... of energy, including sunlight especially the energetic ultraviolet light that could have penetrated all the ay to the Earths surface in the early periods before there as significant ozone, lightning in the atmosphere, or geochemical energy obtained from geothermal heat in ater circulating through hydrothermal vents. All of these energy sources ere available, probably in abundance. There is no need for the energy sources that drove the earliest life to be the same as those that poer life today thus, the complicated chemical mechanisms that drive photosynthesis did not have to be present in the original life. e expect that, under these conditions, the formation of life as relatively straightforard. e also expect that life could originate and continue to exist any place here similar environmental conditions are met. This could mean elsehere in our solar system, or on planets around other stars. A search for life, therefore, is almost tantamount to a search for the basic environmental conditions in hich life could exist. Life on Mars Elsehere in our on solar system, e immediately think of Mars as a possible abode for life. There is abundant geologic evidence on the martian surface to indicate that liquid ater has played an important role in shaping the surface throughout time. The evidence suggests that ater as relatively stable at the martian surface during the first half-billion years recorded in its geology from about 4.0 to 3.5 b.y.a.. If correct, this might suggest that life could have originated on Mars surface at that time. Subsequent to 3.5 b.y.a., hoever, there also is abundant geological evidence for the continued presence of ater. At this later time, the ater as not stable as a liquid at the surface, except perhaps intermittently. Rather, ater as present deep ithin the crust and as released to the surface in catastrophic floods only occasionally. ithin the crust, hoever, the ater ould have been available to support either an origin of life or its continued existence if it had originated earlier. In addition, ithin the crust there as an abundant source of energy from the volcanic activity that has persisted throughout most or all of martian history, and from chemical eathering of the minerals comprising the crust. Life could have originated at the surface on early Mars or in the deep subsurface at any time, and life could exist today. If life is present today, it likely ould be either deep beneath the surface here ater could exist as a liquid several kilometers deep, perhaps or exposed at the surface in any transient vents here hot, volcanically heated ater is released at the surface. Although there is some evidence to suggest that there might be fossils from organisms ithin meteorites from Mars, this evidence is very controversial and is not yet generally accepted. Significantly, even if this meteoritic evidence is rong, the basic argument regarding the possibility of life on Mars ill not change. This is true even though the meteorite findings appear to have reinvigorated the interest in searching for life on Mars. After Mars, other suggestions for an abode for life become more speculative. Life could have arisen on early Venus, hen the Sun as dimmer, temperatures ere loer, and the planet might not yet have undergone a transition to the present thick, hot, greenhouse atmosphere. Of course, any evidence of an early Venusian biosphere ould have been long since obliterated. The moons of Jupiter and Saturn Life also could exist on Europa, a satellite of Jupiter, living in a possible ocean of ater that may lie buried beneath the surface covering of ater-ice. There, melting of the ice ould result from tidal heating generated by Jupiter tides, triggered by gravitational interactions ith Io as they both orbit around Jupiter. If there ere an ocean, tidal heating and decay of radioactive elements ould provide a substantial sou ... Download