Fumarole—A vent, usually in volcanic regions, from which vapors or gases are released.

Geminate recombination—The reaction with each other of two transient molecular entities produced from a common precursor in solution.

Hale-Bopp (comet)—Correctly known as C/1995 O1 (Hale Bopp), it is the brightest comet to appear in the night sky for many decades. Discovered by Alan Hale and Thomas Bopp on July 22, 1995, it reached perihelion on April 1, 1997, and was visible to the naked eye for many months. Its nucleus appears to be very large, about 40 km across.

Heat pulse—A rapid change in ambient temperature conditions over a wide field of view. The temperature change can be higher or lower than that of the normal, ambient, slowly changing temperature.

Infrared—The portion of the electromagnetic spectrum radiation with wavelengths in the range from 750 nm to 0.1 mm.

Interstellar medium (ISM)—The dust, molecular clouds, and neutral hydrogen that lie between the stars of this galaxy, generally in the plane of the Milky Way, but whose density is highly variable.

IP/Halley (comet)—The most famous periodic comet. Its aphelion is beyond the orbit of Neptune, but it returns to the inner solar system every 76 years. Named after the 17th-century British scientist, Edmond Halley, who first recognized its regular pattern of reappearances. Studied by a fleet of spacecraft during its 1986 apparition, including the European Space Agency’s Giotto.

Isomer—One of two or more substances that have the same chemical composition but differ in structural form.

Kerogens—A family of chemical compounds that make up a portion of the organic matter found in sedimentary rocks. They are insoluble in organic solvents, non-oxidizing acids (HCI and HF), and bases because of their very high molecular weight. Each kerogen molecule is formed by the random combination of numerous monomers. When heated, hydrogen-rich kerogens yield crude oil and hydrogen-poor kerogens yield mainly gas.

Kinetic isotope effect—The effect of a difference in mass between two isotopes of the same element, such as a difference in reaction rate, vapor pressure, or equilibrium constant. The term includes effects on molecular or atomic properties; specific nuclear effects such as radioactivity are excluded.

Kuiper belt—A region of the solar system containing icy planetesimals distributed in a roughly circular disk some 40 to 100 AU from the Sun. Pluto’s orbit is believed to circumscribe the innermost region of the Kuiper belt.

Kuiper belt objects (KBOs)—A general name for the bodies found in the Kuiper belt.

Lacustrine—Of or pertaining to lakes.

Murchison (meteorite)—A carbonaceous chondrite, type II (CM2), suspected to be of cometary origin due to its high water content (12 percent).

Neutrino—One of a family of electrically neutral subatomic particles with little or no mass generated during some radioactive decay processes. Because they interact only weakly with matter, neutrinos are extremely difficult to detect.

Organic—Of or relating to any covalently bonded compound containing carbon atoms.