E Glossary
Ablation—
the process whereby material is heated, vaporized, and lost from the surface of a projectile penetrating Earth's atmosphere
Allotrope—
one of two or more existing forms of an element
Astronomical unit (AU)—
mean distance of the Earth from the Sun
Back contamination—
biological contamination of Earth as a result of samples returned from solar system bodies
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 Mars1
Crater palimpsests—
circular features of very little relief that are apparently vestiges of large ancient impacts
Endolithic—
living on the surface of rocks, e.g., lichens
Eukaryotic cells—
cells with a defined nucleus that contains most of a cell's DNA and is enclosed by a membrane, e.g., fungi
Extremophiles—
microorganisms capable of growing under extreme physicochemical conditions such as high temperatures and pressures
Forward contamination—
biological contamination of a solar system body from a sample return mission
Gram-negative bacteria—
bacteria that show a red color from Gram's stain procedure
Gram-positive bacteria—
bacteria that show a purple color from Gram's stain procedure
Hydrolase—
any of a class of enzymes that break hydrogen bonds
Interplanetary debris complex (IDC)—
the ensemble of dust and larger particles (e.g., boulder size) in interplanetary space, generated by the gradual erosion and disintegration of asteroids, comets, and other bodies; components of the complex are manifested in diverse ways (e.g., zodiacal light, IDPs collected in Earth's stratosphere, and so on) and are generally small enough so that their motions are governed not only by gravitational forces but also by radiation forces (e.g., light pressure or Yarkovsky forces) and forces sensitive to particle charge
Kirkwood gap—
a narrow zone in the asteroid belt (generally surrounding a commensurability) that has been depleted of asteroids
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
Laplace orbital resonance—
a commensurability in the mean motions that causes repeated alignment of planetary satellites at identical points in their orbits
Nuclease—
an enzyme that degrades nucleic acids
Oort Cloud—
a spherical zone beyond the outer solar system, extending part way to the nearest star, where long-period comets originate
Poynting-Robertson drag—
forces interplanetary dust particles into Earth-crossing, inwardly spiraling circular orbits around the Sun
Prokaryotic cells—
cells without a defined nucleus, e.g., bacteria
Protease—
an enzyme that degrades proteins
Protoplanetary disk—
another term for the solar nebula at the time it was flattened into a disk, or an analogous disk from which a single planet (e.g., Jupiter) and its satellites were formed
Secular resonance—
occurs where the frequency for an asteroidal orbital precession rate matches a main frequency for planetary eccentricities in Brouwer and van Woerkom's secular theory
SNC meteorites—
achondrites that originate from Mars
Sterilization—
the destruction of all living microorganisms including vegetative forms and spores
Stickney—
the largest crater on Phobos
Tektites—
glassy meteorite-like objects formed when the ejecta of large terrestrial craters was lofted above Earth's atmosphere and then reentered
Tidal heating—
heating of a planet or satellite as a result of the work performed on the object's materials by the flexing due to gravitational interactions between bodies
Trojan asteroids—
asteroids located at the 1/1 mean-motion resonance (commensurability) with Jupiter, librating about the L4 and L5 points 60 degrees ahead of, and behind, Jupiter in its orbit
Vibrios—
comma-shaped bacteria that are common to aquatic environments