NASA's Webb Captures Neptune's Auroras For First Time

Neptune Auroras (Hubble and Webb Image)

Credits/Image: NASA, ESA, CSA, STScI, Heidi Hammel (AURA), Henrik Melin (Northumbria University), Leigh Fletcher (University of Leicester), Stefanie Milam (NASA-GSFC)

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For the first time, NASA’s James Webb Space Telescope has captured bright auroral activity on Neptune. Auroras occur when energetic particles, often originating from the Sun, become trapped in a planet’s magnetic field and eventually strike the upper atmosphere. The energy released during these collisions creates the signature glow.

In the past, astronomers have seen tantalizing hints of auroral activity on Neptune, for example, in the flyby of NASA’s Voyager 2 in 1989. However, imaging and confirming the auroras on Neptune has long evaded astronomers despite successful detections on Jupiter, Saturn, and Uranus. Neptune was the missing piece of the puzzle when it came to detecting auroras on the giant planets of our solar system.

“Turns out, actually imaging the auroral activity on Neptune was only possible with Webb’s near-infrared sensitivity,” said lead author Henrik Melin of Northumbria University, who conducted the research while at the University of Leicester. “It was so stunning to not just see the auroras, but the detail and clarity of the signature really shocked me.”

The data was obtained in June 2023 using Webb’s Near-Infrared Spectrograph. In addition to the image of the planet, astronomers obtained a spectrum to characterize the composition and measure the temperature of the planet’s upper atmosphere (the ionosphere). For the first time, they found an extremely prominent emission line signifying the presence of the trihydrogen cation (H₃⁺), which can be created in auroras. In the Webb images of Neptune, the glowing aurora appears as splotches represented in cyan.

“H₃⁺ has a been a clear signifier on all the gas giants — Jupiter, Saturn, and Uranus — of auroral activity, and we expected to see the same on Neptune as we investigated the planet over the years with the best ground-based facilities available,” explained Heidi Hammel of the Association of Universities for Research in Astronomy, Webb interdisciplinary scientist and leader of the Guaranteed Time Observation program in which the data were obtained. “Only with a machine like Webb have we finally gotten that confirmation.”

The auroral activity seen on Neptune is also noticeably different from what we are accustomed to seeing here on Earth, or even Jupiter or Saturn. Instead of being confined to the planet’s northern and southern poles, Neptune’s auroras are located at the planet’s geographic mid-latitudes — think where South America is located on Earth.

This is due to the strange nature of Neptune’s magnetic field, originally discovered by Voyager 2 in 1989, which is tilted by 47 degrees from the planet’s rotation axis. Since auroral activity is based where the magnetic fields converge into the planet’s atmosphere, Neptune’s auroras are far from its rotational poles.

The ground-breaking detection of Neptune’s auroras will help us understand how Neptune’s magnetic field interacts with particles that stream out from the Sun to the distant reaches of our solar system, a totally new window in ice giant atmospheric science.

From the Webb observations, the team also measured the temperature of the top of Neptune’s atmosphere for the first time since Voyager 2’s flyby. The results hint at why Neptune’s auroras remained hidden from astronomers for so long.

“I was astonished — Neptune’s upper atmosphere has cooled by several hundreds of degrees,” Melin said. “In fact, the temperature in 2023 was just over half of that in 1989.”

Through the years, astronomers have predicted the intensity of Neptune’s auroras based on the temperature recorded by Voyager 2. A substantially colder temperature would result in much fainter auroras. This cold temperature is likely the reason that Neptune’s auroras have remained undetected for so long. The dramatic cooling also suggests that this region of the atmosphere can change greatly even though the planet sits over 30 times farther from the Sun compared to Earth.

Equipped with these new findings, astronomers now hope to study Neptune with Webb over a full solar cycle, an 11-year period of activity driven by the Sun’s magnetic field. Results could provide insights into the origin of Neptune’s bizarre magnetic field, and even explain why it’s so tilted.

“As we look ahead and dream of future missions to Uranus and Neptune, we now know how important it will be to have instruments tuned to the wavelengths of infrared light to continue to study the auroras,” added Leigh Fletcher of Leicester University, co-author on the paper. “This observatory has finally opened the window onto this last, previously hidden ionosphere of the giant planets.”

These observations, led by Fletcher, were taken as part of Hammel’s Guaranteed Time Observation program 1249. The team’s results have been published in Nature Astronomy.

The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

Source: NASA's James Webb Space Telescope/News

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Science: Henrik Melin (Northumbria University)

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NASA's Hubble Celebrates Decade of Tracking Outer Planets

Hubble's Decade-Long Views of the Outer Solar System Planets
Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

OPAL Jupiter Observations
Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

Multiwavelength OPAL Jupiter
Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

Multiwavelength OPAL Saturn
Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

Videos

Evolution of Saturn's Ring Tilt (2018-2024) 

Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Video: Joseph DePasquale (STScI)

OPAL Saturn Observations - August 2024
Credits/Science: NASA, ESA, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Video: Joseph DePasquale (STScI)

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Encountering Neptune in 1989, NASA's Voyager mission completed humankind's first close-up exploration of the four giant outer planets of our solar system. Collectively, since their launch in 1977, the twin Voyager 1 and Voyager 2 spacecraft discovered that Jupiter, Saturn, Uranus, and Neptune were far more complex than scientists had imagined. There was a lot more to be learned.

A NASA's Hubble Space Telescope observation program called OPAL (Outer Planet Atmospheres Legacy) obtains long-term baseline observations of Jupiter, Saturn, Uranus, and Neptune in order to understand their atmospheric dynamics and evolution.

"The Voyagers don't tell you the full story," said Amy Simon of NASA's Goddard Space Flight Center in Greenbelt, Maryland, who conducted giant planet observations with OPAL.

