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comet(s): missions to

from 20 to 50 AU. Their orbits typically have small eccentricities. Both cometary reservoirs are thought to represent primordial solar system material. Comets are distinguished observationally by the emission of gas. As a result, a comet with a dust coating on its surface that inhibits gas production might be classified as an asteroid. Because of this ambiguity, objects such as Chiron, a Centaur asteroid, have now been reclassified as comets. Kuiper belt objects, which are expected to be composed mostly of ice, are classified as comets. Since Pluto and Charon are also composed mainly of ice and are thought to have originally been Kuiper belt objects, they too may be thought of as comets. Triton, although almost certainly a captured Kuiper belt object, orbits a planet and is identified as an icy satellite rather than a comet. Comet nuclei have radii which are typically in the range of 1 to 100 km, although they may be considerably larger. Each time the comet passes the sun, it loses more ice and dirt (if the Earth later passes through this debris, we see a meteor shower) until eventually the comet no longer displays a coma and/or tail. A comet discovered at this point of its evolution is often identified as an asteroid, so some asteroids are probably dead comets. See Oort cloud, Kuiper belt.

comet(s): chemical composition of The volatile material of comets is primarily amorphous water ice but also contains, with some variation in quantity, other simple molecules including a few percent (relative to water) of carbon dioxide (CO2), carbon monoxide (CO), formaldehyde (H2CO), methanol (CH3OH ), and methane (CH4). At a lower level many other molecules have been detected including

NH3, H CN, H NC, C2H6, C2H2, H2S, SO2, and sometimes OCS (Hicks and Fink, 1997). Larger molecules include: H C3N, NH2CH O, H COOH , CH3CN, H NCO, and possibly H COOCH3. The solid materials are probably amorphous silicates, minerals commonly observed in meteorites, such as olivines and pyroxines, and small grains of circumstellar or interstellar origin seen in interplanetary dust particles, such as graphitic or diamond-like carbon grains and silicon carbide. Repeated evaporation of volatiles from successive passages through the inner solar system may result in a

surface layer of complex non-volatile organics and rocky material.

comet(s): dirty iceball or snowball model

Comets are composed of a mixture of volatile and rocky material, what Whipple described as a dirty snowball. It has been estimated that comet Shoemaker–Levy 9, which impacted Jupiter, had a low density and little physical cohesion, suggesting that comets are loosely packed light snowballs with void spaces. Greenberg advocates comets as aggregates of remnant microscopic interstellar ice grains that have experienced little heating and suggests that comets, at least originally, were very homogeneous and similar to one another. Abundances and deuterium enrichments of molecules in the coma and tail of Hale–Bopp and Hyakutake are similar to interstellar values, and the ortho to para ratio (the quantum spin state of the hydrogen atoms

— a measure of the conditions the ice has experienced) of the water from comet Hale–Bopp implies that it formed at and was never warmed above 25 K. Taken together these support the contention that Hyakutake and Hale–Bopp (and by extension other comets) have interstellar heritage and have not experienced much heating. However, comets are notoriously unpredictable in their behavior, which could imply heterogeneity. Weidenschilling has pointed out that comets display complex variations (i.e., outbursts and jetting), and believes that this points to compositional inhomogeneities on the order of tens of meters, indicating that these comet nuclei either formed heterogenously or later became differentiated.

comet(s): missions to Deep Space 1 (http://nmp.jpl.nasa.gov/ds1/) launched in October 1998 and will fly by Comet West– Kohoutek–Ikemura in June 2000, although that is not its primary target. The Stardust mission (http://stardust.jpl.nasa.gov), which launched in early 1999, will use aerogel to collect dust from Comet Wild 2 in 2004 and return the sample to Earth in 2006. The Deep Space-4, Champollion mission (http://nmp.jpl.nasa.gov/st4/) will launch in 2003, meet and land on Comet Tempel 1 in 2006, and return a sample in 2010. The Rosetta Mission (http://www.esoc.esa.de/ external/mso/projects-index.html) will launch

© 2001 by CRC Press LLC

comet, artificial

in 2003, rendezvous with Comet Wirtanen in 2112-13, and RoLand (the lander) will make further measurements on the surface of the comet.

comet, artificial

See barium release.

comminution The breaking up and fragmentation of a rock or other solid. The Earth’s crust is comminuted on a wide range of scales by tectonic process. In California the crust has been fragmented into blocks on scales of microns to tens or hundreds of kilometers. Fault gouge is an example of comminution on the smallest scale.

