Astronomers may now understand why similar planets Uranus and Neptune are different colors. Using the observations of Gemini North Telescope, NASA Infrared Telescope Facility and Hubble Space Telescope, the researchers developed a single atmospheric model that matched the observations of the two planets. The model shows that too much smog on Uranus will accumulate in the planet's stagnant and slow atmosphere, making it look lighter than Neptune's tone.
Neptune and Uranus have many similarities-they have similar mass, size and atmospheric composition-but their appearances are obviously different. At the wavelength of visible light, Neptune is obviously bluer, while Uranus is light cyan. Astronomers now have an explanation for why the two planets are different in color.
New research shows that the thick fog on the two planets is thicker than that on Neptune, and it can "whiten" the appearance of Uranus more than Neptune [1]. If there is no smog in the atmosphere of Neptune and Uranus, their blue colors are almost the same [2].
"This is the first model to fit the observation of reflected sunlight from ultraviolet to near infrared wavelength at the same time," explained Owen, who is the main author of the paper published in Journal of Geophysical Research: Planets. "This is also the first method to explain the visible color difference between Uranus and Neptune."
the team's model consists of three layers of aerosol at different heights [5]. The key layer affecting color is the middle layer, which is a layer of smog particles thicker on Uranus than on Neptune (called Aerosol-2 layer in the paper). The research team suspected that on these two planets, methane ice condensed on the particles in this layer, pulling the particles deep into the atmosphere and forming a methane snow. Because Neptune's atmosphere is more active and turbulent than Uranus, the research team believes that Neptune's atmosphere can stir methane particles into the smog layer more effectively and produce this kind of snow. This eliminates more haze and makes Neptune's haze layer thinner than that on Uranus, which means Neptune's blue color looks stronger.
"We hope that developing this model can help us understand the clouds and smog in the atmosphere of ice giants," commented Mike Wong, an astronomer at the University of California, Berkeley and a team member behind this result. "Explaining the color difference between Uranus and Neptune is an unexpected gain!"
to create this model, Owen's team analyzed the observations of a group of planets, including ultraviolet, visible and near-infrared wavelengths (from .3 to 2.5 microns), which were taken by the Near-infrared Integrated Field Spectrometer (NIFS) on the nearby Gemini North Telescope. Mauna Kea Peak in Hawaii-part of the International Gemini Observatory, a project of the National Science Foundation NOIRLab-and archive data from NASA infrared telescope facilities and NASA/ESA Hubble Space Telescope also located in Hawaii.
the NIFS instrument on Gemini north is particularly important for this result, because it can provide a spectrum for every point in the field of view-measuring the brightness of an object at different wavelengths. This provided the team with a detailed measurement of the reflectivity of the atmospheres of the two planets in the entire planetary disk and a series of near-infrared wavelength ranges.
"Gemini Observatory continues to provide new insights into the nature of our planetary neighbors," said Martin Still, Gemini Program Officer of the National Science Foundation. "In this experiment, Gemini North provides a component in a set of ground and space-based facilities, which are crucial for the detection and characterization of atmospheric smog."
The model also helps to explain the black spots that are occasionally visible on Neptune but less common on Uranus. Although astronomers have realized that there are black spots in the atmosphere of both planets, they don't know which aerosol layer caused these black spots, nor do they know why the aerosol reflectivity of these layers is low. The team's research shows that the deepest darkening of their model will produce black spots similar to those on Neptune and Uranus, thus clarifying these problems.