Hubble's image sharpness is comparable to the Voyager views as they approached the outer planets, and Hubble spans wavelengths from ultraviolet to near-infrared light. Hubble is the only telescope that can provide high spatial resolution and image stability for global studies of cloud coloration, activity, and atmospheric motion on a consistent time basis to help constrain the underlying mechanics of weather and climate systems.

All four of the outer planets have deep atmospheres and no solid surfaces. Their churning atmospheres have their own unique weather systems, some with colorful bands of multicolored clouds, and with mysterious, large storms that pop up or linger for many years. Each outer planet also has seasons lasting many years. (The James Webb Space Telescope's infrared capabilities will be used to probe deep into atmospheres of the outer planets to complement the OPAL observations.)

Following the complex behavior is akin to understanding Earth's dynamic weather as followed over many years, as well as the Sun's influence on the solar system's weather. The four distant worlds also serve as proxies for understanding the weather and climate on similar planets orbiting other stars.

Planetary scientists realized that any one year of data from Hubble, while interesting in its own right, doesn't tell the full story of the outer planets. Hubble's OPAL program has routinely observed the planets once a year when they are closest to the Earth.

"Because OPAL now spans 10 years and counting, our database of planetary observations is ever growing. That longevity allows for serendipitous discoveries, but also for tracking long-term atmospheric changes as the planets orbit the Sun. The scientific value of these data is underscored by the more than 60 publications to date that include OPAL data," said Simon.

This payoff continues to be a huge archive of data that has led to a string of remarkable discoveries to share with planetary astronomers around the world. "OPAL also interfaces with other ground- and space-based planetary programs. Many papers from other observatories and space missions pull in Hubble data from OPAL for context," said Simon.

The team's decade of discovery under Hubble's OPAL program is being presented at the December meeting of the American Geophysical Union in Washington, D.C.

Some Highlights:

Jupiter

Jupiter's bands of clouds present an ever-changing kaleidoscope of shapes and colors. There is always stormy weather on Jupiter: cyclones, anticyclones, wind shear, and the largest storm in the solar system, the Great Red Spot (GRS). Jupiter is covered with largely ammonia ice-crystal clouds on top of an atmosphere that's tens of thousands of miles deep.

Hubble's sharp images track clouds and measure the winds, storms, and vortices, in addition to monitoring the size, shape and behavior of the GRS. Hubble follows as the GRS continues shrinking in size and its winds are speeding up. OPAL data recently measured how often mysterious dark ovals — visible only at ultraviolet wavelengths — appeared in the "polar hoods" of stratospheric haze. Unlike Earth, Jupiter is only inclined three degrees on its axis (Earth is 23.5 degrees). Seasonal changes might not be expected, except that Jupiter's distance from the Sun varies by about 5% over its 12-year-long orbit, and so OPAL closely monitors the atmosphere for seasonal effects. Another Hubble advantage is that ground-based observatories can't continuously view Jupiter for two Jupiter rotations, because that adds up to 20 hours. During that time, an observatory on the ground would have gone into daytime and Jupiter would no longer be visible until the next evening

Saturn

Saturn takes more than 29 years to orbit the Sun, and so OPAL has followed it for approximately one quarter of a Saturnian year (picking up in 2018, after the end of the Cassini mission). Because Saturn is tilted 26.7 degrees, it goes through more profound seasonal changes than Jupiter. Saturnian seasons last approximately seven years. This also means Hubble can view the spectacular ring system from an oblique angle of almost 30 degrees to seeing the rings tilted edge-on. Edge-on, the rings nearly vanish because they are relatively paper-thin. This will happen again in 2025.

OPAL has followed changes in colors of Saturn's atmosphere. The varying color was first detected by the Cassini orbiter, but Hubble provides a longer baseline. Hubble revealed slight changes from year-to-year in color, possibly caused by cloud height and winds. The observed changes are subtle because OPAL has covered only a fraction of a Saturnian year. Major changes happen when Saturn progresses into the next season. Saturn's mysteriously dark ring spokes , which slice across the ring plane, are transient features that rotate along with the rings. Their ghostly appearance only persists for two or three rotations around Saturn. During active periods, freshly formed spokes continuously add to the pattern. They were first seen in 1981 by Voyager 2. Cassini also saw the spokes during its 13-year-long mission, which ended in 2017. Hubble shows that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021. Long-term monitoring shows that both the number and contrast of the spokes vary with Saturn's seasons.

Uranus

Uranus is tilted on its side so that its spin axis almost lies in the plane of the planet's orbit. This results in the planet going through radical seasonal changes along it 84-year-long trek around the Sun. The consequence of the planet's tilt means part of one hemisphere is completely without sunlight, for stretches of time lasting up to 42 years. OPAL has followed the northern pole now tipping toward the Sun.

With OPAL, Hubble first imaged Uranus after the spring equinox, when the Sun was last shining directly over the planet's equator. Hubble resolved multiple storms with methane ice-crystal clouds appearing at mid-northern latitudes as summer approaches the north pole. Uranus' north pole now has a thickened photochemical haze with several little storms near the edge of the boundary. Hubble has been tracking the size of the north polar cap and it continues to get brighter year after year. As northern summer solstice approaches in 2028, the cap may grow brighter still, and will be aimed directly toward Earth, allowing good views of the rings and north pole. The ring system will then appear face-on. Understanding how Uranus changes over time will help in mission planning for NASA's proposed Uranus Orbiter and Probe.

