Moons of Uranus Surprise Scientists in NASA Hubble Study

Uranus and Moons

The five largest moons of Uranus – sometimes called the “classical moons” -- appear in a jagged, roughly diagonal line from top right to bottom left. These are labeled Titania, Oberon, Umbriel, Miranda and Ariel. Also visible is Ariel’s shadow, which is superimposed on Uranus. Faint, ghostly, Saturn-like rings encircle the blue ice giant. Credits/Science: NASA, ESA, STScI, Christian Soto (STScI). Image Processing: Joseph DePasquale (STScI)

Uranus and Moons Compass Image

This image of Uranus and its five classical moons -- Titania, Oberon, Umbriel, Miranda and Ariel -- was captured by the Hubble Space Telescope’s. div style="text-align: justify;">Advanced Camera for Surveys (ACS). The image shows a scale bar, compass arrows, and color key for reference. The five largest moons of Uranus – sometimes called the “classical moons” -- appear in a jagged, roughly diagonal line from top right to bottom left. These are labeled Titania, Oberon, Umbriel, Miranda and Ariel. Also visible is Ariel’s shadow, which is superimposed on Uranus. Faint, ghostly, Saturn-like rings encircle the blue ice giant. Credits/Science: NASA, ESA, STScI, Christian Soto (STScI). Image Processing: Joseph DePasquale (STScI). The scale bar is labeled in miles along the top and kilometers along the bottom. The north and east compass arrows show the orientation of the image on the sky. Note that the relationship between north and east on the sky (as seen from below) is flipped relative to direction arrows on a map of the ground (as seen from above). This image shows visible wavelengths of light that have been translated into visible-light colors. The color key shows which ACS filters were used when collecting the light. The color of each filter name is the visible-light color used to represent the light that passes through that filter. Credits/Science: NASA, ESA, STScI, Christian Soto (STScI). Image Processing: Joseph DePasquale (STScI)

 

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Scientists using NASA’s Hubble Space Telescope went looking for evidence of one phenomenon and found quite another.

The research team studied the four largest moons of the ice giant Uranus, the seventh planet from our Sun, searching for signs of ,. interactions between Uranus' magnetosphere and the surfaces of the moons. (A magnetosphere is a region surrounding a celestial body where particles with an electrical charge are affected by the astronomical object’s magnetic field.)

In particular, the team predicted that, based on interactions with Uranus' magnetosphere, the “leading” sides of these tidally locked moons, which always face in the same direction in which they are orbiting the planet, would be brighter than the “trailing” sides, always facing away. This would be due to radiation darkening of their trailing sides by charged particles such as electrons trapped in Uranus’ magnetosphere.

Instead, they found no evidence for darkening on the moons’ trailing sides, and clear evidence for darkening of the leading sides of the outer moons. This surprised the team and indicates that Uranus’ magnetosphere might not interact much with its large moons, contradicting existing data collected over near-infrared wavelengths.

Hubble’s sharp ultraviolet vision and spectroscopic capabilities were critical for allowing the team to investigate the surface conditions on these moons and uncover the surprising finding.

The Complicated Magnetic Environment of ‘Weird’ Uranus

The four moons in this study — Ariel, Umbriel, Titania, and Oberon — are tidally locked to Uranus, so that they always show the same side to the planet. The side of the moon facing the direction of travel is called the leading hemisphere, while the side that faces backward is called the trailing hemisphere. The thinking was that charged particles trapped along the magnetic field lines primarily hit each moon’s trailing side, which would darken that hemisphere.

“Uranus is weird, so it's always been uncertain how much the magnetic field actually interacts with its satellites,” explained principal investigator Richard Cartwright of the Johns Hopkins University’s Applied Physics Laboratory. “For starters, it is tilted by 98 degrees relative to the ecliptic.”

This means Uranus is dramatically tipped relative to the orbital plane of the planets. It rolls very slowly around the Sun on its side as it completes its 84-Earth-year orbit.

“At the time of the Voyager 2 flyby, the magnetosphere of Uranus was tilted by about 59 degrees from the orbital plane of the satellites. So, there's an additional tilt to the magnetic field,” explained Cartwright.

Because Uranus and its magnetic field lines rotate faster than its moons orbit the planet, the magnetic field lines constantly sweep past the moons. If the magnetosphere of Uranus interacts with its moons, charged particles should preferentially hit the surface of the trailing sides.

These charged particles, as well as our galaxy’s cosmic rays, should darken the trailing hemispheres of Ariel, Umbriel, Titania, and Oberon and possibly generate the carbon dioxide detected on these moons. The team expected that, especially for the inner moons Ariel and Umbriel, the trailing hemispheres would be darker than the leading sides in ultraviolet and visible wavelengths.

