We recognize that our own evolution may represent only one pattern among many and are therefore examining environments where a different type of life may have appeared. However, our understanding of the chemical characteristics of life on our own planet provides a valuable starting point of a search for life elsewhere. Even on Earth, life survives and sometimes thrives under unexpected conditions. For example, some microbes flourish at temperatures of 113°C. Others exist only in caustic acids. Others survive intense radiation in nuclear reactors.
So while Earth organisms are composed of nearly identical macromolecules, these can evolve to cope with a broad range of physical and chemical conditions. This gives us some indication of how life on other planets could differ from that on Earth. Nevertheless, life itself has certain general principles. Living systems must capture energy in some form and obtain nutrients from the environment. They use these inputs to generate the molecular structures they need to maintain and reproduce themselves, and to survive in the face of varying challenges from the environment. Part of the search for extraterrestrial life is to investigate alternative ways that living organisms can implement these functions.
Our initial approach is to look for life that is carbon based and makes use of chemical reactions in the presence of liquid water. Carbon is very broadly dispersed throughout the universe. By forming long chains it is ideally suited to create the complex molecules that sustain life. Water is made up of the commonest element in the universe, hydrogen, and the third most common, oxygen. (The second most common is helium, which is inert.) Strong bonds between water molecules cause them to form networks that enable it to wet surfaces and move through capillaries of organisms. The bonds are formed by electrical charges on opposite sides of the molecule, which can break up other molecules without changing their composition, bringing them into solution to take part in chemical reactions.
Water also has the property of discriminating strongly between fats and water-soluble molecules so as to favor the creation of cell membranes and boundaries. In this way it makes an essential contribution to the formation of cell walls that can enclose and protect groups of interacting biological molecules. While free-floating molecular complexes may have been an early form of life, the formation of cells provided a way of stabilizing molecular systems and enabled them to evolve more complex forms. A fixed unit of evolution is then ready to capture energy from star light or local energy sources, obtain nutrients, and maintain and reproduce itself. The cellular enclosure provides a constant environment where molecules coding the instructions for reproduction can avoid all but the most severe environmental insults. Water also has the valuable property that it remains in liquid form over a wide range of temperatures suitable for both activity and stability of molecules important to life. When it solidifies into ice, the ice floats on top of the water, sheltering life forms in the water below.
The presence of oxygen in a planetary atmosphere is an important indicator of the possible presence of life. It is one of the most distinctive features of Earth’s atmosphere. The chemical surface of a planet will probably remove oxygen from the atmosphere (as rust, for example) unless there is an active process such as plant photosynthesis to replace it. However, by itself the presence of oxygen can be misleading. There are traces of oxygen in the atmosphere of Europa, a moon of Jupiter, but this is thought to be produced in the upper atmosphere by radiation splitting water molecules into oxygen and hydrogen. The hydrogen, being lighter then escapes. Life requires oxygen in the lower atmosphere, which can be difficult to detect directly. A more reliable indication of conditions favoring life is the detection of carbon dioxide, ozone and water appearing together.
Programs searching for extraterrestrial life on planets orbiting other stars initially seek to determine whether a planet is capable of supporting life. These programs represent our the start of our effort to reach a deeper understanding of life and how it arises, and to throw light on whether it is unique to one planet or widely represented in the universe. If we find earth-like planets with conditions suitable for life, a further search for evidence of communications or for industrial gases in their atmospheres may indicate whether a civilization has been present.