Neptune

When Voyager 2 flew by Neptune in 1989, astronomers were mystified by a great dark spot the size of the Atlantic Ocean looming in the atmosphere. Was it long-lived like Jupiter's Great Red Spot? The question remained unanswered until Hubble was able to show in 1994 that such dark storms were transitory, cropping up and then disappearing over a duration of two to six years each. During the OPAL program, Hubble saw the end of one dark spot and the full life cycle of a second one — both of them migrating toward the equator before dissipating. The OPAL program ensures that astronomers won't miss another one.

Hubble observations uncovered a link between Neptune's shifting cloud abundance and the 11-year solar cycle . The connection between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system's farthest major planet. It receives only about 1/1000th as much sunlight as Earth receives. Yet Neptune's global cloudy weather seems to be influenced by solar activity. Do the planet's seasons also play a role?
https://hubblesite.org/contents/news-releases/2024/news-2024-010
The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Source: HubbleSite/News

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Solution to a cosmic mystery—the eccentric orbits of trans-Neptunian objects

Simulation snapshots of model A1.
Credit: Nature Astronomy (2024).
DOI: 10.1038/s41550-024-02349-x

New evidence suggests that billions of years ago, a star may have passed very close to our solar system. As a result, thousands of smaller celestial bodies in the outer solar system outside Neptune's orbit were deflected into highly inclined trajectories around the sun. It is possible that some of them were captured by the planets Jupiter and Saturn as moons.

These findings come from a team of astrophysicists from Forschungszentrum Jülich and Leiden University in the Netherlands. They were published in two studies in the journals Nature Astronomy and The Astrophysical Journal Letters.

When we think of our solar system, we usually assume that it ends at the outermost known planet, Neptune. "However, several thousand celestial bodies are known to move beyond the orbit of Neptune," explains Susanne Pfalzner, astrophysicist at Forschungszentrum Jülich.

It is even suspected that there are tens of thousands of objects with a diameter of more than 100 kilometers. "Surprisingly, many of these so-called trans-Neptunian objects move on eccentric orbits that are inclined relative to the common orbital plane of the planets in the solar system."

Together with her Jülich colleague Amith Govind and Simon Portegies Zwart from Leiden University, Susanne Pfalzner has used more than 3,000 computer simulations to investigate a possible cause of the unusual orbits: could another star have caused the strange orbits of trans-Neptunian objects?

The three astrophysicists found that a distinctive, close flyby of another star can explain the inclined and eccentric orbits of the known trans-Neptunian celestial bodies. "Even the orbits of very distant objects can be deduced, such as that of the dwarf planet Sedna in the outermost reaches of the solar system, which was discovered in 2003.

"And also objects that move in orbits almost perpendicular to the planetary orbits," says Susanne Pfalzner. Such a flyby can even explain the orbits of 2008 KV42 and 2011 KT19—the two celestial bodies that move in the opposite direction to the planets.

Saturn's moon Phoebe is a prime example of the unusual properties of irregular moons. Like many others, it orbits Saturn in the opposite direction. Credit: NASA / JPL

"The best match for today's outer solar system that we found with our simulations is a star that was slightly lighter than our sun—about 0.8 solar masses," explains Pfalzner's colleague Amith Govind. "This star flew past our sun at a distance of around 16.5 billion kilometers. That's about 110 times the distance between Earth and the sun, a little less than four times the distance of the outermost planet Neptune."

However, the scientists' most surprising realization was that the flyby of an alien star billions of years ago could also provide a natural explanation for phenomena closer to home. Susanne Pfalzner and her colleagues found that in their simulations, some trans-Neptunian objects were hurled into our solar system—into the region of the outer giant planets Jupiter, Saturn, Uranus and Neptune.

"Some of these objects could have been captured by the giant planets as moons," says Simon Portegies Zwart from Leiden University. "This would explain why the outer planets of our solar system have two different types of moons."

In contrast to the regular moons, which orbit close to the planet on circular orbits, the irregular moons orbit the planet at a greater distance on inclined, elongated orbits. Until now, there was no explanation for this phenomenon.

"The beauty of this model lies in its simplicity," says Pfalzner. "It answers several open questions about our solar system with just a single cause."

Source: Phys.org

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More information: Susanne Pfalzner et al, Trajectory of the stellar flyby that shaped the outer Solar System, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02349-x

Susanne Pfalzner et al, Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby, The Astrophysical Journal Letters (2024). DOI: 10.3847/2041-8213/ad63a6

Journal information: Astrophysical Journal Letters , Nature Astronomy

Provided By Forschungszentrum Juelich

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Explore further

• Impact of a stellar intruder on our solar system

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New images reveal what Neptune and Uranus really look like

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.

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.

by University of Oxford

Source: Phys.org/News

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More information: Patrick Irwin et al, Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune, Monthly Notices of the Royal Astronomical Society (2023). DOI: 10.1093/mnras/stad3761

Journal information: Monthly Notices of the Royal Astronomical Society

Provided by: University of Oxford

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Mysterious Neptune dark spot detected from Earth for the first time

PR Image eso2314a
Dark spot on Neptune observed with MUSE at ESO’s Very Large Telescope

PR Image eso2314b
Natural view of Neptune taken by MUSE at the VLT

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Videos

PR Video eso2314a

Mysterious Neptune Dark Spot Detected from Earth (ESOcast 265 Light)

PR Video eso2314b

Scanning through different colours of Neptune with MUSE

PR Video eso2314c

Dark spot on Neptune observed with MUSE at the VLT

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Using ESO’s Very Large Telescope (VLT), astronomers have observed a large dark spot in Neptune’s atmosphere, with an unexpected smaller bright spot adjacent to it. This is the first time a dark spot on the planet has ever been observed with a telescope on Earth. These occasional features in the blue background of Neptune’s atmosphere are a mystery to astronomers, and the new results provide further clues as to their nature and origin.