But that’s not what they found. Instead, the leading and trailing hemispheres of Ariel and Umbriel are actually very similar in brightness. However, the researchers did see a difference between the hemispheres of the two outer moons, Titania and Oberon — not the moons they expected.

Like Bugs on a Windshield

Even stranger, the difference in brightness was the opposite of what they expected. The two outer moons have darker and redder leading hemispheres compared with their trailing hemispheres. The team thinks that dust from some of Uranus’ irregular satellites is coating the leading sides of Titania and Oberon.

Irregular satellites are natural bodies that have large, eccentric, and inclined orbits relative to their parent planet’s equatorial plane. Micrometeorites are constantly hitting the surfaces of Uranus’ irregular satellites, ejecting small bits of material into orbit around the planet.

Over millions of years, this dusty material moves inward toward Uranus and eventually crosses the orbits of Titania and Oberon. These outer moons sweep through the dust and pick it up primarily on their leading hemispheres, which face forward. It's much like bugs hitting the windshield of your car as you drive down a highway.

This material causes Titania and Oberon to have darker and redder leading hemispheres. These outer moons effectively shield the inner moons Ariel and Umbriel from the dust, which is why the inner moons’ hemispheres do not show a difference in brightness.

“We see the same thing happening in the Saturn system and probably the Jupiter system as well,” said co-investigator Bryan Holler of the Space Telescope Science Institute. “This is some of the first evidence we’re seeing of a similar material exchange among the Uranian satellites.”

“So that supports a different explanation,” said Cartwright. “That's dust collection. I didn't even expect to get into that hypothesis, but you know, data always surprise you.”

Based on these findings, Cartwright and his team suspect that Uranus' magnetosphere may be fairly quiescent, or it may be more complicated than previously thought. Perhaps interactions between Uranus' moons and magnetosphere are happening, but for some reason, they’re not causing asymmetry in the leading and trailing hemispheres as researchers suspected. The answer will require further investigation into enigmatic Uranus, its magnetosphere, and its moons.

Hubble’s Unique Ultraviolet Vision

To observe the brightnesses of the four largest Uranian moons, the researchers required Hubble’s unique ultraviolet capabilities. Observing targets in ultraviolet light is not possible from the ground because of the filtering effects of Earth’s protective atmosphere. No other present-day space telescopes have comparable ultraviolet vision and sharpness.

“Hubble, with its ultraviolet capabilities, is the only facility that could test our hypothesis,” said the Space Telescope Science Institute’s Christian Soto, who conducted much of the data extraction and analysis. Soto presented results from this study on June 10 at the 246th Meeting of American Astronomical Society in Anchorage, Alaska.

Complementary data from NASA’s James Webb Space Telescope will help to provide a more comprehensive understanding of the Uranian satellite system and its interactions with the planet’s magnetosphere.

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.

The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the science operations center of the Hubble Space Telescope, the science and mission operations centers for the James Webb Space Telescope, and the science operations center for the Nancy Grace Roman Space Telescope. STScI also houses the Barbara A. Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide to the astronomical community a variety of astronomical data archives, and is the data repository for the Hubble, Webb, Roman, Kepler, K2, TESS missions and more. STScI is operated by the Association of Universities for Research in Astronomy in Washington, D.C.

Source: Space Telescope Science Institute (STScI)/Press Release

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20-Year Hubble Study of Uranus Yields New Atmospheric Insights

20 Years of Uranus Observations

The image columns show the change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region darkened going into winter shadow while the north polar region brightened as northern summer approaches. Credits/Image: NASA, ESA, Erich Karkoschka (LPL)

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The ice-giant planet Uranus, which travels around the Sun tipped on its side, is a weird and mysterious world. Now, in an unprecedented study spanning two decades, researchers using NASA’s Hubble Space Telescope have uncovered new insights into the planet's atmospheric composition and dynamics. This was possible only because of Hubble’s sharp resolution, spectral capabilities, and longevity.

The team’s results will help astronomers to better understand how the atmosphere of Uranus works and responds to changing sunlight. These long-term observations provide valuable data for understanding the atmospheric dynamics of this distant ice giant, which can serve as a proxy for studying exoplanets of similar size and composition.

When Voyager 2 flew past Uranus in 1986, it provided a close-up snapshot of the sideways planet. What it saw resembled a bland, blue-green billiard ball. By comparison, Hubble chronicled a 20-year story of seasonal changes from 2002 to 2022. Over that period, a team led by Erich Karkoschka of the University of Arizona, and Larry Sromovsky and Pat Fry from the University of Wisconsin used the same Hubble instrument, STIS (the Space Telescope Imaging Spectrograph), to paint an accurate picture of the atmospheric structure of Uranus.

