C
Glossary


Aitken Basin

Largest impact basin known, located in the Moon’s southern polar region

alpha decay

Radioactive decay of a nucleus by the emission of an alpha particle, the nucleus of a helium-4 atom

amino acid

Class of basic molecules that make up proteins

apoptosis

Cell death

Archean

Period in Earth’s history from 3.8 to 2.5 billion years ago, when life arose

Argyre

Major impact basin on Mars

asteroid

Rocky fragments left over from the formation of the solar system

astrosphere

Zone of interaction between a star and its interstellar environment


beta decay

Radioactive decay of a nucleus by the emission of an electron (or positron) and an electron antineutrino (or electron neutrino)

biosphere

Life zone of Earth, which includes all living organisms and all organic matter that has not yet decomposed

bolide

Meteor that often explodes as a fireball


carbonaceous chondrite

Stony meteorite that contains carbonaceous compounds

Chicxulub

Impact basin in Mexico, which was created at the end of the Cretaceous and the beginning of the Tertiary

chirality

Degree to which an object is different from its mirror image

chromosphere

Visually transparent layer above the Sun’s visible photosphere



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The Astrophysical Context of Life C Glossary Aitken Basin Largest impact basin known, located in the Moon’s southern polar region alpha decay Radioactive decay of a nucleus by the emission of an alpha particle, the nucleus of a helium-4 atom amino acid Class of basic molecules that make up proteins apoptosis Cell death Archean Period in Earth’s history from 3.8 to 2.5 billion years ago, when life arose Argyre Major impact basin on Mars asteroid Rocky fragments left over from the formation of the solar system astrosphere Zone of interaction between a star and its interstellar environment beta decay Radioactive decay of a nucleus by the emission of an electron (or positron) and an electron antineutrino (or electron neutrino) biosphere Life zone of Earth, which includes all living organisms and all organic matter that has not yet decomposed bolide Meteor that often explodes as a fireball carbonaceous chondrite Stony meteorite that contains carbonaceous compounds Chicxulub Impact basin in Mexico, which was created at the end of the Cretaceous and the beginning of the Tertiary chirality Degree to which an object is different from its mirror image chromosphere Visually transparent layer above the Sun’s visible photosphere

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The Astrophysical Context of Life 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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The Astrophysical Context of Life heliosphere Huge magnetic bubble created in interstellar space by the outrushing solar wind and associated magnetic field Hellas Major impact basin on Mars Herschel European Space Agency’s far-infrared and submillimeter telescope project, due to launch in 2007 homochirality Molecules possessing structure with the same handedness hypermutable Genes that have a high rate of mutation impact basin Large crater produced by a meteorite impact in silico Relating to processes that occur in computer simulations in vitro Relating to processes that occur in a laboratory in vivo Relating to processes that occur in living matter isotope Atoms of a chemical element whose nuclei have the same atomic number, Z, but different atomic weights due to different numbers of neutrons K-T boundary Boundary between the Cretaceous and Tertiary periods, about 65 million years ago Kuiper Belt Area of the solar system extending from within the orbit of Neptune (at 30 AU) to some 50 AU from the Sun Lake Acraman Large meteor impact crater in South Australia L-enantiomer Molecule with left-handed chirality L-isomer Molecule with left-handed chirality Long-Duration Exposure Facility NASA experiment involving a satellite that was retrieved after several years to determine the effects of long-term exposure to space L-ribose Left-handed form of a five-carbon sugar LUCA Last Universal Common Ancestor (of life) magma chamber Underground pocket containing molten rock meiosis Reproduction through cell division metals To astronomers, any element except hydrogen or helium mtDNA Mitochondrial DNA NAI NASA Astrobiology Institute NASA U.S. National Aeronautics and Space Administration NSCORT NASA Specialized Centers of Research and Training NEO Near-Earth object, an asteroid or comet that comes relatively close to Earth NIH U.S. National Institutes of Health NSF U.S. National Science Foundation

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The Astrophysical Context of Life obliquity Angle between the plane of a planet’s orbit and that of the planet’s equator Oklo Series of natural fission reactors that were active in the Oklo Valley of Gabon, Africa, about 1.5 billion years ago Oort Cloud Spherical cloud of comets with semimajor axes between 1,000 and 50,000 AU oxygen radical Oxygen atom with an unpaired electron. Radicals are powerful oxidizing agents that can cause structural damage to proteins and nucleic acids. panspermia Hypothesis that life originated elsewhere in the universe and migrated through space to Earth and, potentially, elsewhere photosynthesis Biochemical process by which the energy of light is converted into chemical energy in plants, algae, and certain bacteria phototroph Organism that uses light as its main source of energy phylogenetic Of or related to the evolutionary development of a group of genetically related organisms placer deposit Alluvial deposit of sand and gravel representing stream beds planetesimal Small rocky and icy object that existed at an early stage in the development of the solar system polymerase Enzymes that catalyze the polymerization of nucleic acids polyols Polyhydric alcohols or sugar alcohols polysaccharides Polymers made up of chains of simple sugars. Examples include starch, cellulose, and glycogen protein Complex, high-molecular-weight organic compound that consists of amino acids joined by peptide bonds Pseudomonas fluorescens Common nonpathogenic, gram-negative bacteria that produces a soluble greenish fluorescent pigment, particularly under conditions of low iron availability P-T boundary Geologic boundary between the Permian and Triassic periods, about 250 million years ago pyrimidine dimer Two adjacent pyrimidine nucleotides, usually thymine, in which the pyrimidine residues are covalently joined by a cyclobutane ring. These dimers stop DNA replication. pyrimidines Organic compounds with a heterocyclic ring: two nitrogen atoms taking the place of carbon atoms at positions 1 and 3 relative to each other around the six-member ring radiolarians Single-celled organisms that form protective skeletons, usually made of silicon dioxide ram pressure Force per unit area required to stop the solar wind flow

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The Astrophysical Context of Life redox state Chemical reaction environment determined by the balance of negatively and positively charged atoms replication Reproduction from a template ribose Five-carbon sugar RNA Ribonucleic acid, a molecule involved in transcription of the instructions in DNA. Some theorize that RNA was the precursor to DNA. single-strand break When one strand of the DNA molecule is broken, usually by oxidative stress or certain forms of radiation solar flare Eruption of hot gas and radiation from the Sun’s photosphere solar neighborhood Sun and nearby stars Stardust mission NASA mission to return dust from Comet Wild 2 subduction Geologic process by which one tectonic plate is forced under another Sudbury Ancient impact basin in northern Canada supernova Stellar explosion tars Complex hydrocarbons with high viscosity and melting point tectonics Theory that Earth’s lithosphere exists as separate and distinct plates that float on a fluid-like asthenosphere Titan Largest moon of Saturn TPF NASA’s Terrestrial Planet Finder transition elements Elements located in groups IB to VIIIB of the Periodic Table. They have many of the characteristics of metals. ultraviolet Electromagnetic radiation more energetic than visible light UVB Ultraviolet sunlight that penetrates the ozone layer and reaches Earth’s surface UVC Ultraviolet sunlight with shorter wavelength than UVB that is filtered out by Earth’s ozone layer VPL Virtual Planetary Laboratory, hosted by Caltech and the Jet Propulsion Laboratory volatile Substance with a low evaporation temperature Vredefort 300-km impact crater in South Africa, formed about 2 billion years ago x ray Electromagnetic radiation more energetic than ultraviolet light