DARK HALO - Extended region surrounding a galaxy containing globular clusters and other old stars. The halo has considerable mass but relatively low luminosity, suggesting it contains a lot of dark matter.
DARK ENERGY - Form of energy that is gravitationally repulsive, due to a negative effective pressure. Dark energy is spread almost uniformly throughout space and appears to contribute about 70% of the present energy density of the universe. Its existence was recently inferred from observations of distant Type Ia supernovae from combining CMBR anisotropy measurements with measurements of the density of dark matter. The cosmological constant is a particular form of dark energy.
DARK MANTLING DEPOSITS - Thin layers of ejecta from lunar volcanism covering an older surface. The largest of these areas are near the edges of the lunar mare and cover many thousands of square kilometers. Apollo 17 brought back samples from one such unit, which contained many small spheres of orange and black glass that probably represent quenched droplets of magma. The extent of the dark mantle deposits indicates that some of these spheres may have been thrown hundreds of kilometers. Thus, despite the low gravity and lack of air on the Moon, some of lunar eruptions must have been quite violent, resembling Hawaiian fire fountains, but on a much larger scale.
Many smaller dark mantling units also occur on the Moon, most only a few km in diameter. These are almost always located near the mare or in large crater floors (below). Many also lie along fault lines. Since most have a small central pit or crater, they probably represent small volcanic explosions.
DARK MATTER - Form of matter that does not emit light, absorb light, or otherwise interact with electromagnetic radiation. Its only interactions are gravitational and dark matter particles can clump under the force of gravity (unlike "dark energy" which is evenly distributed throughout space). The existence of dark matter has been inferred from its gravitational effects on the dynamics of luminous tracers in galaxies and galaxy clusters and on the bending of light rays (gravitational lensing). Recent evidence indicates that dark matter contributes about 25% of the critical density for a flat universe, with the remainder, except ~5% ordinary matter, as dark energy. Much of the dark matter resides in halos encompassing luminous galaxies and in clusters of galaxies where it appears to be distributed more smoothly. Evidence from Big Bang Nucleosynthesis (BBN) and the cosmic microwave background radiation indicates that most dark matter is non-baryonic, that is, not composed of quarks. Particle physics theories predict the existence of exotic weakly interacting elementary particles (WIMPs), which are hypothetical candidates for non-baryonic dark matter. Dark matter plays a critical role in shaping the formation of large-scale structure. The bulk of baryons in the universe are also dark, perhaps in the form sub-stellar objects (MACHOs).
DAUBREELITE - Fe-Cr sulfide found in many meteorites; its formula is FeCr2CS4.
DAUGHTER NUCLIDE - Nuclide produced by the radioactive decay of another "parent" nuclide. May be stable or may decay further.
DECAY BRANCHING PERCENTAGE - Nuclide decay rate by a particular decay mode. Some nuclides decay by only one mode, others by more than one mode. For example, 187Pb decays 98% by β decay and 2% by α decay.
DECAY MODE - Disappearance of a radioactive substance due to nuclear emission of an α or β particle, capture of an atomic electron, or spontaneous fission. In rare instances proton, neutron, or light element (for example 14C) emission can occur. When a large amount of decay energy is available, β-delayed emission of neutrons, protons, and other particles may occur.
DEEP IMPACT - Mission study the structure and composition of comets by making a deep crater in Comet Tempel 1 (see http://www.nasa.gov/mission_pages/deepimpact/main/index.html). The flight system consisted of the flyby spacecraft and an impactor. The impactor successfully struck the comet nucleus producing a bright flash as it and part of Tempel 1 vaporized. Subsequently, an ejecta curtain of hot gas and dust emerged and expanded from the crater. Spectrometry revealed a complex mix of silicates, water and organic compounds beneath the surface of the comet.
DEGENERATE MATTER - Highly compressed matter in which the normal atomic structure has broken down and which, because of quantum-mechanical effects, exerts a pressure that is independent of temperature. Bodies with masses <1.4 Msun (e.g., white dwarfs) are supported by electron degeneracy pressure and have densities of ~106 kg/m3. If the mass of a collapsed star >1.4 Msun, gravity will overwhelm electron degeneracy and further collapse ensues. Electrons combine with protons to form neutrons, so producing a neutron star. Because neutrons, like electrons, are subject to the Pauli exclusion principle, at high enough densities ~1014 kg/m3, neutron degeneracy pressure prevents further collapse of the star. For masses >2-3 Msun, even neutron degeneracy can't prevent further collapse, and a black hole is formed.
DENSITY (ρ) - Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much denser at its center than near its surface. Density is commonly expressed as g/cm3, but the correct SI units for density are kg/m3 (1 g/cm3 = 1000 kg/m3).
DENSITY WAVE - Wave, usually spiral, that perturbs the density of gas and solids in a galactic or protoplanetary disk. Spiral density waves can lead to the appearance of ringlets. Spiral density waves have also been proposed as an explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation.
DENSITY PARAMETER (Ω) - Ratio of the actual density of the universe to the critical density.
Closed models of the universe have Ω > 1, flat models have Ω = 1, open models have Ω < 1. Omega can be broken into two components: Ωm and Ωv, which represent density of matter and dark energy. It is now accepted that the dark matter component is substantial and the universe is open, probably accelerating. The diagram shows several sets of Ω values.
