Interstellar dust at Saturn

 Interstellar dust at Saturn

Copyright: ESA; dust grain inset: NASA/JPL; Saturn image: NASA/JPL/Space Science Institute

 Hi-res image

Local interstellar cloud

The local interstellar cloud is an almost empty bubble of gas and dust that our Solar System is travelling through with a distinct direction and speed.

Copyright: ESA

The international Cassini spacecraft has detected the faint but distinct signature of dust coming from outside our Solar System.

Cassini has been flying around the Saturnian system for 12 years, studying the giant planet and its rings and satellites. It has also found millions of ice-rich dust grains with its Cosmic Dust Analyser, the vast majority of which are from icy satellite Enceladus and which make up one of Saturn’s outer rings.

Amongst the grains detected, 36 stick out from the crowd – and scientists conclude they came from beyond our Solar System.

Alien dust in the Solar System is not entirely unexpected. In the 1990s, the ESA/NASA Ulysses mission made the first in-situ discovery of interstellar dust, later confirmed by NASA’s Galileo spacecraft.

The dust was traced back to the local interstellar cloud: an almost empty bubble of gas and dust we are travelling through with a distinct direction and speed.

“From that discovery, we always hoped we would be able detect these interstellar interlopers at Saturn with Cassini: we knew that if we looked in the right direction, we should find them,” says Nicolas Altobelli, ESA’s Cassini project scientist and lead author of the study reporting the results in Science.

“And indeed, on average, we have captured a few per year, travelling at high speed and on a specific path quite different to that of the usual icy grains we collect around Saturn.”

The tiny dust grains were speeding through at over 72 000 km/h, fast enough to avoid being trapped inside the Solar System by Saturn’s – or even the Sun’s – gravity.

Importantly, unlike Ulysses and Galileo, Cassini analysed the composition of the dust for the first time, showing them to be made of a very specific mixture of minerals, not ice.

They all had a surprisingly similar chemical make-up, containing major rock-forming elements like magnesium, silicon, iron and calcium in average cosmic proportions. Conversely, more reactive elements like sulphur and carbon were found to be less abundant compared to the average.

“Cosmic dust is produced when stars die, but with the vast range of types of stars in the Universe we naturally expected to encounter a huge range of dust types over the long period of our study,” says Frank

Postberg, co-author on the paper and co-investigator of Cassini’s dust analyser, of the University of Heidelberg.

“Surprisingly, the grains we’ve detected aren’t old, pristine and compositionally diverse like the stardust grains we find in ancient meteorites,” says Mario Trieloff, a co-author also at the University of Heidelberg.

“They have apparently been made rather uniform through some repetitive processing in the interstellar medium.”

The team speculate that dust in a star-forming region could be destroyed and recondense multiple times as the shockwaves from dying stars passed through, before the resulting similar grains ended up streaming towards our Solar System.

“The long duration of the Cassini mission has enabled us to use it like a micrometeorite observatory, providing us privileged access to the contribution of dust from outside our Solar System that could not have been obtained in any other way,” adds Nicolas.

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Notes for editors 
 

“Flux and composition of interstellar dust at Saturn from Cassini’s Cosmic Dust Analyzer,” by N. Altobelli et al, is published in Science.

The Cassini-Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate in Washington.

The Cosmic Dust Analyser is supported by the German Aerospace Center (DLR); the instrument is managed by the University of Stuttgart, Germany.

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For more information, please contact:

Nicolas Altobelli




ESA Cassini–Huygens Project Scientist



Tel: +34 91 813 1201




Email: nicolas.altobelli@esa.int 

Frank Postberg
Institut für Geowissenschaften, University of Heidelberg, Germany
Email: Frank.Postberg@geow.uni-heidelberg.de

Mario Trieloff
Institut für Geowissenschaften, University of Heidelberg, Germany
Email: Mario.Trieloff@geow.uni-heidelberg.de

Markus Bauer 




ESA Science Communication Officer




Tel: +31 71 565 6799




Mob: +31 61 594 3 954




Email: markus.bauer@esa.int

Source: ESA/ Cassini-Huygens

View over an alien world

View over an alien world

Copyright: ESA/NASA/JPL/University of Arizona

Hi-res JPG  (768.25 kB) - Hi-res JPG (4.20 MB)

At first glance, this scene may look like a reptilian eye or a textured splash of orange paint, but it is actually a fish-eye view of Saturn’s moon Titan. It was acquired at a height of about 5 km as ESA’s Huygens probe, part of the international Cassini–Huygens mission, descended through Titan’s atmosphere before landing.

In the late afternoon of 14 January 2005, engineers and scientists at ESA’s ESOC operations centre in Darmstadt, Germany, waited anxiously for data to arrive from Huygens, which touched down on Titan at around 12:34 GMT – the most distant landing of any craft.

Following its release from NASA’s Cassini on 25 December, Huygens reached Titan’s outer atmosphere after 20 days and a 4 million km cruise. The probe started its descent through Titan’s hazy cloud layers from an altitude of about 1270 km at 10:13 GMT. During the following three minutes Huygens decelerated from 18 000 km/h to 1400 km/h.

A sequence of parachutes then slowed it down to less than 300 km/h. At a height of about 160 km the probe’s scientific instruments were exposed to Titan’s atmosphere. Around 120 km, the main parachute was replaced by a smaller one to complete the descent.

The probe began transmitting data to Cassini four minutes into its descent and continued to transmit after landing at least as long as Cassini was above Titan’s horizon. The signals, relayed by Cassini, were picked up by NASA’s Deep Space Network and delivered immediately to ESOC. The first science data arrived at 16:19 GMT.

Huygens was humankind’s first attempt to land a probe on another world in the outer Solar System. “This is a great achievement for Europe and its US partners in this ambitious international endeavour to explore Saturn system,” said Jean-Jacques Dordain, then ESA’s Director General.

This image is a stereographic (fish-eye) projection taken with the descent imager/spectral radiometer on Huygens.

More information and a high-res TIFF version of the image is available at the NASA JPL website.

ESA Operations - ESOC

Source: ESA/Space Images

Icy Tendrils Reaching into Saturn Ring Traced to Their Source

This collage, consisting of two Cassini images of long, sinuous, tendril-like features from Saturn's moon Enceladus and two corresponding computer simulations of the same, illustrates how well the structures, and the sizes of the particles composing them, can be modeled by tracing the trajectories of tiny, icy grains ejected from Enceladus' south polar geysers. Credit: NASA/JPL-Caltech/Space Science Institute.  › Full image and caption

This graphic plots the source locations of the geysers scientists have located on Enceladus' south polar terrain. Credit: NASA/JPL-Caltech/Space Science Institute.  › Full image and caption

Long, sinuous, tendril-like structures seen in the vicinity of Saturn's icy moon Enceladus originate directly from geysers erupting from its surface, according to scientists studying images from NASA's Cassini spacecraft.

