What Are Terrestrial (Inner) Planets?

These planets, also known as the Inner Planets or Rocky Planets, are the first four closest orbiting planets around the Sun: Mercury, Venus, Earth, and Mars. These planets lie within 1.5 astronomical units (AU) from the Sun. The inner planets are sometimes called "the terrestrial planets" because they share some common characteristics with planet Earth.

The inner planets are composed of silicates, which form their crusts and mantles, and metals (iron and nickel) which form their cores. They all share feautures, such as volcanoes and rift valleys, made by tectonic activity on their surfaces. Venus, Earth, and Mars´s atmospheres have developed sufficiently to generate their own climate.

All the inner planets orbit around the Sun in a nearly circular orbit, and all, except for Venus, share a counterclockwise rotation. Venus´s rotation is retrograde; it rotates on its axis in the opposite direction as the other planets. It has been theorized that one of the many meteorites that have hit Venus might have been so strong that it changed its direction of rotation.

Mercury is the closest planet, at 0.390 AU from the Sun. Mercury´s surface features resemble those of the Moon with a heavily cratered surface which remains untouched due to the lack of erosion , water, or atmosphere on its surface. The small mass of Mercury 3.3 x 10 ^23 kg is insufficient to hold, by gravitation, an atmosphere.Whitout an atmosphere, Mercury is continuously bombarded by ultraviolet radiation and meteoroids. Temperatures on Mercury range from 100 to 700 K.

Mercury´s sidereal period is 88 days; its period of rotation around its own axis is of 59 days. It is believed that, although, there is no atmosphere in Mercury, some regions at the poles which remain constantly in shadows with temperatures as low as 125 K could retain some water ice.

Venus, at 0.7 AU from the Sun, is the second planet in the solar system. It´s similar to Earth in mass with 4.9 x 10^24 Kg. Venus, like the Earth, posesses a thick silicate mantle, enclosing an iron core; Venus´s atmosphere is 90 times denser than that of Earth; its surface temperature reaches 730 K. This extreme temperature is produced by a process known as the greenhouse effect (radiation gets trapped under a shield of gases).

The atmosphere of Venus is composed of 96.5 carbon dioxide (CO2) and the remainder is mostly nitrogen (N2). Most of the CO2 in Venus is produced by vulcanism. The heavy dense clouds in Venus are composed of sulfuric acid (H2SO4). The dense atmosphere of Venus creates a surface pressure equivalent to 90 Earth atmospheres.

Venus sidereal period is 225 days, and its period of rotation on its axis is 243 days; however, Venus´s rotation is retrograde, meaning, it rotates not counterclockwise like the other planets, but clockwise. The Magellan probe launched by NASA on May 4th, 1989 mapped for the first time the surface of Venus using a technique called imaging radar whose objective was to develop a better understanding of the geological structure of the planet.

Earth, at 1 AU from the Sun, is the third planet of the solar system. It´s composed of a silicate mantle with at iron core at its center. Earth is at the right distance from the Sun to have developed a surface temperature which allows for liquid water. Earth´s atmosphere is composed primarily of 78% N2, 21% 02, 0.9% argon, and traces of other gases. Earth is the only inner planet known to have a moon circling around it.

Earth is the only inner planet where plate tectonics has been observed. The mantle layer above its core maintains the crust of the Earth in continuous motion, creating a magnetic field which absorbs the charged particles of the Sun´s solar wind.

Earth´s sidereal period is 1 year; its period of rotation on its own axis is 24 hours. Earth is the only planet in the solar system known to have developed life. The remarkable balance that it has attained have placed it appart from other planets. These and other features of planet Earth makes it one of the most outstanding planets in the solar system.

Mars, at 1.5 AU from the Sun, is the fourth planet in the solar system. Mars´is composed of a mantle of silicates and a core of iron and sulfur. The crust contains sufficient amounts of iron oxide to give Mars its characteristic reddish hue. The atmosphere in Mars is very thin. It consists primarily of 95% CO2, 3% N2, 2% argon, and traces of other gases. temperature in Mars range from 100 to 250 K.

Volcanoes and lava plains are some of the geological characteristics in Mars. Due to the surface gravity 0.38 lower than on Earth, volcanoes can rise spectacularly high. The largest volcano in the solar system (mount Olympus) is 340 miles (547 km) in diameter and 17 miles (27.4 km) high. Another impressive feature on the Martian surface are its canyons; Mariner Canyon extends 2,500 miles(4,000 km) along the Martian equator and is 75 miles (121 km) wide.

Theories suggest that four billion years ago, Mars had a thicker atmosphere that allowed liquid water to exist, forming runoff and outflow channels, believed to be dry river beds.Mars Gloabal Surveyor launched on November 1996 found evidence of the prescence of liquid water, maintaining hopes that life, perhaps microbial life, might have existed on Mars.