Rocky planets

Our solar system consists of the Sun, eight planets which orbit the Sun, dozens of moons orbiting the planets, and a vast number of small bodies which mostly orbit the Sun.

The "innermost" (nearest to the Sun) four planets, Mercury, Venus, Earth and Mars, are collectively called the terrestial (Earth-like), or "rocky" planets, because they all consists of dense rocky and metallic materials.

Mercury

Mercury is the planet nearest the Sun. It is very small, not much larger than the Moon. It's surface is heated by the Sun to a fairly high temperature. Because of it's low gravity and the powerful radiation of the Sun, Mercury has no atmosphere to speak of.

From the Earth, Mercury can be observed soon after sunset or soon before sunrise, because it is never far from the Sun.

Mercury is the densest planet in the solar system, meaning it consists on average of heavier materials than the others. All the terrestial planets have a metallic core in the middle, and Mercury's core is the largest compared to its size.

Venus

Venus is often called the Earth's sister planet. It is similar in size and internal structure. The most significant part of Venus is its massively thick atmosphere, which consists mostly of CO2, a strong greenhouse gas. The atmospheric pressure on the surface of Venus is comparable to deep in the seas of the Earth, and the thick blanket of atmosphere causes a strong greenhouse effect, making the surface temperatures even higher than Mercury's.

When looking at Venus from the Earth, only the cloudy top layers of the atmosphere are seen. The surface of Venus has been photographed only twice, when Soviet space probes landed on the surface. The probes were destroyed quickly by the hostile environment, but sent back a few photographs and some scientific measurements.

The Moon

The Earth's Moon is unsual in the solar system because of its large size. It is not the largest moon in the solar system, but it is the largest compared to the size of the planet it's orbiting. Other planets have moons which are very small compared to the planet.

The Moon always turns the same side towards the Earth. Therefore nothing was known of the other side until spacecraft finally flew past the Moon in the 1960s. The two sides are strikingly different, as the near side is mostly covered in wide flat dark areas known as maria (singular mare, Latin for sea), while the back side is uniformly cratered with practically no maria. This is believed to be because tidal effects caused by the Earth favour the formation of maria on the near side.

The origin on the Moon is somewhat difficult to explain in a satisfying way. The Moon is very large and similar in composition to the Earth's crust. The current theory is that during the formation of the solar system, another protoplanet collided with the Earth, and the Moon was formed from the debris of this massive impact. The theory is not particularly

The Moon is the only celestial body outside of Earth which has been visited by humans. The Apollo 11 mission landed on the Moon in 1969, and five other landings were made in the early 1970s.

Mars

Mars is the fourth terrestial planet. It is smaller in size than Earth and Venus, but larger than Mercury. Of the planets, Mars is the most hospitable to visiting humans, as it has an atmosphere, though it has no oxygen and is very thin.

In the early days of the solar system, Mars has had liquid water on its surface, which is proved by the existence of various minerals which require liquid water to form. The current atmosphere of Mars is too thin and cold for liquid water to exists for long periods of times. Still, there is evidence of terrain shaped by flowing liquids, which are probably temporary flows or floods caused by the melting of ice in the ground.

Mars has seasons like the Earth, and ice caps at the poles that grow and shrink over the course of the year. They consist of water ice and CO2 ice, the latter varying more with the seasons. One year on Mars is a bit less than two Earth years.

Mars has two moons, Phobos and Deimos. They are very small compared to Mars, and are likely to be asteroids captured by Mars's gravity in the distant past.