Biosignature
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A biosignature is any substance – such as an element, isotope, molecule, or phenomenon – that provides scientific evidence of past or present life on a planet. Measurable attributes of life include its complex physical or chemical structures, its use of free energy, and the production of biomass and wastes.
Electron micrograph of microfossils from a sediment core obtained by the Deep Sea Drilling Program
Some researchers suggested that these microscopic structures on the Martian ALH84001 meteorite could be fossilized bacteria.
An image of the plumes of water and ice coming from the surface of Enceladus. Future missions will investigate these geysers to determine the composition and look for signs of life.
Microfossil
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A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.
Microfossils from a deep sea sediment core
A Late Silurian chitinozoan from the Burgsvik beds showing its flask shape
Acritarch from the Weng'an biota c. 570–609 mya