The rocky planets

The four inner planets of the solar system have something in common: they are made of rock and metal and have a solid surface a space probe can set down on. Yet Mercury, Venus, Earth and Mars are completely different worlds. On one you would burn up instantly, on another you would freeze, and on only a single one can you simply breathe.

Four worlds of stone

Mercury is the smallest planet and the closest to the Sun, only about 1.4 times the diameter of our Moon and almost without an atmosphere. That is why its temperature swings wildly: up to 430 degrees Celsius by day, down to minus 180 degrees at night. Venus is almost as big as Earth, which is why it is often called Earth's sister. Earth is the largest rocky planet and the only one with oceans of liquid water. Mars has only about half of Earth's diameter, and its red colour comes from rust, iron oxide in its dust and rock.

Venus, the overheated greenhouse

You might think the planet closest to the Sun must be the hottest. But Venus clearly beats Mercury: its surface is around 465 degrees Celsius everywhere, day and night, at the equator and at the poles. The reason is its extremely dense atmosphere of carbon dioxide. It lets sunlight in but traps the heat like a greenhouse. On top of that, the air presses down with 90 times Earth's air pressure, and the clouds contain sulphuric acid. Venus shows what a runaway greenhouse effect can do.

Mars, the next destination

No planet has been explored as thoroughly as Mars. Rovers roll across its surface, drill into the ground and search for traces of past life. Dried up river valleys and water ice at the poles show that Mars was once warmer and wetter. Today it is a cold desert with a thin carbon dioxide atmosphere and an average of about minus 60 degrees Celsius. At 24.6 hours a Martian day is almost as long as an Earth day, and the volcano Olympus Mons rises about 22 kilometres high, far taller than any mountain on Earth. Two tiny moons, Phobos and Deimos, circle the planet. Many consider Mars the first destination for crewed journeys to another planet.

Why Earth is the exception

Earth got several lucky breaks at once: the right distance from the Sun so that water stays liquid, an atmosphere with oxygen, and a magnetic field that deflects dangerous particles from space. Mercury, Venus and Mars show how easily things can turn out differently. That is exactly why they are so valuable for research: understanding the neighbours means understanding Earth better too.