Image in header: Schematic life cycle of the Earth (homemade image based on diagram by Peter ward, Rare Earth)
So far, we know of only 1 planet with life, and that is our Earth. So, for now, that is also the only ‘case’ we can study. People who register for our Solar System astrobiology course often think it will mainly be about planets other than Earth. They are then sometimes somewhat disappointed that at least half (the first half, of course) will be about Earth. However, that disappointment passes quickly when they discover how fascinating and enlightening the study of (life on) our planet is, and how enlightening and necessary this study is to think meaningfully about possible extraterrestrial life. So again in the written version: first learn about our unique living planet and the major milestones in its history in chapters 1, 2 and 3. Only then will we really understand Chapter 4 (Mars) and Chapter 5 (ice moons).
The wondrous earthly paradise we currently live in is not a representative picture of most of its existence. The image in the header depicts Earth’s entire existence as a 12-hour clockwork. Each hour represents a billion years in the planet’s existence. When our Earth is 11 to 12 billion years old, the Sun will have swollen into a red giant, possibly swallowing up Earth. So we therefore put the end of our planet’s life cycle at 12. The Earth is currently 4.54 billion years old, so we are on the clock now at 4h30 approximately. You can see at that time a green band called “Fanerozoic”. It is the absolute climax of terrestrial life that we are really in the middle of at the moment: the existence of plants and animals. This period of 1 billion years is also called “the age of the animals”. All other periods there are NO plants or animals! And therefore no forests, swamps, savannahs, coral reefs or other amazingly rich ecosystems. In the next two chapters, we will learn to understand why, especially in Chapter 2. Should we humans be able to walk or swim for a while in older periods than the Fanerozoic, we would be surprised at the apparent emptiness. The land consists of bare-looking rocks, and the sea seems empty except for some greenish rocks and some sliminess here and there. Yet even then, the water was full of life, but only microscopically.
Why did it take so long for the complex and multicellular life forms known to us to emerge? And what life forms were there before that? And what characteristics must a planet have to contain life or eventually produce comlpex life forms? And how unique is our Earth with these characteristics? We are going to discover all this in chapters 1 and 2. Let’s start with an overview of the most essential characteristics of our living Earth:
- Water: solid, liquid, gas
- Plate tectonics
- Atmosphere and climate
- Theia/Moon
- Magnetic field
- Location in the Solar System
- Location in the Galaxy
- Impact of meteors