Large spots are common features in the atmospheres of giant planets, the most famous being Jupiter’s Great Red Spot. On Neptune, a dark spot was first discovered by NASA’s Voyager 2 in 1989, before disappearing a few years later. “Since the first discovery of a dark spot, I’ve always wondered what these short-lived and elusive dark features are,” says Patrick Irwin, Professor at the University of Oxford in the UK and lead investigator of the study published today in Nature Astronomy.

Irwin and his team used data from ESO’s VLT to rule out the possibility that dark spots are caused by a ‘clearing’ in the clouds. The new observations indicate instead that dark spots are likely the result of air particles darkening in a layer below the main visible haze layer, as ices and hazes mix in Neptune’s atmosphere.

Coming to this conclusion was no easy feat because dark spots are not permanent features of Neptune’s atmosphere and astronomers had never before been able to study them in sufficient detail. The opportunity came after the NASA/ESA Hubble Space Telescope discovered several dark spots in Neptune's atmosphere, including one in the planet’s northern hemisphere first noticed in 2018. Irwin and his team immediately got to work studying it from the ground — with an instrument that is ideally suited to these challenging observations.

Using the VLT’s Multi Unit Spectroscopic Explorer (MUSE), the researchers were able to split reflected sunlight from Neptune and its spot into its component colours, or wavelengths, and obtain a 3D spectrum [1]. This meant they could study the spot in more detail than was possible before. “I’m absolutely thrilled to have been able to not only make the first detection of a dark spot from the ground, but also record for the very first time a reflection spectrum of such a feature,” says Irwin.

Since different wavelengths probe different depths in Neptune’s atmosphere, having a spectrum enabled astronomers to better determine the height at which the dark spot sits in the planet's atmosphere. The spectrum also provided information on the chemical composition of the different layers of the atmosphere, which gave the team clues as to why the spot appeared dark.

The observations also offered up a surprise result. “In the process we discovered a rare deep bright cloud type that had never been identified before, even from space,” says study co-author Michael Wong, a researcher at the University of California, Berkeley, USA. This rare cloud type appeared as a bright spot right beside the larger main dark spot, the VLT data showing that the new ‘deep bright cloud’ was at the same level in the atmosphere as the main dark spot. This means it is a completely new type of feature compared to the small ‘companion’ clouds of high-altitude methane ice that have been previously observed.

With the help of ESO’s VLT, it is now possible for astronomers to study features like these spots from Earth. “This is an astounding increase in humanity’s ability to observe the cosmos. At first, we could only detect these spots by sending a spacecraft there, like Voyager. Then we gained the ability to make them out remotely with Hubble. Finally, technology has advanced to enable this from the ground,” concludes Wong, before adding, jokingly: "This could put me out of work as a Hubble observer!”

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Notes

[1] MUSE is a 3D spectrograph that allows astronomers to observe the entirety of an astronomical object, like Neptune, in one go. At each pixel, the instrument measures the intensity of light as a function of its colour or wavelength. The resulting data form a 3D set in which each pixel of the image has a full spectrum of light. In total, MUSE measures over 3500 colours. The instrument is designed to take advantage of adaptive optics, which corrects for the turbulence in the Earth’s atmosphere, resulting in sharper images than otherwise possible. Without this combination of features, studying a Neptune dark spot from the ground would not have been possible.

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More Information

This research was presented in a paper titled “Cloud structure of dark spots and storms in Neptune’s atmosphere” to appear in Nature Astronomy (doi: 10.1038/s41550-023-02047-0).

The team is composed of Patrick G. J. Irwin (University of Oxford, UK [Oxford]), Jack Dobinson (Oxford), Arjuna James (Oxford), Michael H. Wong (University of California, USA [Berkeley]), Leigh N. Fletcher (University of Leicester, UK [Leicester]), Michael T. Roman (Leicester), Nicholas A. Teanby (University of Bristol, UK), Daniel Toledo (Instituto Nacional de Técnica Aeroespacial, Spain), Glenn S. Orton (Jet Propulsion Laboratory, USA), Santiago Pérez-Hoyos (University of the Basque Country, Spain [UPV/EHU]), Agustin Sánchez Lavega (UPV/EHU), Lawrence Sromovsky (University of Wisconsin, USA), Amy Simon (Solar System Exploration Division, NASA Goddard Space Flight Center, USA), Raúl Morales-Juberias (New Mexico Institute of Technology, USA), Imke de Pater (Berkeley), and Statia L. Cook (Columbia University, USA).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration for astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as survey telescopes such as VISTA. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates ALMA on Chajnantor, a facility that observes the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.

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Contacts:

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Department of Physics, University of Oxford
Oxford, UK
Tel: +44 1865 272083
Email: patrick.irwin@physics.ox.ac.uk

Michael H. Wong
Center for Integrative Planetary Science, University of California at Berkeley
Berkeley, California, USA
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Email: mikewong@astro.berkeley.edu

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Source: ESO/News

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Neptune's Disappearing Clouds Linked to the Solar Cycle

Neptune Cloud Cover Over Three Decades
Credits: Science: NASA, ESA, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley)

Neptune Cloud Cover and Solar Cycle Over Three Decades 

Credits: Science: NASA, ESA, Erandi Chavez (UC Berkeley), Imke de Pater (UC Berkeley)

Release Images

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Astronomers have uncovered a link between Neptune's shifting cloud abundance and the 11-year solar cycle, in which the waxing and waning of the Sun's entangled magnetic fields drives solar activity.

This discovery is based on three decades of Neptune observations captured by NASA's Hubble Space Telescope and the  W. M. Keck Observatory in Hawaii, as well as data from the Lick Observatory in California.

The link between Neptune and solar activity is surprising to planetary scientists because Neptune is our solar system's farthest major planet and receives sunlight with about 0.1% of the intensity Earth receives. Yet Neptune's global cloudy weather seems to be driven by solar activity, and not the planet's four seasons, which each last approximately 40 years.

