?Why don't Uranus and Neptune display the same color
Uranus and Neptune. Credits: NASA / JPL-Caltech
Uranus and Neptune have almost the same size and the same mass. They also have similar compositions and structures, even turnover rates. However, their color is different. If these two planets are so similar, how can this difference be explained?
Uranus and Neptune, the two outermost (and forgotten ) planets in the Solar System, have a very similar structure. Imagine a small rocky core surrounded by a mantle of water ice, ammonia and methane. Cover everything with a gaseous atmosphere composed mainly of hydrogen, helium and methane. Finally, coat the whole thing in a layered high atmosphere. On paper, these two “ice giantess”, as they are called, therefore look a lot alike, but not quite.
Indeed, their main difference lies in their hue . When Neptune displays an appealing azure blue where several storms loom, Uranus shows us a pale, featureless face. In a study, a team from Oxford University led by planetary physicist Patrick Irwin tried to explain this difference. Note that this work is still awaiting peer review.
A story of mist
For this study, the researchers analyzed observations in the visible and near-infrared regions of the two planets to generate new models of their atmospheric layers.
These two planets offer a layer of photochemical haze (called Aerosol-2). This very common type of structure forms when ultraviolet radiation from the Sun breaks down aerosol particles in the atmosphere. On both planets, this hazy layer appears to be the source of clouds that condense into methane ice at the lower boundary and snow deeper in the atmosphere. The difference is that on Uranus this layer appears to be twice as opaque as on Neptune . This is why the two planets do not show the same face.
“ Since these particles absorb UV, this explains the observed lower UV reflectivity of Uranus which therefore appears paler to the human eye ,” note the authors. “ The lower opacity of Neptune's Aerosol-2 layer also explains why dark spots are easier to observe in the atmosphere than in Uranus. "
The planet Neptune. Credits: NASA / JPL
Beneath the Aerosol-2 layer is a deeper haze layer called Aerosol-1 where methane again evaporates to form haze particles. These then condense into submicron crystals of hydrogen sulfide. It is still unclear why Neptune's Aerosol-2 layer is not as dense as Uranus's . Researchers believe its atmosphere may be better at shedding haze by snowing methane more efficiently than Uranus.
Study details are published on the arXiv preprint server.
Source : websites internet