This result is published online today in a study in the Astronomical Journal, along with additional insights into the nature of the structures.

"We've been able to show that each unique tendril structure can be reproduced by particular sets of geysers on the moon's surface," said Colin Mitchell, a Cassini imaging team associate at the Space Science Institute in Boulder, Colorado, and lead author of the paper. Mitchell and colleagues used computer simulations to follow the trajectories of ice grains ejected from individual geysers. The geysers, which were discovered by Cassini in 2005, are jets of tiny water ice particles, water vapor and simple organic compounds. 

Under certain lighting conditions, Cassini's wide-view images showing icy material erupting from Enceladus reveal faint, finger-like features, dubbed "tendrils" by the imaging team. The tendrils reach into Saturn's E ring -- the ring in which Enceladus orbits -- extending tens of thousands of miles (or kilometers) away from the moon. Since the tendrils were discovered, scientists have thought they were the result of the moon's geysering activity and the means by which Enceladus supplies material to the E ring. But the ghostly features had never before been traced directly to geysers on the surface.

Because the team was able to show that tendril structures of different shapes correspond to different sizes of geyser particles, the team was able to zero in on the sizes of the particles forming them. They found the tendrils are composed of particles with diameters no smaller than about a hundred thousandth of an inch, a size consistent with the measurements of E-ring particles made by other Cassini instruments.

As the researchers examined images from different times and positions around Saturn, they also found that the detailed appearance of the tendrils changes over time. "It became clear to us that some features disappeared from one image to the next," said John Weiss, an imaging team associate at Saint Martin's University in Lacey, Washington, and an author on the paper. 

The authors suspect that changes in the tendrils' appearance likely result from the cycle of tidal stresses -- squeezing and stretching of the moon as it orbits Saturn -- and its control of the widths of fractures from which the geysers erupt. The stronger the tidal stresses raised by Saturn at any point on the fractures, the wider the fracture opening and the greater the eruption of material. The authors will investigate in future work whether this theory explains the tendrils' changing appearance. 

There is even more that can be extracted from the images, the scientists say. "As the supply lanes for Saturn's E ring, the tendrils give us a way to ascertain how much mass is leaving Enceladus and making its way into Saturn orbit," said Carolyn Porco, team leader for the imaging experiment and a coauthor on the paper. "So, another important step is to determine how much mass is involved, and thus estimate how much longer the moon's sub-surface ocean may last." An estimate of the lifetime of the ocean is important in understanding the evolution of Enceladus over long timescales.

Because of its significance to the investigation of possible extraterrestrial habitable zones, Enceladus is a major target of investigation for the final years of the Cassini mission. Many observations, including imaging of the plume and tendril features, and thermal observations of the surface of its south polar geyser basin, are planned during the next couple of years.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory in Pasadena, California, manages the mission for the agency's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena. The Cassini imaging operations center is based at the Space Science Institute in Boulder, Colo.

New images released today can be found at: http://www.ciclops.org/view_event/205

 

More information about Cassini, visit: http://www.nasa.gov/cassini - http://saturn.jpl.nasa.gov

Media Contact

Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches@jpl.nasa.gov

Steve Mullins
Space Science Institute, Boulder, Colo.
720-974-5859
media@ciclops.org

Source: JPL-Caltech/News

Cassini Catches Titan Naked in the Solar Wind

This diagram depicts conditions observed by NASA's Cassini spacecraft during a flyby in Dec. 2013, when Saturn's magnetosphere was highly compressed, exposing Titan to the full force of the solar wind. Image credit: NASA/JPL-Caltech.   › Full image and caption

Researchers studying data from NASA's Cassini mission have observed that Saturn's largest moon, Titan, behaves much like Venus, Mars or a comet when exposed to the raw power of the solar wind. The observations suggest that unmagnetized bodies like Titan might interact with the solar wind in the same basic ways, regardless of their nature or distance from the sun.

Titan is large enough that it could be considered a planet if it orbited the sun on its own, and a flyby of the giant moon in Dec. 2013 simulated that scenario, from Cassini's vantage point. The encounter was unique within Cassini's mission, as it was the only time the spacecraft has observed Titan in a pristine state, outside the region of space dominated by Saturn's magnetic field, called its magnetosphere. 

"We observed that Titan interacts with the solar wind very much like Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of the Institute of Astronomy and Space Physics in Buenos Aires, who led the research with colleagues from the Cassini mission. "We thought Titan in this state would look different. We certainly were surprised," he said.

The solar wind is a fast-flowing gale of charged particles that continually streams outward from the sun, flowing around the planets like islands in a river. Studying the effects of the solar wind at other planets helps scientists understand how the sun's activity affects their atmospheres. These effects can include modification of an atmosphere's chemistry as well as its gradual loss to space.

Titan spends about 95 percent of the time within Saturn's magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles. 

Using its magnetometer instrument, which is akin to an equisitely sensitive compass, Cassini has observed Titan many times during the mission's decade in the Saturn system, but always within Saturn's magnetosphere. The spacecraft has not been able to detect a magnetic field coming from Titan itself. In its usual state, Titan is cloaked in Saturn's magnetic field.

This time the influence of Saturn was not present, allowing Cassini's magnetometer to observe Titan as it interacted directly with the solar wind. The special circumstance allowed Bertucci and colleagues to study the shockwave that formed around Titan where the full-force solar wind rammed into the moon's atmosphere.

At Earth, our planet's powerful magnetic field acts as a shield against the solar wind, helping to protect our atmosphere from being stripped away. In the case of Venus, Mars and comets -- none of which is protected by a global magnetic field -- the solar wind drapes around the objects themselves, interacting directly with their atmospheres (or in the comet's case, its coma). Cassini saw the same thing at Titan. 

Researchers thought they would have to treat Titan's response to the solar wind with a unique approach because the chemistry of the hazy moon's dense atmosphere is highly complex. But Cassini's observations of a naked Titan hinted at a more elegant solution. "This could mean we can use the same tools to study how vastly different worlds, in different parts of the solar system, interact with the wind from the sun," Bertucci said.

Bertucci noted that the list of similarly unmagnetized bodies might include the dwarf planet Pluto, to be visited this year for the first time by NASA's New Horizons spacecraft.

"After nearly a decade in orbit, the Cassini mission has revealed once again that the Saturn system is full of surprises," said Michele Dougherty, principal investigator of the Cassini magnetometer at Imperial College, London. "After more than a hundred flybys, we have finally encountered Titan out in the solar wind, which will allow us to better understand how such moons maintain or lose their atmospheres."

