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Oxygen and Space


Oxygen is the third most abundant element in the universe.  Only Hydrogen and Helium are more abundant.


The Planet Earth has about 21 percent Oxygen in its atmosphere.  Although the limited sample of astronomical bodies we can study fairly easily suggests that this is unusual, the Earth is not the only world with Oxygen in its atmosphere. 

Europa and Ganymede

The atmosphere of Jupiter’s moon Europa is mainly Oxygen, but the air pressure is only about one billionth of the Earth’s air pressure.  So Europa does not really have a lot of Oxygen, although what it has may be very important for life, if it exists, in its sub ice ocean.  The Oxygen on Europa is believed to be produced by the breakdown of water molecules into Hydrogen and Oxygen by electromagnetic radiation.  The Hydrogen escapes from Europa much faster than the Oxygen.

Ganymede is also believed to have a very thin Oxygen atmosphere, with the Oxygen being produced in the same way.

Mars has little uncombined Oxygen in its atmosphere.


The Planet Earth is believed to have had an atmosphere with very little free Oxygen until plants started photosynthesising and using carbon dioxide (CO2) and water (H2O) together with energy from the Sun’s light to produce sugars and give off Oxygen.  Most of the uncombined Oxygen in our atmosphere is believed to be of biological origin.

Oxygen is the most abundant element by mass in the Earth’s crust.  Nearly all the Oxygen in the crust is combined with other elements in compounds.

Reactive Gas

Oxygen is a very reactive gas.  It will react with most metals and many non metals, so if it were not being maintained in some way, we would expect a planet to lose most of its free Oxygen.


It is believed that the presence of large amounts of Oxygen in a planet’s or moon’s atmosphere would be good evidence of photosynthetic life in considerable quantities.

Life, the Universe and Everything

In the Universe, Hydrogen is the most abundant element, Helium is second, Oxygen third and Carbon forth.  The three elements Carbon, Hydrogen and Oxygen are the ones present in the largest amounts in the type of life that we know of.  The forth most important element for our type of life is Nitrogen.  Nitrogen is the seventh most abundant element in the universe.  Perhaps the most important inorganic compound needed for life as we know it is water.  This is a compound of the most abundant element and the third most abundant one.  Hydrogen and Oxygen react readily and can even do this at only 10 degrees above absolute zero.  Water is a very common compound in the universe, possibly even the most abundant compound.

It is remarkably convenient that five of the most important elements for life are so abundant.  Some people even consider that it is strong evidence for a Devine Creator who loves life.  However, as with many things, not everyone agrees and some think that this is looking at the situation the wrong way round.  Life has made use of those elements that are both abundant and have the properties needed for life.  (Helium is very unreactive and does not appear useful for our type of protoplasm.)

Another way of looking at the situation is to consider that the universe has to be suitable for intelligent life because if there were none, we could not be here to observer it.  If other universes exist with other laws, they might not have any life to contemplate the big questions like “What is the answer to life, the universe and everything?”













The Planet Earth has a lot of Oxygen in its Atmosphere.
Picture from NASA Goddard Space Flight Center Image by Reto Stöckli (land surface, shallow water, clouds). Enhancements by Robert Simmon (ocean color, compositing, 3D globes, animation). Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean Group Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Field Center (Antarctica); Defense Meteorological Satellite Program (city lights).
Europa on the left with Ganymede
Image Credit: NASA/JHU-APL/Southwest Research Institute