On mass, composition, and temperature Neptune and Uranus are not far from being twins, but it's not hard to tell the two apart in visible light. Neptune is a deep shade of blue befitting a planet named after the god of the sea, while Uranus is paler and slightly greenish. (If you're going to snigger, this article will give you plenty of opportunities.) So, why do they look so different?
A new paper undergoing peer review attributes the difference to a layer of initially tiny methane ice particles that sink as they grow, eventually forming a sort of snow that then re-evaporates.
All the planets as far out to Saturn, and even some larger asteroids, have been intensely studied at close range by orbiting space probes. The ice giants Uranus and Neptune, on the other hand, have received nothing more than Voyager 2's flybys 36 and 33 years ago respectively to image them using technology from the 1970s when the craft was launched. It's no surprise many astronomers are pushing to find a way back.
In the meantime, Professor Patrick Irwin of the University of Oxford and colleagues have combined Voyager 2's data with measurements taken by Hubble and ground-based telescopes to try to make sense of the pairs' atmospheres. They have presented a model, available to read on arXiv, that might explain the two different shades.
Irwin and co-authors conclude both planets have an atmosphere with a base pressure more than 700 kiloPascals (seven times atmospheric pressure) composed of hydrogen sulfide (the "rotten eggs" gas) ice and photochemical haze. Yes, as Irwin previously showed, Uranus (and Neptune) smells like farts.
Above this is a layer of photochemical haze at pressures 1-2 times sea level on Earth, the authors conclude, which is in turn topped by a layer of the same composition but lower pressure. The haze is produced in the planets' upper atmospheres before sinking to concentrate around the point where methane condenses. When these particles become seeds around which methane nucleates the combination falls as snow until the particles are released to act as nuclei for H2S cloud formation.
The methane, which reflects blue light while absorbing red, accounts for the dominant shade. Uranus has a thicker aerosol layer, causing the color to fade. Neptune, on the other hand, has a thin layer of methane ice particles at a pressure lower than at the top of Mount Everest that enhances the reflection of the blue part of the spectrum.
Neptune in particular is not uniformly colored, with Voyager 2 beaming back images of dark spots, since also seen by Hubble. The authors attribute these to concentrations of material in the deep aerosol layer.
Besides getting to grips with the least-understood planets in our system, the work could provide insight into the abundance of Neptune-sized worlds found orbiting other stars.
[H/T New Scientist]