the transfer of energy, due to volcanic activity from the inside to the surface, could compensate for the planet’s distance from its star, and provide a suitable temperature to support life. Volcanic heat can be responsible for the formation of organic compounds, and as a source of ultra-violet radiation, as well as gases such as ammonia, hydrogen, carbon monoxide and methane. Water is made up of 90 per cent of the gases emitted by a volcano.
The Moon
The first Moon landings decisively ruled out the possibility of independent life on the Moon. After an analysis of Moon soil revealed no signs of life, the quarantine regulations for returning astronauts were lifted. However, strepto-coccus bacteria (found in a camera that had been left by the Surveyor 3 probe in 1966) had survived, and were brought back by the Apollo 12 mission in 1969 (Breuer, 1982: 65).
Long after the first lunar landings revealed the Moon to be a cold, barren, inhospitable place, there were some who still claimed that it was inhabited. Don Wilson (1975, 1979) argued that it was hollow, having been converted into a satellite by extraterrestrials whose own ship was damaged. This theory might survive a brief Moon landing but not the data from moonquake recorders left there by Apollo astronauts which are incompatible with the notion of a hollow Moon. The Moon’s mass is too small to retain an atmosphere, but under suitable protective conditions life could survive.
One school of thought (Zey, 1994) sees settlements on the Moon as early stages in human colonization of the solar system and much of interstellar space. Colonizing the Moon has long featured in science fiction but it is now seriously considered by the USA, Japan and Russia, who envisage a permanent presence of people there early in the twenty-first century. The Moon would provide a base for future expeditions, a nearby testing ground for space habitats, and a possible fusion energy source; for the Moon has a large amount of helium 3, the proposed basic source for fusion power, in its soil. This could be sent back to Earth at low cost. According to Professor Hiromu Momote, of the National Institute of Fusion Science in Nagoya, Japan, the Moon’s abundance of helium 3 – which is not found on Earth – will make it possible to build cheap, simple and clean fusion machines, which may be able to power spacecraft. A Moon colony would provide a superior location for astronomy, low gravity hospital care, and an attractive venue for commerce and tourism.
Further support for proposals for a Moon colony followed the interpretation of data from the Clementine probe of 1994 which recorded an unusual radar reflection near the lunar South Pole. Scientists interpreted this as evidence of ice water. Normally ice arrives on the Moon as a result of cometary impact and it is usually dispersed after exposure to UV radiation which converts it into atoms of oxygen and hydrogen. Sceptics have rejected the suggestion that there may be ice on the Moon on the grounds that similar reflections are found in craters that
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are not in permanent shadow and could not harbour ice. However, if the interpretation of the Clementine data is correct, then ice water may persist in deep craters shielded from solar UV light. Perhaps there is enough to supply a human colony, which would suggest that the Moon is a potentially habitable place. In January 1998, NASA’s Lunar Prospector mission was launched, carrying equipment to measure the amount of hydrogen on the Moon’s surface. An excess of hydrogen in the Moon’s polar regions would be very suggestive of the presence of water-ice.
It was announced on 5 March 1998 that the Lunar Prospector had actually detected water-ice at the North and South Poles of the Moon. Estimates of the amount of water ranged from 11 million to 330 million tonnes (The Times, 6 March 1998: 1). If the more generous estimates are correct, then a human settlement might well be supported, and the possibility of life on the Moon could yet again be raised. However, other reports derived from the Lunar Prospector suggest that ice is in very low concentrations spread widely over the polar regions, amounting to no more than a few crystals in any given location.
Mars
The planet Mars has been the most widely discussed possible site for extra-terrestrial life, as it more closely resembles the Earth than any other planet. As late as the 1960s it was still possible to believe that Mars was a bearer of life, as its variations in light and dark markings were believed to be the response of plants to seasonal change. These light and dark markings are now held to be caused by wind-blown dust on a dry and frozen planet. The Martian tempera- ture is -60ºC. The Martian North Pole consists of water-ice and its South Pole consists of mainly carbon dioxide ice. While its South Pole temperature could drop to -130ºC, temperatures at -15ºC at the equator are possible. The Martian atmosphere is composed of carbon dioxide and a little nitrogen. It is widely believed that Mars was once a site of great physical activity, with large volcanoes and vast rivers, although it might be argued that Mars was never wet, and its valleys were cut out by flowing glacial ice rather than river networks. Mariner 9 surveyed the whole surface with a resolution of one kilometre, but no artefact or large plant was identified on Mars’ barren crater-covered surface. Mars was revealed to be a relatively inhospitable place, an observation which reduced expectations of life being found elsewhere. If there is no life on Mars, where conditions are more similar to Earth than other planets in the solar system, then expectations of the discovery of life elsewhere might be reduced.
No signs of life were conclusively reported after the two Viking landings in 1976, although evidence from them has not been strong enough to completely eliminate the possibility that life, of some kind, persists there. Viking I probe landed on 20 July 1976 at Chryse Planitia, and Viking II at Utopia Planitia on 3 September 1976. The Viking Mars experiments initiated very sophisticated experiments in the search for extraterrestrial life. They revealed that the
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