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WM and brethren our talk on Astronomy tonight is based on Jupiter, the king of planets, and his retinue of moons, or more strictly speaking, satellites. The word planet comes from the original Greek word for wanderer. The planets wandered across the sky against a background of stable stars and Jupiter with its yellow disc was the next most obvious after Venus. Its movement is regular, shifting position slowly but steadily from night to night and easily found in the night sky.

Jupiter is the fifth planet from the Sun and the largest in the Solar System. It was named after the ruler of the gods in Roman mythology and is 1400 times as voluminous as Earth, but is only 318 times as massive. The mean density of Jupiter is therefore only about a quarter that of Earth, indicating that the giant planet must consist of gases and liquids rather than the rocks and metals of which the Earth and the other inner planets are composed. Apparently this huge world consists mostly of the two lightest and most abundant elements in the universe, a composition similar to that of the Sun and other stars. Jupiter may therefore represent a direct condensation of a portion of the primordial solar nebulae – the great cloud of interstellar gas and dust from which the entire solar system formed about 4.6 billion years ago.

Orbiting the Sun at a mean distance 5.2 times as great as that of Earth, Jupiter makes a complete revolution of the Sun in 11.9 years. It takes only 9.9 hours to rotate once on its axis, and this rapid rotation causes an equatorial bulge that is apparent even in telescopic views of the planet. Jupiter is the largest planet in the solar system-bigger than all the others put together-and the weather is on a scale to match. Turbulent winds, fierce lightning and raging storms keep the atmosphere constantly changing. The banded appearance of Jupiter’s clouds indicates different rotational speeds at different latitudes. The colours in those clouds come from traces of compounds formed by ultra-violet light, lightning discharges and heat. Some of these compounds may be similar to the organic molecules that formed on ancient Earth as a prelude to the origin of life.

The first stars which formed from primordial hydrogen and helium produced in the big bang, cannot have had any planets, because there were no heavy elements available from which they could be built up. Planetary systems are all second-generation (or later) systems, made from the previous generations of stars in which heavy elements have been built up by nucleosynthesis and scattered through space in stellar explosions, or supernovae.

As the parent cloud of gas and dust from which our Solar System was being formed, began to shrink, any rotation it possessed made it spin faster and faster, and as the core of the cloud collapsed to form a star, some of the material from which it was forming was held out from the centre of the cloud by residual spin, and the material settled down into a dusty disc around the young star.

Close to the young star, the lightest material in the disc, comprising mainly hydrogen and helium gas, is blown away by the heat of the star and solar radiation. The material left behind is made up of billions of tiny grains of dust that collide and stick together, building up larger lumps. The lumps of matter may be a few millimetres across and are settling into a thinner disc around the star. The process of accretion – lumps growing by sticking together carries on until the original dust grains have become

lumps of rock about one kilometre across, similar to the asteroids that orbit in profusion between Mars and Jupiter today.

Once the pieces of rock reach this size, they begin to tug on each other significantly through gravitation, pulling them into swarms that orbit around the star together, bumping into one another from time to time. Gravitation pulls the pieces more and more tightly together, with the largest lumps (which have the strongest gravitational pull) attracting more and more material, growing to become terrestrial planets and their satellites.

In our own solar system, there are four rocky planets close to the Sun, each formed in the way just described – Mercury, Venus, Earth and Mars. Then there is a belt of cosmic rubble (the asteroid belt), a ring representative in many ways of the kind of material from which the inner planets formed. The material in this ring could never settle down to become a planet itself because it is continuously being disturbed by the gravitational influence of Jupiter, the largest planet in the solar system. Beyond the asteroid belt, there are four “gas giant” planets, Jupiter, Saturn, Uranus and Neptune. These are probably typical of planets that form at large distances from their parent star, planets in which the primordial volatile material has been retained, so that even though they may contain a small rocky core, they are mostly made of gas and ices. Beyond the gas giants, at a great distance, comes small, rocky Pluto, an anomaly, and possibly a comet or asteroid, captured and held in a fixed orbit.