At present, the cloud coverage seen on Neptune is extremely low, with the exception of some clouds hovering over the giant planet's south pole. A University of California (UC) Berkeley-led team of astronomers discovered that the abundance of clouds normally seen at the icy giant's mid-latitudes started to fade in 2019.

"I was surprised by how quickly clouds disappeared on Neptune," said Imke de Pater, emeritus professor of astronomy at UC Berkeley and senior author of the study. "We essentially saw cloud activity drop within a few months," she said.

"Even now, four years later, the most recent images we took this past June still show the clouds haven't returned to their former levels," said Erandi Chavez, a graduate student at the Center for Astrophysics | Harvard-Smithsonian (CfA) in Cambridge, Massachusetts, who led the study when she was an undergraduate astronomy student at UC Berkeley. "This is extremely exciting and unexpected, especially since Neptune's previous period of low cloud activity was not nearly as dramatic and prolonged."

To monitor the evolution of Neptune's appearance, Chavez and her team analyzed Keck Observatory images taken from 2002 to 2022, the Hubble Space Telescope archival observations beginning in 1994, and data from the Lick Observatory in California from 2018 to 2019.

In recent years, the Keck observations have been complemented by images taken as part of the Twilight Zone program and by Hubble's Outer Planet Atmospheres Legacy (OPAL) program.

The images reveal an intriguing pattern between seasonal changes in Neptune’s cloud cover and the solar cycle – the period when the Sun's magnetic field flips every 11 years as it becomes more tangled like a ball of yarn. This is evident in the increasing number of sunspots and increasing solar flare activity. As the cycle progresses, the Sun’s tempestuous behavior builds to a maximum, until the magnetic field beaks down and reverses polarity. Then the Sun settles back down to a minimum, only to start another cycle.

When it's stormy weather on the Sun, more intense ultraviolet (UV) radiation floods the solar system. The team found that two years after the solar cycle's peak, an increasing number of clouds appear on Neptune. The team further found a positive correlation between the number of clouds and the ice giant's brightness from the sunlight reflecting off it.

"These remarkable data give us the strongest evidence yet that Neptune's cloud cover correlates with the Sun’s cycle," said de Pater. "Our findings support the theory that the Sun's UV rays, when strong enough, may be triggering a photochemical reaction that produces Neptune’s clouds."

Scientists discovered the connection between the solar cycle and Neptune's cloudy weather pattern by looking at 2.5 cycles of cloud activity recorded over the 29-year span of Neptunian observations. During this time, the planet's reflectivity increased in 2002 then dimmed in 2007. Neptune became bright again in 2015, then darkened in 2020 to the lowest level ever observed, which is when most of the clouds went away. The changes in Neptune's brightness caused by the Sun appear to go up and down relatively in sync with the coming and going of clouds on the planet. However there is a two-year time lag between the peak of the solar cycle and the abundance of clouds seen on Neptune. The chemical changes are caused by photochemistry, which happens high in Neptune's upper atmosphere and takes time to form clouds.

"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, staff astronomer at Keck Observatory and co-author of the study. "Advances in technology and observations have enabled us to constrain Neptune's atmospheric models, which are key to understanding the correlation between the ice giant's climate and the solar cycle."

However, more work is necessary. For example, while an increase in UV sunlight could produce more clouds and haze, it could also darken them, thereby reducing Neptune's overall brightness. Storms on Neptune rising up from the deep atmosphere affect the cloud cover, but are not related to photochemically produced clouds, and hence may complicate correlation studies with the solar cycle. Continued observations of Neptune are also needed to see how long the current near-absence of clouds will last.

The research team continues to track Neptune's cloud activity. "We have seen more clouds in the most recent Keck images that were taken during the same time NASA's James Webb Space Telescope observed the planet; these clouds were in particular seen at northern latitudes and at high altitudes, as expected from the observed increase in the solar UV flux over the past approximately 2 years," said de Pater.

The combined data from Hubble, the Webb Space Telescope, Keck Observatory, and the Lick Observatory will enable further investigations into the physics and chemistry that lead to Neptune's dynamic appearance, which in turn may help deepen astronomers' understanding not only of Neptune, but also of exoplanets , since many of the planets beyond our solar system are thought to have Neptune-like qualities.

The findings are published in the journal Icarus.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.

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About This Release

Credits: Release: NASA, ESA, STScI, UC Berkeley, Keck Observatory

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Related Links and Documents

• Science Paper: The science paper by Erandi Chavez et al., PDF (23.08 MB)
• NASA's Hubble portal
• W. M. Keck Observatory's Release

Source: HubbleSite/News

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New Webb Image Captures Clearest View of Neptune’s Rings in Decades

Neptune (NIRCam) Labeled

Credits: Image: NASA, ESA, CSA, STScI / Image Processing: Joseph DePasquale (STScI)

Neptune Close Up (NIRCam)
Credits: Image: NASA, ESA, CSA, STScI / Image Processing: Joseph DePasquale (STScI) 

 

Neptune Wide Field (NIRCam)
Credits: Image: NASA, ESA, CSA, STScI / Image Processing: Joseph DePasquale (STScI)

Release Images

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NASA’s James Webb Space Telescope shows off its capabilities closer to home with its first image of Neptune. Not only has Webb captured the clearest view of this distant planet’s rings in more than 30 years, but its cameras reveal the ice giant in a whole new light.

Most striking in Webb’s new image is the crisp view of the planet’s rings – some of which have not been detected since NASA’s Voyager 2 became the first spacecraft to observe Neptune during its flyby in 1989. In addition to several bright, narrow rings, the Webb image clearly shows Neptune’s fainter dust bands.

