Voyager 2/ISS images of Uranus and Neptune released shortly after the Voyager 2 flybys in 1986 and 1989, respectively, compared with a reprocessing of the individual filter images in this study to determine the best estimate of the true colors of these planets. Credit: Patrick Irwin.
Neptune
is fondly known for being a rich blue, and Uranus green—but a new study has
revealed that the two ice giants are actually far closer in color than
typically thought.
The correct shades of the planets have
been confirmed with the help of research led by Professor Patrick Irwin from
the University of Oxford, which has been published today in the Monthly
Notices of the Royal Astronomical Society.
He and his team found that both worlds
are in fact a similar shade of greenish blue, despite the commonly-held belief
that Neptune is a deep azure and Uranus has a pale cyan appearance.
Astronomers have long known that most
modern images of the two planets do not accurately reflect their true colors.
The misconception arose because images captured of both planets during the 20th
century—including by NASA's Voyager 2 mission, the only spacecraft to fly past
these worlds—recorded images in separate colors.
The single-color images were later
recombined to create composite color images, which were not always accurately
balanced to achieve a "true" color image, and—particularly in the
case of Neptune—were often made "too blue."
In addition, the early Neptune images
from Voyager 2 were strongly contrast enhanced to better reveal the clouds,
bands, and winds that shape our modern perspective of Neptune.
Professor Irwin said, "Although the
familiar Voyager 2 images of Uranus were published in a form closer to 'true'
color, those of Neptune were, in fact, stretched and enhanced, and therefore
made artificially too blue. Even though the artificially-saturated color was
known at the time among planetary scientists—and the images were released with captions explaining
it—that distinction had become lost over time. Applying our model to the
original data, we have been able to reconstitute the most accurate
representation yet of the color of both Neptune and Uranus."
Uranus as seen by HST/WFC3 from
2015-2022. During this sequence the north pole, which has a paler green color,
swings down towards the Sun and Earth. In these images the equator and latitude
lines at 35N and 35S are marked. Credit: Patrick Irwin
In
the new study, the researchers used data from Hubble Space Telescope's Space
Telescope Imaging Spectrograph (STIS) and the Multi Unit Spectroscopic Explorer
(MUSE) on the European Southern Observatory's Very Large Telescope. In both
instruments, each pixel is a continuous spectrum of colors.
This means that STIS and MUSE
observations can be unambiguously processed to determine the true apparent
color of Uranus and Neptune. The researchers used these data to re-balance the
composite color images recorded by the Voyager 2 camera, and also by the Hubble
Space Telescope's Wide Field Camera 3 (WFC3).
This revealed that Uranus and Neptune
are actually a rather similar shade of greenish blue. The main difference is
that Neptune has a slight hint of additional blue, which the model reveals to
be due to a thinner haze layer on that planet.
The
study also provides an answer to the long-standing mystery of why Uranus's
color changes slightly during its 84-year orbit of the sun. The authors came to
their conclusion after first comparing images of the ice giant to measurements
of its brightness, which were recorded by the Lowell Observatory in Arizona
from 1950–2016 at blue and green wavelengths.
These
measurements showed that Uranus appears a little greener at its solstices (i.e.
summer and winter), when one of the planet's poles is pointed towards our star.
But during its equinoxes—when the sun is over the equator—it has a somewhat
bluer tinge.
Part
of the reason for this was known to be because Uranus has a highly unusual
spin. It effectively spins almost on its side during its orbit, meaning that
during the planet's solstices either its north or south pole points almost
directly towards the sun and Earth. This is important, the authors said,
because any changes to the reflectivity of the polar regions would therefore
have a big impact on Uranus's overall brightness when viewed from our planet.
What
astronomers were less clear about is how or why this reflectivity differs. This
led the researchers to develop a model which compared the spectra of Uranus's
polar regions to its equatorial regions. It found that the polar regions are
more reflective at green and red wavelengths than at blue wavelengths, partly
because methane, which is red-absorbing, is about half as abundant near the
poles than the equator.
Animation of seasonal changes in color on Uranus
during two Uranus years (one Uranus year is 84.02 Earth years), running from
1900 to 2068 and starting just before southern summer solstice, when Uranus's
south pole points almost directly towards the Sun. The left-hand disk shows the
appearance of Uranus to the naked eye, while the right-hand disk has been color
stretched and enhanced to make atmospheric features clearer. In this animation,
Uranus's spin has been slowed down by over 3000 times so that the planetary
rotation can be seen, with discrete storm clouds seen passing across the
planet's disk. As the planet moves towards its solstices a pale polar 'hood' of
increasing cloud opacity and reduced methane abundance can be seen filling more
of the planet's disk leading to seasonal changes in the overall color of the
planet. The changing size of Uranus's disk is due to Uranus's distance from the
Sun changing during its orbit. Credit: Patrick Irwin, University of Oxford
However, this wasn't enough to
fully explain the color change, so the researchers added a new variable to the
model in the form of a "hood" of gradually thickening icy haze that
has previously been observed over the summer sunlit pole as the planet moves
from equinox to solstice.
Astronomers think this is likely to
be made up of methane ice particles. When simulated in the model, the ice
particles further increased the reflection at green and red wavelengths at the
poles, offering an explanation as to why Uranus is greener at the solstice.
Professor Irwin said, "This is
the first study to match a quantitative model to imaging data to explain why
the color of Uranus changes during its orbit. In this way, we have demonstrated
that Uranus is greener at the solstice due to the polar regions having reduced
methane abundance but also an increased thickness of brightly scattering methane ice particles."
Dr. Heidi Hammel, of the
Association of Universities for Research in Astronomy (AURA), who has spent
decades studying Neptune and Uranus but was not involved in the study, said,
"The misperception of Neptune's color, as well as the unusual color changes
of Uranus, have bedeviled us for decades. This comprehensive study should
finally put both issues to rest."
The ice giants Uranus and Neptune
remain a tantalizing destination for future robotic explorers, building on the
legacy of Voyager in the 1980s.
Professor Leigh Fletcher, a
planetary scientist from the University of Leicester and co-author of the new
study, said, "A mission to explore the Uranian system—from its bizarre
seasonal atmosphere, to its diverse collection of rings and moons—is a high
priority for the space agencies in the decades to come."
However, even a long-lived
planetary explorer, in orbit around Uranus, would only capture a short snapshot
of a Uranian year.
"Earth-based studies like this, showing how Uranus's appearance and color has changed over the decades in response to the weirdest seasons in the solar system, will be vital in placing the discoveries of this future mission into their broader context," Professor Fletcher added.
Source: New images reveal what Neptune and Uranus really look like (phys.org)
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