The Inner Planets Chapter 16-2

The Motion of Planets The motion of a body as it orbits another body in space is called revolution. The time it takes to revolve around the sun once is called its period of revolution. The spin of an object in space is called rotation The amount of time it takes for an object to rotate once is called its period of rotation.

The Inner Planets The inner planets are more closely spaced than the outer planets. The inner planets are called terrestrial planets because they are small, dense, and rocky, like the earth.

Mercury- Closest to the Sun Mercury is less massive, so gravity is less. A day on Mercury is 59 Earth days. A year is only 88 Earth days long. Surface temperature is -173 to 427 *C. Distance from the sun is 3.2 light-minutes or .39 AU.

Venus- Earth’s Twin? It is more similar to Earth than any other planet, same size, mass, & density. It rotates in the opposite direction as Earth (clockwise or retrograde). Its rotation is so slow that its day is longer than its year! The atmosphere of Venus is the densest of the inner planets, 90x that of Earth’s. It atmosphere is carbon dioxide & acids. Venus has the hottest surface temperature of all the planets, with an average of 464*C.

Earth- An Oasis in Space Earth formed at just the right distance from the sun. The temperatures are warm enough to keep water from freezing, but cool enough to keep it from boiling away. Water provides a means for much of the chemistry that living things depend on.

Mars- The Red Planet Other than Earth, it is the most studied planet. It is a cold planet (-123 to 37*C) because of its thin atmosphere and its distance from the sun (1.52 AU). Because the atmosphere is thin, it has less pressure than Earth. A Martian day is 34 min. longer than Earth’s, and its year is 322 days long. Liquid water doesn’t exist, only frozen water in its polar icecaps.

Mars (con’t) The largest mountain in our solar system is a shield volcano on Mars. Olympus Mons is about the size of Arizona. In Jan. 2004, the exploration rovers landed on Mars. They have found strong evidence that water once existed on the surface of the planet.