Image in header: Solar wind is deflected by Earth’s magnetic field. Source: ESA.
Planet Earth was formed 4.54 billion years ago. We denote the time of formation by -4.54 GA. The G stands for Giga (= Billion) and the A for Annum(= years). You will see that in the literature it is usually written 4.54 GA, and not -4.54 GA. So that means so many years ago. If it is about millions of years, then MA (Mega) is used instead of GA, and for thousands of years, KA (Kilo) is used.
Life emerged on Earth fairly quickly (discussed further in the course), and has not disappeared until today. This means that Earth life has experienced 4 billion years of evolution without being erased, even though there have been threatening periods. The global climate and other conditions (cosmic radiation, air and water composition, incident meteorites, volcanism, etc.) have remained within livable limits all this time. This has allowed amazingly rich ecosystems to develop on Earth with very complex life forms. Earth life itself has also played a very large role in stabilising the climate, making the planet an even more stable system. Achieving that took a lot of time without being destroyed.
Yet the Earth’s habitability is also finite. We will see further in the course that there will come a time when first complex life, and later even single-celled simple life will eventually disappear on our planet. Of the total existence time of our planet – just over 10 billion years – there is actually only 1 billion years with plants and animals. Our current earthly paradise is temporary, and is sometimes called “the age of the animals”. And so for that 1 billion to be possible, it took 3-4 billion years of sufficient stability to precede it.
Mars and Venus both probably also started out as habitable planets. But both became – at least on the surface – very unlivable in the first part of their existence. Life forms as we know them cannot survive on their surface because of deadly dangers constantly present:
- Mars: extremely low temperatures, no liquid water and extreme dehydration, extremely low atmospheric pressure, high doses of radiation from the Sun and other cosmic sources.
- Venus: extremely high temperatures, no water, extremely high pressure, volcanic violence, extremely acid rain.
The subsurface of Mars is somewhat more protected from these dangers, and is therefore considered a place where extraterrestrial life might still be possible to a limited extent. We discuss this further in part 4 of the course. Even at high altitudes in the atmosphere of Venus, a very small chance of +/- habitable places still seems theoretically possible.
Things are actually worse on other planets in our Solar System. However, there are places where conditions have prevailed for billions of years that presumably closely resemble the deep sea of the young Earth. It’s about the subterranean oceans on some of the icy moons around Jupiter and Saturn. We discuss this in part 5.
