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The following HTML text is provided to enhance online readability. Many aspects of typography translate only awkwardly to HTML. Please use the page image as the authoritative form to ensure accuracy. clay Generic term for an aggregate of hydrous silicate particles less than 4 µm in diameter comet 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 Darwin 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 dichroism Differential absorption of left- and right-hand circularly polarized light in a material DNA Deoxyribonucleic acid, the molecule that carries genetic information DOE U.S. Department of Energy dose 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 D-ribose 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 endogenous Organic matter synthesized on Earth and necessary for the appearance of life (see exogenous) 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 Hadean Era in Earth’s history 4.5 to 3.8 billion years ago, before life is thought to have arisen
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