Jupiter is the senior member of the Sun’s family because of its volume and mass, and the number of planetary satellites under its influence. After the Sun, Jupiter exerts the most influence over the rest of the Solar System. The Sun is nearly one thousand times as massive as the rest of the Solar System combined, that is, it holds 99.9% of all the mass, whilst Jupiter holds more mass than the rest of the planets, their satellites, all the asteroids and comets combined. However, Jupiter is not a miniature Sun and it is not self-luminous, as was originally thought.

Jupiter is believed to have a small core of metals and silicates as it has swallowed all asteroids and other cosmic rubble in its path, and has a core temperature of approximately 30,000 degrees at its centre. Surrounding the core is a thick layer of hydrogen making up almost all of the mass and volume of the planet. The hydrogen in the upper layers is in the conventional or gaseous state, but beneath this, it is under such enormous gravitational attraction that the hydrogen is liquid with the lowest hydrogen layer being in a metallic state that is highly electrically conductive. Above all this comes the atmosphere comprising clouds of different kinds: water droplets; ice crystals; ammonia crystals; methane, helium and other compounds.

When comet Shoemaker-Levy 9 crashed into the planet in July 1994, the collision stirred up the planet’s atmosphere, heating interior gases to incandescence and bringing them to the surface. Astronomers captured images of these gases with telescopes based on Earth and in space. They used spectroscopes to analyse the gases in order to verify and expand their knowledge of the composition of Jupiter’s atmosphere. Still more knowledge was gained from the entry into Jupiter’s atmosphere of a probe that was part of the Galileo unmanned mission to the planet. In December 1995 Galileo went into orbit around Jupiter after a six-year flight and the entry probe separated from the main craft and entered the atmosphere, deploying parachutes to slow it down. For an hour it transmitted data to the mother ship above as it descended approximately 160 kilometres below the visible cloud tops before being crushed by the atmospheric pressure. The data was relayed to Earth from the mother ship above, which continued its investigation of the Jovian or, Galilean system.

In the atmosphere there are dark belts visible from Earth where gases are descending into the depths, and bright zones where gases are rising. The great red spot has been under observation for centuries, since 1631 in fact so that it is much more long-lived than most of the other Jovian spots. At its greatest extent it can be 30,000 miles long by 6,000 miles wide. This massive storm of a cyclonic nature has a distinctive red colour which must be explained in terms of the chemistry of the atmosphere

Jupiter seems to radiate twice-as-much energy as it receives from the Sun. The source of this energy appears to be a very slow gravitational contraction of the entire planet. This is the way in which stars form, however Jupiter would need to be almost 100 times as massive to produce a temperature at its core high enough to release nuclear energy in reactions like that which power the Sun and other stars.

Planetary physicists had expected Jupiter’s composition to be very similar to that of the primordial gas cloud from which the solar system formed – a composition that survives in today’s Sun. After initial uncertainty, the proportion of helium was confirmed to be about 24%, close to the amount in the Sun. Proportions of heavier elements, such as carbon, nitrogen and sulphur, are rather greater than in the Sun, probably because they have been increased by billions of years of bombardment from meteoroids and comets.

Jupiter is a source of radio waves and had long been known to be associated with a powerful magnetic field. The Pioneer spacecraft showed that the field was even stronger than expected and much more complex than that of earth. Moreover, Jupiter is encircled by zones of intense radiation which would be lethal to any astronaut foolhardy enough to approach the planet too closely. Pioneer 10 passed within 80,000 kilometres of the upper clouds and very nearly lost all of its instrumentation because of that radiation. Jupiter’s magnetic field is generated from deep within its ‘metallic’ layers. At the top of the atmosphere this is 14 times stronger than Earth’s magnetic field. Its polarity is opposite to that of Earth. The Jovian magnetic field is responsible for huge radiation belts of trapped charged particles that encircle the planet out to a distance of ten million kilometres, or six million miles.

