Where Do Hydrocarbons Come From?

We now know with certainty that there are two worlds in our solar system that are rich with hydrocarbons.

Where'd they come from?

Now ... don't fall for the old saw that on Earth "they come from decaying and metamorphosizing plant matter". That explains where purer, relatively disorganized, carbon minerals like coal come from.

Hydrocarbons need a somewhat different explanation. Further, hydrocarbons seem to be being created currently on Titan. Why not on Earth?

We can't be absolutely certain, but it is reasonable to assume that they are not being created by plant decay and metamorphosis on Titan. This is not to say that there might not be life on Titan, but the earthly story requires the deposition of a lot of dead plant material, and the deposition of even more stuff to bury that and heat it up. This seems unlikely on Titan.

So, what is going on? Let me speculate.

Let's think about gravity and heat a bit. Heavenly bodies like planets and moons hold atmospheres for two reasons. First, the pull of gravity can keep gases closer to the planet - bigger planets hold gas more easily, and they hold the heaviest gases most easily. Secondly, heat can drive gases off of a planet - just like a boiling kettle.

Now, let's go through the roster of gases. The elemental ones that are common in the universe are hydrogen, helium, nitrogen, oxygen, and argon in lightest to heaviest order.  Then their are the common hydrogen compounds methane, ammonia, and water - all with weights between helium and nitrogen molecules. Then there are a few gases with more complex formulas or rarer occurence: the hydrogen-carbon-nitrogen compounds (cyanides), the nitrogen-oxygen compounds, and the carbon-oxygen compounds. The most important of these is carbon dioxide, which is also the heaviest gas listed so far.

And now we have the roster of worlds. Start with Earth - big enough and cool enough to hold nitrogen, but not hydrogen or helium. That means it would hold methane, but that burns off in our oxygen rich atmosphere. It holds water vapor easily too (but the ammonia is too unstable). Venus is a little smaller and gets about twice as much solar radition, which burns off everything but carbon dioxide. Mercury is even smaller and hotter and loses everything. Mars is colder and smaller than Earth, and holds mostly carbon dioxide. The four gas giants are big enough to retain hydrogen and helium, and these are the big components in their atmospheres. And who cares about Pluto anyway ... more of a dog than a planet anyway.

Where does Titan fit in? Well, it's bigger than Mercury but smaller than Mars - and a lot colder than the latter. Cold enough to retain the nitrogen that Mars has trouble with, but not helium. Apparently also cold enough to retain some methane and water (again, ammonia is more unstable).

The only problem is that Titan's orbit is outside the magnetosphere of Saturn. So, it is bombarded with ionizing radiation that constantly breaks down its methane. The same process makes ozone in Earth's upper atmosphere. But our oxygen is not lost in this process because it is constantly being regenerated by plants on our surface.

We're really left with two unusual possibilities. That Titan is a huge ball of methane rich stuff. That's encouraging, because it means that Earth's hydrocarbons might be celestial rather than terrestrial in source. Alternatively, some chemical process inside Titan is generating new methane constantly. That seems really odd, but there is at least a theory that this process may be occurring on Earth also. Either way, this tends to point away from the sky-is-falling story some people tell about fossil fuel use on Earth.

I don't know what the answer is, but since you read this far my money is on a lost tribe of cows living on Titan (a la Battlestar Galactica), wandering the free range orange landscape, farting up a storm. ;)

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