Sky & Telescope - March 2025 - 62

Neptune's Clouds
Neptune
10−4
200
10−3
to be scattered back to us observers.
Neptune is slightly bluer than Uranus. Recent cloud mod100
10−2
Haze
10−1
els
explain why: To match the observations, the cloud deck at
a depth of about 2 bars needs to be thicker on Uranus than
on Neptune. Neptune's thinner cloud deck allows sunlight
to penetrate deeper into its atmosphere before being reflected
back out; this leads to more absorption by the red-absorbing
gaseous methane and thus makes the planet bluer.
Even prior to the 1989 Voyager 2 flyby, we knew Neptune
Haze
100
CH2
had an active atmosphere. Throughout the 1980s, I spent
many nights studying this distant world with the University
of Hawai'i 2.2-meter telescope on Mauna Kea. The Hubble
Space Telescope had not yet launched, and advanced methods
to remove the distortion from Earth's atmosphere in images
had not yet arrived on the astronomical scene. Even with the
best telescopes on the planet, I could only rarely catch clear
glimpses of Neptune's tiny disk, when Earth's atmosphere
was still and calm.
Nevertheless, I could see bright features traversing the
H2
101
−100
NH4
102
100
200
Temperature (kelvin)
pNEPTUNE'S SKY A layer of methane clouds (CH4) floats above a
deeper layer of hydrogen sulfide (H2S). Models suggest that farther
down, ammonium hydrosulfide (NH4HS) clouds may exist. High-altitude
hazes help obscure deeper regions. Discrete white clouds appear between
0.1 and 0.6 bar. The black line traces the atmosphere's conditions.
ate myriad discrete clouds and banded structure. This is the
atmosphere we see at wavelengths visible to the human eye.
Whether the boundaries between the atmosphere, mantle,
and core are sharp - like the boundary between oil and water
- or more gradual remains unknown: We don't have enough
observations to accurately model Neptune's interior structure
with high confidence.
Methane Clouds and Layers of Haze
Let's instead consider the atmosphere that we see: the global
layer of pale blue clouds that serves as the background to
bright white storms, dark spots, and banded structure.
One of the most common misconceptions about Neptune's
atmosphere is that its color is a deep azure blue. This
misunderstanding resulted from the Voyager 2 images, which
were processed to enhance the visibility of fainter clouds and
banded structure. In fact, Neptune's true color is closer to a
pale aquamarine. This color results when sunlight scatters off
Neptune's atmosphere: Traces of methane gas absorb the red
wavelengths of light, leaving the green and blue wavelengths
62 MARCH 2025 * SKY & TELESCOPE
300
SH
−200
400
S
planet's disk as it rotated rapidly. (Neptune takes 16 hours
to make one rotation.) So, I expected a good show when
Voyager 2 arrived.
Even so, both I and others were stunned by the panoply of
clouds, bands, and raging winds that Voyager revealed.
On Earth, the atmosphere is primarily molecular nitrogen,
while water is the weather wonder-worker: Water's transformation
through ice, liquid, and gas phases creates our myriad
clouds, rain, and hail. Similarly, although Neptune's atmosphere
is primarily hydrogen, methane acts as the primary
condensable gas for the visible clouds. These clouds form at
extremely cold temperatures, near 60K (−213°C, or −352°F),
and at a pressure level in Neptune's skies of about 1 to 2 bars,
which is similar to Earth's ambient surface pressure (1 bar).
In other words, the white puffy clouds are likely methaneice
crystals that have condensed out of the methane gas in
Neptune's atmosphere, much like Earth's white puffy clouds
condense from water vapor in our warmer atmosphere.
Newer models of the atmosphere include observations at
near-infrared wavelengths, which scatter off high-altitude
clouds. These models suggest that above the main visible
cloud deck extends a thin aerosol haze, which reaches high
up into the planet's stratosphere. This haze layer, unlike anything
we see on Earth, is almost certainly due to photochemistry:
Sunlight interacting with gases like methane creates
other molecules composed of carbon, hydrogen, and oxygen
- compounds such as ethane, acetylene, carbon monoxide,
and diacetylene, all of which we've detected in trace amounts.
If we could strip away Neptune's outer layers to see what
lies underneath, we would find yet another, deeper layer
of clouds. This layer extends down to a pressure of about
5 bars, the same pressure experienced at the depth limit of
recreational scuba diving. We think this deep layer is made
of clouds of hydrogen sulfide (H2S) as well as photochemical
byproducts from the higher layers. These byproducts may rain
down from above, or perhaps huge clouds in the deep layer
Pressure (bars)
Altitude (km)
BEATRIZ INGLESSIS / S&T, SOURCE: A. SÁNCHEZ-LAVEGA ET AL. / THE
ASTRONOMY AND ASTROPHYSICS REVIEW 2023
Sky & Telescope - March 2025

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