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The orange sky

Titan's colour is made of its own methane. Sunlight breaks it, the pieces link up, and the haze they form stops blue light first.

See it in the tour: chapter into titan, shot 3.1.

Methane does not last

Methane is 5.65% of the air near the ground physics. High in the atmosphere, ultraviolet sunlight at 121.6 nm, the solar Lyman-alpha line, and charged particles break it apart physics. At the present rate the air's methane is destroyed in about 30 million years physics. That is brief on a planet's clock, so something must resupply it physics.

**Titan from orbit.** An opaque orange ball against the dark, the haze too thick to see through.

From methane to haze

The broken pieces link up. Cassini's instruments traced the chain from methane and nitrogen through molecules of 80 to 350 daltons to heavy, negatively charged molecules of about 8,000 daltons physics. They form near 1,000 km, far higher than anyone had assumed before the flybys physics.

Those molecules grow into particles and sink. Haze runs from the top of the air all the way to the ground physics. A detached layer floated near 500 km in 2004 to 2007, was gone by 2012, and is not there on the reference date physics.

What a tholin is

Carl Sagan and Bishun Khare coined the word in 1979 for the brown organic solids that form when energy passes through mixtures of methane and other simple gases physics. By convention the lab material is the tholin and Titan's own particles are the haze physics. Both are organic, and both carry nitrogen.

Why the sky is orange

The haze stops light unevenly. Its opacity falls steeply with wavelength, as the wavelength to the power minus 2.34 physics. Blue is blocked highest in the column and red reaches deepest, so the limb looks blue and the disc looks orange illustrative. On the way down, blue light is stopped about 160 km up and red about 131 km up illustrative. Only perhaps 10% of the sunlight that reaches Titan gets to the ground physics.

Sunlight destroys methane in the upper air and builds ethane and haze from the pieces. Left alone, the methane would be gone in about 30 million years, so something resupplies it. Rain returns what is left to the seas, and the settlement lifts it out by the barge-load. physics design

Where it all goes

Haze settles on the ground and the seas. Ethane, the other product, condenses and falls into the seas physics. Rain brings back the methane that has not been destroyed physics. The settlement lifts methane and ethane out of the sea by the barge-load. See Sorting the sea and Plastics from the sky.

Key numbers

Quantity	Value	Basis
Methane near the surface	5.65%	physics
Photolysis wavelength	121.6 nm	physics
Methane lifetime without a source	about 30 million years	physics
Heavy ions	about 8,000 daltons near 1,000 km	physics
Detached haze	about 500 km (2004 to 2007), absent on the reference date	physics
Haze opacity against wavelength	wavelength to the power −2.34	physics
Blue / red stopped at	about 160 km / 131 km	illustrative
Sunlight reaching the ground	perhaps 10%	physics