Uranus' atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. The methane gives Uranus its cyan color by absorbing the red wavelengths of sunlight.

The Hubble team observed Uranus four times in the 20-year period: in 2002, 2012, 2015, and 2022. They found that, unlike conditions on the gas giants Saturn and Jupiter, methane is not uniformly distributed across Uranus. Instead, it is strongly depleted near the poles. This depletion remained relatively constant over the two decades. However, the aerosol and haze structure changed dramatically, brightening significantly in the northern polar region as the planet approaches its northern summer solstice in 2030.

Uranus takes a little over 84 Earth years to complete a single orbit of the Sun. So, over two decades, the Hubble team has only seen mostly northern spring as the Sun moves from shining directly over Uranus’ equator toward shining almost directly over its north pole in 2030. Hubble observations suggest complex atmospheric circulation patterns on Uranus during this period. The data that are most sensitive to the methane distribution indicate a downwelling in the polar regions and upwelling in other regions.

The team analyzed their results in several ways. The image columns show the change of Uranus for the four years that STIS observed Uranus across a 20-year period. Over that span of time, the researchers watched the seasons of Uranus as the south polar region (left) darkened going into winter shadow while the north polar region (right) brightened as it began to come into a more direct view as northern summer approaches.

The top row, in visible light, shows how the color of Uranus appears to the human eye as seen through even an amateur telescope.

In the second row, the false-color image of the planet is assembled from visible and near-infrared light observations. The color and brightness correspond to the amounts of methane and aerosols. Both of these quantities could not be distinguished before Hubble's STIS was first aimed at Uranus in 2002. Generally, green areas indicate less methane than blue areas, and red areas show no methane. The red areas are at the limb, where the stratosphere of Uranus is almost completely devoid of methane.

The two bottom rows show the latitude structure of aerosols and methane inferred from 1,000 different wavelengths (colors) from visible to near infrared. In the third row, bright areas indicate cloudier conditions, while the dark areas represent clearer conditions. In the fourth row, bright areas indicate depleted methane, while dark areas show the full amount of methane.

At middle and low latitudes, aerosols and methane depletion have their own latitudinal structure that mostly did not change much over the two decades of observation. However, in the polar regions, aerosols and methane depletion behave very differently.

In the third row, the aerosols near the north pole display a dramatic increase, showing up as very dark during early northern spring, turning very bright in recent years. Aerosols also seem to disappear at the left limb as the solar radiation disappeared. This is evidence that solar radiation changes the aerosol haze in the atmosphere of Uranus. On the other hand, methane depletion seems to stay quite high in both polar regions throughout the observing period.

Astronomers will continue to observe Uranus as the planet approaches northern summer.

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: Space Telescope Science Institute (STScI)/News

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NASA's Hubble, New Horizons Team Up for a Simultaneous Look at Uranus

Hubble and New Horizons Views of Uranus

In this image, two three-dimensional shapes (top) of Uranus are compared to the actual views of the planet from NASA's Hubble Space Telescope (bottom left) and NASA's New Horizon's spacecraft (bottom right). These two missions recently simultaneously observed the gas giant, comparing high-resolution images from Hubble to the smaller view from New Horizons. This combined perspective will help researchers learn more about what to expect while imaging planets around other stars with future observatories.
The gas giant planets in our solar system have dynamic and variable atmospheres with changing cloud cover. By knowing the details of what the clouds on Uranus looked like from Hubble, researchers are able to verify what is interpreted from the New Horizons data.

While it was clear the cloud features were not changing with the planet's rotation, Uranus appeared dimmer in the New Horizons data than expected.

Researchers found this has to do with how the planet reflects light at a different phase than what Hubble can see. This showed that exoplanets may be dimmer than predicted at partial and high phase angles, and that the atmosphere reflects light differently at partial phase. Credits Science: NASA, ESA, STScI, Samantha Hasler (MIT), Amy Simon (NASA-GSFC), New Horizons Planetary Science Theme Team/Image Processing: Joseph DePasquale (STScI), Joseph Olmsted (STScI)

New Horizons Looking Back Towards Uranus (Artist's Concept)

This illustration shows NASA's New Horizons spacecraft's view of our solar system from deep in the Kuiper Belt. New Horizons is currently at an estimated distance of more than 5 billion miles from Earth. The probe was 6.5 billion miles away from Uranus when it recently observed the planet. In this study, researchers used the gas giant as an exoplanet proxy, comparing high-resolution images from NASA's Hubble Space Telescope to the smaller view from New Horizons to learn more about what to expect while imaging planets around other stars. Credits Artwork? NASA, ESA, Christian Nieves (STScI), Ralf Crawford (STScI), Greg Bacon (STScI)

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NASA's Hubble Space Telescope and New Horizons spacecraft simultaneously set their sights on Uranus recently, allowing scientists to make a direct comparison of the planet from two very different viewpoints. The results inform future plans to study like types of planets around other stars.

