Two primary definitions of a bio-signature:
Any phenomenon produced by life (either modern or ancient). Two sub-definitions: Definitive Biosignature: A phenomenon produced exclusively by life. Due to its unique biogenic characteristics, a definitive biosignature can be interpreted without question as having been produced by life. Potential Biosignature: A phenomenon that may have been produced by life, but for which alternate abiotic origins may also be possible.
(see reference section for link)
A chemical or physical marker indicating the presence of life, especially something sought in geologic formations or in an extraterrestrial environment. Also called biomarker.*
As one might imagine, the definitions give a certain amount of latitude in interpretation—almost too much.
Methane, carbon dioxide, and water are indicators for the presence of life on the Earth—as well as possible biomarkers on other planets. So, if we were at the outer edges the Solar System—peering towards the Earth and wondered perhaps if we could discern the methane signature. The answer would be yes—and a whole lot of other molecules. The manner in which we could obtain a biosignature would be to wait until the Earth were to pass in front of the Sun–then analyze the sun-light that “passes” through Earth’s atmosphere. Thus, the analyzed light can yield information about what molecules are present: the spectrum.
The figure below depicts a simulated spectrum of an earth-like planet
Legend for color spectrum:
carbon dioxide (black)
nitric acid (teal)
Rayleigh (opacity of atmosphere)
This is the first part of a series devoted to discerning biosignatures from space.
Visible Earth caption was deleted & should read:
This narrow-angle color image of the Earth, dubbed ‘Pale Blue Dot’, is a part of the first ever ‘portrait’ of the solar system taken by Voyager 1. The spacecraft acquired a total of 60 frames for a mosaic of the solar system from a distance of more than 4 billion miles from Earth and about 32 degrees above the ecliptic. From Voyager’s great distance Earth is a mere point of light, less than the size of a picture element even in the narrow-angle camera. Earth was a crescent only 0.12 pixel in size. Coincidentally, Earth lies right in the center of one of the scattered light rays resulting from taking the image so close to the sun. This blown-up image of the Earth was taken through three color filters — violet, blue and green — and recombined to produce the color image. The background features in the image are artifacts resulting from the magnification.
First Definition of Biosignature:
( Steele, Andrew, The Astrobiology Field Laboratory. U.S.A.: the Mars Exploration Program Analysis Group (MEPAG) – NASA. pp. 72. http://mepag.jpl.nasa.gov/reports/AFL_SSG_WHITE_PAPER_v3.doc ) via Wikipedia
Second Definition of Biosignature:
Dictionary Reference: American Heritage Dictionary of the English Language 11th edition