In addition to our Moon and the two tiny satellites of Mars, a further 31 natural satellites are known in the outer Solar System. They are in orbit around the four giant planets: Mercury, Venus and Pluto have no known satellites whereas Jupiter has fourteen, Saturn has ten, Uranus five and Neptune two. Five satellites – lo, Ganymede, Callisto, Titan and Triton – are bigger than the Moon, and of these Ganymede and Titan (certainly), and Callisto and Triton (possibly), are larger than Mercury. A number of other satellites are larger than Ceres, the largest asteroid. At the other extreme are bodies about ten kilometers across, at the limit of what can be detected by present-day techniques. In addition the rings of Saturn consist of what are effectively myriads of tiny satellite!}.

Jupiter has fourteen known satellites which have been numbered in order of discovery. The innermost five, Amalthea, Io, Europa, Ganymede and Callisto, move in circular orbits in Jupiter’s equatorial plane. They appear to rotate on their axes in the same period as they revolve around Jupiter, so that they keep the same face always turned towards the planet. The remaining nine satellites can be divided into two groups on the basis of their orbital elements: (a) XIV, XIII, VI, X, VII, (b) XII, XI, VIII, IX. The members of each group have similar mean distances from Jupiter, periods, and orbital inclinations. All nine are at great distances from Jupiter so that their orbits are significantly perturbed by the Sun. The members of the outer group move in retrograde orbits. Almost nothing is known about these nine bodies because of their small sizes, and the radii are very rough estimates based on their observed brightness. The small satellites are probably captured asteroids, rather than true satel¬lites formed at the same time as, and close to Jupiter. The inner¬most satellite, Amalthea, moves in an orbit only 1.54 Jupiter radii from the planetary surface, where observations are severely hampered by glare from Jupiter.

The remaining four satellites are all comparable in size to the Moon. They are commonly referred to as the GALILEAN SATELLITES after Galileo, who discovered three of them on 7 January 1610; six days later he noticed all four at the same time. Galileo was probably the first person to realize what it was that he had seen but he was not the first to see the satellites as is commonly stated elsewhere. This honour belongs to Simon Marius (or Meyer), who saw them ten days before Galileo, and who proposed the names by which these satellites are known today: I0, EUROPA, GANYMEDE and CALLISTO.

All four Galilean satellites are relatively bright objects and, were it not for the glare of Jupiter, they would be visible to the naked eye. A very few people claim that they can see the satellites clearly and distinguish them individually from the general glare. Most of us are not sufficiently keen-sighted to do this but can possibly see them as an extension of the light of the planet whenever two or more are situated on the same side of the planet.

The biggest Galilean satellite, Ganymede, is larger than Mercury, and Callisto is almost the same size. lo is larger than the Moon and the smallest of the four, Europa, is not much smaller. In 1971 lo occulted the star ? Sco C and hi 1972 Ganymede occulted SAO 186800. Observations of these events gave the diameter of lo accurate to within about 5 km and of Ganymede to within 100 or 200 km. Every six years, the Earth passes through the plane of the satellites’ orbits and mutual occultations are then visible. From observations of these in 1973 the diameters of Europa and Ganymede have now been obtained to within 50 to 100km and of Callisto to within 200km. Actual values of the radii together with other physical data. These data include masses of the satellites obtained from the nights of the Pioneer spacecraft in 1973 and 1974.

The occultation of SAO 186800 by Ganymede also provided the first direct evidence of an atmosphere on a Galilean satellite. The density is uncertain but it is definitely very much less than at the surface of the Earth. Another occultation, this time of a radio transmitter on board Pioneer 10, showed the existence of free electrons in Io’s atmosphere and also of an atmosphere of neutral particles having a surface pressure between about 10-8 and 10-10 atmospheres.

Io is eclipsed by Jupiter’s shadow in every orbit. As it emerges from the shadow into the sunlight it is sometimes seen to be about 0.1 magnitude brighter than usual. This excess brightness was first noticed in 1964. It does not occur after every eclipse and when it does it disappears after about 15 minutes. It was suggested that the cause was the condensation of part of an atmosphere as lo cooled in the darkness of an eclipse. The condensed material could form a bright layer on the surface which would only gradually evaporate when the satellite re-emerged into the sunlight. How¬ever the Pioneer 10 results show that lo does not have a sufficiently dense atmosphere for such an explanation to be possible. In any case the observations of the excess brightness are difficult to make because of the proximity of Jupiter

Another oddity of Io is the yellow glow surrounding it which was discovered in 1974. This glow extends outwards from lo for several times its diameter and is due to the characteristic light of sodium (emission in the D line). Io’s orbit is within Jupiter’s magnetosphere and the surface of the satellite is continuously bombarded by the energetic particles in Jupiter’s radiation belts. If the surface of lo is covered at least partly with compounds of sodium, such as common salt (sodium chloride), the bombarding particles could knock sodium atoms off the surface to form a cloud around the satellite where they could then emit their characteristic yellow light. Observations also show a corresponding cloud of hydrogen, although this fills the whole of lo’s orbit, forming a torus, instead of being confined to the immediate vicinity of the satellite.

Observations of the visible and infrared sunlight reflected by the Galilean satellites and an examination of the fraction reflected at different wavelengths show that the surface properties vary considerably from satellite to satellite. Europa and Ganymede are coated largely with water frost at a temperature of about 150K and with grains about 0.1 mm across. The proportion of water frost in the .surface layer is around 75 per cent and 60 per cent respectively for these two satellites whereas it is at most a minor constituent for Io and callisto Io has a very high albedo at near infrared wavelengths: between I and 3 ? m it is over 90 percent .This can be explained by a surface layer of minerals containing a large amount of water of crystallization. This is consistent with the requirement for a proportion of sodium compounds to explain the D-line glow.

Little known about surface features on the satellites. Faint markings can be seen from the Earth through large telescopes; these show that each of the four satellites keeps one face turned towards Jupiter as it revolves around it. Pioneer 10 took photo¬graphs of Ganymede which seem to show a bright region near its south pole. This might be an ice cap, possibly composed of methane rather than water. One Pioneer 11 photograph showed an ice cap on Callisto quite clearly.

Ganymede and Callisto have mean densities of 2000 and 1740 kgm-3 respectively. If they were balls of rock their densities would be at least 3000 kg m-3. These two satellites must therefore contain sizeable quantities of low-density frozen solid or liquid material such as water ammonia, and possibly, methane. This material probably forms a thick mantle around a rocky core of hydrated silicates. These mantles will be liquid if there is even a small amount of radioactive decay in their interiors. lo and Euro pa both have a higher mean density, around 3600 kg m”8 in each case, and could contain a high proportion of rock so that they have a large core and a relatively thin mantle. All four should have at least a thin solid crust. Except on lo, this may be ice, although on Callisto there must be additional dark material on the surface itself in order to explain the low albedo.

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