“It has been three decades since we last saw those faint, dusty bands, and this is the first time we’ve seen them in the infrared,” notes Heidi Hammel, a Neptune system expert and interdisciplinary scientist for Webb. Webb’s extremely stable and precise image quality permits these very faint rings to be detected so close to Neptune.

Neptune has fascinated researchers since its discovery in 1846. Located 30 times farther from the Sun than Earth, Neptune orbits in the remote, dark region of the outer solar system. At that extreme distance, the Sun is so small and faint that high noon on Neptune is similar to a dim twilight on Earth.

This planet is characterized as an ice giant due to the chemical make-up of its interior. Compared to the gas giants, Jupiter and Saturn, Neptune is much richer in elements heavier than hydrogen and helium. This is readily apparent in Neptune’s signature blue appearance in Hubble Space Telescope images at visible wavelengths, caused by small amounts of gaseous methane.

Webb’s Near-Infrared Camera (NIRCam) images objects in the near-infrared range from 0.6 to 5 microns, so Neptune does not appear blue to Webb. In fact, the methane gas so strongly absorbs red and infrared light that the planet is quite dark at these near-infrared wavelengths, except where high-altitude clouds are present. Such methane-ice clouds are prominent as bright streaks and spots, which reflect sunlight before it is absorbed by methane gas. Images from other observatories, including the Hubble Space Telescope and the W.M. Keck Observatory, have recorded these rapidly evolving cloud features over the years.

More subtly, a thin line of brightness circling the planet’s equator could be a visual signature of global atmospheric circulation that powers Neptune’s winds and storms. The atmosphere descends and warms at the equator, and thus glows at infrared wavelengths more than the surrounding, cooler gases.

Neptune’s 164-year orbit means its northern pole, at the top of this image, is just out of view for astronomers, but the Webb images hint at an intriguing brightness in that area. A previously-known vortex at the southern pole is evident in Webb’s view, but for the first time Webb has revealed a continuous band of high-latitude clouds surrounding it.

Webb also captured seven of Neptune’s 14 known moons. Dominating this Webb portrait of Neptune is a very bright point of light sporting the signature diffraction spikes seen in many of Webb’s images, but this is not a star. Rather, this is Neptune’s large and unusual moon, Triton.

Covered in a frozen sheen of condensed nitrogen, Triton reflects an average of 70 percent of the sunlight that hits it. It far outshines Neptune in this image because the planet’s atmosphere is darkened by methane absorption at these near-infrared wavelengths. Triton orbits Neptune in an unusual backward (retrograde) orbit, leading astronomers to speculate that this moon was originally a Kuiper belt object that was gravitationally captured by Neptune. Additional Webb studies of both Triton and Neptune are planned in the coming year.

The James Webb Space Telescope is the world's premier space science observatory. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

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Release: NASA, ESA, CSA, STScI

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Space Telescope Science Institute, Baltimore, Maryland

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Source: NASA's James Webb Space Telescope/News

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Hubble Helps Explain Why Uranus and Neptune Are Different Colours

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Hubble’s Observations of Uranus and Neptune in 2021

 

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Diagram of the Atmospheres of Uranus and Neptune 

 

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Hubble’s Observation of Uranus in 2021

 

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Hubble’s Observation of Neptune in 2021

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Videos

PR Video heic2209a

Space Sparks Episode 15: Hubble Helps Explain Why Uranus and Neptune Are Different Colours

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Astronomers may now know why Uranus and Neptune are different colours. Using observations from the NASA/ESA Hubble Space Telescope, as well as the Gemini North telescope and the NASA Infrared Telescope Facility, 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.

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 is a rich, deep azure hue whereas Uranus is a distinctly pale shade of cyan. Astronomers now have an explanation for why the two planets are different colours.

New research suggests that a layer of concentrated haze that is present on both planets is thicker on Uranus than on Neptune and therefore ‘whitens’ Uranus’s appearance more than Neptune’s [1]. If there was no haze in the atmospheres of Neptune and Uranus, both would appear almost equally blue as a result of blue light being scattered in their atmospheres [2]. 

This conclusion comes from a model [3] 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 [4]. Previous investigations of these planets’ upper atmospheres had focused on the appearance of the atmosphere at only specific wavelengths. However, this new model consists of multiple atmospheric layers and matches observations from both planets across a wide range of wavelengths. The new model also includes haze particles within deeper layers that had previously been thought to contain only clouds of methane and hydrogen sulphide ices. 

“This is the first model to simultaneously fit observations of reflected sunlight from ultraviolet to near-infrared wavelengths,” explained 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 colour between Uranus and Neptune.”

The team’s model consists of three layers of aerosols at different heights [5]. The key layer that affects the colours 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, with the result that the blue colour of Neptune looks stronger.

“We hoped that developing this model would help us understand clouds and hazes in the ice giant atmospheres,” commented Mike Wong, an astronomer at the University of California, Berkeley, and a member of the team behind this result. “Explaining the difference in colour between Uranus and Neptune was an unexpected bonus!” 

To create this model, Irwin’s team analysed archival data spanning several years from the NASA/ESA Hubble Space Telescope. This spectrographic data was obtained with Hubble’s Space Telescope Imaging Spectrograph (STIS), covering a broad range of wavelengths from ultraviolet through to visible and infrared (0.3–1.0 micrometres). It was complemented with data from ground-based telescopes: a set of new observations from the Gemini North telescope, and archival data from the NASA Infrared Telescope Facility, both located in Hawai‘i.

