A system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.

Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo Galilei in 1610.

A substance, such as oxygen or hydrogen, that has no fixed shape and that can expand without limit.

The outer solar system planets: Jupiter, Saturn, Uranus, and Neptune, composed mostly of hydrogen, helium and methane gas, and have a density less than 2 grams per cubic centimeter.

An ellipsoid or sphere-shaped "net" that is used as a type of base-map or control to correlate co-ordinates on a flat image with their actual location on the spherical planetary body in order to obtain accurate measurements from images.

The expression of the internal and external processes and events that affect a planetary body.

The time extending from the end of the formative period of the Earth to the beginning of human history.

A body of rock (or ice, in Europa's case) that has a distinct origin and consists of dominant, unifying features that can be easily recognized and mapped.

A direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface. In other words, the satellite and planet move at the exactly the same speed.

A depressed segment of the crust of the earth or a celestial body (as the moon) bounded on at least two sides by faults.

The acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.

Refers to the difference in the acceleration of gravity from one side of an object to another, because gravity is a force that changes with distance between two points.

The line of intersection of the surface of a sphere and any plane which passes through the center of the sphere.

Increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide and water vapor are the major factors). Very important on Venus and Earth but very weak on Mars.

The on-site gathering of data at a particular location, for the purpose of calibrating and interpreting remotely acquired data.