Planetary Atmospheres
The same principle applies to planets. Early Earth had a reducing atmosphere, along with Mars and Venus. This proved to be a good environment for Cyanobacteria to evolve the first photosynthetic metabolic pathways which gradually increased the oxygen portion of the atmosphere, changing it to what is known as an oxidizing atmosphere. With increased levels of oxygen, the evolution of the more efficient aerobic respiration was enabled, allowing animal life to evolve and thrive.
In contrast, the constant bombardment with hydrogen in the solar wind means that interplanetary space is reducing. For example, the Moon is directly exposed to solar wind, such that sodium is reduced and evaporated to produce the sodium tail of the Moon (see atmosphere of the Moon).
Read more about this topic: Reducing Atmosphere
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