Inspired oxygen: present, past, and future.

As earthlings, we take the oxygen in the air that we breathe for granted. Few people realize that this easy access to oxygen makes us unique in the whole universe. Nowhere else in our planetary system or in distant stars has stable oxygen ever been detected. However, the present plentiful supply of oxygen in our atmosphere was not always there. Long after the Earth was formed some 4.5 billion years ago, the Po2 in the atmosphere was near zero, and it remained so for millions of years. But about 2 billion years ago, the Po2 dramatically increased to as high as 200 mmHg during the Great Oxygen Event, due to the activity of microorganisms, the cyanobacteria. Subsequently, the oxygen level fell to the intermediate values that we have today. Here, we also look to the future, for example, the next 50 years. This period will be special because it will include the beginnings of human space exploration, initially to the Moon and Mars. Neither of these has atmospheric oxygen. Nevertheless, plans to visit and live on both of these are developing rapidly. We consider the fascinating problems of how to ensure that sufficient oxygen will be available for the groups of people. Although it is interesting to discuss these issues now, we can expect that major advances will be made in the next few years.