General Astronomy/The Terrestrial Planets

The Solar System consists of eight planets orbiting a star - the Sun. The eight planets fall into two distinct groups. The four innermost planets are very close to the sun, are formed of rocky materials, and are relatively small. The four outermost planets are far from the Sun, are formed of gasses, and are relatively large. The Earth is one of the four innermost planets, so they are called the terrestrial planets (Terra is latin for earth, so terrestrial means "earth-like". The four outer planets are called the Jovian (Jupiter-like) planets.

The terrestrial planets are, in order of distance from the Sun, Mercury, Venus, Earth and Mars. All of these planets are made of rocky substances, primarily nickel, iron, and silicon. The Jovians are composed mostly of hydrogen and helium gas. The terrestrials orbit the Sun at distances between approximately 0.3 and 1.5 Astronomical Units from the Sun (an Astronomical Unit or AU is defined as the average distance between the Earth and the Sun, so the Earth is 1 A. U. from the Sun). In contrast, the Jovian planets spread from 5AU to 30 AU. The terrestrials are fairly small; the Earth is the largest of the four, and Mercury is only about 1/4 the diameter of the Earth. The Jovians are relatively large, from 4 to 11 times the size of the Earth.

All of the terrestrial planets are presumed to have formed through the same process, so all should share common properties. The terrestrial planets are different than the Jovians because a planet close to a star forms under different conditions than if it is far from the star. The terrestrial planets formed separately in different orbits, so there are significant differences between the four. The four terrestrials have many things in common, but each one is unique in its particular size, atmosphere, orbit, and so on.

Mercury
General Information

Mercury is the closest planet to the Sun. Its orbit is a mere 88 Earth-days, meaning that in the time it takes for the Earth to make one full revolution around the Sun, Mercury makes a little over four revolutions.

Solar days on Mercury are extremely long. It takes two orbits about the Sun for Mercury to complete one full rotation around its axis relative to the sun, so most points on the planet experience direct sunlight for a (Mercurial) year at a time.

Mercury has a highly elliptical orbit, finding itself a mere 0.313 AU from the sun at Perihelion (closest point to the Sun) and at 0.459 AU at Aphelion (farthest point from the Sun). This stretched orbit, combined with a thin and nebulous atmosphere, result in Mercury having wild temperature swings: temperatures during the day tend to exceed 430 degrees °C, while temperatures during Mercurial night tend to drop to a frigid -180 °C.

Mercury has a mantle far thinner than its core, so it is the second-densest planet and has surface gravity as strong as that of Mars.

Venus
Venus is the second planet from the sun, and the only terrestrial planet to rotate in a retrograde manner. Venus has a sidereal day (rotation period relative to the distant stars) lasting about 243.0 Earth-days, or about 1.08 times its orbital period of 224.7 Earth-days. Because of its retrograde rotation, its solar day is "only" about 116.8 Earth-days, and it has about 1.9 solar days per orbital period.

The surface temperature of Venus is highly uniform, about 462 °C (about 736 K/864 °F). The surface of Venus is thought to have been completely resurfaced by volcanic activity 300 to 500 million years ago.

Normal telescopes cannot penetrate the thick clouds of sulfuric acid droplets, but radar can be used to map out the topography of the land.

Earth
The planet Earth is the planet on which all known life in the universe lives. It is the third planet from the sun and the largest terrestrial planet.

Life on Earth is made possible by its massive reserves of liquid water. No other body in the solar system is known to have massive quantities of liquid water, though several Jovian moons are suspected to have oceans under their crust.

The Earth's only natural satellite, the Moon, is the largest satellite in the solar system relative to its planet. This means that the Moon and Earth are sometimes considered to be a double planet. The Moon is also the only celestial body aside from the Earth that has been set foot upon by humans.

Mars
The highest mountain on Mars is Olympus Mons. The northern hemisphere is dominated by volcanic plains; the south by heavily-cratered highlands. The surface is very dusty, and covered with dunes. The sky is a pinkish-orange, from dust storms.

Mars has a chaotic terrain. Astronomers believe it was created when magma melted underground permafrost, causing the ground to collapse. Valley networks are believed to have been caused by rain, and water flowing under ice.

Dried lake beds indicate the presence of water in Mars' past. Another indicator is calcium carbonate, found in a Martian meteor in 1996. Yet another is the presence of outflow channels (believed to have been caused by underground permafrost melted by magma) and alluvial fans. Evidence of seepage flows have also been found. These occur when water breaks through ice and cascades down a cliff.

Martian craters are different from the craters found on the Moon, Venus, or Earth. They're surrounded by mud flows.

The first to map the surface of Mars was Italian astronomer Giovanni Schiaperelli in 1877. He observed a series of lines crisscrossing the Martian landscape, which he called canali (channels). Some astronomers translated canali as canal, implying that they had been built by intelligent beings. This idea was advanced by Percival Lowell in his books Mars and Its Waterways (1903) and Mars as the Abode of Life (1908). It was later discovered that the Martian canals don't actually exist, and were very likely an optical illusion.