Jupiter, the fifth planet in the solar system, has a composition close to that of the Sun, but its mass is insufficient to trigger nuclear reactions and form a star. Its hydrogen atmosphere combined with other rare elements gives brightly coloured clouds agitated by violent storms caused by an intense emission of heat.
Unlike telluric planets and like the other three gas giants, Jupiter does not have a solid surface: it is a ball of gas - essentially hydrogen and helium - surrounding a core probably composed of iron, rock and silicates, which are probably mingled with “water ice” ammonia and methane.
Jupiter has a magnetic field, a magnetosphere and an ionosphere, and there are intense radio frequency emissions. As on Earth, there are polar auroras in high latitude regions.
1. Structure of the planet
Jupiter, like the other giant planets in the solar system, is profoundly different from the telluric planets: Mercury, Venus, Earth and Mars have solid surfaces and are several thousand kilometres in diameter with a thin surrounding atmosphere that in the case of Mercury is tenuous. Jupiter, on the other hand, is an enormous ball of gas, composed essentially of hydrogen and helium, like the Sun and the other stars. The majestic images that we see through the telescope and that are sent back by space probes are of the outer layers of clouds. These clouds hide the deep structure of the planet, but with modern techniques for measuring the electromagnetic radiation reflected or emitted by the planet, the precise location of the space probes passing close by and the application of the laws of physics, we can gain an extraordinarily precise idea of the interior of the planet.
Analysis of the planet's ultraviolet, visible, infrared and radio radiation using observations from Earth and from devices on board space probes has made it possible to determine the temperature and composition of the outer layers of Jupiter over a thickness of around 2000 kilometres, which is, however, small compared with Jupiter's 70 000 kilometre radius. What would a traveller see on descending towards Jupiter armed with the necessary - and indestructible - means of investigation?
Coming from interplanetary space and going towards the centre of the planet, our traveller would first of all encounter a high and extremely tenuous atmosphere composed mainly of hydrogen, where the temperature is of the order of 1500 kelvins. He would then enter a region where the pressure is of the order of 1 millionth of Earth's sea level atmospheric pressure, below which turbulence is strong enough for the continuous mixing of the various atmospheric components. The temperature here is around 37 kelvins; it continues to decrease as we descend. From then on, the atmosphere is composed of about 90 % of molecular hydrogen (H2) and nearly 10 % of helium. There is also a small amount of methane (CH4) - of the order of 0.1 % - and still smaller amounts of acetylene (C2H2) and ethane (C2H6); the latter two gases are produced in the high atmosphere by solar ultraviolet radiation, which breaks the methane molecules into pieces that later recombine to make more complex hydrocarbon molecules. Acetylene and ethane are the only hydrocarbons that have been detected with certainty, but there are probably others in very small quantities. From analysis of the data from the probes, ethylene (C2H4), benzene (C6H6) and methylacetylene (C3H4) should also be present.
Still descending, to levels where the pressure is of the order of a few thousandths of an atmosphere, the traveller would detect ammonia (NH3) in infinitesimal quantities but nevertheless sufficient to be detected from Earth-orbiting astronomical observation satellites. He would also find a thin mist of small particles of diameter less than a micrometre, whose nature is as yet unknown (they could be small crystals of ammonia or hydrocarbon particles in the solid or liquid state). On arriving at a level where the pressure is close to a tenth of an atmosphere, the traveller would encounter temperatures of the order of 12 kelvins in a region called the tropopause, and from here on the temperature rises continuously up to the centre of the planet. At this level, the amount of ammonia increases extremely quickly up to a few ten thousandths at around 0.6 atmospheres.
