Vast regions of dark dunes also extend across Titan's exotic landscape, especially around its equatorial regions. Unlike Earth's sand, the "sand" that creates Titan's dunes is composed of dark grains of hydrocarbon that resemble coffee grounds. The tall linear dunes of this misty moisty moon-world appear to be quite similar to those seen in the desert of Namibia in Africa. Because Titan's surface is pockmarked by relatively few impact craters, its surface is considered to be quite young. Older surfaces display heavier cratering than more youthful surfaces, whose craters have been "erased" by resurfacing. This resurfacing is caused by processes that cover the scars left by old impacts as time goes by. Our own planet is similar to Titan in this respect. The craters of Earth are erased by the ongoing processes of flowing liquid (water on Earth), powerful winds, and the recycling of Earth's crust as a result of plate-tectonics. These processes also occur on Titan, but in modified forms. In particular, the shifting of the ground resulting from pressures coming from beneath (plate tectonics), also appear to be at work on this veiled moon-world. However, planetary scientists have not seen signs of plates on Titan that are analogous to those of our own planet.
A moon is defined as a natural satellite that orbits a larger body--such as a planet--that, in turn, orbits a star. The moon is kept in its position both by the gravity of the object that it circles, as well as by its own gravity. Some planets are orbited by moons; some are not. Some dwarf planets--such as Pluto--possess moons. In fact, one of Pluto's moons, named Charon, is almost half the size of Pluto itself, and some planetary scientists think that Charon is really a chunk of Pluto that was torn off in a disastrous collision with another object very long ago. In addition, some asteroids are also known to be orbited by very small moons.
Ganymede, and four other moons dwelling in our Sun's family, possess liquid water beneath their frigid crusts of ice. The others are Saturn's moons, Titan and Enceladus, and two other Galilean moons of Jupiter--Europa and Callisto. Planetary scientists think the oceans of Europa and Enceladus are in contact with rock--thus making these two moons high-priority targets for future astrobiology missions.