Facultative anaerobe

—An organism with the capacity to grow in both the presence and the absence of oxygen. See Aerobe and Anaerobe.

Forward contamination

—The biological contamination of an extraterrestrial body by terrestrial organisms inadvertently carried aboard a spacecraft. See Back contamination.

Gram-positive bacterium

—A bacterium that shows a purple color from Gram’s stain procedure. The structure of the bacterium’s cell wall determines its ability to retain the dye used in the Gram-stain procedure.


—A measure of radiation exposure defined in terms of the total amount of energy absorbed per unit mass of the absorbing material. One gray is equal to 1 joule of energy deposition per kilogram of the target material. Because the amount of energy absorbed depends on the nature of the target material, the unit is often qualified to indicate the nature of the target. One gray is equal to 100 rad.


—An organism that survives by the ingestion and breakdown of complex organic materials. See Autotroph.


—A bacterium that produces acetate as an end product and can live as both a chemoautotroph (like methanogens) and a chemoheterotroph.

Hydrothermal vents

—Springs of hot seawater on the deep ocean floor. They are formed when cold seawater seeps through cracks in the ocean floor, circulates through volcanically heated rock, and returns to the seafloor rich in dissolved minerals.

LET (linear energy transfer)

—The energy lost per unit length of path when ionizing radiation passes through a material.


—The volume of space surrounding a planetary body that is under the dynamical influence of that body’s magnetic field.


—A prokaryote that produces methane via the reduction of either carbon monoxide or carbon dioxide in an anerobic environment.


—A theory suggesting that life on a given planet may have been seeded by life originating on another planetary body.

PCR (polymerase chain reaction)

—A relatively quick and sensitive technique used to detect and generate copies of specific DNA fragments. See DNA.


—The process by which certain organisms use the energy derived from sunlight to sustain their metabolism.


—Pertaining to the relationships between different organisms. Such relationships are typically based on comparisons between the DNA sequences of different organisms.


—Organisms such as the Bacteria and the Archaea whose cells lack a nuclear membrane and other organelles. See Eukaryote.


—Organisms that have a maximum growth temperature of 20 °C, an optimal growth temperature of 15 °C or lower, and a minimum growth temperature of 0 °C or lower.


— Organisms that have a maximum growth temperature above some 25 to 30 °C and a minimum growth temperature of 5 °C or lower.

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