Generic term for an aggregate of hydrous silicate particles less than 4 µm in diameter


Relatively small planetary body similar to an asteroid but composed largely of ice

cosmic rays

High-energy particles originating outside Earth

cross strand exchange

Process in which bases along one double helix strand separate and link across to a separate double helix and vice versa


European Space Agency’s large infrared-wavelength space interferometer

Deep Impact

NASA mission to collide a spacecraft to create a deep crater in Comet Tempel 1

Deinococcus radiodurans

Gram-positive aerobic bacteria, the most radiation-resistant organism known


Differential absorption of left- and right-hand circularly polarized light in a material


Deoxyribonucleic acid, the molecule that carries genetic information


U.S. Department of Energy


Absorbed amount of radiation

dose rate

Rate at which tissue absorbs radiation

double-strand break

Break in both strands of the DNA molecule, usually caused by oxidative stress or certain forms of radiation


Right-handed form of a five-carbon sugar, found in all living cells

early stars

Stars that formed primarily from hydrogen and helium and are deficient in heavier elements


Organic matter synthesized on Earth and necessary for the appearance of life (see exogenous)


Organic matter delivered from space and necessary for the appearance of life (see endogenous)

Exploration Initiative

U.S. multiagency Vision for Space Exploration

field reversal

Periodic reversal of the magnetic field of a star or planet

flare star

Star that sporadically flares in brightness by an order of magnitude or more

gamma ray

Most energetic form of electromagnetic radiation

gamma-ray burst

Energetic cosmic event that produces a burst of gamma rays

habitable zone

Radial zone in which certain parameters thought to be necessary for life are met


Era in Earth’s history 4.5 to 3.8 billion years ago, before life is thought to have arisen

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