- Biosignature
A Biosignature, generally, is a measurable phenomenon that indicates the presence of
life . The termbiomarker is sometimes used as a synonym.Biosignatures in Biology
In
biology ,biochemistry , and biomedical research, biosignatures are biological indicators obtainable through assays (such as a blood sample) which can be used to ascertain facts about an individual's health or disease state. Peptides andantibodies are examples of biochemical constructs that can be used to measure biosignatures and thus determine if, for example, a person has been infected with a particular disease, prior to their actually exhibiting symptoms.Biosignatures in Geochemistry
In
Geochemistry ,Geobiochemistry , andGeomicrobiology , biosignatures are often used in a more precise fashion to determine "which" livingorganism s are or were present in a sample. For example, the particularfatty acids measured in a sample can indicate which types of bacteria andarchaea live in that environment. When used in this sense, geochemists often prefer the termbiomarker .For example, long-chain
fatty alcohol s with more than 23 atoms are produced byplankton icbacteria and are their molecular marker insediment s. [http://www.cyberlipid.org/simple/simp0003.htm]At the same time the presence of straight-chain
lipids in the form ofalkanes ,alcohols anfatty acids with 20-36 carbon atoms and a predominance of odd-over-even chain lengths for the alkanes or even-over-odd chain lengths for the alcohols and fatty acids in soils, sediments or peat deposits are an indication of input originating from theepicuticular wax of higher terrestrial plants.Biosignatures in Astrobiology
In
Astrobiology , a biosignature is a sign of the presence of life in general and is usually studied with an eye towards identifying and detecting extraterrestrial life. Often, biosignatures in this context are chemical: the detection ofDNA in the Martian soil oroxygen in anextrasolar planet 's atmosphere would likely be interpreted as diagnostic evidence of life. (Oxygen is highly reactive with many minerals and metals, and is generally assumed to persist in an atmosphere only if lifeforms are actively producing it, as photosyntheticplants andcyanobacteria do on Earth.)Biosignatures need not be chemical, however. The shape and size of certain objects may potentially indicate the presence of life. For example, tiny
magnetite crystals in the Martianmeteorite ALH84001 were the longest-debated of several potential biosignatures in that specimen because it was believed until recently that only bacteria could create crystals of their specific shape. Other potential biosignatures studied in the ALH84001 meteorite included putativenanofossils , tiny rocklike structures whose shape was a potential biosignature because it resembled known bacteria. Most scientists ultimately concluded that these were far too small to be fossilized cells.From this point of view, even the hypothetical radio signatures that
SETI scans for would be anelectromagnetic biosignature, since a message from intelligent aliens would certainly demonstrate the existence of extraterrestrial life.The Viking Missions to Mars
The Viking Missions to
Mars in the 1970s conducted the only thorough experiments to date which were explicitly designed to look for biosignatures on another planet. Each of the twoViking landers carried three life-detection experiments which looked for signs of biochemicalmetabolism .More details can be found in the entry on the
Viking Biological Experiments .External links
* [http://www.biosignatures.org Biosignatures.org] - at the Biodesign Institute at Arizona State University
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