common envelope A binary star system enters a common envelope phase when one, or both, of the stars in the binary overfills its Rochelobe and the cores orbit within one combined stellar envelope. Common envelope phases occur in close binaries where the Roche-lobe overflowing star expands too rapidly for the accreting star to incorporate the accreting material. Most common envelope phases occur when a star moves off the main sequence and expands toward its giant phase (either in Case B or Case C mass transfer phases). The standard formation scenarios of many short-period binary systems (e.g., low-mass X-ray binaries, double neutron stars) require a common envelope phase which tightens the orbital separation and ejects the common envelope.

common envelope binary Binary stars that are so close to one another that both fill their respective Roche surfaces, resulting in a common envelope that surrounds both stars.

comoving frame In general relativity coordinates are just labels for space-time points and have no a priori physical meaning. It is, however, possible to associate those labels to part of the matter present in the universe, in which case one has a material realization of a reference frame. Since such coordinates follow the matter in its motion, the corresponding frame is said to be comoving. A fundamental requisite is that the trajectories of the objects considered do not cross at any point; otherwise the map of coordinates would become singular in such points. A useful, but not necessary, property is that those objects interact only gravitation-

ally, so as to track geodesics in the background spacetime.

compact group of galaxies Isolated group of galaxies for which the separation between the galaxies is comparable to the size of the galaxies themselves; groups of galaxies isolated by Hickson from the Palomar Observatory Sky Survey according to three criteria: (1) there are at least four members whose magnitudes differ by less than three magnitudes from the magnitude of the brightest member; (2) if RG is the radius of the circle on the sky containing all group members, then the distance to the nearest galaxy outside the group must be larger than 3 RG (in other words, the group must be reasonably isolated and not an obvious part of a larger structure); (3) the mean surface brightness within RG should be brighter than 26 mag per square second of arc, i.e., the group must not contain vast empty sky areas and hence should be “compact”.

compaction As rocks are buried to a greater depth in a sedimentary basin, the “lithostatic” pressure increases. This causes the rock to compact. The void space, or “porosity” of the rock, decreases with increasing depth, and the density increases.

compact steep spectrum radio sources

A class of radio sources which includes radio galaxies and quasars unresolved at resolution 2 arcsecs. They are differentiated from other core-dominated radio sources by showing a steep radio spectrum. Observations at higher resolution show that compact steep spectrum radio sources are either classical lobe dominated sources whose lobe size is less than the size of the galaxy or quasars with a core single-side jet morphology. In both cases, the radio morphology appears often to be disrupted and irregular. Compact steep spectrum radio sources are thought to be young radiogalaxies which are expanding or, alternatively, radio sources in which the expansion of the radio plasma is hampered by interstellar or intergalactic medium.

compensation In geophysics, the positive mass of major mountain belts is compensated by the negative mass of crustal mountain roots. The crustal rocks are lighter than the mantle rocks be-

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Compton scattering

neath. In a compensated mountain belt, the total mass in a vertical column of rock is equal to the total mass in the adjacent lowlands. A mountain belt behaves like a block of wood floating on water.

In cosmological scenarios where topological defects are suddenly formed, both the geometry of spacetime and ordinary matter and radiation fluids are perturbed in such a way as to satisfy theoretical conservation laws imposed by general relativity theory. Matching conditions between times before and after the relevant phase transition during which defects are generated require that cosmic fluid perturbations compensate for the energy density inhomogeneity of space in the presence of the defect. See cosmic phase transition, cosmic topological defect, Kibble mechanism.

compensation depth The depth of the Earth at which the overlying rocks are assumed to exert a constant pressure. Below this depth lateral variations in density are assumed to be small.

composite volcanos Composite volcanos, also called stratovolcanos, are steep-sided volcanic cones consisting of alternating layers of lava flows and ash deposits. Eruptions from composite volcanos are often very explosive and deadly, as evidenced by the eruptions of volcanos such as Vesuvius in Italy, Mt. St. Helens in the United States, and Mt. Pinatubo in the Philippines. Composite volcanos are found near subduction zones on Earth and are produced by silica-rich magmas moving upwards from the subducted plate. The high silica content of the magmas allows them to be very viscous and retain much gas, which leads to the explosive nature of the eruptions.

compound channel A channel or river in which the equation for flow area vs. depth exhibits a discontinuity.

compressibility The ratio of the fractional change in volume dV/V (volumetric strain) in response to a change in pressure dp, that is,

β =

 

1

dV

= −

1

 

 

 

 

 

 

 

V

 

dp

ρ dp

where ρ is density. Isothermal or adiabatic changes of V and p yield slightly different values of β. β is a modulus of elasticity, and its reciprocal is the bulk modulus.

Compton, Arthur H. Physicist (1892– 1962). His studies in X-rays led him to discover the Compton effect, that is the change in wavelength of a photon when it is scattered by a free electron. The discovery of the Compton effect confirmed that electromagnetic waves had both wave and particle properties.