Astronomers used Uranus as a proxy for similar planets beyond our solar system, known as exoplanets, comparing high-resolution images from Hubble to the more-distant view from New Horizons. This combined perspective will help scientists learn more about what to expect while imaging planets around other stars with future telescopes.

"While we expected Uranus to appear differently in each filter of the observations, we found that Uranus was actually dimmer than predicted in the New Horizons data taken from a different viewpoint," said lead author Samantha Hasler of the Massachusetts Institute of Technology in Cambridge and New Horizons science team collaborator.

Direct imaging of exoplanets is a key technique for learning about their potential habitability, and offers new clues to the origin and formation of our own solar system. Astronomers use both direct imaging and spectroscopy to collect light from the observed planet and compare its brightness at different wavelengths. However, imaging exoplanets is a notoriously difficult process because they're so far away. Their images are mere pinpoints and so are not as detailed as the close-up views that we have of worlds orbiting our Sun. Researchers can also only directly image exoplanets at "partial phases," when only a portion of the planet is illuminated by their star as seen from Earth.

Uranus was an ideal target as a test for understanding future distant observations of exoplanets by other telescopes for a few reasons. First, many known exoplanets are also gas giants similar in nature. Also, at the time of the observations, New Horizons was on the far side of Uranus, 6.5 billion miles away, allowing its twilight crescent to be studied—something that cannot be done from Earth. At that distance, the New Horizons view of the planet was just several pixels in its color camera, called the Multispectral Visible Imaging Camera.

On the other hand, Hubble, with its high resolution, and in its low-Earth orbit 1.7 billion miles away from Uranus, was able to see atmospheric features such as clouds and storms on the day side of the gaseous world.

"Uranus appears as just a small dot on the New Horizons observations, similar to the dots seen of directly-imaged exoplanets from observatories like Webb or ground-based observatories," added Hasler. "Hubble provides context for what the atmosphere is doing when it was observed with New Horizons."

The gas giant planets in our solar system have dynamic and variable atmospheres with changing cloud cover. How common is this among exoplanets? By knowing the details of what the clouds on Uranus looked like from Hubble, researchers are able to verify what is interpreted from the New Horizons data. In the case of Uranus, both Hubble and New Horizons saw that the brightness did not vary as the planet rotated, which indicates that the cloud features were not changing with the planet’s rotation.

However, the importance of the detection by New Horizons has to do with how the planet reflects light at a different phase than what Hubble, or other observatories on or near Earth, can see. New Horizons showed that exoplanets may be dimmer than predicted at partial and high phase angles, and that the atmosphere reflects light differently at partial phase.

NASA has two major upcoming observatories in the works to advance studies of exoplanet atmospheres and potential habitability.

"These landmark New Horizons studies of Uranus from a vantage point unobservable by any other means add to the mission's treasure trove of new scientific knowledge, and have, like many other datasets obtained in the mission, yielded surprising new insights into the worlds of our solar system," added New Horizons principal investigator Alan Stern of the Southwest Research Institute.

NASA's upcoming Nancy Grace Roman Space Telescope, set to launch by 2027, will use a coronagraph to block out a star's light to directly see gas giant exoplanets. NASA's Habitable Worlds Observatory , in an early planning phase, will be the first telescope designed specifically to search for atmospheric biosignatures on Earth-sized, rocky planets orbiting other stars.

"Studying how known benchmarks like Uranus appear in distant imaging can help us have more robust expectations when preparing for these future missions," concluded Hasler. "And that will be critical to our success." Launched in January 2006, New Horizons made the historic flyby of Pluto and its moons in July 2015, before giving humankind its first close-up look at one of these planetary building block and Kuiper Belt object, Arrokoth, in January 2019. New Horizons is now in its second extended mission, studying distant Kuiper Belt objects, characterizing the outer heliosphere of the Sun, and making important astrophysical observations from its unmatched vantage point in distant regions of the solar system.

The Uranus results are being presented this week at the 56th annual meeting of the American Astronomical Society Division for Planetary Sciences, in Boise, Idaho. 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, Colorado, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, Maryland, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

The Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, built and operates the New Horizons spacecraft and manages the mission for NASA's Science Mission Directorate. Southwest Research Institute, based in San Antonio and Boulder, Colorado, directs the mission via Principal Investigator Alan Stern and leads the science team, payload operations and encounter science planning. New Horizons is part of NASA's New Frontiers program, managed by NASA's Marshall Space Flight Center in Huntsville, Alabama.