A gas called phosphine (PH3) also appears which, although in modest amounts (less than 1 millionth), absorbs infrared radiation enormously, as does ammonia. At around 0.3 - 0.5 atmospheres pressure, the traveller would discover a layer of white clouds like terrestrial cirrus clouds, composed of crystals of ammonia of a size of up to 100 micrometres. This cloudy layer is not too opaque in the domain of the visible, and from Earth, coloured clouds lying at a deeper level can be seen through it, probably at a pressure of 2 or 3 atmospheres. In contrast, the ammonia "cirrus" clouds strongly absorb infrared radiation, blocking the radiation from hotter layers lying at greater depths. However, the ammonia layer is not consistent and, at various places on Jupiter, for example in the equatorial region, it is not very dense, or is non-existent, allowing 5 micrometre infrared radiation to reach us. But the coloured clouds are opaque to both infrared and visible light. Their nature is still unknown: are they ammonium sulphide (NH4SH), phosphorus compounds, or even complex organic compounds? To answer this question we have to await the analysis of the results obtained during the descent of a probe into Jupiter's atmosphere (the Galileo mission).
Around 3 or 4 atmospheres, the traveller would begin to detect other atmospheric compounds such as water vapour, germane (GeH4), carbon dioxide (CO). Other as yet undetected minor components are doubtless present in very small quantities. From 4 or 5 atmospheres, around 27 kelvins, visible or infrared radiation can no longer provide any information, but radio frequency radiation from these layers can still be detected from the ground with using large telescopes. Beyond around 40 atmospheres pressure, around 32 kelvins, we have no more direct information. We are entering the domain of the internal structure, the subject of complex theories, about which a few words need to be spoken before delving deeper into the mysteries of Jupiter.
Three kinds of information set constraints on the theories on the internal structure of Jupiter. Firstly, there are the respective proportions of the two major constituents of Jupiter, hydrogen and helium; these proportions have been accurately measured by the Voyager probes in the outer atmosphere. Secondly, measurements in the infrared have shown that Jupiter emits 1.7 times more energy than it receives from the Sun; in other words, there is a source of energy at the centre of Jupiter that is producing of the order of 70% of the energy received from the Sun; the presence of this internal source sets a value for the central temperature. Finally, like all massive bodies, the planet radiates a gravitational field around itself; this field is not symmetrical and its variations disturb the trajectories of space probes; the deviations from symmetry of the gravitational field that can be deduced from this give information about the distribution of mass inside the planet.
Back to our imaginary traveller. Dropping below Jupiter's visible clouds, he would doubtless find himself in more complex clouds. And with the ever increasing temperature, he would begin to find - still in very small amounts compared with the hydrogen and helium, which remain uniformly mixed - various compounds that become volatile (compounds of carbon, nitrogen, silicon, magnesium, sulphur etc.). The pressure, too, gets higher and higher, reaching values well beyond those that can be achieved in laboratories on Earth. Nevertheless, the compounds remain fluid and not solid, because of the relatively high temperatures. At around 2 million atmospheres and 10 000 kelvins, a radical change occurs, however: the hydrogen becomes monatomic and metallic, i.e. its density and conductivity suddenly become much higher. As a result, the local density increases sharply. It is thought that, contrary to what happens on Saturn, the helium remains mixed with the metallic hydrogen due to the high temperatures prevailing in this region of Jupiter. For the same reasons, the metallic hydrogen is in liquid form, not solid.
Continuing down, the traveller reaches the fantastic level of 45 million atmospheres and 20 000 kelvins at a distance of about 57 000 kilometres beneath Jupiter's visible clouds. It is believed that this is where the upper limit of the solid core of the planet should be located, originally formed by the accretion of grains and dust in the primitive nebula. This core is probably composed of silicates, metals and perhaps frozen water, ammonia and even methane. At the time of accretion, the core heated considerably. It is the remains of this primordial heat that is supposed to be the origin of the observed internal energy source of Jupiter.