Compton cooling The reduction of energy of a free electron, due to its interaction with a photon. If the kinetic energy of the electron is sufficiently high compared to the incoming photon, the energy of the incoming photon plus part of the kinetic energy of the electron is redirected as a photon of higher energy. Named after Arthur H. Compton (1892–1962).

Compton reflection The Compton scattering of hard X-ray radiation by a layer of dense and thick matter, such as the surface of a star or of an accretion disk. Hard X-ray radiation is scattered off the surface of the layer after having lost part of its energy. Compton reflection creates a distinguishing spectral feature, an enhancement in the spectral energy distribution between 10 and 50 keV. Such features have been detected in the spectra of several Seyfert-1 galaxies and of a galactic object, the black hole candidate Cyg X- 1. See Seyfert galaxies, Cygnus X1.

Compton scattering The inelastic scattering of high energy photons by charged particles, typically electrons, where energy is lost by the photon because of the particle recoil. A photon carries momentum, part of which is exchanged between the photon and the particle. Conservation of energy and momentum yields an increase in the photon wavelength (and hence a decrease in photon energy) as measured in the initial rest frame of the electron equal to

λ λ0 = λC(1 cos θ) ,

where λ0 is the wavelength of the incident photon, θ is the angle between the initial and final direction of propagation of the photon, and λC is a constant, called the Compton wavelength, and

© 2001 by CRC Press LLC

computational relativity

defined by λC = h/mc, where h is the Planck constant, m the particle mass, and c the speed of light. In the case of scattering by electrons, λC = 0.02426 Å. If λ λC, then the energy exchange is irrelevant, and the scattering is elastic (Thomson scattering). Compton scattering by electrons occurs for photons in the X-ray domain. See inverse Compton effect, Thomson scattering.

computational relativity

Numerical relativ-

ity.

 

conditional unstability

The atmosphere is

said to be conditionally unstable when the lapse rate is between the adiabatic lapse rate and moist lapse rate. In this case, parcels displaced downward will be restored, whereas saturated parcels displaced upward will continue to move upward.

conducting string In cosmology, possible topological defects include conducting cosmic strings. In generic grand unified models, one may have couplings between the cosmic stringforming Higgs field and fermionic fields, and the vanishing of Higgs-generated fermionic mass terms in the core of the defect allows the existence of fermionic zero modes carrying currents along the string. Alternately, bosonic conductivity arises when charged boson fields acquire non-zero expectation values in the string cores. These currents are persistent, and the vortex defects containing them are called conducting cosmic strings. They are also often referred to as superconducting strings, for it can be seen that the electric (or other) current they carry is dissipationless.

The production of equilibrium currentcarrying string loop configurations called vortons may contribute to the dark-matter density of the universe. Such loops might also serve as seeds for the generation of primordial magnetic fields. See current carrier (cosmic string), cur- rent generation (cosmic string), fermionic zero mode, vorton, Yukawa coupling, Witten conducting string.

conduction (1.) Transport of electric current. (2.) Transfer of heat without the flow of particles from one part of a medium to another by the transfer of energy from one particle to the next

and by lattice oscillations (phonons) in a solid. This flow of heat is directed by temperature gradients in the medium. In the solar corona, the conductive flux, Fcond , is directly proportional to the temperature gradient for classical Spitzer conduction, via Fcond = κ0 T , where κ0 is the coefficient of thermal conductivity parallel to the magnetic field.

conductive heat transfer Transfer of heat due purely to a temperature difference. Heat conduction is a diffusive process, in which molecules transmit kinetic energy to other molecules by colliding with them. Fourier’s law of heat conduction states

q = −λ · T

where q is the heat flux vector, λ is the thermal conductivity (tensor), and T is the temperature gradient. The minus sign indicates that heat is transferred by conduction from higher temperature to lower temperature regions.

conductivity of water The ionic content of water enhances the electric conductivity of water (strongly temperature-dependent). Since most of the dissolved solids in natural waters are present in the form of ions, the easy-to-measure conductivity is a practical way to estimate salinity. Instead of salinity, often conductivity normalized to T = 20 or 25C is used in lakes to express the concentration of ionic content. (Conductivity in ocean and fresh water is in the range of 50 70 mS/cm and 50 500 µS/cm, respectively.)

conformal infinity In relativity there exists a conformal isometry of Minkowski spacetime with a region of an Einstein universe. Conformal infinity I is the boundary of this region. The boundary I has the topology of a light cone, with the vertex points i± (timelike future/past infinity) missing. Future/past conformal infinity I± is the set of future/past endpoints of null geodesics in I. See null infinity.

conformal tensor Weyl tensor.

congruence A family of curves at least in some small region, such that one and only one curve passes through each point in the region.

© 2001 by CRC Press LLC

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