Source: HubbleSite/News

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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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NASA's Webb Rings in the Holidays with the Ringed Planet Uranus

Uranus Close-up (NIRCam image)
Credits: Image: NASA, ESA, CSA, STScI

Uranus Wide (NIRCam Image)
Credits: Image: NASA, ESA, CSA, STScI

Uranus Wide (Compass NIRCam Image)
Credits: Image: NASA, ESA, CSA, STScI

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NASA’s James Webb Space Telescope recently trained its sights on unusual and enigmatic Uranus, an ice giant that spins on its side. Webb captured this dynamic world with rings, moons, storms, and other atmospheric features – including a seasonal polar cap. The image expands upon a two-color version released earlier this year, adding additional wavelength coverage for a more detailed look.

With its exquisite sensitivity, Webb captured Uranus’ dim inner and outer rings, including the elusive Zeta ring – the extremely faint and diffuse ring closest to the planet. It also imaged many of the planet’s 27 known moons, even seeing some small moons within the rings.

In visible wavelengths as seen by Voyager 2 in the 1980s, Uranus appeared as a placid, solid blue ball. In infrared wavelengths, Webb is revealing a strange and dynamic ice world filled with exciting atmospheric features.

One of the most striking of these is the planet’s seasonal north polar cloud cap. Compared to the Webb image from earlier this year, some details of the cap are easier to see in these newer images. These include the bright, white, inner cap and the dark lane in the bottom of the polar cap, toward the lower latitudes.

Several bright storms can also be seen near and below the southern border of the polar cap. The number of these storms, and how frequently and where they appear in Uranus’s atmosphere, might be due to a combination of seasonal and meteorological effects.

The polar cap appears to become more prominent when the planet’s pole begins to point toward the Sun, as it approaches solstice and receives more sunlight. Uranus reaches its next solstice in 2028, and astronomers are eager to watch any possible changes in the structure of these features. Webb will help disentangle the seasonal and meteorological effects that influence Uranus’s storms, which is critical to help astronomers understand the planet’s complex atmosphere.

Because Uranus spins on its side at a tilt of about 98 degrees, it has the most extreme seasons in the solar system. For nearly a quarter of each Uranian year, the Sun shines over one pole, plunging the other half of the planet into a dark, 21-year-long winter.

With Webb’s unparalleled infrared resolution and sensitivity, astronomers now see Uranus and its unique features with groundbreaking new clarity. These details, especially of the close-in Zeta ring, will be invaluable to planning any future missions to Uranus.

Uranus can also serve as a proxy for studying the nearly 2,000 similarly sized exoplanets that have been discovered in the last few decades. This “exoplanet in our backyard” can help astronomers understand how planets of this size work, what their meteorology is like, and how they formed. This can in turn help us understand our own solar system as a whole by placing it in a larger context.

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 the Canadian Space Agency.

Source: NASA's James Webb Space Telescope/News

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NASA Scientists Make First Observation of a Polar Cyclone on Uranus

NASA scientists used microwave observations to spot the first polar cyclone on Uranus, seen here as a light-colored dot to the right of center in each image of the planet. The images use wavelength bands K, Ka, and Q, from left. To highlight cyclone features, a different color map was used for each. Credits: NASA/JPL-Caltech/VLA

This image of Uranus was taken by NASA’s Voyager 2 spacecraft in 1986.
Credits: NASA/JPL-Caltech/VLA

 

Scientists used ground-based telescopes to get unprecedented views, thanks to the giant planet’s position in its long orbit around the Sun.

For the first time, NASA scientists have strong evidence of a polar cyclone on Uranus. By examining radio waves emitted from the ice giant, they detected the phenomenon at the planet’s north pole. The findings confirm a broad truth about all planets with substantial atmospheres in our solar system: Whether the planets are composed mainly of rock or gas, their atmospheres show signs of a swirling vortex at the poles.

Scientists have long known that Uranus’ south pole has a swirling feature. NASA’s Voyager 2 imaging of methane cloud tops there showed winds at the polar center spinning faster than over the rest of the pole. Voyager’s infrared measurements observed no temperature changes, but the new findings, published in Geophysical Research Letters, do.

Using huge radio antenna dishes of the Very Large Array in New Mexico, they peered below the ice giant’s clouds, determining that the circulating air at the north pole seems to be warmer and drier – the hallmarks of a strong cyclone. Collected in 2015, 2021, and 2022, the observations went deeper into Uranus’ atmosphere than any before.

“These observations tell us a lot more about the story of Uranus. It’s a much more dynamic world than you might think,” said lead author Alex Akins of NASA’s Jet Propulsion Laboratory in Southern California. “It isn’t just a plain blue ball of gas. There’s a lot happening under the hood.”

