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Hubble watches spoke season on Saturn

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Planet Saturn with bright white rings, multi-colored main sphere, and moons Mimas, Dione, and Enceladus. Spoke features on the left and right sides of the rings appear like faint grey smudges against the rings’ bright backdrop, about midway from the planet to the rings’ outer edge. Above the rings plane, the planet’s bands are shades of red, orange and yellow, with bright white nearer the equator.

Hubble watches spoke season on Saturn

Planet Saturn with bright white rings, multi-colored main sphere, and moons Mimas, Dione, and Enceladus. Spoke features on the left and right sides of the rings appear like faint grey smudges against the rings’ bright backdrop, about midway from the planet to the rings’ outer edge. Above the rings plane, the planet’s bands are shades of red, orange and yellow, with bright white nearer the equator.
This photo of Saturn was taken by the NASA/ESA Hubble Space Telescope on 22 October 2023, when the ringed planet was approximately 1365 million kilometres from Earth. Hubble’s ultra-sharp vision reveals a phenomenon called ring spokes.
Saturn’s spokes 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.
In 1981, NASA’s Voyager 2 first photographed the ring spokes. Hubble continues observing Saturn annually as the spokes come and go. This cycle has been captured by Hubble’s Outer Planets Atmospheres Legacy (OPAL) program that began nearly a decade ago to annually monitor weather changes on all four gas-giant outer planets.
Hubble’s crisp images show that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021 but only on the morning (left) side of the rings. Long-term monitoring shows that both the number and contrast of the spokes vary with Saturn’s seasons. Saturn is tilted on its axis like Earth and has seasons lasting approximately seven years.
This year, these ephemeral structures appear on both sides of the planet simultaneously as they spin around the giant world. Although they look small compared with Saturn, their length and width can stretch longer than Earth’s diameter!
The OPAL team notes that the leading theory is that spokes are tied to interactions between Saturn’s powerful magnetic field and the sun. Planetary scientists think that electrostatic forces generated from this interaction levitate dust or ice above the ring to form the spokes, though after several decades no theory perfectly predicts the spokes. Continued Hubble observations may eventually help solve the mystery.
Credit: Credit: NASA, ESA, STScI, A. Simon (NASA-GSFC)

This photo of Saturn was taken by the NASA/ESA Hubble Space Telescope on 22 October 2023, when the ringed planet was approximately 1365 million kilometres from Earth. Hubble’s ultra-sharp vision reveals a phenomenon called ring spokes.

Saturn’s spokes 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.

In 1981, NASA’s Voyager 2 first photographed the ring spokes. Hubble continues observing Saturn annually as the spokes come and go. This cycle has been captured by Hubble’s Outer Planets Atmospheres Legacy (OPAL) program that began nearly a decade ago to annually monitor weather changes on all four gas-giant outer planets.

Hubble’s crisp images show that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021 but only on the morning (left) side of the rings. Long-term monitoring shows that both the number and contrast of the spokes vary with Saturn’s seasons. Saturn is tilted on its axis like Earth and has seasons lasting approximately seven years.

This year, these ephemeral structures appear on both sides of the planet simultaneously as they spin around the giant world. Although they look small compared with Saturn, their length and width can stretch longer than Earth’s diameter!

The OPAL team notes that the leading theory is that spokes are tied to interactions between Saturn’s powerful magnetic field and the sun. Planetary scientists think that electrostatic forces generated from this interaction levitate dust or ice above the ring to form the spokes, though after several decades no theory perfectly predicts the spokes. Continued Hubble observations may eventually help solve the mystery. This image was created with Hubble data from proposal 16995 (A. Simon).

Planet Saturn with bright white rings, multi-colored main sphere, and moons Mimas, Dione, and Enceladus. Spoke features on the left and right sides of the rings appear like faint grey smudges against the rings’ bright backdrop, about midway from the planet to the rings’ outer edge. Above the rings plane, the planet’s bands are shades of red, orange and yellow, with bright white nearer the equator.
This photo of Saturn was taken by the NASA/ESA Hubble Space Telescope on 22 October 2023, when the ringed planet was approximately 1365 million kilometres from Earth. Hubble’s ultra-sharp vision reveals a phenomenon called ring spokes.
Saturn’s spokes 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.
In 1981, NASA’s Voyager 2 first photographed the ring spokes. Hubble continues observing Saturn annually as the spokes come and go. This cycle has been captured by Hubble’s Outer Planets Atmospheres Legacy (OPAL) program that began nearly a decade ago to annually monitor weather changes on all four gas-giant outer planets.
Hubble’s crisp images show that the frequency of spoke apparitions is seasonally driven, first appearing in OPAL data in 2021 but only on the morning (left) side of the rings. Long-term monitoring shows that both the number and contrast of the spokes vary with Saturn’s seasons. Saturn is tilted on its axis like Earth and has seasons lasting approximately seven years.
This year, these ephemeral structures appear on both sides of the planet simultaneously as they spin around the giant world. Although they look small compared with Saturn, their length and width can stretch longer than Earth’s diameter!
The OPAL team notes that the leading theory is that spokes are tied to interactions between Saturn’s powerful magnetic field and the sun. Planetary scientists think that electrostatic forces generated from this interaction levitate dust or ice above the ring to form the spokes, though after several decades no theory perfectly predicts the spokes. Continued Hubble observations may eventually help solve the mystery.
Credit: NASA, ESA, STScI, A. Simon (NASA-GSFC)

 

Press release from ESA Hubble.

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JWST adds another ringed world with new image of Uranus

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Zoomed-in image of Uranus ringed atmosphere Webb JWST

The James Webb Space Telescope adds another ringed world with new image of Uranus

Webb’s infrared image highlights the planet’s dramatic rings and dynamic atmosphere.