At least 63 significant satellites of Jupiter have now been discovered, but as recently as 1978 it was believed that Jupiter only had 13 satellites. The four largest satellites were discovered by Galileo in 1610, and named Io, Europa, Ganymede and Callisto, each of which is about the size of our moon or, even larger. His telescopic observations revealed sometimes two, sometimes three and sometimes four specks of light along Jupiter’s equatorial plane, one side of the planet, or the other. These observations supported his theory that satellites revolved around a planet, and planets revolved around the Sun.

Recent measurements have shown that the mean density of the largest satellites follows the trend apparent in the solar system, Io and Europa close to Jupiter are dense and rocky like the inner planets, while Ganymede and Callisto at greater distances, are composed largely of water ices and have lower densities. During the formation of both planets and satellites, proximity to the central body (the Sun or Jupiter) evidently prevented the more volatile substances from condensing.

Some of the smaller and outermost satellites move in a wrong-way or retrograde orbit indicating that they are probably captured asteroids instead of bona-fide satellites formed at the same time as Jupiter.

Once it was realised that that there were four large satellites orbiting Jupiter, Cassini drew up schedules of predicted eclipses. Cassini and Romer however, noticed from continuing observations that the predicted eclipses were not always precisely right, because when Jupiter was closest to Earth, the eclipses occurred a few minutes too early and when Jupiter was at its furthest, a few minutes too late. Romer pursued the discrepancies and theorised that light might not be instantaneous as previously thought but travel at a finite speed.

In 1675 Romer calculated the speed of light as 186,000 miles per second. We now know that the true value is 186,282 miles per second. Thus careful observations and clever reasoning assisted in finding, and proving, one of the greatest, and most important of scientific yardsticks.

In 1772 the Director of Berlin Observatory, Johann Bode drew attention to a strange ‘law’ (now known as Bode’s Law), concerning planetary distances from each other.

According to Bode’s Law there seemed to be a planet missing from between Mars and Jupiter. Astronomers turned their telescopes to that part of the heavens and began a systematic search. On 1st Jan 1801 Piazzi discovered Ceres the first asteroid or minor planet. Shortly thereafter others were found, but none the size of a normal planet. Ceres with a diameter of 700 miles is much the largest of the minor planets. It is still not clear if they represent the remains of a pre-existing planet, or represent debris left over when the main planets were formed. To add to the conjecture there exists a vast belt of cosmic rubble known as the asteroid belt, situated between Mars and Jupiter. Possibly a planet between Mars and Jupiter could not have formed because of the enormous gravitational attraction of Jupiter which causes perturbations to orbits of planets, satellites, asteroids and comets.

There are two groups of asteroids known as the Trojan Asteroids that occupy the Lagrange Points in Jupiter’s orbit. The Lagrange points are positions of stability in an orbit where they form an equilateral triangle with respect to the planet and the Sun. The effect of Jupiter’s huge gravitational attraction is to control these asteroids like no other planet and they continue to travel in these areas of equilibrium.

The four largest or Galilean satellites are of great interest to astronomers because of their size and because humans will most likely land on them after Mars. They are more solid than Jupiter which has no solid surface itself and can therefore support a landing, well away from the dangers of the main planet. There is a considerable range in densities indicating differing conditions when they were created. The Galilean satellites then are large and interesting bodies in their own right and since it is impossible to land directly on Jupiter will probably serve as a future base from which to observe Jupiter, and perhaps, colonise that portion of our solar system.

There is another compelling reason for using a Galilean satellite as a base, and that is water, the substance of life. There seems to be extensive ices and possibly oceans under some of the crusts and with this material,future astronauts will have a resource that can be broken down to provide oxygen for breathing and hydrogen for fuel. It might not happen in our lifetime but the way we are treating the environment of this our planet it may not be habitable too much longer unless radical steps are taken to change our profligate lifestyle wherein we are wasting our resources. Mankind may very well have to relocate all, or at least some, of its people off-planet and then where will we go? If we have not mastered the technology to create an artificial self-sustaining satellite, then we will have to utilise natural satellites and modify them to suit the requirements of mankind and they could very well be near Jupiter.

I hope that this talk will provide you with some stimulus to learn more of the king of planets and to listen attentively to future scientific discoveries as, and when, they are announced as they could affect the future of the human race.

VW Bro Robert Taylor