Aphelion  -  The farthest point from the Sun in a solar orbit.

Asteroid belt  -  The region between Mars and Jupiter where most asteroids in our solar system are found.

Asteroids  -  Small planetoids distributed widely throughout the solar system. Their biggest concentration is found between Mars and Jupiter.

Bar  -  Unit of atmospheric pressure measurements. One bar is the pressure measured on Earth at sea level.

Basaltic lava  -  Lava made of a rock called basalt. Basalt is richer in iron and magnesium silicates than continental rock.

Tycho Brahe  -  Danish astronomer (1546-1601) who built the largest naked eye astronomical observatory in modern Europe. He made countless observations of the position of Mars and other celestial bodies. Read more about Brahe.

Carbonates  -  Rocks containing the chemical group CO₃.

Cirrus clouds  -  High, wispy clouds made of tiny water ice crystals.

Convection  -  A type of motion found in a gas or liquid when there is a temperature difference between separate regions. For instance, in boiling pot of water, the water closer to the flame becomes warmer and, correspondingly, becomes less dense. The hot water therefore rises to the surface, pushing the warmer and cooler water into contact and then pushing the cooler water down. This mechanism exchanges heat between warmer and cooler regions.

Copernicus  -  Polish astronomer (1473-1543) who first theorized the heliocentric system in modern times. The heliocentric system is the model where planets revolve around the Sun. Read more about Copernicus and the Scientific Revolution Revolution.

Core  -  The core of a planet is its central, spherical portion. It can be divided in two regions. The inner region is made of a mixture of nickel (Ni) and iron (Fe), while the external portion is made of iron (Fe) and sulfur (S).

Crust  -  The external, solid portion of the planet, literally its 'skin.' Like the mantle, it is made of silicate minerals. The crust's silicates are less dense than are those of the mantle.

Density  -  Mass per unit volume.

Dissociated  -  A molecule is dissociated when it is broken into its atomic constituents.

Dry ice  -  The frozen form of carbon dioxide.

Eccentric  -  An ellipse is eccentric if it is quite elongated. Eccentricity is the ratio between the distance from the center to a focus of an ellipse and the length of its major axis.

Escape velocity  -  The velocity required for an object to escape the gravitational pull of another object (such as a planet or star).

Erosion  -  The wearing down of rocks due to the action of water and wind.

Freezing point  -  The temperature at which a liquid or a vapor freezes.

Mars Global Surveyor  -  Orbital probe that provided images of the entire surface of Mars during 1997-2000.

Greenhouse effect  -  The visible light of the Sun passes through the atmosphere and heats the ground. The heated ground radiates that energy back into space in the form of infrared light, invisible to human eye. Greenhouse gases, like water vapor and carbon dioxide, trap infrared radiation, making it more difficult for the air and ground to cool off.

Hot spots  -  Small regions of the crust where there are volcanoes, fueled by magma coming from far down in the mantle.

Hubble Space Telescope  -  A large orbiting telescope above the Earth's atmosphere. Its mirror measures 2.5 m.

Impact basin  -  A very large crater--so large that the rim is of negligible height compared to the crater's diameter.

Inner planets  -  The four rocky planets closest to the Sun (Mercury, Venus, Earth, and Mars) are considered inner planets.

Johannes Kepler  -  German astronomer (1571-1630), his mentor was Tycho Brahe. He completed Brahe's observations of the position of Mars and was finally able to pin down its orbit. He realized that planets follow elliptical, rather than circular, orbits around the Sun. He was an ardent proponent of the heliocentric model. Read more about Kepler.

Percival Lowell  -  American astronomer (1855-1916); the director of the Flagstaff observatory.

Magnetite  -  Iron oxide rock that can become a permanent magnet.

Mantle  -  The mantle is the portion of a planet beneath the crust and above the core. It is mostly made of silicates, though these minerals are more dense on average than those in the crust. The mantle exists mostly in a semi-solid viscous phase, allowing very slow convection currents.

Mariner 9  -  NASA's previous Mariner missions had sent only a few images back to Earth. The Mariner 9 probe, which consisted solely of an orbiter, gave us the first reasonably complete picture of Mars from its orbit. It was launched in May, 1971.

Meteoric craters  -  Craters due to the impact of extraterrestrial rocks, ranging in size from a few centimeters to kilometers.

Meteorites  -  Extraterrestrial rocks that fall through a planet's atmosphere and reach its surface.

Nano-bacteria  -  Bacteria 100 times smaller than normal bacteria.

Isaac Newton  -  British physicist (1643-1727) and mathematician, he invented modern mechanics and wrote the law of gravity, which, for most applications, is still used today in its original form. SparkNote on Newton.

Oxidation  -  The chemical reaction between metals and oxygen in which new compounds are formed.

Ozone layer  -  The region of the Earth's atmosphere where molecules of ozone (O₃) capture the ultraviolet radiation from the Sun, preventing it from reaching the ground.

Pathfinder  -  1997 mission to Mars that sent a lander to Mars's surface, equipped with a rover that explored the ground in the vicinity of the landing site. Pathfinder obtained data on the composition of the soil and the types of rock in that region of the planet.

Perihelion  -  The point closest to the Sun in a solar orbit

Plate tectonics  -  The particular tectonic theory that explains the geology of Earth's crust. Earth's crust is divided in mobile sections, moving at the rate of a few centimeters per year. New crust is formed or destroyed at the boundaries between plates. In places such as the San Andreas Fault, two plates slide by each other moving in opposite directions. Continents are carried on top of the tectonic plates.

Rifts  -  Regions of a planet's crust where the crust is itself being created. On Earth, rifts are an active force in plate tectonics. The new crust slowly moves away from the fissure as two plates moving in opposite directions.

Giovanni Schiapparelli  -  Italian astronomer (1835-1910) who made planetary observations.

Sedimentation  -  The process by which sand and silt is deposited in successive layers.

Seismic  -  Having to do with quakes.

Silicates  -  Minerals containing silicon oxide and metals, for example Fe₂SiO₄.

Stratosphere  -  The layer of the atmosphere above the troposphere; the troposphere is where most weather phenomena occur.

Sublimation  -  The volatilization of a solid directly into a gas, without apparently passing through the liquid faze.

Surface gravity  -  The acceleration due to gravity at planet's surface.

Tilt  -  The angle that measures the direction of the axis as compared to the plane of the orbit.

Ultraviolet radiation  -  Light with a wavelength shorter than the violet, invisible to human eye.

Uplifting  -  A process by which portions of the crust are lifted due to tectonic forces.

Viking 1 and 2  -  Operating in 1975-76, these two probes consisted of both an orbiter and a lander. The landers probed for life and provided data about Mars's soil and atmosphere.