There is an interesting discussion about whether there could be life on Saturn's moon Titan. (For example here.) The life could use reaction of hydrogen and acetylene which are being produced by photolysis in Titan's atmosphere. I would like to know what total energy is available for such life per square meter per day, or at least how much hydrogen is produced in Titan's atmosphere per day.

There are some estimates in article McKay and Smith, Icarus, 2005, but I have problem translating the quantities they use for the estimate to total amount of hydrogen produced per unit of time or energy produced per unit of time.


1 Answer 1


I can give a rough answer to your question and some brief thoughts on the paper, and I invite correction from anyone smarter than me. Your article talks about energy per mole, but energy of production on a daily basis would be a product of solar energy.

To produce hydrogen in Titan's atmosphere requires UV light, from this article, UV light of 1600 Angstroms (160 NM) (Source)


Methane is a carbon atom surrounded by four hydrogen atoms, and it can be broken apart by ultraviolet light at wavelengths of about 1600 Angstroms. The fragments, or radicals, that are produced from this process are very chemically reactive. They’re things like CH, CH2, and in some cases CH3.

Only a small percentage of the sun's light is in that range. 8%-10% of the sun's light is in the UV range, but maybe only 1%-2% in the upper UV3 range of 160 NM. (Source)

The same Wiki article, using Saturn as a guide, Saturn receives between 13.4 and 16.7 Watts per square meter (Source), so figure an estimate of 15 watts per square meter total solar energy, 1%-2% of that strong enough to pull a hydrogen atom off Methane, so 0.15 - 0.3 watts per square meter. That might not sound like much, but on a moon the size of titan, that's a lot of square meters so the low wattage isn't a big deal. That's a peak estimate though as some of the hydrogen would recombine with the CH3 it split from, some energy would likely be converted to heat, and perhaps the biggest problem, mentioned in the article above, a share of the hydrogen, perhaps the lion's share would simply be lost off the planet.

In the atmospheres of the giant planets, the hydrogen stays around because of the high gravity of those planets. Their atmospheres are primarily hydrogen anyway, and after the fragmentation occurs, the products sink into the deeper atmosphere, and methane is reconstituted. It’s a complete chemical cycle.

On Titan, as far as we understand, that does not occur. Because the gravity is low, hydrogen should escape. The Voyager ultraviolet spectrometer saw a corona of hydrogen around Titan, which is a good indication that hydrogen is escaping. Now, if hydrogen is going away, the products that can be made from the methane are going to have a higher carbon to hydrogen ratio than methane itself, and you’re not going to be able to remake methane. So as far as we understand the photochemistry, Titan should be destroying methane and making more carbon-rich products, the simplest of which are acetylene – C2H2 – and ethane – C2H6. These are made directly from methane

This is my problem with the article. I don't see any way the hydrogen finds it's way down to the planet's surface to be used in the way the article suggests, life in the lakes of Titan. It's an interesting idea and the people who wrote it are probably PHD educated and I'm not, but I suspect the majority of hydrogen produced high in Titan's atmosphere would remain high in it's atmosphere and over time, lost to the solar wind.

It does raise the question, because Titan should lose it's Methane over time, why does it still have so much Methane - a very good question.

Lets look at the timing. Huygens landed on Titan January 14th, 2005.

Your article was written 14 January 2005; revised 18 April 2005

and here's an article from a few months later, November 2005.


The origin of methane in Titan's atmosphere is a mystery because it gets broken down by sunlight and particle radiation from space in the upper atmosphere. If surface lakes and pools were the only source, all of Titan's methane would be lost by this mechanism in less than a hundred million years, a short time for a moon that's been around since the formation of the solar system 4.5 billion years ago.� Components of the methane molecules react with each other and atmospheric nitrogen. As they descend, they form larger and heavier molecules that comprise the orange haze that blankets the moon. Because Titan is very cold (292 degrees below zero F, or minus 180 degrees Celsius) these heavy compounds condense and rain out on the surface.

"We have determined that Titan's methane is not of biological origin, so it must be replenished by geologic processes on Titan, perhaps venting from a supply in the interior that could have been trapped there as the moon formed,"

So, I'd chalk this one up to timing. January-April 2005, life on Titan was a viable theory, which has since been de-theoried, or, whatever the correct term is.


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