From The American Heritage® Student Science Dictionary
, The American Heritage® Children's Science Dictionary
, and other sources.
asteroid Any of numerous small, often irregularly shaped bodies that orbit the sun. Asteroids range from several hundred miles in diameter to the size of a speck of dust. Most are found in the region between the orbits of Mars and Jupiter.
atmosphere The mixture of gases that surround the Earth, some other planet, or another celestial body. It is held by the force of gravity and forms various layers at different heights.
crater 1. A bowl-shaped depression at the top of a volcano or at the mouth of geyser. 2. A shallow, bowl-shaped hole in the surface, formed by an explosion or by the impact of a body, such as a comet or asteroid.
magnetic field The space around a magnet within which its magnetism can affect other objects. The molten core of a planet can act like a magnet, generating a magnetic field.
Mercury The smallest planet and the one closest to the sun. Mercury's surface is covered with mountains, craters, ridges, and valleys. It orbits the sun once every 88 days. Because this planet appears to move across the sky faster than any other planet, early observers named it after the Roman god Mercury, a swift-footed messenger.
molten Rocks, metals, or other materials that have melted into a liquid as the result of exposure to extreme heat.
planet A celestial body that does not produce its own light, is larger than an asteroid, and is illuminated by light from a star, such as the sun, around which it revolves. In our solar system, there are eight known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Plutolong thought a planetwas downgraded to "dwarf planet" status. Dwarf planets are not considered true planets by International Astronomical Union.
Copyright © 2002, 2003 Houghton-Mifflin Company
. All rights reserved. Used with permission.