astrophysicist: A scientist who works in an area of astronomy that deals with understanding the physical nature of stars and other objects in space.
atom: The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.
computer model: A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
core: Something — usually round-shaped — in the center of an object.
crust: (in geology) Earth's outermost surface, usually made from dense, solid rock (in planetary science) the outermost surface of rocky planets, dwarf planets and natural satellites.
data: Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.
element: A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
iron: A metallic element that is common within minerals in Earth’s crust and in its hot core. This metal also is found in cosmic dust and in many meteorites.
isotopes: Different forms of an element that vary somewhat in mass (and potentially in lifetime). All have the same number of protons in their nucleus, but different numbers of neutrons.
mantle: (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.
mass: A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.
metal: Something that conducts electricity well, tends to be shiny (reflective) and is malleable (meaning it can be reshaped with heat and not too much force or pressure).
model: A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
moon: The natural satellite of any planet.
orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body.
Pacific: The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.
plate tectonics: The processes governing the movements of massive pieces that make up Earth’s outer layer, which is called the lithosphere. Those processes cause the rock masses to rise from inside Earth, travel along its surface, and sink back down.
ratio: The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
scenario: A possible (or likely) sequence of events and how they might play out.
tectonic: Surface activity on a large rocky body (such as a planet or moon) as liquid rock flows up to the surface where it solidifies, then slowly drifts atop molten rock, carrying surface features with it.
Theia: (in astronomy) The name of a hypothetical protoplanet, named for the Greek goddess of sight, who was also the supposed mother of the moon goddess Selene. If this protoplanet existed, the Mars-sized rocky world would have died in a violent collision with Earth, some 4.5 billion years ago. Part of the debris from it — and Earth — might have eventually collected to form a new celestial object: Earth’s moon.
theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.