The clouds observed on Neptune have almost completely disappeared for the
first time in over three decades of measurements. With the exception of the
south pole, clouds are almost nonexistent in images of the huge blue planet
taken between 1994 and 2022 from Maunakea on Hawaii Island by the W. M. Keck
Observatory and from orbit by NASA's Hubble orbit Telescope.
The data, which were reported in the journal Icarus, also point to a
relationship between the solar cycle and Neptune's evaporating clouds. This
is an unexpected finding considering that Neptune is the most distant main
planet from the sun and only gets 1/900th of the sunlight that Earth
receives on a daily basis.
2019 saw the mid-latitudes of the ice giant lose some of its usual
profusion of clouds, according to scientists at the University of California
(UC) Berkeley.
Imke de Pater, senior author of the study and emeritus professor of
astronomy at UC Berkeley, remarked, "I was shocked by how swiftly clouds
dissipated on Neptune. "We basically noticed a decline in cloud activity
within a few months."
"Even four years later, the images we took this past June showed the clouds
haven't returned to their former levels," said Erandi Chavez, a PhD student
at the Center for Astrophysics at Harvard University who oversaw the project
as an undergraduate astronomy student at UC Berkeley. This is really
exciting and surprising, especially considering how brief and mild the prior
low cloud activity on Neptune was.
Chavez and her team used images from the Hubble Space Telescope (since
1994) and the second-generation Near-Infrared Camera (NIRC2) at Keck
Observatory, along with observations from Lick Observatory (2018–2019) and
the Hubble Space Telescope (since 1994), to track the evolution of Neptune's
appearance.
In recent years, photos from the Keck Observatory's Twilight Observing
Program and Hubble Space Telescope photographs from the Outer Planet
Atmospheres Legacy (OPAL) program have been used to supplement the
observations made by the Keck Observatory.
The information showed an intriguing correlation between variations in
Neptune's cloud cover and the solar cycle, which is the time during which
the sun's magnetic field alternates directions every 11 years and results in
variations in solar radiation levels. About two years later, additional
clouds start to emerge on Neptune as a result of the sun's increased
ultraviolet (UV) light emissions, particularly the powerful hydrogen
Lyman-alpha emission. The scientists also discovered a link between the
quantity of clouds and the brightness of the ice giant as reflected
sunlight.
According to de Pater, "These extraordinary data provide us with the
strongest evidence yet that Neptune's cloud cover correlates with the sun's
cycle." The hypothesis that the sun's UV radiation, when powerful enough,
may be starting a photochemical process that creates Neptune's clouds is
supported by our observations.
The 2.5 cycles of cloud activity seen throughout the 29-year period of
Neptunian observations are used to infer the relationship between the solar
cycle and the cloudy weather pattern on Neptune. During this time, the
planet's reflectivity increased in 2002 (brightness maxima), decreased in
2007 (brightness minima), brightened once more in 2015, darkened to the
lowest level ever seen in 2020, which coincided with the disappearance of
the majority of the clouds.
The variations in Neptune's brightness brought on by the sun seem to rise
and set roughly in time with the arrival and departure of clouds from the
planet.
Given the intricacy of other aspects, additional research is needed to
fully understand this association. For instance, while an increase in UV
rays may increase clouds and haze, it may also darken them, lowering
Neptune's total brightness. Storms on Neptune that rise up from the deep
atmosphere have an impact on the cloud cover but are unrelated to clouds
that are created by photochemistry, which may make connection studies with
the solar cycle more difficult. In order to determine how long the present
near-absence of clouds will endure, more observations of Neptune are
required.
This finding adds to the thrilling discoveries of the extraordinarily
turbulent atmosphere of the blue-hued world, which includes methane clouds
being thrashed around by supersonic winds—the highest wind speeds yet
observed in our solar system. During its 1989 flyby of Neptune, NASA's
Voyager 2 spacecraft obtained one of the first and most spectacular photos,
depicting a gigantic storm system known as the "Great Dark Spot." Since
then, more storms and dark spots have been observed, including a major storm
in the equatorial region in 2017 and a sizable dark spot in the northern
hemisphere in 2018.
"It's fascinating to be able to use telescopes on Earth to study the
climate of a world more than 2.5 billion miles away from us," said Carlos
Alvarez, a staff astronomer at Keck Observatory and a co-author of the
study. We have been able to restrict Neptune's atmospheric models, which are
essential to understanding the relationship between the ice giant's climate
and the solar cycle, thanks to technological advancements as well as our
Twilight Observing Program.
The study group is still monitoring the cloud activity around Neptune. The
most current pictures were taken in June 2023 at the same time as NASA's
James Webb Space Telescope (JWST) was taking pictures in the near- and
mid-infrared.
According to the reported increase in solar UV output during the previous
two years, "we have seen more clouds in the most recent images, in
particular at northern latitudes and at high altitudes," said de
Pater.
The combined JWST and Keck Observatory data will allow for more in-depth
research into the physics and chemistry that underlies Neptune's dynamic
appearance, which might improve astronomers' knowledge of both exoplanets
and Neptune.