 

Following in the footsteps of the Neptune image released in 2022, the NASA/ESA/CSA 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 demonstrate the observatory’s unprecedented sensitivity by revealing 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 a nearly 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 at the northern pole, which is visible here; Uranus’s 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, shown here in blue and orange, respectively. The planet displays a blue hue in the resulting representative-colour image.

When Voyager 2 looked at Uranus, its camera saw an almost featureless blue-green ball at visible wavelengths. At infrared wavelengths, and with Webb’s greater sensitivity, we see more detail, showing how dynamic the atmosphere of Uranus really is.

On the right side of the planet is 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 vanishes in the autumn; these Webb data will help scientists understand the currently mysterious mechanism behind this feature. Webb has revealed a surprising aspect of the polar cap: a subtle enhanced brightening at the centre of the cap. The sensitivity of Webb’s NIRCam and the longer wavelengths it can see may explain why we can see this enhanced polar feature of Uranus when it has not been seen with other powerful telescopes like the NASA/ESA Hubble Space Telescope and the Keck Observatory.

At the edge of the polar cap lies a bright cloud and a few fainter extended features can be seen just beyond the cap’s edge; a second very bright cloud is seen at the planet’s left limb. Such clouds are typical for Uranus at infrared wavelengths, and are likely connected to storm activity.

This planet is characterised as an ice giant because of 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 as seen by 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.

 

Press release from ESA JWST

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Saturn’s hexagon: the most extensive system of haze layers in the solar system

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Saturn hexagon

The most extensive system of haze layers ever observed in the solar system have been discovered and characterised on the planet Saturn

High-resolution images obtained by the Cassini spacecraft were used for this purpose by the Planetary Science Group at the University of the Basque Country

Saturn hexagon
High-resolution images of Saturn’s Hexagon obtained by the Cassini spacecraft. Credits: UPV/EHU

 

A rich variety of meteorological phenomena take place in the extensive hydrogen atmosphere of the planet Saturn, a world about ten times the size of the Earth. They help us to better understand those that operate in a similar way in the Earth’s atmosphere.  Featuring among them due to its uniqueness is the well-known “hexagon”, an amazing wave structure that surrounds the planet’s polar region and whose shape looks as if it had been drawn by a geometrician.

Discovered in 1980 by NASA’s Voyager 1 and 2 spacecraft, it has been observed without interruption since then, despite the planet’s long, strong cycle of seasons. A fast, narrow jet stream flows inside this gigantic planetary wave where winds reach maximum speeds of about 400 km/h. Yet, strangely enough, the wave itself remains almost static; in other words, it barely shifts with respect to the planet’s rotation. All these properties mean that the “hexagon” is a highly attractive phenomenon for meteorologists and planet atmosphere researchers.

Cassini, which was in orbit around the planet between 2004 and 2017, took a vast quantity of images from a whole range of distances from the planet and viewing angles. In June 2015 its main camera obtained very high-resolution images of the planet’s limb which are capable of solving details of between 1 and 2 km; they captured the hazes located above the clouds that shape the hexagonal wave. In addition, it used many colour filters, from ultraviolet to near infrared, thus enabling the composition of these hazes to be studied. To complete this study, images produced by the Hubble Space Telescope taken 15 days later and showing the hexagon not on the limb but seen from above were also used. “The Cassini images have enabled us to discover that, just as if a sandwich had been formed, the hexagon has a multi-layered system of at least seven mists that extend from the summit of its clouds to an altitude of more than 300 km above them,” said Professor Agustín Sánchez-Lavega, who led the study.  “Other cold worlds, such as Saturn’s satellite Titan or the dwarf planet Pluto, also have layers of hazes, but not in such numbers nor as regularly spaced out”.

The vertical extent of each haze layer is between approximately 7 and 18 km thick, and according to the spectral analysis, they contain minute particles with radii of the order of 1 micron. Their chemical composition is exotic for us, because, owing to the low temperatures in Saturn’s atmosphere ranging between 120° C and 180° C below zero, they could comprise hydrocarbon ice crystallites, such as acetylene, propyne, propane, diacetylene or even butane in the case of the highest clouds.

Another aspect studied by the team is the regularity in the vertical distribution of the hazes. The hypothesis put forward is that the hazes are organised by the vertical propagation of gravity waves that produce oscillations in the density and temperature of the atmosphere, a well-known phenomenon on the Earth and on other planets. The researchers raise the possibility that it could be the very dynamics of the hexagon itself and its powerful jet stream that may be responsible for the formation of these gravity waves. On the Earth, too, waves of this type produced by the undulating jet stream travelling at speeds of 100 km/h from West to East in the mid-latitudes have been observed. The phenomenon could be similar on both planets, even though the peculiarities of Saturn mean that it is the only case in the solar system. This is an aspect that remains subject to future research.

Saturn's hexagon
Santiago Pérez-Hoyos, Agustín Sánchez-Lavega, Teresa del Río-Gaztelurrutia and Ricardo Hueso. Credits: UPV/EHU

About the authors at the UPV/EHU  

Agustín Sánchez-Lavega is professor of physics at the UPV/EHU-University of the Basque Country, head of the GCP-Planetary Science Group and holder of the 2016 Euskadi Award for Research.  Teresa del Río-Gaztelurrutia and Ricardo Hueso are tenured lecturers, and Santiago Pérez-Hoyos is a permanent research doctor; they all belong to the GCP.

bibliographic reference

 

Press release on Saturn’s hexagon from the University of the Basque Country.

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