The new research is published today in the journal Geophysical Review Letters.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The magnetometer team is based at Imperial College, London, U.K. 

More information about Cassini:  http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov

Media Contact

Preston Dyches
NASA's Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston.dyches@jpl.nasa.gov

Source: JPL-Caltech/News

 

Cassini’s view of Jupiter’s southern hemisphere

Cassini’s view of Jupiter’s southern hemisphere

Copyright: NASA/JPL/Space Science Institute

This Cassini image shows Jupiter from an unusual perspective. If you were to float just beneath the giant planet and look directly up, you would be greeted with this striking sight: red, bronze and white bands encircling a hazy south pole. The multicoloured concentric layers are broken in places by prominent weather systems such as Jupiter’s famous Great Red Spot, visible towards the upper left, chaotic patches of cloud and pale white dots. Many of these lighter patches contain lightning-filled thunderstorms.

Jupiter has very dramatic weather – the planet’s axis is not as tilted (towards or away from the Sun) as much as Earth’s so it does not have significant seasonal changes, but it does have a thick and tumultuous atmosphere filled with raging storms and chaotic cloud systems.

These clouds, formed from dense layers of ammonia crystals, are tugged, stretched and tangled together by Jupiter’s turbulence and strong winds, creating vortices and hurricane-like storms with wind speeds of up to 360 km per hour.

The Great Red Spot is actually an anticyclone that has been violently churning for hundreds of years. It was at one stage large enough to contain several Earth-sized planets but recent images from the Hubble Space Telescope show it to be shrinking. There are other similarly striking storms raging in both Jupiter’s cool upper atmosphere and hotter lower layers, including a Great Dark Spot and Oval BA, more affectionately nicknamed Red Spot Jr.

Jupiter’s south pole is at the very centre of this image, visible as a murky grey-toned circle. This patch is not as detailed as the rest of the planet because Cassini had to peer through a lot more atmospheric haze in the polar region, making it harder to see.

This polar map is composed of 18 colour images taken by the narrow-angle camera on NASA’s Cassini spacecraft during a flyby on 11–12 December 2000. This map is incredibly detailed; the smallest visible features in this image are about 120 km across. There is also an accompanying map of the planet’s north pole. In 2016, NASA’s Juno spacecraft will arrive at Jupiter and start to beam back images of the planet’s poles.

The Cassini–Huygens mission, launched in 1997 as a joint endeavour of ESA, NASA and Italy’s ASI space agency, flew past Venus, Earth and Jupiter en route to observe Saturn, its moons and rings. Observations with Cassini have given us an unprecedented understanding of the Saturnian system. ESA’s Juice mission aims to do the same for Jupiter. Planned for launch in 2022, the spacecraft will reach Jupiter in 2030 and begin observing the planet and three of its moons – Ganymede, Callisto and Europa. Previous flybys of these moons have raised the exciting prospect that some of them might harbour subsurface liquid oceans and conditions suitable to support some forms of life.

Juice was recently given the green light to continue to the next stage of development.

Source: ESA

NASA Identifies Ice Cloud Above Cruising Altitude on Titan

This cloud in the stratosphere over Titan’s north pole (left) is similar to Earth’s polar stratospheric clouds (right). NASA scientists found that Titan’s cloud contains methane ice, which was not previously thought to form in that part of the atmosphere. Cassini first spotted the cloud in 2006. Image Credit:  L. NASA/JPL/U. of Ariz./LPGNantes; R. NASA/GSFC/M. Schoeberl

NASA scientists have identified an unexpected high-altitude methane ice cloud on Saturn's moon Titan that is similar to exotic clouds found far above Earth's poles.

This lofty cloud, imaged by NASA's Cassini spacecraft, was part of the winter cap of condensation over Titan's north pole. Now, eight years after spotting this mysterious bit of atmospheric fluff, researchers have determined that it contains methane ice, which produces a much denser cloud than the ethane ice previously identified there.

"The idea that methane clouds could form this high on Titan is completely new," said Carrie Anderson, a Cassini participating scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. "Nobody considered that possible before."

Methane clouds were already known to exist in Titan's troposphere, the lowest layer of the atmosphere. Like rain and snow clouds on Earth, those clouds form through a cycle of evaporation and condensation, with vapor rising from the surface, encountering cooler and cooler temperatures and falling back down as precipitation. On Titan, however, the vapor at work is methane instead of water.

The newly identified cloud instead developed in the stratosphere, the layer above the troposphere. Earth has its own polar stratospheric clouds, which typically form above the North Pole and South Pole between 49,000 and 82,000 feet (15 to 25 kilometers) -- well above cruising altitude for airplanes. These rare clouds don't form until the temperature drops to minus 108 degrees Fahrenheit (minus 78 degrees Celsius).

Other stratospheric clouds had been identified on Titan already, including a very thin, diffuse cloud of ethane, a chemical formed after methane breaks down. Delicate clouds made from cyanoacetylene and hydrogen cyanide, which form from reactions of methane byproducts with nitrogen molecules, also have been found there.

But methane clouds were thought unlikely in Titan's stratosphere. Because the troposphere traps most of the moisture, stratospheric clouds require extreme cold. Even the stratosphere temperature of minus 333 degrees Fahrenheit (minus 203 degrees Celsius), observed by Cassini just south of the equator, was not frigid enough to allow the scant methane in this region of the atmosphere to condense into ice.

What Anderson and her Goddard co-author, Robert Samuelson, noted is that temperatures in Titan's lower stratosphere are not the same at all latitudes. Data from Cassini's Composite Infrared Spectrometer and the spacecraft's radio science instrument showed that the high-altitude temperature near the north pole was much colder than that just south of the equator.

It turns out that this temperature difference -- as much as 11 degrees Fahrenheit (minus 12 degrees Celsius) -- is more than enough to yield methane ice.

Other factors support the methane identification. Initial observations of the cloud system were consistent with small particles composed of ethane ice. Later observations revealed some regions to be clumpier and denser, suggesting that more than one ice could be present. The team confirmed that the larger particles are the right size for methane ice and that the expected amount of methane -- one-and-a-half percent, which is enough to form ice particles -- is present in the lower polar stratosphere.

The mechanism for forming these high-altitude clouds appears to be different from what happens in the troposphere. Titan has a global circulation pattern in which warm air in the summer hemisphere wells up from the surface and enters the stratosphere, slowly making its way to the winter pole. There, the air mass sinks back down, cooling as it descends, which allows the stratospheric methane clouds to form.

"Cassini has been steadily gathering evidence of this global circulation pattern, and the identification of this new methane cloud is another strong indicator that the process works the way we think it does," said Michael Flasar, Goddard scientist and principal investigator for Cassini's Composite Infrared Spectrometer (CIRS).

