The recipe for the distinctive colour of Jupiter’s Great Red Spot has long been a mystery, but now researchers have an idea of how it got so red. Start with an ammonia-rich cloud deck and add intense solar radiation for eons.

Ammonia forms the top layer of Jupiter’s atmosphere. Below that are clouds of ammonium hydrosulphide, a compound that breaks up at temperatures found on Earth but is stable on Jupiter. The lowest clouds are water. All three gases form white clouds, so what gives them a red hue?

Mark Loeffler at Northern Arizona University in Flagstaff suspects high-energy radiation from space splits molecules in the clouds, with the fragments combining to form complex compounds that look red to the eye.

Competing reds:He found that irradiating ammonium hydrosulphide with high-energy protons produces coloured granules. They look green when the sample is held at -113°C, slightly cooler than the cloud tops, and look reddish when cooled to -223°C, colder than the clouds.

When the granules are compared to the light spectrum coming from the Great Red Spot, Loeffler says some of the visible wavelengths match up to his low-irradiance experiments. He suspects the green in his samples may come from a sulphur radical produced in the lab.

Robert Carlson at the Jet Propulsion Laboratory in California says a better mix of ingredients is ammonia and the hydrocarbon acetylene. Last year, he reported that irradiating this mix with ultraviolet light produced reddish material that more closely matched the red spot spectrum. And when blended with white clouds, it matches shades of red seen across Jupiter’s surface.

The missing piece: Sulphur is a much more abundant ingredient in Jupiter’s atmosphere than acetylene, but Carlson says the hydrocarbon is concentrated in the right place to redden the planet’s iconic spot. Acetylene forms when sunlight splits methane molecules in the upper atmosphere, then drifts down to the ammonia clouds where solar ultraviolet can trigger further reactions to produce the reddish compounds.

Carlson says the red spot is darker than the rest of the cloud deck because its ammonia clouds rise higher than those on the rest of the planet, so they encounter more acetylene molecules to make more of the red stuff.

The red material Carlson made “has optical properties that are an excellent match to the spectrum of the Great Red Spot,” says Larry Sromovsky of the University of Wisconsin-Madison. By adjusting particle size and concentration, their model could match the visible spectra of other reddish clouds on Jupiter, unlike Loeffler’s material.

The problem, Sromovsky says, is that for Jupiter’s Great Red Spot to be as red as it is, the gas giant’s atmosphere would have to produce far more of this reddish material than it possibly can. The missing piece could be reddish organic molecules called tholins, which are found on Titan and many other cold or icy bodies in the solar system, but have not been identified on any planet other than Pluto.