The study of the composition of Jupiter is important in more than one way. Normally, gaseous molecules in the atmospheres of planets tend to escape through their own movement - brownian movement - the more so the higher the atmospheric temperature; on the other hand, the gravitational attraction of the planet tends to oppose this escape. In the case of Jupiter, gravity is strong (around three times Earth's) and the temperature of the outer layers is much lower than for telluric planets, so that even the lightest molecules cannot escape from the atmosphere. As a result, Jupiter's atmosphere must now be the same as when the planet was formed around 4.5 billion years ago. In other words, by determining the current composition of Jupiter, we have access to the primitive nebula from which it is believed the whole solar system was formed. We can thus know the composition of the interstellar medium at that location in our galaxy 4.5 billion years ago.
Two of the elements making up the interstellar medium that can be measured on Jupiter - hydrogen and deuterium - are particularly interesting from a cosmological point of view. This is because the Big Bang theory predicts that these two gases were essentially produced during the first three minutes of the existence of our universe. Later, helium was also produced inside stars during their evolution. Some of these stars end their life by exploding: these are supernovae. In doing so, they enrich the interstellar medium with the materials they produced, especially helium. The proportion of helium in the interstellar medium is increasing constantly with time. A measurement of the abundance of helium on Jupiter therefore gives a higher value than the abundance of primordial helium. This higher value, measured during the Voyager mission, is of the order of 24% by mass, which is in agreement with the upper limits deduced from the observation of very old galaxies.
What is still more important is the measurement of deuterium in Jupiter. This element, also formed during the Big Bang, is destroyed in stars. Supernova explosions therefore enrich the interstellar medium in all of the elements except deuterium. As a result, the relative proportion of deuterium - for example relative to hydrogen - is decreasing constantly with time. Now, at the moment, interstellar deuterium can only be measured within our galaxy, i.e. we can only have access to the value of the amount of deuterium at the current time. The measurement on Jupiter therefore provides precious information, giving a second point 4.5 billion years ago on the deuterium evolution curve and a value lower than the primordial abundance.
The measurements of the abundance of deuterium obtained during the Voyager mission seem to confirm that the deuterium/hydrogen ratio has decreased slightly since the birth of the solar system, in agreement with the predictions of the model of deuterium abundance evolution over time.
Using a model like this, we can also go right back to the abundance of deuterium when it was produced in the Big Bang. Using the theoretical model of this primordial explosion, we can deduce the density of protons and neutrons (which are called nucleons or baryons) in the universe. From this density value, cosmological models draw fundamental conclusions on the structure of the universe, which, according to the models, is open, i.e. it will continue expanding forever. This result would, however, be put into doubt if experiments being carried out in large particle accelerators prove - as some experiments are already suggesting - that the elementary particle called a neutrino has a mass. As neutrinos are much more abundant than protons and neutrons, the total density of the universe would be much greater. The universe could then be closed, i.e. after having continued its present expansion for a certain time, it would contract again until it reached its initial size ("the Big Crunch").
Two scenarios are currently envisaged for the formation of Jupiter. In the first scenario, we assume that in the region of Jupiter and the other giant planets, quite large fragments (of the order of several thousand times the current radius of Jupiter) of the primitive nebula condensed and formed giant gaseous protoplanets. Later, a core would have formed from particles of iron and silicates that were already in the nebula falling towards the centre of the protoplanet. In this scenario, the atmospheric composition of the giant planets would be similar to the Sun's, if we assume that the primitive nebula had the same composition at its centre and at its edge. In particular, carbon, nitrogen and oxygen - which are the most abundant elements in the universe after hydrogen and helium - should be in the same proportions relative to the hydrogen in the atmospheres of Jupiter and the Sun. This is not what is observed; the carbon/hydrogen ratio in all the giant planets and, it appears, the nitrogen/hydrogen in Jupiter and Saturn are higher than in the Sun.
In another scenario, the giant planets are assumed to have formed in two phases. In a first phase, a core was formed by the concentration of grains floating in the primitive nebula. These grains were composed of iron and silicates, but also, because of the low temperatures in the nebula at its edge, of frozen water, ammonia and methane. The core grew until it reached a critical mass of the order of ten times the mass of the Earth. The heat released during this process could have partially re-vaporised the frozen materials. When the nucleus reached the critical mass, it attracted the surrounding materials of the primitive nebula composed mainly of hydrogen and helium that could not condense because that would have required extremely low temperatures. In this way, in the second phase, the atmospheres of Jupiter and the other giant planets were formed, in which carbon, nitrogen and oxygen could, after re-vaporisation of the frozen materials in the atmosphere, be enriched in comparison with the Sun.
