Why Astronomers Are Sniffing Around The Methane Gas Of Uranus

While Neptune and Uranus are similar in many ways, new observations have revealed why Uranus might be the paler of the two. The new research suggests the reason for the difference in color lies in a haze that exists on both planets and is thicker on Uranus.

Neptune and Uranus are similar in mass, size, and atmospheric compositions. However, Neptune seems to have a noticeable bluer color, while Uranus appears to be a shade of cyan. Astronomers believe they may have figured out why this color difference exists, and it comes down to haze.

The new observations from Gemini Observatory seem to indicate that a layer of concentrated haze that exists on the two planets is thicker on Uranus. This thicker haze is believed to "whiten" Uranus' appearance and makes it seem paler than Neptune. The research suggests that if the haze did not exist, the two planets would appear almost equally blue.

An international team led by Patrick Irwin, Professor of Planetary Physics at Oxford University, created a model to "describe aerosol layers in the atmospheres of Neptune and Uranus." Prior research focused on the planets' upper atmospheres at specific wavelengths. The new model of the atmospheric layers includes comparisons of both planets at a wide range of wavelengths. It also takes into account haze particles within deeper layers that were previously believed to only contain clouds of methane and hydrogen sulfide ices.

"This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths," remarked Irwin, who is also the lead author of a paper in the Journal of Geophysical Research Planets. "It's also the first to explain the difference in visible color between Uranus and Neptune."

The diagram above displays three layers of aerosols in the atmospheres of Uranus and Neptune. The deepest layer (the Aerosol-1-layer) is composed of a thick mixture of hydrogen sulfide ice and particles produced by interactions between the planets' atmosphere and sunlight.

Research suggests that the key layer that produces the difference in colors is the middle layer, which is a layer of haze particles that is thicker on Uranus. The thought is that methane ice condenses onto the particles in this layer on both planets, pulling the particles farther into the atmosphere, resulting in a shower of methane snow. However, Neptune has a more active and turbulent atmosphere than Uranus and this is believed to make Neptune's atmosphere more efficient at "churning up particles into the haze layer and producing this snow". The result is a reduction in haze on Neptune, and thus giving it its bluer color.

The Near-Infrared Integral Field Spectrometer (NIFS) was particularly important in the research. It was capable of providing spectra-measurements of how bright an object is at different wavelengths, for each point in its field of view. This ability gave researchers detailed measurements of how reflective both planets' atmospheres are "across both the full disk of the planet and across a range of near-infrared wavelengths."

Along with detailing why the planets differ in color, the model created also helps to explain the dark spots that are at times visible on Neptune and not as frequently on Uranus. The new data helps shed a bit of light on which layer is responsible for the dark spots, as well as why the layers are less reflective. The research shows that a darkening of the deepest layer of the model would produce dark spots similar to those spotted on Neptune and perhaps Uranus.

Top Image Credit: NOIRLab