DESTRUCTIVE INTERFERENCE - Result when two waves overlap such that they are completely out ofphase. When destructive interference occurs, the two waves are subtracted from each other. If the two waves have equal amplitude before colliding they will completely cancel each other.
DETECTOR - Any device used to sense the passage of a particle or photon (x-ray, γ-ray, etc.). X-rays can be detected using sealed-gas proportional, gas-flow proportional detectors, and Li-drifted Si semiconductor detectors. A Li-drifted Ge detector is used to count γ-rays in the laboratory.
DEUTERIUM - Isotope of hydrogen whose nucleus contains one proton and one neutron. As a trace element formed during the nucleosynthesis epoch of the Big Bang, deuterium is an important indicator of the baryon density in the universe. The larger the density, the smaller the amount of deuterium formed in Big Bang nucleosynthesis.
DIAMOND - One of the naturally occurring forms of carbon found in meteorites. Each C atom is bonded through covalent sp3 hydrid orbitals to four others. The strength of the C-C bonds makes diamond the hardest substance in terms of resistance to scratching.
DIAPLECTIC GLASS - Natural glass formed by shock transformation from any of several minerals (more commonly feldspar) without melting. It is found only in association with meteorite impact craters and crater ejecta. Diaplectic plagioclase glass is called maskelynite.
DIFFERENTIATION - Process by which an originally homogeneous planetary or asteroidal body separated into regions of different composition, such as core, mantle and crust. Heating of the primordial mixture of stony minerals, metals, and sulfides produced dense metallic liquids which sank to become planetary or asteroidal cores. Less dense silicate magmas rose and solidified to become basaltic or, on Earth, granitic crust. Material with neutral buoyancy formed planetary mantles.
DIFFUSION - Movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). Diffusion, the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena.
Because diffusion is thermally activated, coefficients for diffusion by a single mechanism at different temperatures may be described by an Arrhenius equation:
Data may be fit by a straight line on a graph of log D (m2/s) as a function of l/T (K-1). The resulting line has a slope of ΔH/(2.303R), where R is the gas constant (8.3143 J/mole K) and ΔH (J/mole) is the "activation energy."
DIMICT BRECCIAS - Breccia made up of only two lithologies. The term is usually applied to a common type of rock collected on the Apollo 16 mission that consists of anorthosite (light color) and mafic (dark, Fe rich) crystallized impact melt in a mutually intrusive textural relationship. These are also termed "dilithologic breccias.".
DIOGENITE (DIO) - Achondrite composed mostly of Mg-rich orthopyroxene (En67-77), with minor amounts of olivine (Fa27-35) and plagioclase (An60-91). Pyroxene are usually coarse-grained, indicating that diogenites represent intrusive rocks that formed in magma chambers within the deeper regions of 4 Vesta. The Tatahouine meteorite, a unique diogenite that fell in Tunisia in 1931, is renowned for its green, centimeter-sized pyroxene crystals. Diogenites belong to the HED group, which also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from space (a realization forgotten for the next 2,000 years).
DISTAL EJECTA - Impact ejecta found at distances more than 5 crater radii from the rim of the source crater.
DOME - Rounded volcanic structure produced by eruption or intrusion of viscous high-silica magma. Domes are common on Earth, but rare on the Moon. Lunar domes are mostly fairly small, few more than a few hundred meters high or more than 10-15 km across. They are also somewhat irregular in outline and few show any large central pit or vent structures, but many do have very small central pits or craters. Most lunar domes appear to be basaltic and not formed like Earth domes from silicic lavas. Instead, lunar domes may mark places where the erupted basalts were just barely molten and consequently had high viscosity.
DOPPLER EFFECT - Change in frequency of a wave (light, sound, etc.) due to the relative motion of source and receiver. Approaching objects have their wavelengths shortened. Receding objects have emitted wavelengths lengthened.
DOPPLER SHIFT - Result of the relative motion of the light emitting object and the observer. If the source of light is moving away, the wavelength of the light is stretched (shifted towards lower frequencies). If the source of light is approaching, the wavelength of the light is compressed (shifted towards higher frequencies). These effects, called redshift and blueshift, respectively, are together known as Doppler shifts. The shift in the wavelength, λobserved - λrest, is given by a simple formula (v = velocity, c = speed of light):
DUST - Grains of carbon and silicate ~0.1-1.0 μm in size. Dust grains are a major component of the interstellar medium. Dust blocks visible light causing interstellar extinction and scatters incident starlight, particularly blue light (which has a wavelength comparable to the dust grain's size), causing reddening. Cooling of interstellar gas and dust clouds occurs by infrared emission from the dust.
DWARF GALAXY - Relatively small galaxy, typically containing ~107 stars. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere at a distance of ~50 kpc, are two irregular dwarf galaxies gravitationally bound to the Milky Way. The latter contains ~1011 stars.
DYNAMO THEORY - Branch of magnetohydrodynamics concerned with self-excitation of magnetic fields in any large rotating mass of conducting fluid in motion (usually turbulent). Self-exciting dynamo action is believed to account for magnetic fields at the planetary, stellar, and galactic scales.