Not only did the team examine the spectra of the planets, they also made use of some of the many images Hubble has taken of the two planets with its Wide Field Camera 3 (WFC3) instrument. Hubble provides excellent views of the distinctive atmospheric storms shared by both planets known as ‘dark spots’, which astronomers have been aware of for many years. It wasn't known exactly which atmospheric layers were disturbed by dark spots to make them visible to Hubble. The model produced by the team explains what gives the spots a dark appearance, and why they are more easily detectable on Uranus compared to Neptune.

The authors thought that a darkening of the aerosols at the deepest layer of their model would produce dark spots similar to those seen on Neptune and perhaps Uranus. With the detailed images from Hubble, they could check and confirm their hypothesis. Indeed, simulated images based on that model were seen to closely match the WFC3 images of both planets, producing dark spots visible at the same wavelengths. The same thick haze in the Aerosol-2 layer on Uranus that causes its lighter blue colour is believed also to obscure these dark spots more often than on Neptune.

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Notes

[1] This whitening effect is similar to how clouds in exoplanet atmospheres dull or ‘flatten’ features in the spectra of exoplanets.

[2] This process — referred to as Rayleigh scattering — is what makes the sky blue here on Earth. Rayleigh scattering occurs predominantly at shorter, bluer wavelengths; the red light scattered from the haze and air molecules is more absorbed than the blue light by methane molecules in the atmosphere of the planets. On Earth, it is nitrogen molecules in the atmosphere that scatter most of the light in this way, while on Neptune and Uranus hydrogen is the main scattering molecule.

[3] A scientific model is a computational tool used by scientists to test predictions about a phenomenon that would be impossible to test in the real world.

[4] An aerosol is a suspension of fine droplets or particles in a gas. Common examples on Earth include mist, soot, smoke, and fog. On Neptune and Uranus, particles produced by sunlight interacting with elements in the atmosphere (photochemical reactions) are responsible for aerosol hazes in these planets’ atmospheres.

[5] The deepest layer (referred to in the paper as the Aerosol-1 layer) is thick and is composed of a mixture of hydrogen sulphide ice and particles produced by the interaction of the planets’ atmospheres with sunlight. The top layer is an extended layer of haze (the Aerosol-3 layer) similar to the middle layer but more tenuous. On Neptune, large methane ice particles also form above this layer.

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More Information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Gemini North is one half of the international Gemini Observatory, which is a Program of NSF's NOIRLab.

This research was presented in the paper “Hazy blue worlds: A holistic aerosol model for Uranus and Neptune, including Dark Spots” to appear in the Journal of Geophysical Research: Planets.

The team is composed of P. G. J. Irwin (Department of Physics, University of Oxford, UK), N. A. Teanby (School of Earth Sciences, University of Bristol, UK), L. N. Fletcher (School of Physics & Astronomy, University of Leicester, UK), D. Toledo (Instituto Nacional de Tecnica Aeroespacial, Spain), G. S. Orton (Jet Propulsion Laboratory, California Institute of Technology, USA), M. H. Wong (Center for Integrative Planetary Science, University of California, Berkeley, USA), M. T. Roman (School of Physics & Astronomy, University of Leicester, UK), S. Perez-Hoyos (University of the Basque Country, Spain), A. James (Department of Physics, University of Oxford, UK), J. Dobinson (Department of Physics, University of Oxford, UK).

The observations were conducted as part of the following Hubble observing programmes: spectra of Neptune with HST/STIS, 9330 (PI: E. Karkoschka); spectra of Uranus with HST/STIS, 9035 (PI: E. Karkoschka), 12894 (PI: L. Sromovsky), 14113 (PI: L. Sromovsky); imaging of Uranus and Neptune with HST/WFC3, 13937 and 15262 (PI: A. Simon).

Image credit: NASA, ESA, A. Simon (Goddard Space Flight Center), and M. H. Wong (University of California, Berkeley) and the OPAL team

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Links

• Images of Hubble 
• Science paper
• NOIRLab release

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Contacts:

Patrick Irwin
University of Oxford
United Kingdom
Email: patrick.irwin@physics.ox.ac.uk

Bethany Downer
ESA/Hubble Chief Science Communications Officer
Email: Bethany.Downer@esahubble.org

Source:ESA/Hubble/News

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ESO telescope captures surprising changes in Neptune's temperatures

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Thermal images of Neptune taken between 2006 and 2020 

 

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Thermal images of Neptune taken between 2006 and 2020 (different layout) 

 

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Neptune from the VLT and Hubble

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Videos

 

PR Video eso2206a
Evolution of thermal images from Neptune

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An international team of astronomers have used ground-based telescopes, including the European Southern Observatory’s Very Large Telescope (ESO’s VLT), to track Neptune’s atmospheric temperatures over a 17-year period. They found a surprising drop in Neptune’s global temperatures followed by a dramatic warming at its south pole.

“This change was unexpected,” says Michael Roman, a postdoctoral research associate at the University of Leicester, UK, and lead author of the study published today in The Planetary Science Journal. “Since we have been observing Neptune during its early southern summer, we expected temperatures to be slowly growing warmer, not colder.”

Like Earth, Neptune experiences seasons as it orbits the Sun. However, a Neptune season lasts around 40 years, with one Neptune year lasting 165 Earth years. It has been summertime in Neptune’s southern hemisphere since 2005, and the astronomers were eager to see how temperatures were changing following the southern summer solstice.

Astronomers looked at nearly 100 thermal-infrared images of Neptune, captured over a 17-year period, to piece together overall trends in the planet’s temperature in greater detail than ever before.

These data showed that, despite the onset of southern summer, most of the planet had gradually cooled over the last two decades. The globally averaged temperature of Neptune dropped by 8 °C between 2003 and 2018. 

The astronomers were then surprised to discover a dramatic warming of Neptune’s south pole during the last two years of their observations, when temperatures rapidly rose 11 °C between 2018 and 2020. Although Neptune’s warm polar vortex has been known for many years, such rapid polar warming has never been previously observed on the planet.