Like Earth's stratospheric clouds, this methane cloud was located near the winter pole, above 65 degrees north latitude. Anderson and Samuelson estimate that this type of cloud system -- which they call subsidence-induced methane clouds, or SIMCs for short -- could develop between 98,000 to 164,000 feet (30 to 50 kilometers) in altitude above Titan's surface.

"Titan continues to amaze with natural processes similar to those on the Earth, yet involving materials different from our familiar water," said Scott Edgington, Cassini deputy project scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "As we approach southern winter solstice on Titan, we will further explore how these cloud formation processes might vary with season."
The results of this study are available online in the journal Icarus.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, Pasadena, manages the mission for NASA's Science Mission Directorate in Washington. The CIRS team is based at Goddard. The radio science team is based at JPL.

More information about Cassini is available at the following sites: http://www.nasa.gov/cassini  -  http://saturn.jpl.nasa.gov

Elizabeth Zubritsky
NASA's Goddard Space Flight Center, Greenbelt, Maryland
301-614-5438
elizabeth.a.zubritsky@nasa.gov

Preston Dyches
NASA's Jet Propulsion Laboratory, Pasadena, California
818-354-7013
preston.dyches@jpl.nasa.gov

Source:  NASA's  - Cassini-Huygens Mission

Titan's swirling polar cloud is cold and toxic

Spectral map of Titan

A map of Titan, the largest moon of Saturn, obtained with the Cassini Visual and Infrared Mapping Spectrometer (VIMS). The insert shows a natural-colour view (also from Cassini) of the southern polar vortex, a huge swirling cloud that was first observed in 2012.

Three components are evident in the VIMS map, and they are shown in different colours according to the different characteristics of their light. These are the surface of Titan (shown in orange), the atmospheric haze along the limb (green) and the polar vortex (blue).

When observed with VIMS, the southern polar vortex shows a remarkable difference with respect to other portions of Titan’s atmosphere: a signature of frozen hydrogen cyanide molecules (HCN). This discovery suggests that the atmosphere of Titan’s southern hemisphere is cooling much faster than expected.

The VIMS infrared image was processed by Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research. Copyright: NASA/JPL-Caltech/ASI/University of Arizona/SSI/Leiden Observatory & SRON

The international Cassini mission has revealed that a giant, toxic cloud is hovering over the south pole of Saturn’s largest moon, Titan, after the atmosphere has cooled in a dramatic fashion. 

Scientists analysing data from the mission found that this giant polar vortex contains frozen particles of the toxic compound hydrogen cyanide. 

“The discovery suggests that the atmosphere of Titan’s southern hemisphere is cooling much faster than we expected,” says Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research, lead author of the study published in the journal Nature. 

Unlike any other moon in the Solar System, Titan is shrouded by a dense atmosphere dominated by nitrogen, with small amounts of methane and other trace gases. Almost 10 times further from the Sun than Earth, Titan is very cold, allowing methane and other hydrocarbons to rain onto its surface to form rivers and lakes.

Like Earth, Titan experiences seasons as it makes its 29-year orbit around the Sun along with Saturn. Each of the four seasons lasts about seven Earth years and the most recent seasonal switch occurred in 2009, when summer transitioned to autumn in the southern hemisphere. 

In May 2012, images from Cassini revealed a huge swirling cloud, several hundred kilometres across, taking shape at the south pole.  

Titan’s changing seasons

Artist’s impression of the change in observed atmospheric effects before, during and after equinox in 2009. The Titan globes also provide an impression of the detached haze layer that extends all around the moon (blue). 

During the first years of Cassini’s exploration of the Saturnian system, Titan sported a ‘hood’ of dense gaseous haze (white) in a vortex above its north pole, along with a high-altitude ‘hot spot’ (red). During this time the north pole was pointed away from the Sun. 

At equinox both hemispheres received equal heating from the Sun. Afterwards, the north pole tilted towards the Sun, signalling the arrival of spring, while the southern hemisphere tilted away from the Sun and moved into autumn. 

After equinox and until 2011 there was still a significant build up of trace gases over the north pole, but the vortex and hot spot had almost disappeared. Instead, similar features began developing at the south pole, which are still present today. 

These observations are interpreted as a large-scale reversal in the single pole-to-pole atmospheric circulation cell of Titan immediately after equinox, with an upwelling of gases in the summer hemisphere and a corresponding downwelling in the winter hemisphere.  Copyright: ESA/AOES

This polar vortex appears to be an effect of the change of season, with large amounts of air being heated by sunlight during the northern spring and flowing towards the southern hemisphere. 

A puzzling detail about this swirling cloud is its altitude, some 300 km above Titan's surface, where scientists thought it was too warm for clouds to form. 

“We really didn’t expect to see such a massive cloud so high in the atmosphere,” says Dr de Kok. 

Keen to understand what could give rise to this mysterious cloud, the scientists turned to the rich data from Cassini. After careful scrutiny, they found an important clue in the spectrum of sunlight reflected by Titan’s atmosphere. 

A spectrum splits the light from a celestial body into its constituent colours, revealing signatures of the elements and molecules that are present. The Visual and Infrared Mapping Spectrometer on Cassini takes spectra at many different points on Titan, mapping the distribution of the chemical compounds in its atmosphere and on its surface. 

“The light coming from the polar vortex showed a remarkable difference with respect to other portions of Titan’s atmosphere,” says Dr de Kok. “We could clearly see a signature of frozen hydrogen cyanide molecules – HCN.” 

Vortex on Titan close up

A true-colour image of the south pole vortex observed in Titan’s atmosphere at about 200–300 km altitude, as seen during a Cassini flyby of Saturn’s largest moon on 27 June 2012. Since equinox in August 2009, the seasons have been changing, becoming spring in the northern hemisphere and autumn in the southern hemisphere. The formation of the vortex over the south pole indicates the effect of the changing seasons on the circulation pattern in Titan’s atmosphere, specifically with cooler air sinking down from warmer, high altitudes. 

The images were obtained with the Cassini spacecraft narrow-angle camera at a distance of approximately 484,000 kilometres from Titan. Copyright: NASA/JPL–Caltech/Space Science Institute

As a gas, HCN is one of the molecules present in small amounts in the nitrogen-rich atmosphere of Titan. However, finding these molecules in the form of ice was very surprising, as HCN can condense to form frozen particles only if the atmosphere is as cold as –148ºC. 

“This is about 100ºC colder than predictions from current theoretical models of Titan’s upper atmosphere,” explains co-author Nick Teanby from the University of Bristol, UK. 

“To check whether such low temperatures were actually possible, we investigated a second set of observations from Cassini’s Composite Infrared Spectrometer, which allows us to measure atmospheric temperature at different altitudes.” 

