absorption band: a section of the spectrum of light from a source from which some or all of the light is missing; caused by the material located in between the observer and the original light source that absorbs light at very distinct colors or wavelengths.
albedo: the proportion of incident sunlight that is reflected by the surface of a planet or moon.
algae: aquatic, photosynthetic organisms that can live in freshwater or seawater; lack roots, stems, and leaves; can exist as single, microscopic cells or as multicellular, macroscopic organisms.
ALH 84001: (Allan Hills 84001), a meteorite from Mars, collected by Roberta Score in Antarctica in 1984, that may show fossil evidence for ancient life on Mars.
ancient valley networks: features on the ancient Martian surface, some as much as 0.6 miles (1 kilometer) in width and 600 feet (several hundred meters) in depth, that resemble river valley networks on Earth.
areography: term, akin to geography, coined in the nineteenth century for the mapping of the surface of Mars (after Ares, Greek god of war).
areology: the study of the planet Mars.
canale: Italian word (singular) for channel; can be used to refer to a canal constructed by human engineers, a body of water as broad and deep as the English Channel, or a mountain gully; first applied to Mars in 1858 by Father Angelo Secchi and made popular in the 1880s in Giovanni Schiaparelli’s maps of Mars (plural, canali).
Cepheid: a star whose brightness increases and decreases periodically, and whose period of brightness changes is directly correlated with the maximum brightness of the star, such that the brightest Cepheids have long periods (about 100 days) and the faintest Cepheids have periods of less than a day; used by astronomers to determine distances to clusters of stars and galaxies that have within them at least one Cepheid.
chlorophyll: a molecule in plants that absorbs light in the process of photosynthesis; by absorbing most colors of visible light except green, which is reflected, chlorophyll gives plants their green color.
deuterium: a heavy hydrogen atom, containing both a proton and a neutron in the nucleus, whereas normal hydrogen has only a single proton in the nucleus.
diogenite: a meteorite that formed as igneous rock that cooled slowly, deep inside a large parent-body, such as a moon, planet, or asteroid.
gemination: the process by which, according to the claims of Giovanni Schiaparelli, the supposed canals on Mars suddenly changed from single canals to double canals.
heavy water: a water molecule (H2O) in which both hydrogen atoms have been replaced with deuterium atoms (thus, D2O); semi-heavy water would have the form HDO.
lichen: long cells of fungi, many of which contain multiple nuclei, connected end-to-end to form long, tubular filaments with cell walls receiving structural support from chitin, a carbohydrate polymer molecule; live symbiotically with photosynthetic cells, usually green algae but sometimes an ancient form of bacteria called cyanobacteria.
lithophile: an element that bonds easily and readily with oxygen, thereby forming oxides and silicates; found concentrated in the mantle and crust (rather than the core) of Earth.
macrobe: word coined by Carl Sagan for a hypothesized life-form on Mars (or possibly elsewhere in the universe) that is large enough to be obvious in photographs taken from a camera (rather than a microscope or other specialized detection equipment) on the surface of a planet.
metaphysics: an ancient branch of philosophy that attempts to explain everything about the world (motion, space, time, substance, existence) on the basis of fundamental principles determined by rational thought, rather than on the basis of knowledge gained from experiments and observations of the physical universe.
methane: a molecule made up of one carbon atom and four hydrogen atoms (CH4).
methanogenic bacteria: anaerobic (living in environments absent of free oxygen) bacteria that produce methane gas as a by-product of their energy metabolism.
moss: a small, nonvascular land plant that reproduces with spores rather than flowers and seeds and that absorbs water and nutrients mainly through leaves rather than roots.
opposition: an alignment of two planets on the same side of the Sun, such that the Sun and the two planets form a straight line, and so the second planet, when viewed from Earth, is seen in exactly the opposite direction as the Sun and the full disk of the planet is illuminated by sunlight.
organic molecule: a molecule that contains one or more carbon atoms and carbon-hydrogen (C-H) bonds; molecules associated with all living things on Earth, including DNA, fats, sugars, proteins, and enzymes, are organic molecules.
outflow channels: features on the ancient Martian surface, some as much as tens of miles wide and nearly a thousand miles in length, that appear to have been carved by the quick melting and catastrophic release of enormous volumes of water.
photographic plate: a piece of glass coated with light-sensitive chemicals that, when exposed to light collected by a telescope and then bathed in the right darkroom chemical baths, yields images of celestial objects; used by professional astronomers from the 1890s through the 1970s.
plurality of worlds hypothesis: A medieval and Renaissance-era idea that claimed that every star, planet, or moon in the universe (all of them being “worlds”) is inhabited by intelligent beings that are able to worship God.
precipitable water vapor: the total depth of water on the surface of a planet if all of the moisture in the atmosphere condensed onto the surface in liquid form.
principle of plenitude: twentieth-century American intellectual historian Arthur Lovejoy identified the principle of plenitude as the idea that “no genuine potentiality of being can remain unfulfilled, that the extent and abundance of the creation must be as great as the possibility of existence . . . that the world is better, the more things it contains.”a
semi-heavy water: see heavy water.
siderophile: an element that tends not to form bonds with oxygen and sulfur and that is readily soluble in molten iron; siderophiles are concentrated in the core (rather than the mantle or crust) of Earth.
sol: one day (the time from sunrise to the next sunrise) on Mars, equal to 24 hours, 39 minutes, 35.244 seconds; this length of time results from the combination of Mars’s actual rotation period and the forward motion of Mars in its orbit around the Sun.
spectroscopy: channeling a beam of light from any source of light through a prism or reflecting the light off a grating, which spreads the light out into its constituent colors, allowing one to study the details of brightness and faintness of the different colors.
spectrum: the detailed rainbow of light obtained by allowing light from a source to pass through a prism or reflect off a grating; the spectrum of a celestial object, in addition to showing light across a breadth of colors, will show some colors brighter and others fainter than average.
terraforming: to change the environment, in particular the contents and temperature of the atmosphere, of another planet into one resembling Earth, such that it becomes habitable for humans.
terrestrial fractionation line: a way of distinguishing material from the Earth-Moon system from material from other planets, that utilizes the way in which isotopes of a single element like oxygen separate (or fractionate) as a result of processes like evaporation or chemical bonding.
a Arthur O. Lovejoy, 1971, The Great Chain of Being: A Study of the History of an Idea (Cambridge, MA: Harvard University Press).