The outer planets have more satellites than the terrestrial planets. There are 16 Jovian satellites, but many are small. These satellites, like planets around the Sun, receive a considerable amount of energy from Jupiter, which might be regarded as a mini-stellar system. The space shuttle Atlantis has undertaken observations of the Jovian satellites since July 1995. The four largest are the size of terrestrial planets and are now regarded as planets in their own right, but none have appreciable atmospheres, and they appear to be inhospitable. Nevertheless they reveal astonishing diversity in their surfaces which suggests different evolutionary development. The outermost three, Europa, Ganymede and Callisto, are icebound and airless. In 1997 the Galileo spacecraft identified molecules containing carbon and nitrogen on the surfaces of Ganymede and Callisto. Data from the Galileo spacecraft have been interpreted as radio waves emanating from Ganymede. Io has active volcanoes, is hot and without water, but it is the only one with an atmosphere, although it is almost exclusively composed of sulphur dioxide ejected from the volcanoes. This volcanic heat source could maintain life, although it has eliminated water, which means that it is unlikely to produce hydrothermal conditions for life.
The Voyager probes in 1979 revealed evidence of massive meteoric impacts on Callisto, which is pockmarked with ice craters and is probably the most cratered object in the solar system. This satellite is marginally smaller than the planet Mercury and recent data from the Galileo probe suggests that it has an internal liquid water ocean. Europa and Ganymede have distorted orbits with resulting tidal patterns which could generate heat. There is a possibility that a liquid water ocean exists under the icy surface of Europa. Although its surface temperature is -145ºC, and there is no active volcanism, released images from the space probe Galileo, in August 1996, suggested that the ice crust might not be very thick, and underneath may be oceans of liquid water, heated by Europa’s core or the tidal forces caused by Jupiter’s gravitational pull, which flood the outer surface from time to time. Jupiter’s mass is capable of raising tides in excess of 30 metres each day of 85 hours. So the main question to be resolved is: how thick is Europa’s icy surface? According to one school of thought, the ice is extremely thick, between 10 and 15 kilometres deep. Another standpoint appeals to close-up photographs of Europa from the Galileo probe which indicate regions where the water has actually risen through cracks in the ice, thus supporting the theory that beneath the ice there is warm liquid water. If the surface of the ice is thin with several cracks in it, then chemicals could find their way into the ocean and signs of life might be observed near the surface. It is believed that this liquid ocean could support microbial life and even larger life-forms. Limited analogies may be drawn with life in icy conditions on Earth. But if Europa is deeply sealed with ice, such that oxygenated chemicals of the sort that maintain sea creatures in deep sea vents on Earth would not penetrate down from the surface, then Europa would not be a very likely candidate for evidence of life. Predictions that life could develop on Europa suffered a setback in 1999 when sulphuric acid was detected on the planet’s surface. However, some
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scientists saw this in a favourable light, as sulphuric oxidants are energy sources for life on Earth.
Europa is roughly the same size as Earth’s Moon and is a good candidate for exploration. The Europa Observer, scheduled for launch in 2003, will use long-range equipment similar to that used to observe hidden lakes beneath the Antarctic ice sheet in order to penetrate the layer of ice which covers Europa. This radar survey will be followed by a robotic lander which can cut through the ice to enable a more detailed survey.
Saturn
Saturn is smaller, but broadly speaking similar to Jupiter, consisting of about 80 per cent of Jupiter’s diameter. Like Jupiter, Saturn is a large rotating gas planet consisting mainly of hydrogen with no surface but with a probability of a hot interior. Saturn has stronger winds than Jupiter, but lower temperatures because of its distance from the Sun. The outer planets appear to be too far away from the Sun to provide the environmental conditions that would support life. However, several of the planetary satellites appear to have some of the environmental conditions suitable for life, such as the presence of liquid water and access to relevant biogenic elements. Saturn has at least seven satellites with diameters between 400 and 1,500 kilometres, the largest of which is Titan, which is about half the size of the Earth and often regarded as a planet in its own right. There are fourteen other satellites which are very small, with diameters less than 300 kilometres, and little is known of them.
In size, Titan is between Mercury and Mars. It possesses an atmosphere consisting primarily of nitrogen, which is thick and suggestive of greenhouse warming. It has an icy surface where water-ice is most abundant and there are oceans of liquid methane, liquid hydrocarbons and a single giant continent the size of Australia. Its temperature is -170ºC. It may be a site for prebiotic chemistry. Substantial organic chemistry has already occurred on Titan, of the sort that has been associated with models of prebiotic Earth by Miller and Urey. Suggestions have been made that Titan could therefore be a model for the origins of life on Earth. However, if life on Earth did not arise this way, and was brought about from migration, meteors, various exotic modes of dissemination, or in fact developed out of hypothermal systems wholly dissimilar to that depicted by Miller and Urey, then Titan might not be such a useful laboratory model for the origins of life on Earth. A model, after all, is only meaningful insofar as it is abundantly clear what it is a model of.
At present Titan does not have a biosphere, but with warming it could be a potential site for life. NASA and the European Space Agency launched a spacecraft, Cassini, in October 1997, which will begin its four-year exploration of Saturn’s moon system in June 2004, but will concentrate primarily on Titan.
It is the destiny of the Sun to become a red giant star in about 6 billion years time. This process will engulf the inner planets and will incinerate the Earth,
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