Observations from Hubble Space Telescope, the NASA Infrared Telescope and
the Gemini Observatory, reveal that excess haze on Uranus makes it paler
than Neptune and that dark spots are caused by a darkening of a second
deeper cloud/haze layer.
Astronomers may now understand why the similar planets Uranus and Neptune
are different colors. Using observations from the Hubble Space Telescope,
the NASA Infrared Telescope Facility, and the Gemini North telescope,
researchers have developed a single atmospheric model that matches
observations of both planets. The model reveals that excess haze on Uranus
builds up in the planet's stagnant, sluggish atmosphere and makes it appear
a lighter tone than Neptune. The model also reveals the presence of a
second, deeper layer that, when darkened, can account for dark spots in
these atmospheres, such as the famous Great Dark Spot (GDS) observed by
Voyager 2 in 1989.
Neptune and Uranus have much in common—they have similar masses, sizes, and
atmospheric compositions—yet their appearances are notably different. At
visible wavelengths Neptune has a distinctly bluer color than Uranus and
astronomers now have an explanation for why this might be.
New research suggests that a layer of concentrated haze that exists on both
planets is thicker on Uranus than a similar layer on Neptune and "whitens"
Uranus's appearance more than Neptune's. If there were no haze in the
atmospheres of Neptune and Uranus, both would appear almost equally blue.
This conclusion comes from a model that an international team led by Patrick
Irwin, Professor of Planetary Physics at Oxford University, developed to
describe aerosol layers in the atmospheres of Neptune and Uranus. Previous
investigations of these planets' upper atmospheres had focused on the
appearance of the atmosphere at only specific wavelengths. However, this new
model, consisting of multiple atmospheric layers, matches observations from
both planets across a wide range of wavelengths simultaneously. The new
model also includes haze particles at deeper layers that had previously been
thought to contain only 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," explains Professor
Irwin, who is the lead author of a paper presenting this result in the
Journal of Geophysical Research: Planets. "It's also the first to explain
the difference in visible color between Uranus and Neptune."
The team's model consists of three layers of aerosols at different heights.
The key layer that affects the colors is the middle layer, which is a layer
of haze particles (referred to in the paper as the Aerosol-2 layer) that is
thicker on Uranus than on Neptune. The team suspects that, on both planets,
methane ice condenses onto the particles in this layer, pulling the
particles deeper into the atmosphere in a shower of methane snow. Because
Neptune has a more active, turbulent atmosphere than Uranus does, the team
believes Neptune's atmosphere is more efficient at churning up methane
particles into the haze layer and producing this snow. This removes more of
the haze and keeps Neptune's haze layer thinner than it is on Uranus, making
Neptune bluer than Uranus.
"We hoped that developing this model would help us understand clouds and
hazes in the ice giant atmospheres," comments Mike Wong, an astronomer at
the University of California, Berkeley, and a member of the team behind this
result. "Explaining the difference in color between Uranus and Neptune was
an unexpected bonus!"
To create this model, Professor Irwin's team analyzed a set of observations
of the planets encompassing ultraviolet, visible, and near-infrared
wavelengths (from 0.3 to 2.5 micrometers) taken with the NASA/ESA Hubble
Space Telescope, the NASA Infrared Telescope Facility located near the
summit of Maunakea in Hawai'i, and the Gemini North Telescope, also located
in Hawai'i.
The model also helps explain the dark spots that are occasionally visible on
Neptune and more sporadically on Uranus. While astronomers were already
aware of the presence of dark spots in the atmospheres of both planets, they
didn't know which aerosol layer was causing these dark spots or why the
aerosols at those layers were less reflective. The team's research sheds
light on these questions by showing that a darkening of the particles in the
deepest layer of their model would produce dark spots very similar to those
seen on Neptune and occasionally on Uranus.
Reference:
P.G.J. Irwin et al, Hazy blue worlds: A holistic aerosol model for Uranus and
Neptune, including Dark Spots, Journal of Geophysical Research: Planets
(2022).
DOI: 10.1029/2022JE007189.
Tags:
Space & Astrophysics