The rings of Jupiter - Statistics
The rings of Jupiter were discovered on 4 March 1979 by cameras on the Voyager-1 probe; the density of the rings appears to be a billion times less than those of Saturn, explaining why, being very close to the bright planet, they had never before been observed from Earth: it would have been as difficult to detect them as a faint candle beside a lighthouse. Observed in the infrared at a wavelength of 2.2 micrometres (where methane, abundant in Jupiter's atmosphere, is virtually opaque), the ratio of the brightness of the rings to the brightness of the planet is greatly increased and they can be detected from Earth, which was achieved five days after their discovery by Voyage-1. This discovery explained why, during its Jupiter fly-past five years earlier, Pioneer-11 had observed sudden variations in the number of charged particles in orbit around Jupiter at certain distances from the planet; some scientists had suggested that Jupiter had as yet undiscovered satellites or rings at the locations where the number of high energy particles decreased; five years later, they were proved right!
The discovery of Jupiter's rings two years after Uranus' rings showed that rings around giant planets are a natural phenomenon. Like those of Saturn and Uranus, Jupiter’s rings have sharp edges and nearby satellites; however, they are much more tenuous and different. For the moment, we do not know the size or nature of the particles in this ring: they are inside Jupiter's magnetosphere and are probably charged. There are four components: a bright ring about 6000 kilometres in width is extended outwards by a very bright edge about 800 kilometres wide. Towards the interior, more dispersed material extends to the summit of Jupiter's clouds; everything is enveloped in a very tenuous halo.
The satellites of Jupiter
The first of Jupiter’s moons were discovered in 1610, when Galileo Galilei observed the Galilean moons (Io, Europa, Ganymede and Callisto), the four large satellites in the Jovian system. This was the first observation of moons other than the Earth's. However, a previous observation may have been made in 362 BCE by the Chinese astronomer Gan De.
During the following four centuries, before the space age, eight other satellites were discovered: Amalthea (1892), Himalia (1904), Elara (1905), Pasiphaë (1908), Sinope (1914), Lysithea et Carme (1938), and Ananke (1951). During the 1970s, two more satellites were observed from Earth: Leda (1974) and Themisto (1975), which were then lost and found again in 2000.
Before space probes were sent to the neighbourhood of Jupiter, there were thus 13 known satellites (or 14 including Themisto). With the Voyager missions, which flew by the Jovian system in 1979, three new moons were discovered: Metis and Thebe in March 1979 from Voyager 1 photographs, Adrastea in July 1979 by Voyager 2.
Between 1979 and 1999 no new satellites of Jupiter were discovered. On 6 October 1999, the Space watch programme discovered what was at first thought to be a new asteroid, 1999 UX18, but which was quickly identified as a new Jupiter moon, Callirrhoe.
A year later, between 23 November and 5 December 2000,f Scott S. Sheppard and David C. Jewitt of the University of Hawaii and their team began a systematic search for small irregular moons around Jupiter using two of the thirteen telescopes on the summit of Mauna Kea in Hawaii: the Subaru (8.3 m diameter) and the Canada-France-Hawaii (3.6 m).
47 satellites were discovered between 2000 and 2006, with distant eccentric orbits that are inclined and retrograde; they are on average 3 kilometres in diameter, the largest reaching barely 9 km. They are all thought to be captured asteroids or comets, possibly broken up into several pieces.
In 2010, 66 moons of Jupiter were known, the record for the solar system. There may still be other undiscovered smaller moons (less than 1 km in diameter).
The Galilean satellites of Jupiter, with views of their surfaces.