...ccessible form, and, for some of them, on photosynthesis to get access to energy. These organisms are much more complicated than e imagine the first life forms ould have been. Life did not exist, hoever, for one-half to one billion years folloing the formation of the Earth. The earliest half-billion years on the Earth ere marked by the continued influx of impacting objects left over from the formation of the planets. Even today, these objects are capable of dramatically affecting the terrestrial environment hen they collide ith the Earth. Back then, hen larger objects ere present, they ere capable of sterilizing the Earth completely. There ere an estimated five or so impacts subsequent to about 4.5 b.y.a. that ere capable of completely sterilizing the Earths surface. The early environment on the Earth, therefore, as not conducive to the continuing existence of life, and the earliest life may not have been able to grab a foothold until sometime after around 4.2 or 4.0 b.y.a. Thus, life may have taken as little as a hundred million years, and no more than about a half-billion years, to form once the environment became sufficiently clement to allo it. The rapidity ith hich life originated on the Earth has important implications for the process of forming life. It tells us that the formation of life is not a difficult event, but, rather, it is a relatively straightforard consequence of natural events on the planet. This is consistent ith our current vie that life originated through chemical and geochemical processes, starting ith organic molecules in a et surface or near-surface environment and using energy from some chemical source to build them into more complicated molecules. Even though e do not yet understand the specific processes that led to the origin of life, e can easily see that simple chemical processes can lead to more complicated molecules and, eventually, to life. The ingredients for life On Earth, e imagine that life really needed only a fe key ingredients to get started. Liquid ater is one such substance. It is difficult for us to imagine that life could exist ithout liquid ater. Even ith this requirement, e still see that life could be idespread throughout the universe if e allo for the possibility that a different liquid also could hold the key to life, then life could be even more idely distributed. The second ingredient is access to the necessary biogenic elements, such as C, H, O, N, and so on. This is not a very limiting factor, though, since e expect these elements to be very idespread throughout the universe, to be incorporated into planets during their formation, and to be readily available at the surfaces of geologically active planets. A source of organic molecules as required for the origin of life on Earth, and presumably ould be required for life elsehere as ell. On Earth, organics could have come from one or more of several different sources. These include the Earths atmosphere, here they could form from energetic processes such as lightning in a slightly reducing atmosphere the so-called Urey-Miller process hydrothermal vents at the bottoms of the early oceans, heated by the extremely active volcanism that ould have been present then, here organics could form by a chemical slide toard equilibrium as very hot ater cools off once injected into the oceans or from organic molecules that ere present in dust and planetesimals accreting onto the Earth. Most likely, all of these sources contributed to the prebiotic supply of organic molecules. Finally, a source of energy is needed to poer life. The energy causes the molecules in the environment to react, moving them out of their natural state of chemical equilibrium. As they move back toard equilibrium, they can release chemical energy to poer other chemical reactions, thereby providing usable energy for biota. Again, there are several possible sources 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 in...
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