Accretion - The process whereby colliding particles adhere to one another causing the body to become larger. The planets are thought to have formed by the accretion of planetesimals.
Angular Momentum - The angular momentum of a point mass can be expressed in the equation L = mvr where "L" represents the angular momentum; "m"represents mass; and "v" represents velocity in a circular path having a radius of "r". The angular momentum conservation principle states that the angular momentum of a system must remain constant. Thus, if the value for "r" decreases, the value for "v" must increase.
Atmosphere - A layer of gases gravitationally bound to a planet.
Atom - The smallest whole unit of matter. Atoms are made of positively charge protons and neutral neutrons which are located in the nucleus, and negatively charged electrons which orbit the nucleus.
Atomic Number - The number of protons in the nucleus of an atom. For example, the atomic number of hydrogen is 1 because it has 1 proton in its nucleus. The identity of an atom is determined solely by its atomic number.
Atomic Weight - The sum of the protons and neutrons in an atom. On the periodic table it is expressed as a decimal since it represents the average of all isotopes of that element.
Condensation - The process whereby a gas changes to the liquid state as a result of cooling. Planetesimals formed as a result of the condensation of the solar nebula. Because the solar nebula varied in temperature with distance from the evolving Sun, different materials were able to condense at different distances.
Daughter Element - The element produced by the radioactive decay of a parent atom.
Differentiate - The new element produced as the result of radioactive decay.
Diatomic - Term used for molecules consisting of 2 identical atoms. The molecular symbol for diatomic oxygen is O2.
Ecliptic - The orbital plane of the Earth.
Element - A specific type of atom, identified by the number of protons in the nucleus. There are 92 naturally occurring elements.
Escape Velocity - The speed at which a object must travel in order to escape the gravitational pull of a planet.
Gravitational Energy - Energy released due to the gravitational collapse of material. This mechanism is a critical part of star formation since it eventually will produces temperatures sufficiently high for thermonuclear reactions to begin.
Half Life - The time it takes for fifty percent of the existing radioactive atoms to decay into daughter atoms.
Inclination of Orbit - The angle between the orbital plane of a planet and the ecliptic.
Kinetic Energy - Energy of motion. A ball rolling down a hill has kinetic energy. Atoms and molecules, although we cannot see them, also have kinetic energy. They are constantly in motion. How fast they are moving is directly related to temperature, the higher the temperature, the faster they move. If the kinetic energy of an atmopheric gas molecule is high enough, it may reach escape velocity and escape the gravitational pull of a planet.
Molecular Weight - The combined weight of all of the atoms that make up a molecule.
Nebula - A massive cloud of dust and gas, There are three types of nebulae: absorption (dark) nebulae, reflection nebulae and emission nebulae.
Orbital Plane - The plane that coincides with the orbit of a planet. The orbital plane of the earth is referred to as the "ecliptic".
Ozone - The triatomic form of oxygen O3 that naturally forms in the stratosphere. It helps to shield the Earth from harmful solar radiation by absorbing unltraviolet energy.
Parent Element - The original radioactive element which through the decay process gives rise to a new element called the daughter.
Primary Atmosphere - An atmosphere composed of primordial hydrogen and helium. The jovian planets have primary atmospheres.
Protosun - A star still in its formative stages, prior to when it becomes a Main Sequence Star. While it is the hottest portion of the solar nebula (approximately 1500 degrees Kelvin), it is still cooler than an ordinary star . The term refers to a star before nuclear reactions are initiated.
Radioactive Decay - The process whereby an unstable nucleus spontaneously breaks down, releasing energy and particles from the nucleus. The result is the creation of a new nucleus. Eventually, through the process of continuous decay, a stable, non-radioactive nucleus is formed.
Secondary Atmosphere - An atmosphere that evolves around a terrestrial planet after the loss of any primary atmosphere. A secondary atmosphere evolves from gasses released from the interior of the planet as a result of volcanic outgassing. Such atmospheres consist of carbon dioxide (CO2), nitrogen (N2) and water (H2O). The carbon dioxide atmospheres of Venus and Mars are representative of secondary atmospheres.
Sink - A mechanism that helps to remove carbond dioxide from an atmosphere. On the Earth the three sinks are the oceans, chemical reactions in seawater that generate rocks such as limestone (CaC03) and dolomite (CaMgCO3), and photosynthesis by plants.
Solar Nebula - A rotating disk of dust and gas out of which a solar system is thought to evolve.
Super Nova - A violent explosion that occurs at the end of the evolution of a high mass star. The energy released during the explosion is equivalent to the energy released by 100 billion stars.
Tertiary Atmosphere - An atmosphere that evolves from a secondary atmosphere. It involves the removal of carbon dioxide and the generation of diatomic oxygen (O2 and triatomic oxygen (O3 (ozone). The Earth has a tertiary atmosphere.
T-Tauri Star - An early stage in the evolution of a star prior to the onset of nuclear fusion. It is characterized by its variable output of energy.
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1995 - 2007 P. Noeller All Rights Reserved Updated: February 25, 2007 |
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