Unfortunately, no such readings were taken in 2012 at this cloud’s altitude, but the scientists looked at data from other dates, probing the atmosphere above and below the vortex. 

These data showed that the southern hemisphere has been cooling rapidly, making it possible to reach the low temperature needed to form the giant toxic cloud seen on the south pole. 

This fast cooling of the southern atmosphere may be a consequence of the atmospheric circulation, which has been drawing large masses of gas towards the south ever since the change of season in 2009. As the HCN gas becomes more concentrated, its molecules shine brightly at infrared wavelengths, cooling the surrounding air in the process. 

Another factor contributing to this cooling is the reduced exposure to sunlight on Titan’s southern hemisphere.

“This surprising result shows how much we are still learning about Titan’s weather and the complex dynamics of its atmosphere,” says Nicolas Altobelli, Cassini–Huygens Project Scientist at ESA. “We can look forward to more fascinating discoveries from Cassini in the next few years, as it continues to monitor the seasonal changes on Saturn and its moons.” 

Notes for Editors

“HCN ice in Titan’s high-altitude southern polar cloud,” by R. J. de Kok et al. is published in the journal Nature on 2 October 2014; doi: 10.1038/nature13789 

The results are reported by R.J. de Kok, Leiden Observatory and SRON Netherlands Institute for Space Research, The Netherlands; N.A. Teanby, University of Bristol, UK; L. Maltagliati and S. Vinatier, LESIA-Observatoire de Paris, CNRS, UPMC Université Paris 06, Université Paris-Diderot, France; and P.G.J. Irwin, University of Oxford, UK. 

The Cassini–Huygens mission is a cooperative project of NASA, ESA and Italy’s ASI space agency. Launched in 1997, Cassini arrived in the Saturn system in 2004 and is studying the ringed planet and its moons. The Huygens probe was released from the main spacecraft and, in 2005, parachuted through the atmosphere to the surface of Saturn’s largest moon, Titan. 

Cassini’s initial four-year mission to explore the Saturn System covered the period July 2004 to June 2008, when Saturn and its moons were experiencing northern winter and southern summer. The first extended mission, called the ‘Cassini Equinox Mission’, was completed in September 2010. This included the spring equinox, on 11 August 2009, when winter was followed by spring in the northern hemisphere and summer was followed by autumn in the southern hemisphere.

A second extended mission, the ‘Cassini Solstice Mission’, will continue until September 2017. This will allow scientists to study the Saturnian system until after the next seasonal change, the summer solstice in May 2017, which will mean the arrival of northern summer and southern winter.

NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA’s Science Mission Directorate, Washington, D.C., USA.

For further information, please contact:

Markus Bauer 

ESA Science and Robotic Exploration Communication Officer
Phone: +31 71 565 6799

Mobile: +31 61 594 3 954

Email: markus.bauer@esa.int

Nicolas Altobelli

ESA Cassini–Huygens Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34 91 813 1201
Email: nicolas.altobelli@sciops.esa.int

Remco J. de Kok
Leiden Observatory
Leiden, The Netherlands
and SRON Netherlands Institute for Space Research
Utrecht, The Netherlands
Phone: +31 88 7775725
Email: R.J.de.Kok@sron.nl

Nick Teanby
University of Bristol, UK
Phone: +44 117 3315006
Email: N.Teanby@bristol.ac.uk

Source: ESA- Cassini-Huygens

Bright Clumps in Saturn Ring Now Mysteriously Scarce

Cassini spied just as many regular, faint clumps in Saturn's narrow F ring, like those pictured here, as Voyager did, but it saw hardly any of the long, bright clumps that were common in Voyager images. Image credit: NASA/JPL-Caltech/SSI.  Full image and caption

A map of Saturn's F ring from 2006 shows one of the few bright, extended clumps (indicated by a green box) seen during six years of observation by Cassini. Image credit: NASA/JPL-Caltech/SSI.  Hi-res image  -  enlarge image

Compared to the age of the solar system -- about four-and-a-half billion years -- a couple of decades are next to nothing. Some planetary locales change little over many millions of years, so for scientists who study the planets, any object that evolves on such a short interval makes for a tempting target for study. And so it is with the ever-changing rings of Saturn.

Case in point: Saturn's narrow, chaotic and clumpy F ring. A recent NASA-funded study compared the F ring's appearance in six years of observations by the Cassini mission to its appearance during the Saturn flybys of NASA's Voyager mission, 30 years earlier. The study team found that, while the overall number of clumps in the F ring remained the same, the number of exceptionally bright clumps of material plummeted during that time. While the Voyagers saw two or three bright clumps in any given observation, Cassini spied only two of the features during a six-year period. What physical processes, they wondered, could cause only the brightest of these features to decline sharply?

While a variety of features in Saturn's many rings display marked changes over multiple years, the F ring seems to change on a scale of days, and even hours. Trying to work out what is responsible for the ring's tumultuous behavior is a major goal for ring scientists working on Cassini.

"Saturn's F ring looks fundamentally different from the time of Voyager to the Cassini era," said Robert French of the SETI Institute in Mountain View, California, who led the study along with SETI Principal Investigator Mark Showalter. "It makes for an irresistible mystery for us to investigate."

The researchers hypothesize that the brightest clumps in the F ring are caused by repeated impacts into its core by small moonlets up to about 3 miles (5 kilometers) wide, whose paths around Saturn lie close to the ring and cross into it every orbit. They propose that the diminishing number of bright clumps results from a drop in the number of these little moonlets between the Voyager and Cassini eras.

As for what might have caused the moonlets to become scarce, the team has a suspect: Saturn's moon Prometheus. The F ring encircles the planet at a special location, near a place called the Roche limit -- get any closer to Saturn than this, and tidal forces from the planet's gravity tear apart smaller bodies. "Material at this distance from Saturn can't decide whether it wants to remain as a ring or coalesce to form a moon," French said. Prometheus orbits just inside the F ring, and adds to the pandemonium by stirring up the ring particles, sometimes leading to the creation of moonlets, and sometimes leading to their destruction. 

Every 17 years, the orbit of Prometheus aligns with the orbit of the F ring in such a way that its influence is particularly strong. The study team thinks this periodic alignment might spur the creation of many new moonlets. The moonlets would then crash repeatedly through the F ring, like cars in a Hollywood high-speed chase, creating bright clumps as they smash across lanes of ring material. Fewer clumps would be created as time goes by, because the moonlets themselves are eventually destroyed by all the crashes.

As with any good scientific hypothesis, the researchers offer a way to test their ideas. It happens that the Voyager encounters with Saturn occurred a few years after the 1975 alignment between Prometheus and the F ring, and Cassini was present for the 2009 alignment. If the moon's periodic influence is indeed responsible for creating new moonlets, then the researchers expect that Cassini would see the F ring return to a Voyager-like number of bright clumps in the next couple of years.