Uranus is showing off more these days, thanks to the planet’s position in orbit. It’s a long haul around the solar system for this outer planet, taking 84 years to complete a full lap, and for the last few decades the poles weren’t pointed toward Earth. Since about 2015, scientists have had a better view and have been able to look deeper into the polar atmosphere.

Ingredients for a Cyclone

The cyclone on Uranus, compactly shaped with warm and dry air at its core, is much like those spotted by NASA’s Cassini at Saturn. With the new findings, cyclones (which rotate in the same direction their planet rotates) or anti-cyclones (which rotate in the opposite direction) have now been identified at the poles on every planet in our solar system except for Mercury, which has no substantial atmosphere.

But unlike hurricanes on Earth, cyclones on Uranus and Saturn aren’t formed over water (neither planet is known to have liquid water), and they don’t drift; they’re locked at the poles. Researchers will be watching closely to see how this newly discovered Uranus cyclone evolves in the coming years.

“Does the warm core we observed represent the same high-speed circulation seen by Voyager?” Akins asked. “Or are there stacked cyclones in Uranus’ atmosphere? The fact that we’re still finding out such simple things about how Uranus’ atmosphere works really gets me excited to find out more about this mysterious planet.”

The National Academies’ 2023 Planetary Science and Astrobiology Decadal Survey prioritized exploring Uranus. In preparation for such a mission, planetary scientists are focused on bolstering their knowledge about the mysterious ice giant’s system.

Gretchen McCartney
Jet Propulsion Laboratory, Pasadena, Calif.
818-393-6215
gretchen.p.mccartney@jpl.nasa.gov 

Karen Fox / Alana Johnson
NASA Headquarters, Washington
301-286-6284 / 202-358-1501
karen.c.fox@nasa.gov / alana.r.johnson@nasa.gov

Editor: Tony Greicius

Source: NASA/Uranus

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NASA’s Webb Scores Another Ringed World with New Image of Uranus

Uranus (NIRCam Image)
Credits: Science: NASA, ESA, CSA, STScI
Image Processing: Joseph DePasquale (STScI)

Release Images

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Following in the footsteps of the Neptune image released in 2022, NASA’s James Webb Space Telescope has taken a stunning image of the solar system’s other ice giant, the planet Uranus. The new image features dramatic rings as well as bright features in the planet’s atmosphere. The Webb data demonstrates the observatory's unprecedented sensitivity for the faintest dusty rings, which have only ever been imaged by two other facilities: the Voyager 2 spacecraft as it flew past the planet in 1986, and the Keck Observatory with advanced adaptive optics.

The seventh planet from the Sun, Uranus is unique: It rotates on its side, at roughly a 90-degree angle from the plane of its orbit. This causes extreme seasons since the planet’s poles experience many years of constant sunlight followed by an equal number of years of complete darkness. (Uranus takes 84 years to orbit the Sun.) Currently, it is late spring for the northern pole, which is visible here; Uranus’ northern summer will be in 2028. In contrast, when Voyager 2 visited Uranus it was summer at the south pole. The south pole is now on the ‘dark side’ of the planet, out of view and facing the darkness of space.

This infrared image from Webb’s Near-Infrared Camera (NIRCam) combines data from two filters at 1.4 and 3.0 microns, which are shown here in blue and orange, respectively. The planet displays a blue hue in the resulting representative-color image.

When Voyager 2 looked at Uranus, its camera showed an almost featureless blue-green ball in visible wavelengths. With the infrared wavelengths and extra sensitivity of Webb we see more detail, showing how dynamic the atmosphere of Uranus really is.

On the right side of the planet there’s an area of brightening at the pole facing the Sun, known as a polar cap. This polar cap is unique to Uranus – it seems to appear when the pole enters direct sunlight in the summer and vanish in the fall; these Webb data will help scientists understand the currently mysterious mechanism. Webb revealed a surprising aspect of the polar cap: a subtle enhanced brightening at the center of the cap. The sensitivity and longer wavelengths of Webb’s NIRCam may be why we can see this enhanced Uranus polar feature when it has not been seen as clearly with other powerful telescopes like the Hubble Space Telescope and Keck Observatory. At the edge of the polar cap lies a bright cloud as well as a few fainter extended features just beyond the cap’s edge, and a second very bright cloud is seen at the planet’s left limb. Such clouds are typical for Uranus in infrared wavelengths, and likely are connected to storm activity.

This planet is characterized as an ice giant due to the chemical make-up of its interior. Most of its mass is thought to be a hot, dense fluid of "icy" materials – water, methane and ammonia – above a small rocky core.

