Page 51 - Curriculum Visions Dynamic Book

 The contrast between the rocky planets and the gas giants.
and encased in a skin of ice. Similarly, Saturn’s largest moon, Titan, has an atmosphere that is denser than that of the Earth.
Comets and asteroids
There is much more uniformity among the comets and asteroids, suggesting that they are made of materials that more closely resemble the debris from which the planets and moons first formed. That is particularly true of the comets, whose black, sooty ice nuclei seem never to have changed.
The asteroids and comets travel on very different paths. The asteroids, of which the largest are nearly 1,000 km across, mainly occur between Mars and Jupiter. Some have orbits that occasionally bring them across the orbit of the Earth. In this way there is a very real—if small—danger of collisions between large asteroids and planets like the Earth.
Collisions with small asteroids are quite common.
They produce meteors that sometimes burn up in the atmosphere and sometimes crash onto the surface to form features like meteor craters. The cratered surfaces of the Moon and many planets testify to the fact that such impacts have been a common experience throughout the history of the Solar System.
It is widely believed, for instance, that one large meteor collision was responsible for global climate change and the destruction of the dinosaurs, among other species.
The comets are the most distant things in the Solar System. They are grouped beyond Pluto and orbit in various directions, so that they make a shell of sooty bodies at the near edge of the Solar System completely invisible to any telescope. There are thought to be two such shells, named the Kuiper belt and the Oort cloud.
The larger by far is the Oort cloud. Bodies in the Oort cloud may stay in their paths for billions of years. But
asteroid Any of the many small objects within the Solar System.
atmosphere The envelope of gases that surrounds the Earth and other bodies in the universe.
coma The blurred image caused by light bouncing from a collection of dust and ice particles escaping from the nucleus of a comet.
comet Asmallobject,oftendescribed as being like a dirty snowball, that appears to be very bright in the
night sky and has a long tail when it approaches the Sun.
core The central region of a body.
crater A deep bowl-shaped depression in the surface of a body formed by the high-speed impact of another, smaller body.
density A measure of the amount of matter in a space.
kuiper belt A belt of planetesimals (small rocky bodies, one kilometer to hundreds of kilometres across) much closer to the Sun than the Oort cloud.
meteor A streak of light (shooting star) produced by a meteoroid as it enters the Earth’s atmosphere.
nucleus (pl. nuclei) The centermost part of something, the core.
oort cloud A region on the edge of the Solar System that consists of planetesimals and comets that did not get caught up in planet making.
For more on comets and asteroids see Volume 4: Rocky planets.
51