"Cassini's continued presence at Saturn gives us an interesting opportunity to test this prediction," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study. "Whatever the result, we're certain to learn something valuable about how rings, as well as planets and moons, form and evolve."

The study by French and colleagues was published in the online edition of the Journal Icarus on July 15, 2014.

NASA's Jet Propulsion Laboratory manages the Voyager and Cassini-Huygens missions for NASA's Science Mission Directorate at NASA Headquarters in Washington.

More information about Cassini is available at: http://www.nasa.gov/cassini  - http://saturn.jpl.nasa.gov
 
Preston Dyches 818-354-5011
Jet Propulsion Laboratory, Pasadena, Calif.
preston.dyches@jpl.nasa.gov

Seth Shostak 650-960-4530
SETI Institute, Mountain View, Calif.
seth@seti.org

Source: JPL-Caltech

Titan's subsurface reservoirs modify methane rainfall

Titan's subsurface reservoirs

Credit: ESA/ATG medialab

Lakes on Titan

Credit: NASA/JPL/USGS

The international Cassini mission has revealed hundreds of lakes and seas spread across the icy surface of Saturn's moon Titan, mostly in its polar regions. These lakes are filled not with water but with hydrocarbons, a form of organic compound that is also found naturally on Earth and includes methane. While most of the liquid in the lakes is thought to be replenished by rainfall from clouds in the moon's atmosphere, the cycling of liquid throughout Titan's crust and atmosphere is still not well understood.

A recent study led by Olivier Mousis at the Université de Franche-Comté, France, and involving colleagues at Cornell University and NASA's Jet Propulsion Laboratory in the USA, probed the hydrological cycle of Titan by examining how Titan's methane rainfall would interact with icy materials within underground reservoirs. They found that the formation of materials called clathrates changes the chemical composition of the rainfall runoff that fills these hydrocarbon reservoirs, leading to the formation of reservoirs of propane and ethane that may feed into some rivers and lakes.

"We knew that a significant fraction of the lakes on Titan's surface might be connected with hidden bodies of liquid beneath Titan's crust, but we just didn't know how they would interact", says Mousis. "Now, we've modelled the moon's interior in great detail, and have a better idea of what these hidden lakes or oceans could be like."

Mousis and colleagues modelled how a subsurface reservoir of liquid hydrocarbons would diffuse throughout Titan's porous icy crust. They found that this diffusion could cause a new reservoir – formed from clathrates - to form where the bottom of the original reservoir meets layers of non-porous ice.

Clathrates are compounds in which water forms a crystal structure with small cages that trap other substances like methane and ethane. On Earth, clathrates that contain methane are found in some polar and ocean sediments. On Titan, the surface pressure and temperature allow clathrates to form when liquid hydrocarbons come into contact with water ice, a main component of the moon's crust. These clathrates could remain stable as far down as several kilometres below the surface of Titan.

"One of the interesting properties of clathrates is that they cause fractionation – in this case, they trap and split molecules into a mix of liquid and solid phases," adds Mousis. Because of this, astronomers have suggested that clathrates may be responsible for many unusual phenomena on Titan, including the depletion of the heavy noble gases in the moon's atmosphere, and variations in the moon's polar radius.

Titan's subsurface clathrate reservoirs would interact with and fractionate the liquid methane within the original underground hydrocarbon lake, slowly changing its composition. Eventually, subsurface lakes that had come into contact with the clathrate layer would mainly be composed of either propane or ethane, depending on the type of clathrate that had formed.

Importantly, this would continue up to Titan's surface. Lakes fed by these propane or ethane subsurface reservoirs would show the same kind of composition, whereas those fed by rainfall would be different and contain methane, nitrogen, and trace amounts of argon and carbon monoxide. "This means we would be able to look at the composition of the surface lakes and learn something about what is happening deep underground," says Mousis.

The Cassini Solstice mission, an extension of Cassini that runs until 2017, will give scientists a chance to explore Titan's surface lakes even more closely by performing an additional 54 close flybys of the Saturnian moon.

"Understanding Titan's hydrological cycle is one of the most important objectives of Cassini's extended mission," says ESA's Cassini-Huygens project scientist Nicolas Altobelli. "The changing seasons on Titan mean that soon we can again explore the lake-filled region at its north pole, and maybe spot seasonal phenomena we haven't seen before. This is crucial to getting a better understanding of what lies hidden beneath Titan's surface."

MORE INFORMATION

"Equilibrium composition between liquid and clathrate reservoirs on Titan" by O. Mousis et al. is published in the journal Icarus, Volume 239, 1 September 2014; doi: 10.1016/j.icarus.2014.05.032

The results are reported by Olivier Mousis, Université de Franche-Comté, France; Mathieu Choukroun and Christophe Sotin, Jet Propulsion Laboratory, USA; and Jonathan I. Lunine, Center for Radiophysics and Space Research, Cornell University, USA.

The Cassini–Huygens mission is a cooperative project of NASA, ESA and the Italian Space Agency (ASI). Launched in 1997, Cassini arrived in the Saturn system in 2004 and is studying the ringed planet and its moons. The Huygens probe was released from the main spacecraft and, in 2005, parachuted through the atmosphere to the surface of Saturn's largest moon, Titan.

NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C.

CONTACTS

Olivier Mousis
Université de Franche-Comté, France
Phone: +33-381-666-921
Mob: +33 660 853 392 
Email: olivier@obs-besancon.fr

Nicolas Altobelli
ESA Cassini-Huygens Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency
Phone: +34-91-813-1201
Email: nicolas.altobelli@sciops.esa.int

Source: ESA/Cassini-Huygens

Saturn’s rainbow rings

Saturn’s rainbow rings
Copyright: NASA/JPL/University of Colorado
JPG - TIF

This colourful cosmic rainbow portrays a section of Saturn’s beautiful rings, four centuries after they were discovered by Galileo Galilei.

Saturn’s rings were first observed in 1610. Despite using his newly created telescope, Galileo was confounded by what he saw: he referred to the peculiar shapes surrounding the planet as “Saturn’s children”. Only later did Christiaan Huygens propose that the mysterious shapes were actually rings orbiting the planet. These were named in the order in which they were discovered, using the first seven letters of the alphabet: the D-ring is closest to the planet, followed by C, B, A, F, G and E.

The data for this image, which shows the portion of the C-ring closest to Saturn on the left, with the B-ring beginning just right of centre, were acquired by Cassini’s Ultraviolet Imaging Spectrograph, or UVIS, as the spacecraft entered into orbit around Saturn on 30 June 2004.

