Biosignature

A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, or molecule  – or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts. A biosignature (sometimes called chemical fossil or molecular fossil) is any substance – such as an element, isotope, or molecule  – or phenomenon that provides scientific evidence of past or present life. Measurable attributes of life include its complex physical or chemical structures and its use of free energy and the production of biomass and wastes. A biosignature can provide evidence for living organisms outside the Earth and can be directly or indirectly detected by searching for their unique byproducts. In general, biosignatures can be grouped into ten broad categories: Determining if a potential biosignature is worth being investigated is a fundamentally complicated process. Scientists must consider any and every possible alternate explanation before concluding that something is a true biosignature. This includes investigating the minute details that make other planets unique and being able to understand when there is a deviation from the expected non-biological processes present on a planet. In the case of a planet with life, it is possible that these differences can be extremely small or not present at all, adding to the difficulties of discovering a biosignature. Years of scientific studies have culminated in three criteria that a potential biosignature must meet in order to be considered viable for further research: Reliability, survivability, and detectability. A biosignature must be able to dominate over all other processes that may produce similar physical, spectral, and chemical features. When investigating a potential biosignature, scientists must be careful to consider all other possible origins of the biosignature in question. There are many forms of life which are known to mimic geochemical reactions. In fact, one of the theories on the origin of life involves molecules figuring out how to catalyze geochemical reactions to exploit the energy being released by them. These are some of the earliest known metabolisms (see methanogenesis). In a case such as this, scientists might search for a disequilibrium in the geochemical cycle, which would point to a reaction happening more or less often than it should. A disequilibrium such as this could be interpreted as an indication of life. A biosignature must be able to last for long enough so that a probe, telescope, or human can be able to detect it. A consequence of a biological organism's use of metabolic reactions for energy is the production of metabolic waste. In addition, the structure of an organism can be preserved as a fossil and we know that some fossils on Earth are as old as 3.5 billion years. These byproducts can make excellent biosignatures since they provide direct evidence for life. However, in order to be a viable biosignature, a byproduct must subsequently remain intact so that scientists may discover it.