OXYGEN

1. The size and consequent gravitational force of the inner rocky planets imposes relatively low escape velocities (on Earth it’s 11 km/s) meaning they are unable to retain the abundant, light elemental gases hydrogen and helium. Venus and the Earth are nearly the same size while Mars is less than half the size of Earth. Venus and Mars have atmospheres dominated by CO2. Earth, uniquely in our solar system, retains an atmosphere composed predominantly of N2 and O2.
2. Earth formed 4.6 billion years ago from the gravitational accretion of a broiling mixture of interstellar gases and dust. At this stage it had almost no atmosphere and the surface was molten. As Earth cooled, an atmosphere formed mainly from gases spewed forth from active volcanoes.
3. 7 billion years ago, blue-green cyanobacteria flourished in the Earth’s oceans. They made gaseous, or free, oxygen by the photolysis of water, utilising the abundant energy source of ubiquitous UV light.
4. Until about 430 million years ago, most aerobic organisms lived in the ocean and exploiting oxygen dissolved in seawater. Terrestrial life then appears in the form of small plants and invertebrates that had evolved the ability to harness oxygen in the gas phase directly from the atmosphere.