Mercury is the nearest planet to the sun. Its surface is comparable to that of Earth's moon, with smooth plains and deep craters. Like the moon, it is covered by a thin layer of silicates. Silicates are compounds containing silicon and oxygen. Sand, for instance, is a silicate. Mercury's solid surface makes it one of the terrestrial planets. Mercury's core is thought to be much like that of Earth, composed primarily of iron and nickel.
Venus is the second nearest planet to the sun. Like Earth, Venus is a terrestrial planet. In fact, Venus has been called "Earth's twin" because its composition and size are so similar to Earth's. Venus' outer layer, or crust, is thought to be composed of silicate rocks. It is also thought to have an inner core of iron or nickel. According to Universe Today, the composition of Venus's middle layer is unknown. This middle layer is also called the mantle.
For obvious reasons, Earth is the planet about which scientists know the most. Earth is a terrestrial planet. In fact, the name terrestrial comes from the Latin word terra, which means "earth" or "land." Earth's mostly silicate composition---with a core of iron and nickel---makes it a terrestrial planet as well. Earth is the largest of the rocky planets and the only one known to support life. It is also the only one to have liquid water on its surface.
Mars is the fourth planet from the sun and the last of the terrestrial planets. Mars's outer layer contains rocks that are rich in iron oxide. Since iron oxide is essentially rust, the surface of Mars has a red tinge. For this reason, Mars is also known as "the red planet." These red colored rocks make mars a terrestrial planet. Scientists believe that long ago, there were vast quantities of liquid water on the surface of Mars. It is thought that this water evaporated or froze beneath the planet's surface.