Biosignature

A Biosignature, generally, is a measurable phenomenon that indicates the presence of life. The term biomarker is sometimes used as a synonym.

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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 and antibodies 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, and Geomicrobiology, biosignatures are often used in a more precise fashion to determine which living organisms are or were present in a sample. For example, the particular fatty acids measured in a sample can indicate which types of bacteria and archaea live in that environment. When used in this sense, geochemists often prefer the term biomarker.

For example, long-chain fatty alcohols with more than 23 atoms are produced by planktonic bacteria and are their molecular marker in sediments. [1]

At the same time the presence of straight-chain lipids in the form of alkanes, alcohols an fatty 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 the epicuticular 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 of DNA in the Martian soil or oxygen in an extrasolar 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 photosynthetic plants and cyanobacteria 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 Martian meteorite 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 putative nanofossils, 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 an electromagnetic 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 two Viking landers carried three life-detection experiments which looked for signs of biochemical metabolism.

More details can be found in the entry on the Viking Biological Experiments.