The Inner Planets

The four planets closest to the Sun, Mercury, Venus, Earth and Mars, are grouped together as the inner planets. They have rocky surfaces, and are often called the terrestrial planets. These planets are small and closely spaced, as compared to the outer planets.

Mercury is the smallest planet, located closest to the Sun. Mercury revolves on its axis very slowly. A single Mercury day is about 59 Earth days long. Mercury does not have any moons or atmosphere. Its surface is covered with craters. A magnetic field suggests that it has an iron core.

Venus is the second closest planet to the Sun. It is often called Earth's sister or twin planet due to similar composition, gravity and size. Venus rotates very slowly in a clockwise direction, called retrograde rotation. Similar to the planet Mercury, Venus does not have any moons.

Venus is surrounded by a very dense atmosphere composed of carbon dioxide and sulfuric acid clouds. Due to the greenhouse effect, Venus has the hottest surface temperature of any planet. The air pressure on Venus is 92 times that on Earth. Craters and active volcanoes cover the surface of the planet. From a geologic perspective, Venus is considered a young planet.

The Earth is the third planet from the Sun. The Earth's distance from the Sun allows for moderate temperatures that keep most surface water in liquid form. The presence of liquid water and an oxygen-rich atmosphere make Earth suitable for millions of living organisms.

Mars, the fourth planet from the Sun, has two moons. A day on Mars is slightly longer than a day on Earth, and it takes about 687 days for Mars to revolve once around the Sun. Space probes and rovers have successfully gathered much information about Mars.

Mars has a very thin atmosphere composed primarily of carbon dioxide. The surface is cold, dry, and dusty, with frozen ice caps found at the poles. Landforms and rover samples indicate that in the past there were large amounts of liquid water on the surface. Mars has a volcanic history, and the largest volcano in the solar system, Olympus Mons, is found there.