Why does Titan's atmosphere not start to burn?

Question

If Titan has a methane atmosphere and seas of methane, then why doesn't some meteoroid / chemical reaction ignite and blow the whole thing?

Answer 1

Titan is one of Saturn's moons. Titan has a dense atmosphere, at about 1.5 bars. It also seems to have lakes of liquid methane.

For a conventional combustion, you would need a good Methane-Oxygen mix. Every combustion is basically an oxidization. Apart from very energetic events, such as SL9 hitting Jupiter, you will need quite a bit of oxygen to get a nice, explosive fireball in Titan's atmosphere.

According to Wikipedia, you need two Oxygen molecules for every Methane molecule you want to burn.

Answer 2

This question is little different from asking why, if the Sun is a ball of Hydrogen, it hasn't already exploded (or burned out). It is difficult for me to take it seriously. But I'll try.

Methane and Oxygen are gases and will react at room temperature and pressure in certain concentration ranges and when exposed to a spark (or other high energy density discharge; a flame, friction, etc.). We call these limits the LEL and UEL (abbreviations for Upper and Lower Explosion Limits) for hydrogen which change as the atmospheric concentration of Oxygen changes. A tragic example of this was the enormously stupid decision by NASA to use pure oxygen in the Apollo space capsules. During testing of Apollo 1, three astronauts burned to death in a flash fire in which the extreme oxygen concentration allowed almost all plastic materials (wire jackets, clothing, etc.) to burn intensely. An urban myth I have heard, but can neither confirm nor contradict, is that if you held a cigarette lighter under your finger in a pure O2 atmosphere, it would start to burn, and continue to burn even after the lighter was put out.

The temperature of a gas is an indication of how fast the atoms/molecules are going. As you lower the temperature, the average speed declines. For the $\mathrm{CH_4+O_2}$ reaction to proceed, you need the methane and the oxygen molecules' collision to impact hard enough to break chemical bonds. As temperature drops, this becomes increasingly unlikely. OTOH, the reaction also releases energy, so if there is enough energy both from the speed of the molecules and the energy released from the reaction products to induce more that one additional reaction, then you will have a chain reaction and the result will depend on whether the supply of both molecule types is large enough to sustain the reaction. It can self-quench, stay in a steady state (form a flame) or explode. Note that while on Earth, the most common reaction in the air is using oxygen as the oxidizer (one of the reactants), methane can react with many other gasses, for instance $\mathrm{F_2}$, fluorine or $\mathrm{Cl_2}$, Chlorine.

Since there is virtually no oxidizer present in Titan's atmosphere, methane can't burn, even if the temperature, pressure, and concentration were high enough.