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air pressure: The force exerted by the weight of air molecules. 

Antarctica: A continent mostly covered in ice, which sits in the southernmost part of the world. 

arid: A description of dry areas of the world, where the climate brings too little rainfall or other precipitation to support much plant growth. 

asteroid: A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt. 

astronaut: Someone trained to travel into space for research and exploration. 

astrophysicist: A scientist who works in an area of astronomy that deals with understanding the physical nature of stars and other objects in space. 

atmosphere: The envelope of gases surrounding Earth, another planet or a moon. 

cyanobacteria: A type of bacteria that can convert carbon dioxide into other molecules, including oxygen. 

engineer: A person who uses science and math to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need. 

eruption: (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice. 

evaporate: To turn from liquid into vapor. 

fiction: (adj. fictional) An idea or a story that is made-up, not a depiction of real events. 

fluctuation: (v. fluctuate) Some type of change in a pattern or signal that varies at irregular intervals and often by amounts that are hard to predict. 

gene: (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves. 

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity. 

greenhouse: A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry. 

greenhouse effect: The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants as greenhouse gases. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk. 

greenhouse gas: A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas. 

liquid: A material that flows freely but keeps a constant volume, like water or oil. 

Mars: The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s. 

methane: A hydrocarbon with the chemical formula CH₄ (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 80 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas. 

microbe: Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell. 

microbiology: The study of microorganisms, principally bacteria, fungi and viruses. Scientists who study microbes and the infections they can cause or ways that they can interact with their environment are known as microbiologists. 

mineral: Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns). (in physiology) The same chemicals that are needed by the body to make and feed tissues to maintain health. 

muscle: A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue. 

NASA: Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system. 

nitrogen: A colorless, odorless and nonreactive gaseous element that forms about 78 percent of Earth’s atmosphere. Its scientific symbol is N. Nitrogen is released in the form of nitrogen oxides as fossil fuels burn. It comes in two stable forms. Both have 14 protons in the nucleus. But one has 14 neutrons in that nucleus; the other has 15. For that difference, they are known, respectively, as nitrogen-14 and nitrogen-15 (or ¹⁴N and ¹⁵N). 

orbit: The curved path of a celestial object or spacecraft around a galaxy, star, planet or moon. One complete circuit around a celestial body. 

oxygen: A gas that makes up about 21 percent of Earth’s atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism). 

perchlorate: This naturally occurring chemical is a potentially cancer-causing component of certain jet fuels, explosives and fertilizers. In animals, this pollutant can perturb levels of thyroid hormones. It also appears capable of acting like an androgen (a male sex hormone). 

photosynthesis: (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water. 

pressure: Force applied uniformly over a surface, measured as force per unit of area. 

radiation: (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space. 

Red Planet: A nickname for Mars. 

replicate: (in biology) To copy something. When viruses make new copies of themselves — essentially reproducing — this process is called replication. (in experimentation) To copy an earlier test or experiment — often an earlier test performed by someone else — and get the same general result. Replication depends upon repeating every step of a test, one by one. If a repeated experiment generates the same result as in earlier trials, scientists view this as verifying that the initial result is reliable. If results differ, the initial findings may fall into doubt. Generally, a scientific finding is not fully accepted as being real or true without replication. 

risk: The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.) 

salt: A compound made by combining an acid with a base (in a reaction that also creates water). The ocean contains many different salts — collectively called “sea salt.” Common table salt is a made of sodium and chlorine. 

science fiction: A field of literary or filmed stories that take place against a backdrop of fantasy, usually based on speculations about how science and engineering will direct developments in the distant future. The plots in many of these stories focus on space travel, exaggerated changes attributed to evolution or life in (or on) alien worlds. 

solar: Having to do with the sun or the radiation it emits. It comes from sol, Latin for sun. 

solar system: The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets. 

star: The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become hot enough, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star. 

suffocate: To be unable to breathe, or to cause a person or other organism to be unable to breathe. 

technology: The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts. 

toxic: Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity. 

Venus: The second planet out from the sun, it has a rocky core, just as Earth does. Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit). 

volcano: A place on Earth’s crust that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. This plumbing system can become more complex over time. This can result in a change, over time, to the chemical composition of the lava as well. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock. 

water vapor: Water in its gaseous state, capable of being suspended in the air.