Reece Flawn, of King's High School, asks :-

Why is Earth the only planet in our solar system with enough oxygen for humans to survive?

Frank Andrews, an astronomer at the Carter Observatory, Wellington, responded.

Firstly Earth has an atmosphere because it is the right size for it's gravity to be strong enough to hold onto the heavier gases such as oxygen, carbon dioxide and water vapour. The lighter gases such as hydrogen and helium have been able to escape. The moon is much less massive than the earth so all gasses have escaped and it has no atmosphere. Mercury is too small and close to the sun to have held on to any atmosphere.

Early in their formation the large rocky planets such as Earth, Venus and Mars had atmospheres dominated by carbon dioxide thrown up during extensive volcanic activity. Venus was too hot and Mars too cold for life as we know it to become established. On earth simple green plants evolved which produce their food by photosynthesis. Sunlight gives the energy for carbon dioxide and water (hydrogen oxide) to chemically combine giving oxygen off as a waste product. This oxygen has built up in the earth's atmosphere until it is sufficient for oxygen breathing animals such as ourselves to evolve and survive. We breath in oxygen which chemically reacts (burns) with our food to produce energy and carbon dioxide as a waste product.

Animals and plants depend on each other. Oxygen is a very reactive gas and combines with many substances, for example with iron to form rust. If it were not for the green plants there would be no oxygen in the earth's atmosphere and therefore no humans. The carbon dioxide that was the main component of the earth's early atmosphere has gradually been converted into living plants and the animals that eat them (biomass) and into fossil remains. Coal and oil are fossil remains of plants and contain the very large amounts of carbon dioxide which they trapped during their life. Similarly, rocks such as marble and limestone are mostly calcium carbonate and lock up large amounts of carbon dioxide trapped when sea animals such as shell fish and corals built up their skeletons.

Hence the size of the earth, its distance from the sun, geological events and life itself has made the atmosphere of the earth suitable for modern life forms. The other planets of our solar system never had the necessary balance of these conditions.