I'm piecing together the biology and ecosystem of a Titan-like cryoplanet. And I want it to have clear or nearly-clear skies instead of Titan's complex hydrocarbon haze, so that my ethane-based lifeforms can use visible light for photosynthesis.

Some planetary info:

  • Magnetic field about two thirds the strength of Earth's.
  • 120-150 kPa surface pressure.
  • Atmospheric composition:
  • 90-95% Nitrogen.
  • 5-10% Methane or Hydrogen, or a mix.
  • 1-4% Ethane (due to evaporation from lakes, seas and oceans).
  • Trace amounts (< 1%) of oxygen or some other oxidizing gas or liquid. Emitted by plants as a waste-product.

What compounds could form either naturally or biologically that could function as a transparent UV-absorbing layer? I.e. an ozone layer equivalent.

Is less than 1% oxygen or other oxidizing gas (at 120-150 kPa) sufficient to form an ozone layer able to absorb 90-99% of UV-C and UV-B light? Or are greater percentages of oxygen required?

Could methane naturally react to form transparent UV-C and UV-B absorbing compounds that would be in sufficient concentration to act as an ozone-layer equivalent?

  • 1
    $\begingroup$ Welcome to Worldbuilding SE, FutureBoy, & congratulations on a well researched question. Excuse me. You want to add 1% oxygen to an atmosphere with 5-10% methane and 1-4% ethane? This might be a fire hazard. $\endgroup$
    – a4android
    Aug 27, 2016 at 12:09
  • $\begingroup$ Thanks. The oxygen is in concentrations of less than 1%. Figure that's low enough that nothing would burn :) $\endgroup$
    – Rene
    Aug 27, 2016 at 18:57

2 Answers 2


Good old fashioned water vapor, that's H2O, will do the trick. At high altitudes incoming UV can split water molecules into hydrogen and oxygen. The oxygen can in turn be ionized and form ozone. if the water content of the atmosphere is increased this will lead naturally to the formation of an ozone layer.

Considering the OP wants a Titan-like planet with a clear sky it's worth looking at the cause of the murky skies of real-life Titan.

The orange color as seen from space must be produced by other more complex chemicals in small quantities, possibly tholins, tar-like organic precipitates.[13] The hydrocarbons are thought to form in Titan's upper atmosphere in reactions resulting from the breakup of methane by the Sun's ultraviolet light, producing a thick orange smog.

This suggests that assuming methane can act as a UV absorber as a way of keeping skies clear won't work. In fact, methane absorbing UV is making Titanian skies smog filled.

So we're back to water vapor and its capacity to form an ozone layer.

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    $\begingroup$ Nice answer, but at Titanian temperatures water would be frozen solid. So something, perhaps cryovolcanic eruptions, would have to loft water-ice aerosols into the upper atmosphere. Workable... Still I think a gaseous layer would be better. $\endgroup$
    – Rene
    Aug 27, 2016 at 18:59
  • $\begingroup$ Yes liquid water will be frozen solid. This doesn't prevent water vapor being in the atmosphere. Water vapor is a gas & mainly free water molecules. Ice volcanoes are the most straight forward to loft water molecules into the upper atmosphere. UV splits the water molecules in the upper atmosphere into oxygen & hydrogen. The oxygen will become ozone. $\endgroup$
    – a4android
    Aug 28, 2016 at 4:32
  • $\begingroup$ Maybe... Why doesn't this occur on Titan? $\endgroup$
    – Rene
    Aug 28, 2016 at 5:41
  • $\begingroup$ Good question. It looks like there's very little water vapor in the Titanian atmosphere. Therefore, if there was considerable ice vulcanism (lots of ice volcanoes) on your Titan-like world there could be enough water vapor. Low gravity could mean eruptions reach higher into its atmosphere than would be the case on Earth. $\endgroup$
    – a4android
    Aug 28, 2016 at 6:05
  • $\begingroup$ How much cryovulcanism would be required though? How much ice would need to be lofted into the upper atmosphere? $\endgroup$
    – Rene
    Aug 28, 2016 at 22:51

The phtotosynthesis could use some narrow notch in the haze spectrum, where it is transparent. Look at the history of trying to explore Venus and Titan for discussions about discovering such windows and building instruments to match.

Likewise the eyes of any locals would be evolved to take advantage of the kind of light actually available.

Note that the sky can be frosted and not provide a coherent image of what’s on the other side, but still transmit a useful amount of light. You'll get a general glow rather than a tight spotlight of the sun.

  • $\begingroup$ Titan's haze absorbs blue light, leaving only red and some yellow that can make it through. I did consider plants could use near-infrared (which does go through the haze). However if I take that route then my planet would be only a featureless orange ball in space :/ $\endgroup$
    – Rene
    Aug 27, 2016 at 19:04

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