“Our data cover less than half of a Neptune season, so no one was expecting to see large and rapid changes,” says co-author Glenn Orton, senior research scientist at Caltech’s Jet Propulsion Laboratory (JPL) in the US.

The astronomers measured Neptune’s temperature using thermal cameras that work by measuring the infrared light emitted from astronomical objects. For their analysis the team combined all existing images of Neptune gathered over the last two decades by ground-based telescopes. They investigated infrared light emitted from a layer of Neptune’s atmosphere called the stratosphere. This allowed the team to build up a picture of Neptune’s temperature and its variations during part of its southern summer.

Because Neptune is roughly 4.5 billion kilometres away and is very cold, the planet’s average temperature reaching around –220°C, measuring its temperature from Earth is no easy task. “This type of study is only possible with sensitive infrared images from large telescopes like the VLT that can observe Neptune clearly, and these have only been available for the past 20 years or so,” says co-author Leigh Fletcher, a professor at the University of Leicester.

Around one third of all the images taken came from the VLT Imager and Spectrometer for mid-InfraRed (VISIR) instrument on ESO’s VLT in Chile’s Atacama Desert. Because of the telescope’s mirror size and altitude, it has a very high resolution and data quality, offering the clearest images of Neptune. The team also used data from NASA’s Spitzer Space Telescope and images taken with the Gemini South telescope in Chile, as well as with the Subaru Telescope, the Keck Telescope, and the Gemini North telescope, all in Hawai‘i. 

Because Neptune’s temperature variations were so unexpected, the astronomers do not know yet what could have caused them. They could be due to changes in Neptune’s stratospheric chemistry, or random weather patterns, or even the solar cycle. More observations will be needed over the coming years to explore the reasons for these fluctuations. Future ground-based telescopes like ESO’s Extremely Large Telescope (ELT) could observe temperature changes like these in greater detail, while the NASA/ESA/CSA James Webb Space Telescope will provide unprecedented new maps of the chemistry and temperature in Neptune’s atmosphere.

“I think Neptune is itself very intriguing to many of us because we still know so little about it,” says Roman. “This all points towards a more complicated picture of Neptune’s atmosphere and how it changes with time.”

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More Information

This research was presented in the paper “Sub-Seasonal Variation in Neptune’s Mid-Infrared Emission” published today in The Planetary Science Journal (doi:10.3847/PSJ/ac5aa4).

The team is composed of M. T. Roman and L. N. Fletcher (School of Physics and Astronomy, University of Leicester, UK), G. S. Orton (Jet Propulsion Laboratory/California Institute of Technology, California, USA), T. K. Greathouse (Southwest Research Institute, San Antonio, TX, USA), J. I. Moses (Space Science Institute, Boulder, CO, USA), N. Rowe-Gurney (Department of Physics and Astronomy, Howard University, Washington DC, USA; Astrochemistry Laboratory, NASA/GSFC, Greenbelt, MD, USA; Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, USA), P. G. J. Irwin (University of Oxford Atmospheric, Oceanic, and Planetary Physics, Department of Physics Clarendon Laboratory, Oxford, UK), A. Antuñano (UPV/EHU, Escuela Ingernieria de Bilbao, Spain), J. Sinclair (Jet Propulsion Laboratory/California Institute of Technology, California, USA), Y. Kasaba (Planetary Plasma and Atmospheric Research Center, Graduate School of Science, Tohoku University, Japan), T. Fujiyoshi (Subaru Telescope, National Astronomical Observatory of Japan, HI, USA), I. de Pater (Department of Astronomy, University of California at Berkeley, CA, USA), and H. B. Hammel (Association of Universities for Research in Astronomy, Washington DC, USA).

The European Southern Observatory (ESO) enables scientists worldwide to discover the secrets of the Universe for the benefit of all. We design, build and operate world-class observatories on the ground — which astronomers use to tackle exciting questions and spread the fascination of astronomy — and promote international collaboration in astronomy. Established as an intergovernmental organisation in 1962, today ESO is supported by 16 Member States (Austria, Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom), along with the host state of Chile and with Australia as a Strategic Partner. ESO’s headquarters and its visitor centre and planetarium, the ESO Supernova, are located close to Munich in Germany, while the Chilean Atacama Desert, a marvellous place with unique conditions to observe the sky, hosts our telescopes. ESO operates three observing sites: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope and its Very Large Telescope Interferometer, as well as two survey telescopes, VISTA working in the infrared and the visible-light VLT Survey Telescope. Also at Paranal ESO will host and operate the Cherenkov Telescope Array South, the world’s largest and most sensitive gamma-ray observatory. Together with international partners, ESO operates APEX and ALMA on Chajnantor, two facilities that observe the skies in the millimetre and submillimetre range. At Cerro Armazones, near Paranal, we are building “the world’s biggest eye on the sky” — ESO’s Extremely Large Telescope. From our offices in Santiago, Chile we support our operations in the country and engage with Chilean partners and society.

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Contacts:

Michael Roman
School of Physics and Astronomy, University of Leicester
Leicester, UK
Email: m.t.roman@leicester.ac.uk

Glenn Orton
Caltech’s Jet Propulsion Laboratory (JPL)
Pasadena, California, US
Email: glenn.s.orton@jpl.nasa.gov

Leigh Fletcher
School of Physics and Astronomy, University of Leicester
Leicester, UK
Tel: + 44 (0)116 252 3585
Email: leigh.fletcher@le.ac.uk

Bárbara Ferreira
ESO Media Manager
Garching bei München, Germany
Tel: +49 89 3200 6670
Cell: +49 151 241 664 0
Email: press@eso.org

Source: ESO/News

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