Science Questions with Surprising Answers
Answers provided by
Dr. Christopher S. Baird

Why doesn't the planet Uranus explode if it contains so much hydrogen and methane?

Category: Chemistry      Published: April 3, 2015

uranus
Public Domain Image, source: NASA/STSI.

The planet Uranus indeed contains a significant amount of hydrogen and methane, both highly flammable gases. However, the burning of methane or hydrogen requires oxygen. Simply put, there is no free oxygen on the planet Uranus. On earth, we are so immersed in oxygen that we tend to take it for granted. Many chemical reactions that require oxygen seem to just happen automatically on earth: metals rust, forests catch fire, and candles burn. We may be tempted to ignore oxygen's role in a chemical reaction since it seems to be always there. But oxygen is not always present. If I place metal in a jar containing only argon, it will not rust. If I place a lit candle in a jar with no oxygen, its flame will go out. There is an easy demonstration you can do at home to convince yourself of this fact. Get a bowl and fill it about a quarter full with baking soda and vinegar. The baking soda will react with the vinegar, releasing carbon dioxide gas that fills up the bowl and pushes the oxygen out. If you are careful to not disturb the bowl, then the carbon dioxide will stay in the bowl and keep the oxygen out since carbon dioxide is heavier than oxygen. Now, light a match and slowly move it into the bowl. The moment that the match hits the invisible oxygen-depleted pocket of carbon dioxide gas in the bowl, it will immediately go out. This simple demonstration makes the role of oxygen in combustion obvious.

Methane is the main component of natural gas, which is used as the fuel in many household furnaces and ovens. Household gas leaks are so dangerous because methane is so explosive. However, combustion of methane requires oxygen. Therefore, methane is only explosive and flammable as long as there is oxygen or some other oxidizing agent present. Methane consists of four hydrogen atoms bonded symmetrically to a central carbon atom. When it burns with oxygen, it produces mostly carbon dioxide and water according to the formula:

CH4 + 2 O2 → CO2 + 2 H2O + energy

Hydrogen is the simplest chemical element, containing just one proton and one electron (and occasionally a neutron or two). Under standard conditions, two hydrogen atoms bond to each other to form a hydrogen molecule, H2. In the presence of oxygen, hydrogen is highly flammable, combusting to form water molecules according to the formula:

2 H2 + O2 → 2 H2O + energy

An example of hydrogen burning is the Space Shuttle Main Engines. The External Tank of the Space Shuttle contained giant tanks of hydrogen and oxygen. These chemicals were pumped into the Main Engines where they were mixed and burned, providing the thrust that helped propel the Space Shuttle into space.

The atmosphere of the planet Uranus contains mostly hydrogen, helium, and methane. Interestingly, the methane in the atmosphere is what gives Uranus its distinctive blue color. Since Uranus contains effectively zero free oxygen, the hydrogen and methane in the atmosphere does not burn or explode.

Topics: Uranus, combustion, hydrogen, methane, oxygen