It's safe to say that we know a few things about Jupiter’s colossal, faintly cider-hued anticyclone. We know, for example, that it’s about 1.3 times as wide as the Earth, and it’s up to 100 times deeper than Earth’s oceans. As pointed out recently by a few other outlets, however, we aren’t really sure why its Great Red Spot is actually red – but why?
Astrochemistry is exactly what it sounds like: the study of off-world chemical compounds and their constituent parts. Direct sampling using space probes and robotic rovers aside, it’s often fairly difficult to scoop up a distant object’s atmosphere, but there are sneaky ways around our lonely planet’s segregation from the rest of the cosmos.
Spectroscopy allows us to parse apart different electromagnetic wavelengths as they head our way. By picking up on specific, clearly defined absorption and emission lines, we can deduce the chemical composition of the objects emitting said radiation. Indeed, by using a spectroscope during a total eclipse on August 18, 1868, a French astronomer by the name of Pierre Jules Cesar Janssen first discovered helium.
This is also how we know what the majority of Jupiter’s atmosphere is composed of: the Juno probe, for example, has two such spectrometers, an infrared and an ultraviolet one. At the same time, Galileo was equipped with a mass spectrometer, which directly sampled and parsed through the Jovian atmosphere.
Thanks to these techniques, we know that Jupiter’s atmosphere – including its Great Red Spot – contains a lot of hydrogen and helium, as well as methane, hydrogen sulfide, ammonia, and others. We still don’t know the chemistry of Jupiter’s atmosphere in detail, though, and that’s where terrestrial laboratory experiments come into it.
As pointed out by The Conversation, back in the 1970s, it was thought that phosphorous was the reason for the spot’s paint job. Red phosphorous is most certainly real, and it emerges from the somewhat unstable white phosphorus at moderately high temperatures.
Jupiter’s atmosphere is frigid, though – its average is a chilly -145°C (-234°F) – so the only other viable way to get it there was for another phosphorous compound, phosphine, to break down when exposed to radiation. Experiments carried out in the 1980's tried to replicate this, but comprehensively failed to do so. In fact, it only turns yellow under these conditions.
Phosphorous has largely been ruled out these days, but more suitable chemical candidates have cropped up since.
A 2014 NASA announcement explained that it might be a combination of simple ammonia and acetylene. If you attack these in Jupiter-like conditions in a laboratory with ultraviolet radiation, you get a reddish material – one that matches spectroscopy conducted by NASA’s Cassini mission.
This not only suggests that the reddish color only exists right at the top of the spot, where it’s most exposed to sunlight. This idea also contradicts a separate hypothesis, which suggests that the coloring compounds are upwelling at depth.
More recent experiments suggest it may be something more complex.
Below a higher layer of Jovian ammonia, there exists ammonium hydrosulfide. This compound, which is unstable on Earth but hunky-dory on Jupiter, is white. As discovered by Northern Arizona University, though – and as highlighted by New Scientist – if you keep it at extremely cold temperatures and bombard it with solar radiation, red-tinted granules appear.
There’s a lingering problem, though. Space.com notes that, when irradiated at temperatures matching those of Jupiter’s clouds, ammonium hydrosulfide turns into a green salt; only at colder temperatures does it turn the desired color of red. Although warming these green salts up robs them of their viridian hues, it’s still not clear why there’s a mismatch here with what’s going on in Jupiter.
So, for now, the Great Red Spots color remains an enigma – one that these experiments will continually try to solve.