Respiration—The process by which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for biological processes.

Serpentinization—A metamorphic process in which ultrabasic rocks react with water to create a variety of hydrous, magnesium-iron phyllosilicate minerals known collectively as serpentine. The process is endothermic and results in the liberation of hydrogen, methane, and hydrogen sulfide.

SLiME—Subsurface lithoautotrophic microbial ecosystem.

SNC meteorites—The family of shergottite, nakhlite, and chassignite stony meteorites believed to have originated on Mars.

SRB—Sulfate-reducing bacteria.

Stereoisomers—Isomers that differ only in the arrangement of their atoms in space.

Stratigraphic horizon—A layer within a planet’s crust that formed in a specific geological epoch.

Strecker synthesis—The synthesis of -amino acids by the reaction of an aldehyde or ketone with a mixture of ammonium chloride and sodium cyanide followed by acid hydrolysis of the amino nitriles formed.

Stromatolites—Lithified sedimentary growth structures formed by the trapping of sedimentary grains by microorganisms.

Tholins—Complex polymeric substances formed on the surfaces of the icy bodies of the outer solar system by the irradiation of organic compounds. The arrival of these compounds on comets could provide the basic building blocks of life.

TRL—Technology readiness level, a measure of the technical maturity of an instrument or mechanism. The higher the TRL, the more mature the technology.

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