Uranus has 13 known rings and 11 of them are visible in this Webb image. Some of these rings are so bright with Webb that when they are close together, they appear to merge into a larger ring. Nine are classed as the main rings of the planet, and two are the fainter dusty rings (such as the diffuse zeta ring closest to the planet) that weren’t discovered until the 1986 flyby by Voyager 2. Scientists expect that future Webb images of Uranus will reveal the two faint outer rings that were discovered with Hubble during the 2007 ring-plane crossing.

Webb also captured many of Uranus’s 27 known moons (most of which are too small and faint to be seen here); the six brightest are identified in the wide-view image. This was only a short, 12-minute exposure image of Uranus with just two filters. It is just the tip of the iceberg of what Webb can do when observing this mysterious planet. Additional studies of Uranus are happening now, and more are planned in Webb’s first year of science operations.

In 2022, the National Academies of Sciences, Engineering, and Medicine identified Uranus science as a priority in its 2023-2033 Planetary Science and Astrobiology decadal survey.

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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Christine Pulliam
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Source: NASA's James Webb Space Telescope/News

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Hubble Monitors Changing Weather and Seasons at Jupiter and Uranus

Jupiter (Nov. 2022 and Jan. 2023)
Credits: Science: NASA, ESA, STScI, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

Uranus (Nov. 2014 and Nov. 2022)
Credits: Science: NASA, ESA, STScI, Amy Simon (NASA-GSFC), Michael H. Wong (UC Berkeley)
Image Processing: Joseph DePasquale (STScI)

Release Images

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Ever since its launch in 1990, NASA's Hubble Space Telescope has been an interplanetary weather observer, keeping an eye on the largely gaseous outer planets and their ever-changing atmospheres. NASA spacecraft missions to the outer planets have given us a close-up look at these atmospheres, but Hubble's sharpness and sensitivity keeps an unblinking eye on a kaleidoscope of complex activities over time. In this way Hubble complements observations from other spacecraft such as Juno, currently orbiting Jupiter; the retired Cassini mission to Saturn, and the Voyager 1 and 2 probes, which collectively flew by all four giant planets between 1979 and 1989.

Inaugurated in 2014, the telescope's Outer Planet Atmospheres Legacy (OPAL) Program has been providing us with yearly views of the giant planets. Here are some recent images:

Jupiter

[left]—The forecast for Jupiter is stormy weather at low northern latitudes. A prominent string of alternating storms is visible, forming a "vortex street" as some planetary astronomers call it. This is a wave pattern of nested anticyclones and cyclones, locked together like in a machine with alternating gears moving clockwise and counterclockwise. If the storms get close enough to each other, in the very unlikely event of a merger, they could build an even larger storm, potentially rivaling the current size of the Great Red Spot. The staggered pattern of anticyclones and cyclones prevents individual storms from merging. Activity is also seen interior to these storms; in the 1990s Hubble didn't see any cyclones or anticyclones with built-in thunderstorms, but these storms have sprung up the last decade. Strong color differences indicate that Hubble is seeing different cloud heights and depths as well.

The orange moon Io photobombs this view of Jupiter's multicolored cloud tops, casting a shadow toward the planet's western limb. Hubble's resolution is so sharp that it can see Io's mottled-orange appearance, related to its numerous active volcanoes. These volcanoes were first discovered when the Voyager 1 spacecraft flew by in 1979. The moon's molten interior is overlaid by a thin crust through which the volcanoes eject material. Sulfur takes on various hues at different temperatures, which is why Io's surface is so colorful. This image was taken on November 12, 2022.

[right]—Jupiter's legendary Great Red Spot takes center stage in this view. Though this vortex is big enough to swallow Earth, it has actually shrunken to the smallest size it has ever been over observation records dating back 150 years. Jupiter's icy moon Ganymede can be seen transiting the giant planet at lower right. Slightly larger than the planet Mercury, Ganymede is the largest moon in the solar system. It is a cratered world with a mainly water-ice surface with apparent glacial flows driven by internal heat. (This image is smaller in size because Jupiter was 81,000 miles farther from Earth when the photo was taken). This image was taken on January 6, 2023.

Uranus

Planetary oddball Uranus rolls on its side around the Sun as it follows an 84-year orbit, rather than spinning in a more-vertical position as Earth does. Uranus has a weirdly tipped "horizontal" rotation axis angled just eight degrees off the plane of the planet's orbit. One recent theory proposes that Uranus once had a massive moon that gravitationally destabilized it and then crashed into it. Other possibilities include giant impacts during planetary formation, or even giant planets exerting resonant torques on each other over time. The consequences of the planet's tilt are that for stretches of time lasting up to 42 years, parts of one hemisphere are completely without sunlight. When the Voyager 2 spacecraft visited during the 1980s, the planet's south pole was pointed almost directly at the Sun. Hubble's latest view shows the northern pole now tipping toward the Sun.