UVIS, as its name suggests, carries out observations in ultraviolet wavelengths. During the Saturn orbit insertion manoeuvre, when Cassini flew closest to the rings, UVIS could resolve features up to 97 km across. The region shown in this image spans about 10 000 km.

The variation in the colour of the rings arises from the differences in their composition. Turquoise-hued rings contain particles of nearly pure water ice, whereas reddish rings contain ice particles with more contaminants.

Saturn’s prominent and complex ensemble of rings is the best studied in the Solar System, but it is still not known how the rings formed. One suggestion is that they formed at the same time as the planet and that they are as old as the Solar System. Another idea is that they formed when icy material was pulled from another body into Saturn’s gravitational field, in which case the rings could be younger than the planet.

One thing is sure: as Cassini searches for answers it is providing amazing images of these rainbow rings.

The Cassini–Huygens mission is a cooperative project of NASA, ESA and Italy’s ASI space agency.

This image was first published at the NASA Cassini website, in 2004.

Source: ESA

 

NASA Cassini Images May Reveal Birth of a Saturn Moon

The disturbance visible at the outer edge of Saturn's A ring in this image from NASA's Cassini spacecraft could be caused by an object replaying the birth process of icy moons.  Full image and caption

NASA's Cassini spacecraft has documented the formation of a small icy object within the rings of Saturn that may be a new moon, and may also provide clues to the formation of the planet's known moons.

Images taken with Cassini's narrow angle camera on April 15, 2013, show disturbances at the very edge of Saturn's A ring -- the outermost of the planet's large, bright rings. One of these disturbances is an arc about 20 percent brighter than its surroundings, 750 miles (1,200 kilometers) long and 6 miles (10 kilometers) wide. Scientists also found unusual protuberances in the usually smooth profile at the ring's edge. Scientists believe the arc and protuberances are caused by the gravitational effects of a nearby object. Details of the observations were published online today (April 14, 2014) by the journal Icarus.

The object is not expected to grow any larger, and may even be falling apart. But the process of its formation and outward movement aids in our understanding of how Saturn's icy moons, including the cloud-wrapped Titan and ocean-holding Enceladus, may have formed in more massive rings long ago. It also provides insight into how Earth and other planets in our solar system may have formed and migrated away from our star, the sun.

"We have not seen anything like this before," said Carl Murray of Queen Mary University of London, the report's lead author. "We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right."

The object, informally named Peggy, is too small to be seen in images so far. Scientists estimate it is probably no more than about a half mile (about a kilometer) in diameter. Saturn's icy moons range in size depending on their proximity to the planet -- the farther from the planet, the larger. And many of Saturn's moons are composed primarily of ice, as are the particles that form Saturn's rings. Based on these facts, and other indicators, researchers recently proposed that the icy moons formed from ring particles and then moved outward, away from the planet, merging with other moons on the way.

"Witnessing the possible birth of a tiny moon is an exciting, unexpected event," said Cassini Project Scientist Linda Spilker, of NASA's Jet Propulsion Laboratory in Pasadena, Calif. According to Spilker, Cassini's orbit will move closer to the outer edge of the A ring in late 2016 and provide an opportunity to study Peggy in more detail and perhaps even image it.

It is possible the process of moon formation in Saturn's rings has ended with Peggy, as Saturn's rings now are, in all likelihood, too depleted to make more moons. Because they may not observe this process again, Murray and his colleagues are wringing from the observations all they can learn.

"The theory holds that Saturn long ago had a much more massive ring system capable of giving birth to larger moons," Murray said. "As the moons formed near the edge, they depleted the rings and evolved, so the ones that formed earliest are the largest and the farthest out." 

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology, manages the mission for NASA's Science Mission Directorate in Washington. 

To view an image of the Saturn ring disturbance attributed to the new moon, visit: http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18078

For more information about Cassini, visit these sites: http://www.nasa.gov/cassini http://saturn.jpl.nasa.gov

Jane Platt
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0880
jane.platt@jpl.nasa.gov

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov

 

NASA Space Assets Detect Ocean inside Saturn Moon

Gravity measurements by NASA's Cassini spacecraft and Deep Space Network suggest that Saturn's moon Enceladus, which has jets of water vapor and ice gushing from its south pole, also harbors a large interior ocean beneath an ice shell, as this illustration depicts. Image credit: NASA/JPL-Caltech.  Full image and caption

NASA's Cassini spacecraft and Deep Space Network have uncovered evidence Saturn's moon Enceladus harbors a large underground ocean of liquid water, furthering scientific interest in the moon as a potential home to extraterrestrial microbes.

Researchers theorized the presence of an interior reservoir of water in 2005 when Cassini discovered water vapor and ice spewing from vents near the moon's south pole. The new data provide the first geophysical measurements of the internal structure of Enceladus, consistent with the existence of a hidden ocean inside the moon. Findings from the gravity measurements are in the Friday, April 4 edition of the journal Science.

"The way we deduce gravity variations is a concept in physics called the Doppler Effect, the same principle used with a speed-measuring radar gun," said Sami Asmar of NASA's Jet Propulsion Laboratory in Pasadena, Calif., a coauthor of the paper. "As the spacecraft flies by Enceladus, its velocity is perturbed by an amount that depends on variations in the gravity field that we're trying to measure. We see the change in velocity as a change in radio frequency, received at our ground stations here all the way across the solar system."

The gravity measurements suggest a large, possibly regional, ocean about 6 miles (10 kilometers) deep, beneath an ice shell about 19 to 25 miles (30 to 40 kilometers) thick. The subsurface ocean evidence supports the inclusion of Enceladus among the most likely places in our solar system to host microbial life. Before Cassini reached Saturn in July 2004, no version of that short list included this icy moon, barely 300 miles (500 kilometers) in diameter.

"This then provides one possible story to explain why water is gushing out of these fractures we see at the south pole," said David Stevenson of the California Institute of Technology, Pasadena, one of the paper's co-authors. 

Cassini has flown near Enceladus 19 times. Three flybys, from 2010 to 2012, yielded precise trajectory measurements. The gravitational tug of a planetary body, such as Enceladus, alters a spacecraft's flight path. Variations in the gravity field, such as those caused by mountains on the surface or differences in underground composition, can be detected as changes in the spacecraft's velocity, measured from Earth. 

The technique of analyzing a radio signal between Cassini and the Deep Space Network can detect changes in velocity as small as less than one foot per hour (90 microns per second). With this precision, the flyby data yielded evidence of a zone inside the southern end of the moon with higher density than other portions of the interior. 

The south pole area has a surface depression that causes a dip in the local tug of gravity. However, the magnitude of the dip is less than expected given the size of the depression, leading researchers to conclude the depression's effect is partially offset by a high-density feature in the region, beneath the surface.

