused by charged particles moving in curves somewhat like the centrifugal effect, and show there are electrons moving almost at the speed of light. From Earth’s perspective this radiation fluctuates up and down according to whether we’re facing the planet’s magnetic equator or not.
This image is a painting made for Carl Sagan’s 1980 TV series ‘Cosmos’ and will be removed on request. As well as providing a fairly accurate image of what the planet looks like at cloud top level, it also illustrates Sagan’s speculations regarding life there. Although I am restraining myself from commenting on life elsewhere in the Solar System, the image without the organisms is still interesting. There are diffuse crystals of ammonia in the blue sky creating a halo around the rather smaller-looking Sun. A vortex can be seen towering over the scene to the left, with a bank of white clouds to the right, and there are a number of smaller vortices visible. Then there are long, almost straight clouds winding off into the distance, and of course on Jupiter the horizon would be several times further away than on Earth at around fifteen kilometres, although the clouds make it difficult to judge. Sagan proposed three ecological niches of organisms. There are “sinkers”, aerial phytoplankton which survive by photosynthesis and gradually sink into the depths of the planet, reproducing as they go until conditions kill them, “floaters”, somewhat jellyfish-like and balloon-like floating herbivores several kilometres across who can be seen in this image, and “hunters”, one of which can be seen at bottom right, who have a kind of retro, 1930s quality to them but look a little like Art Deco biplanes with round heads and sharp projections at the front. Asimov and Arthur C Clarke both believed that Jupiter was actually even more suitable for life than Earth, although the former’s belief was based on an earlier model of the planet which posited a vast, deep ocean beneath the clouds.
Right, so that’s Jupiter. I’ll probably do Io next.
A Box Of Chocolates 