[left]—This is a Hubble view of Uranus taken in 2014, seven years after northern spring equinox when the Sun was shining directly over the planet's equator, and shows one of the first images from the OPAL program. Multiple storms with methane ice-crystal clouds appear at mid-northern latitudes above the planet's cyan-tinted lower atmosphere. Hubble photographed the ring system edge-on in 2007, but the rings are seen starting to open up seven years later in this view. At this time, the planet had multiple small storms and even some faint cloud bands.

[right]—As seen in 2022, Uranus' north pole shows a thickened photochemical haze that looks similar to the smog over cities. Several little storms can be seen near the edge of the polar haze boundary. Hubble has been tracking the size and brightness of the north polar cap and it continues to get brighter year after year. Astronomers are disentangling multiple effects—from atmospheric circulation, particle properties, and chemical processes—that control how the atmospheric polar cap changes with the seasons. At the Uranian equinox in 2007, neither pole was particularly bright. 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. This image was taken on November 10, 2022.

About Hubble

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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University of California, Berkeley, Berkeley, California

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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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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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How do ice giants maintain their magnetic fields?

Illustration of Neptune's interior purchased from Shutterstock

Figure illustrating how the experiments were performed, revealing two forms of superionic ice
Courtesy of Vitali Prakapenka.

Washington, DC—A layer of “hot,” electrically conductive ice could be responsible for generating the magnetic fields of ice giant planets like Uranus and Neptune. New work from Carnegie and the University of Chicago’s Center for Advanced Radiation Sources reveals the conditions under which two such superionic ices form. Their findings are published in Nature Physics.

As all school children learn, water molecules are made up of two hydrogen atoms and one oxygen atom—H20. As the conditions in which water exists change, the organization and properties of these molecules are affected. We can see this in our everyday lives when liquid water is boiled into steam or frozen into ice.

The molecules that comprise ordinary ice that you might find in your drinking glass or on your driveway in winter arranged in a crystalline lattice held together by hydrogen bonds between the hydrogen and oxygen atoms. Hydrogen bonds are highly versatile. This means that ice can exist in a striking diversity of different structures—at least 18 known forms—which emerge under increasingly extreme environmental conditions.

Of particular interest is so-called superionic ice, formed at very high pressures and temperatures, in which the traditional water molecule bonds are shifted, allowing the hydrogen molecules to float freely in an oxygen lattice. This mobility makes the ice capable of conducting electricity almost as well as a metallic material.

Observations of hot, superionic ice created in the lab have led to contradictory results and there has been a great deal of disagreement about the exact conditions under which the new properties emerge.

“So, our research team, led by the University of Chicago’s Vitali Prakapenka, set out to use multiple spectroscopic tools to map changes in ice’s structure and properties under conditions ranging up to 1.5 million times normal atmospheric pressure and about 11,200 degrees Fahrenheit,” explained Carnegie’s Alexander Goncharov.

By doing this, the scientists—also including Nicholas Holtgrewe formerly of Carnegie, now at the Food and Drug Administration in St Louis, and Sergey Lobanov, formerly of Carnegie, now at the GFZ German Research Center for Geosciences—were able to pinpoint the emergence of two forms of superionic ice, one of which they suggest could be found in the interiors of ice giant planets Uranus and Neptune.

“In order to probe the structure of this unique state of matter under very extreme conditions—heated by a laser and compressed between two diamonds—we used the Advanced Photon Source’s brilliant high-energy synchrotron x-ray beam, which was focused down to about 3 micrometers, 30 times smaller than a single human hair,” said Prakapenka, explaining the work done using the facility’s GSECARS beamline. “These experiments are so challenging that we had to run a few thousand of them over a decade to get enough high-quality data to solve the long-standing mystery of high-pressure, high-temperature behavior of ice under conditions relevant to giant planet interiors.”

“Simulations have indicated that the magnetic fields of these two planets are generated in thin, fluid layers found at relatively shallow depths,” Goncharov added. “The conductivity of superionic ice would be able to accomplish this type of field generation and one of the two structures we revealed could exist under the conditions found in these magnetic field-generating zones.”

Further study is needed to understand the conductive properties and viscosity of these ice phases under ice giant-interior conditions.

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This work was supported by the U.S. National Science Foundation, the Army Research Office, the Deep Carbon Observatory, the Helmholtz Young Investigators Group, and the Carnegie Institution for Science. This work was performed at GeoSoilEnviroCARS, Advanced Photon Source, Argonne National Laboratory.

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Scientific Area:
Earth & Planetary Science

Reference to Person:
Alexander Goncharov

Reference to Department:
Earth and Planets Laborator

Source: Carnagie Science/News

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