Glossary

Albedo: The fraction of incoming electromagnetic radiation that an object reflects back into space.

Auroras: Transient and dynamic rings or curtains of light near magnetic poles, produced when charged particles from the solar wind interact with atmospheric gases after being guided by a global magnetic field.

Canali: The Italian word for “channels,” once used to describe dark lines thought to exist on Mars’ surface—later shown to be an optical illusion.

Conjunction (inferior, superior): The apparent meeting of two celestial bodies in the sky, such as the Moon and Sun at new moon. Because Mercury and Venus orbit inside Earth’s orbit, they experience two kinds of conjunctions: inferior (when between Earth and the Sun) and superior (when on the far side of the Sun).

Core: The innermost layer of a planet, typically composed of metal (iron and nickel).

Crust: The outermost solid layer of a planet.

Curiosity Rover: NASA’s mobile science laboratory on Mars, operating since 2012. Curiosity is equipped to analyze rock and soil samples on-site to study the planet’s past habitability.

Dynamo effect: The geophysical process by which global magnetic fields are generated through the convection of electrically conducting fluids within a planet’s or star’s interior.

Global magnetic field: The large-scale, planet-wide magnetic field generated by internal electrical currents, as opposed to localized crustal magnetism.

Greenhouse effect: The process in which a planet absorbs visible sunlight and re-emits it as infrared radiation, which is then partially trapped by greenhouse gases in the atmosphere, warming the surface.

Greenhouse gas: A gas that is transparent to visible light but absorbs and re-emits infrared radiation (see greenhouse effect).

Habitability: The potential of a planet or moon to support life. A habitable world need not be inhabited but must possess conditions, such as liquid water, that could sustain life.

Habitable zone: The region around a star where surface temperatures and pressures on a planet allow liquid water to exist stably through the greenhouse effect.

Mantle: The layer of rock between a planet’s core and crust, where convection drives tectonic and volcanic activity.

Mariner missions: A series of NASA–Jet Propulsion Laboratory probes (1962–1973) that investigated Mercury, Venus, and Mars, pioneering planetary exploration.

Mars Reconnaissance Orbiter (MRO): NASA spacecraft orbiting Mars since 2006, using high-resolution imaging and spectrometry to study the planet’s surface and atmosphere.

MAVEN mission: NASA’s Mars Atmosphere and Volatile EvolutioN mission, launched in 2014 and still active, studying how the solar wind strips gases from Mars’ upper atmosphere.

Opposition: The alignment in which an outer planet lies opposite the Sun in the sky as seen from Earth—its closest approach and brightest appearance.

Photodissociation: A chemical reaction in which molecular bonds are broken by the absorption of photons.

Plate tectonics: The geological theory describing how a planet’s crust is divided into moving plates that shift atop the mantle, reshaping the surface over time.

Runaway greenhouse effect: A feedback loop in which the greenhouse effect amplifies itself—higher temperatures cause more greenhouse gases to accumulate, which in turn cause further warming.

Tharsis bulge: A massive volcanic plateau on Mars formed by long-lived mantle plumes that produced large shield volcanoes such as Olympus Mons and the Tharsis Montes.

Volcanoes (composite, shield): Openings in a planet’s crust through which lava, ash, and gases escape from the interior. Shield volcanoes form from fluid lava flows and have broad, gentle slopes; composite volcanoes form at plate tectonic boundaries and have steeper profiles built from alternating layers of ash and lava.