Rocky+Neighbours

Mercury, Venus, Earth and Mars
The four planets of the inner solar system are all rocky planets, and because of this are sometimes referred to as terrestrial planets (Terra was the Roman god assciated with earth, being responsible for earthquakes and the productivity of farm land).

These four plants are all composed of rock, with a solid surface (e.g. on which spaceships can land). They have thin atmospheres, in some cases bordering on nothing, and are relatively small in size compared to the outer planets. The inner planets also have few moons and no ring systems.

So the inner planets share some similarities, but they are also wildly different from each other. Some of their key characteristics are summarised below.

Mercury
At about 4,900 km in diameter, Mercury is less than half of the diameter of the Earth, and is the smallest of the inner planets (indeed it is the smallest of the major planets in the Solar System, but still more than twice the diameter of the largest dwarf planet). This planet is the closest to the Sun, has no moons and no atmosphere. The lack of an atmosphere means that the temperature in the day soars to 430 o C as the unprotected surface is baked by the Sun, then plunges to -180 o C at night as there is no 'blanket' to keep the heat leaching into space. Each day and night lasts some 59 Earth days, so it is a harsh environment of long periods of baking followed by long periods of intense cold (colder than any of the other planets as far out as Saturn). It takes 88 days to orbit the Sun.

[|Mercury - BBC material]

Venus
Fractionally smaller than Earth, Venus is sometimes referred to as Earth's ugly sister. It has the closet orbit to the Earth, but its surface is hidden from our sight by a thick atmosphere of carbon dioxide more than 30 kilometres thick (although this is still thin compared to the planet's diameter of about 12,100 km). The pressure of this thick atmosphere (90 Earth atmospheres) would be enough to crush a human in seconds (not to mention a deep sea submarine!). The thick layer of greenhouse gases also keeps the surface temperature of Venus relatively constant, but at a whopping 464 o C on average, making the surface of Venus hotter than any other planet in our solar system. Like Mercury, Venus has no moons. Its surface is covered in volcanoes and plains of larva and we are still looking for evidence of whether the volcanoes are still active. Venus is the only planet in the solar system that spins the opposite way to Earth, so that the Sun rises in the west and sets in the east, but it spins so slowly that its day (243 Earth days) is longer than its year (225 days).

[|Venus - BBC material]

Earth
Our home planet measures approximately 12,800 km in diameter and is 150 million km (1.5 x 10 11 m) from the Sun. Earth has the largest moon relative to its size of all the major planets (although one of Pluto's moons, Charon, is relatively larger). The distinguishing feature of the Earth, which is so important to life, is the presence of liquid water on the surface. The Earth has much less evidence of asteroid impacts that other planets, although this is due to the active geology of the Earth, which results in the surface rock being replaced and remoulded over time.

The Earth's equatorial plane is tilted at an angle of 23.5o to the plane of the Earth's orbit around the Sun. The axis or rotation is therefore angled towards or away from the Sun at any point in time. This accounts for the varying length of days throughout the year, as well as the changing seasons. For example, in the northern hemisphere, during the summer the north pole of the Earth is tilted towards the Sun. This has two effects: (1) the Sun's rays hit the atmosphere at a steeper angle than in the winter, this means the same energy arriving is spread over a smaller area, and (2) the Sun is above the horizon for longer, so more energy will arrive over the course of a day.

Note that the rotation axis itself wobbles, but with a period of around 26,000 years, so its effect is not noticeable.

[|Earth - BBC material]

Earth's moon is the most obvious object in the night sky, and has an important effect on the Earth as it drives the tides. See the separate page on the moon here (link).

Mars
About half the diameter of the Earth, Mars has an atmosphere made up primarily of carbon dioxide and argon. Known as the red planet, early observations of this planet raised expectations about what we would find on other planets very high. This was in a large part due to an Italian astronomer Giovanni Schiaparelli who, in 1877, saw thin faint lines criss-crossing the surface. He described them as 'canali', the Italian word for channels. This caught the public imagination who pictured canal building civilisations. Martians were, however, not found. Instead the first probe to photo the planet (Mariner 4), sent back pictures of a cratered surface, much like the moon - suggesting that Mars does not have tectonic activity like the Earth, and finding none of Giovanni's 'canals'. Later probes and landers identified that Mars has four huge volcanoes, including Olympus Mons, a 25 km high monster - the largest volcano in the solar system. Mars orbits the Sun with a year lasting 687 days, and has a day similar in length to our own. Its surface temperature -125 o C to 25 o C.

More recently robotic rovers sent to Mars have confirmed the presence of water ice under the surface [|Pathfinder]. There is currently (@ February 2012) a new lander on the way to Mars which is significantly larger than anything sent before [|Curiosity - Mars Science Laboratory].

[|Mars - BBC material]


 * Activity:**

The rocky planets can be compared with the gaseous planets with the attached table (teacher and student versions)

Student version (blank table) [|120123 Rocky vs Gaseous planets - fact sheet (student).pdf]

Teacher version (completed table) [|120123 Rocky vs Gaseous planets - fact sheet (teacher).pdf]