Commensurability—A location (e.g., in the asteroid belt) where a body orbits with a period that is a simple fraction (e.g., 2/5 or 1/3) of the period of another large body (e.g., Jupiter) where resonant effects can build up.

Containment—Physical and biological isolation and handling of returned samples as specified for samples returned from Mars.

COSPAR (Committee on Space Research of the International Council for Science)—An international body responsible for setting planetary protection policies.

COSPAR categories—Categories, I-V, are a series of rules and requirements that have to be met depending on the object visited and the type of mission.

Category I—Includes any mission to a target body that is not of direct interest for understanding the process of chemical evolution or the origin of life. No protection of such bodies is warranted, and no planetary protection requirements are imposed by this policy.

Category II—Includes types of missions to those target bodies where there is significant interest relative to the process of chemical evolution and the origin of life, but where there is only a remote chance that contamination carried by a spacecraft could jeopardize future exploration. The requirements are for simple documentation only. Preparation is required for these flight projects primarily to outline intended or potential impact targets, provide brief pre- and post-launch analyses detailing impact strategies, and supply a post-encounter and end-of-mission report that will provide the location of impact if such an event occurs.

Categories III and IV—Include certain types of missions to a target body of interest with respect to chemical evolution and/or origin of life or for which scientific opinion posits a significant chance of contamination that could jeopardize future biological experiments. For Category III, mostly flyby and orbiter missions, the requirements consist of documentation (more involved than that for Category II) and some implementing procedures, including trajectory biasing, the use of clean rooms during spacecraft assembly and testing, and possibly bioburden reduction. For Category IV, mostly probe and lander missions, the requirements include rather detailed documentation (more involved than that for Category III), including a bioassay to enumerate the bioburden, an analysis of the probability of contamination, an inventory of the bulk constituent organics, and an increased number procedures governing implementing.

Category V—Includes Earth-return missions. The concern for these missions is the protection of the terrestrial system, Earth and the Moon. For solar system bodies deemed by scientific opinion to have no indigenous life forms, a subcategory, “unrestricted Earth return,” is defined. For all other Category V missions, in a subcategory defined as “restricted Earth return,” the highest degree of concern is expressed as the absolute prohibition of destructive impact upon return, the need for containment throughout the return phase of all returned hardware that directly contacted the target body or unsterilized material from the body, and the need for containment of any unsterilized sample collected and returned to Earth.

Cryovolcanism—A low-temperature analog of silicate volcanism in which a volatile, such as water, ammonia, or methane, plays the role of lava on the surface of a planetary body.

Diapir—A dome or anticlinal fold in which a mobile plastic core has ruptured the more brittle overlying rock.

DNA (deoxyribonucleic acid)—A polymer of nucleotides connected via a sugar-phosphate backbone. This complex biomolecule encodes genetic information in all terrestrial organisms.

Downwelling—The downward movement of material of a body driven by buoyancy forces, as in the case of a high-density fluid sinking beneath a low-density fluid.

Ejecta—Material thrown up from an event, such as an impact.

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