Kuiper belt—A torus-shaped volume beyond the orbit of Neptune populated by bodies ranging up to many hundreds of kilometers in size; the source region for most short-period comets.

Lacustrine—Relating to lakes.

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

Mesophilic—Preferring moderate temperatures.

NASA—National Aeronautics and Space Administration.

NRC—National Research Council.

Outgassing—The emanation of gases from within an object.

Pg—Probability of growth.

Phenotype—An organism’s observed characteristics or traits that result from the expression of the organism’s genes, environmental factors, and the interaction between the two.

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

Pluvial—Involving abundant rainfall.

Psychrophile—An organism that has 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.

Psychrotolerant organism—An organism that has a maximum growth temperature of 35°C, an optimal growth temperature of 15°C or lower, and a minimum growth temperature of 0°C or lower.

Rad—Unit of absorbed dose of radiation defined in terms of the total amount of energy absorbed per unit mass of the absorbing material. One rad is equal to 100 erg of energy deposition per gram 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, e.g., 5 krad [water] per month.

Radiation-resistant organism—An organism that can survive and grow following acute exposure to radiation.

Radiolysis—The breakdown of molecules as a result of exposure to ionizing radiation.

Redox couples—A coupled series of chemical reactions driven by the simultaneous loss of electrons from one species (oxidation) and the gain of electrons from a second species (reduction).

Regolith—The layer of fragmented, incoherent rocky debris on the surface of a planetary body.

Retrograde—Rotational or orbital motion in the direction opposite to that of Earth.

RNA (ribonucleic acid)—A polymer of nucleotides connected via a sugar-phosphate backbone. It plays an important role in protein synthesis and other chemical activities in cells.

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