"The Cassini gravity measurements show a negative gravity anomaly at the south pole that however is not as large as expected from the deep depression detected by the onboard camera," said the paper's lead author, Luciano Iess of Sapienza University of Rome. "Hence the conclusion that there must be a denser material at depth that compensates the missing mass: very likely liquid water, which is seven percent denser than ice. The magnitude of the anomaly gave us the size of the water reservoir."

There is no certainty the subsurface ocean supplies the water plume spraying out of surface fractures near the south pole of Enceladus, however, scientists reason it is a real possibility. The fractures may lead down to a part of the moon that is tidally heated by the moon's repeated flexing, as it follows an eccentric orbit around Saturn.

Much of the excitement about the Cassini mission's discovery of the Enceladus water plume stems from the possibility that it originates from a wet environment that could be a favorable environment for microbial life. 

"Material from Enceladus' south polar jets contains salty water and organic molecules, the basic chemical ingredients for life," said Linda Spilker, Cassini's project scientist at JPL. "Their discovery expanded our view of the 'habitable zone' within our solar system and in planetary systems of other stars. This new validation that an ocean of water underlies the jets furthers understanding about this intriguing environment." 

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for NASA's Science Mission Directorate in Washington. 

For more information about Cassini, visit: http://www.nasa.gov/cassini   and  http://saturn.jpl.nasa.gov.

Jane Platt 
Jet Propulsion Laboratory, Pasadena, Calif. 
818-354-0880 
jane.platt@jpl.nasa.gov

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown@nasa.gov

Brian Bell
California Institute of Technology, Pasadena
626-395-5832
bpbell@caltech.edu

NASA Spacecraft Get a 360-Degree View of Saturn's Auroras

Ultraviolet and infrared images from NASA's Cassini spacecraft and Hubble Space Telescope show active and quiet auroras at Saturn's north and south poles. Full caption 

The dark region seen on the face of the sun at the end of March 2013 is a coronal hole (just above and to the right of the middle of the picture), which is a source of fast solar wind leaving the sun.  Image Credit: NASA/SDO/AIA. Full image and caption

While the curtain-like auroras we see at Earth are green at the bottom and red at the top, NASA's Cassini spacecraft has shown us similar curtain-like auroras at Saturn that are red at the bottom and purple at the top. Image Credit: NASA/JPL-Caltech/SSI.  Full image and caption

NASA trained several pairs of eyes on Saturn as the planet put on a dancing light show at its poles. While NASA's Hubble Space Telescope, orbiting around Earth, was able to observe the northern auroras in ultraviolet wavelengths, NASA's Cassini spacecraft, orbiting around Saturn, got complementary close-up views in infrared, visible-light and ultraviolet wavelengths. Cassini could also see northern and southern parts of Saturn that don't face Earth.

The result is a kind of step-by-step choreography detailing how the auroras move, showing the complexity of these auroras and how scientists can connect an outburst from the sun and its effect on the magnetic environment at Saturn.

"Saturn's auroras can be fickle -- you may see fireworks, you may see nothing," said Jonathan Nichols of the University of Leicester in England, who led the work on the Hubble images. "In 2013, we were treated to a veritable smorgasbord of dancing auroras, from steadily shining rings to super-fast bursts of light shooting across the pole."

The Hubble and Cassini images were focused on April and May of 2013. Images from Cassini's ultraviolet imaging spectrometer (UVIS), obtained from an unusually close range of about six Saturn radii, provided a look at the changing patterns of faint emissions on scales of a few hundred miles (kilometers) and tied the changes in the auroras to the fluctuating wind of charged particles blowing off the sun and flowing past Saturn.

"This is our best look yet at the rapidly changing patterns of auroral emission," said Wayne Pryor, a Cassini co-investigator at Central Arizona College in Coolidge, Ariz. "Some bright spots come and go from image to image. Other bright features persist and rotate around the pole, but at a rate slower than Saturn's rotation."

The UVIS images, which are also being analyzed by team associate Aikaterini Radioti at the University of Liege, Belgium, also suggest that one way the bright auroral storms may be produced is by the formation of new connections between magnetic field lines. That process causes storms in the magnetic bubble around Earth. The movie also shows one persistent bright patch of the aurora rotating in lockstep with the orbital position of Saturn's moon Mimas. While previous UVIS images had shown an intermittent auroral bright spot magnetically linked to the moon Enceladus, the new movie suggests another Saturn moon can influence the light show as well.

The new data also give scientists clues to a long-standing mystery about the atmospheres of giant outer planets.

"Scientists have wondered why the high atmospheres of Saturn and other gas giants are heated far beyond what might normally be expected by their distance from the sun," said Sarah Badman, a Cassini visual and infrared mapping spectrometer team associate at Lancaster University, England. "By looking at these long sequences of images taken by different instruments, we can discover where the aurora heats the atmosphere as the particles dive into it and how long the cooking occurs."

The visible-light data have helped scientists figure out the colors of Saturn's auroras. While the curtain-like auroras we see at Earth are green at the bottom and red at the top, Cassini's imaging cameras have shown us similar curtain-like auroras at Saturn that are red at the bottom and purple at the top, said Ulyana Dyudina, an imaging team associate at the California Institute of Technology, Pasadena, Calif.

The color difference occurs because Earth's auroras are dominated by excited nitrogen and oxygen molecules, and Saturn's auroras are dominated by excited hydrogen molecules.

"While we expected to see some red in Saturn's aurora because hydrogen emits some red light when it gets excited, we also knew there could be color variations depending on the energies of the charged particles bombarding the atmosphere and the density of the atmosphere," Dyudina said. "We were thrilled to learn about this colorful display that no one had seen before."

Scientists hope additional Cassini work will illuminate how clouds of charged particles move around the planet as it spins and receives blasts of solar material from the sun.

"The auroras at Saturn are some of the planet's most glamorous features – and there was no escaping NASA's paparazzi-like attention”, said Marcia Burton, a Cassini fields and particles scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif., who is helping to coordinate these observations. "As we move into the part of the 11-year solar cycle where the sun is sending out more blobs of plasma, we hope to sort out the differences between the effects of solar activity and the internal dynamics of the Saturn system."

There is still more work to do. A group of scientists led by Tom Stallard at the University of Leicester is busy analyzing complementary data taken during the same time window by two ground-based telescopes in Hawaii -- the W.M. Keck Observatory and NASA's Infrared Telescope Facility. The results will help them understand how particles are ionized in Saturn's upper atmosphere and will help them put a decade of ground-based telescope observations of Saturn in perspective, because they can see what disturbance in the data comes from Earth's atmosphere.

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov