NASA Cassini: Europa's atmosphere is thinner than previously thought

Data collected by NASA's Cassini spacecraft during its 2001 flyby of Jupiter shows that Europa's tenuous atmosphere is thinner than had been thought.

Europa is considered one of the most exciting destinations in the Solar System for future exploration because it shows strong indications of having an ocean beneath its icy crust.

Long, linear cracks and ridges crisscross Europa's surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re-frozen into new patterns.

Colour variations across the surface are associated with differences in geologic feature type and location.

The polar regions are bluer than the more equatorial latitudes, which appear more white. This colour variation is thought to be due to differences in ice grain size in the two locations.

Europa has a crust made up of blocks, which are thought to have broken apart and 'rafted' into new positions, as shown in the image on the left. 

Image Credit: NASA /JPL /University of Arizona

Europa is surrounded by very tenuous hot, excited gas. Indications of possible plume activity were reported in 2013 by researchers using NASA's Hubble Space Telescope.

Data collected by Cassini's ultraviolet imaging spectrograph (UVIS) as Cassini sped through the Jupiter system en route to Saturn, shows that most of the plasma around Europa originates not from the moon itself, but from volcanoes on the nearby moon Io.

Cassini's ultraviolet imaging spectrograph (UVIS)

The researchers calculate that Europa contributes 40 times less oxygen than previously thought to its surrounding environment, making it less likely that the moon is regularly venting plumes of water vapour high into orbit.

"Our work shows that researchers have been overestimating the density of Europa's atmosphere by quite a bit," said Don Shemansky, a Cassini UVIS team member with Space Environment Technologies, who led the study.

The moon's tenuous atmosphere, which was already thought to be millions of times thinner than Earth's atmosphere, is actually about 100 times less dense than those previous estimates.

The data shows no evidence of plume activity occurring at the time of the flyby, so if there is plume activity, it is likely intermittent.

Ongoing plume activity at Europa, as Cassini has observed at Saturn's moon Enceladus, would inject large amounts of water vapour into the area around Europa's orbit if the plumes were large enough, but that is not what UVIS observed.

"It is certainly still possible that plume activity occurs, but that it is infrequent or the plumes are smaller than we see at Enceladus," said Amanda Hendrix, a Cassini UVIS team member with the Planetary Science Institute, who co-authored the new study.

Missions that visited Jupiter prior to Cassini provided strong indications that Io is the major contributor of material to the environment around Jupiter, and indicated a hot, low density plasma surrounding Europa. The new results confirm that. "Io is the real monster here," Shemansky said.

"Europa is a complex, amazing world, and understanding it is challenging given the limited observations we have," said Curt Niebur, Outer Planets program scientist at NASA Headquarters.

"Studies like this make the most of the data we have and help guide the kinds of science investigations NASA should pursue in the future."

The Hubble Space Telescope is currently conducting an extensive six-month long survey looking for plume activity, and NASA is studying various possible Europa missions for future exploration.

Nasa Cassini: Enceladus a small speck before enormous Saturn

Enceladus (visible in the lower-left corner) is but a speck before enormous Saturn 

Credit: Nasa Cassini

Enceladus (visible in the lower-left corner of the image) is but a speck before enormous Saturn, but even a small moon can generate big waves of excitement throughout the scientific community.

Enceladus, only 313 miles (504 kilometers) across, spurts vapour jets from its south pole.

The presence of these jets from Enceladus has been the subject of intense study since they were discovered by NASA's Cassini. Their presence may point to a sub-surface water reservoir.

This view looks toward the unilluminated side of the rings from about 2 degrees below the ringplane.

The image was taken with the Cassini spacecraft wide-angle camera on Oct. 20, 2014 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 752 nanometers.

The view was obtained at a distance of approximately 589,000 miles (948,000 kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 26 degrees. Image scale is 35 miles (57 kilometers) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency.

The 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.

The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.

The imaging operations center is based at the Space Science Institute in Boulder, Colo.

The Cassini imaging team homepage is at ciclops.org .

Credit: NASA /JPL-Caltech /Space Science Institute 

Europa's salty lakes may harbour simple life forms - video

Jupiter's moon Europa is thought to have a vast ocean beneath its frozen surface.

NASA Cassini and other past missions have shown proof of salty water, which from our experience, has life-bearing potential.

Exploration of Europa is still stated as a high priority for NASA and is definitely a place of interest for science.

The video is presented by Kevin Hand, Astrobiologist and Deputy Chief Scientist at JPL.

The puzzling, fascinating surface of Jupiter's icy moon Europa looms large in this newly-reprocessed colour view, made from images taken by NASA's Galileo spacecraft in the late 1990s. 

This is the colour view of Europa from Galileo that shows the largest portion of the moon's surface at the highest resolution. 

The view was previously released as a mosaic with lower resolution and strongly enhanced colour. 

To create this new version, the images were assembled into a realistic colour view of the surface that approximates how Europa would appear to the human eye. 

The scene shows the stunning diversity of Europa's surface geology. Long, linear cracks and ridges crisscross the surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re-frozen into new patterns. 

Colour variations across the surface are associated with differences in geologic feature type and location. 

For example, areas that appear blue or white contain relatively pure water ice, while reddish and brownish areas include non-ice components in higher concentrations. 

The polar regions, visible at the left and right of this view, are noticeably bluer than the more equatorial latitudes, which look more white. 

This colour variation is thought to be due to differences in ice grain size in the two locations. Images taken through near-infrared, green and violet filters have been combined to produce this view. 

The images have been corrected for light scattered outside of the image, to provide a color correction that is calibrated by wavelength. 

Gaps in the images have been filled with simulated color based on the colour of nearby surface areas with similar terrain types. 

This global colour view consists of images acquired by the Galileo Solid-State Imaging (SSI) experiment on the spacecraft's first and fourteenth orbits through the Jupiter system, in 1995 and 1998, respectively.

Image scale is 2 miles (1.6 kilometers) per pixel. North on Europa is at right. 

Credit: NASA/JPL-Caltech/SETI Institute

NASA Cassini image of the swirling clouds on Saturn

Nature is an artist, and this time she seems to have let her paints swirl together a bit.

What the viewer might perceive to be Saturn's surface is really just the tops of its uppermost cloud layers.

Everything we see is the result of fluid dynamics.

Astronomers study Saturn's cloud dynamics in part to test and improve our understanding of fluid flows.

Hopefully, what we learn will be useful for understanding our own atmosphere and that of other planetary bodies.

This view looks toward the sunlit side of the rings from about 25 degrees above the ringplane.

The image was taken in red light with the NAC (Narrow Angle Camera) Cassini spacecraft on Aug. 23, 2014.

The view was acquired at a distance of approximately 1.1 million miles (1.8 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 127 degrees. Image scale is 7 miles (11 kilometers) per pixel.

NASA Cassini Sails into New Ocean Adventures on Titan

Cassini radar data reveal the depth of a liquid methane/ethane sea on Saturn's moon Titan near the mouth of a large, flooded river valley.

Image Credit: NASA/JPL-Caltech/ASI/Cornell

Cassini's radar instrument images show that a bright feature appeared in Kraken Mare, Titan's largest sea.

Image Credit: NASA/JPL-Caltech/ASI/Cornell

NASA's Cassini mission continues its adventures in extraterrestrial oceanography with new findings about the hydrocarbon seas on Saturn's moon Titan.

During a flyby in August, the spacecraft sounded the depths near the mouth of a flooded river valley and observed new, bright features in the seas that might be related to the mysterious feature that researchers dubbed the "magic island."

The findings are being presented this week at the Division for Planetary Sciences Meeting of the American Astronomical Society held in Tucson, Arizona.

To the delight of Cassini scientists, two new bright features appeared in Titan’s largest sea, Kraken Mare, during the August 21 flyby.

In contrast to a previously reported bright, mystery feature in another of Titan's large seas, Ligeia Mare, the new features in Kraken Mare were observed in both radar data and images from Cassini's Visible and Infrared Mapping Spectrometer (VIMS).

Having observations at two different wavelengths provides researchers with important clues to the nature of these enigmatic objects.

The VIMS data suggest the new features might have similarities to places in and around the seas that the Cassini team has interpreted as waves or wet ground.

The observations support two of the possible explanations the team thinks are most likely, that the features might be waves or floating debris.

Unfortunately for mystery lovers, the August Titan flyby marked the final opportunity for Cassini's radar to observe Kraken Mare.

However, the spacecraft is scheduled to observe the original "magic island" feature in Ligeia Mare once more, in January 2015.

The August Titan flyby also included a segment designed to collect altimetry (or height) data, using the spacecraft's radar instrument along a 120-mile (200-kilometer) shore-to-shore track of Kraken Mare.

For a 25-mile (40-kilometer) segment of this data along the sea's eastern shoreline, Cassini's radar beam bounced off the sea bottom and back to the spacecraft, revealing the sea's depth in that area.

This region, which is near the mouth of a large, flooded river valley, showed depths of 66 to 115 feet (20 to 35 meters).

Cassini will perform this experiment one last time in January 2015, to try to measure the depth of Punga Mare.

Punga Mare is the smallest of three large seas in Titan's far north, and the only sea whose depth has not been observed by Cassini.

Scientists think that, for the areas in which Cassini did not observe a radar echo from the seafloor, Kraken Mare might be too deep for the radar beam to penetrate.

Alternatively, the signal over this region might simply have been absorbed by the liquid, which is mostly methane and ethane.

The altimetry data for the area in and around Kraken Mare also showed relatively steep slopes leading down to the sea, which also suggests the Kraken Mare might indeed be quite deep.

Methane Ice Cloud Floats High Above Saturn's Moon Titan

NASA's Cassini probe imaged a cloud in the stratosphere over the north pole of Saturn's moon Titan during a flyby in December 2006.

Credit: NASA /JPL /University of Arizona /LPGNantes

In a celestial surprise, NASA's Cassini spacecraft has identified a cloud of methane ice high in the stratosphere of Saturn's huge moon Titan.

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

Anderson and her colleagues spotted the methane cloud hovering over Titan's north pole in images taken by Cassini in December 2006, when it was winter in the moon's northern hemisphere. (The north is now shifting from spring into summer.)

Researchers had seen methane clouds on Titan before, but in the troposphere, the lowest part of the moon's thick, nitrogen-rich atmosphere.

While wispy clouds of ethane and several other materials have been observed in the stratosphere, this region had been regarded as not quite cold enough to support the existence of methane clouds.

NB: Cloud formation requires colder temperatures at higher altitudes, because the air higher up contains less moisture, researchers said.

This view was based on previous measurements taken just south of Titan's equator, which returned stratospheric temperatures of around minus 333 degrees Fahrenheit (minus 203 degrees Celsius).

But more recent Cassini data show that the stratosphere is patchy, with temperatures as low as minus 344 F (minus 209 C) in places, researchers said. And those frigid patches are cold enough for methane ice particles to form.

The methane cloud likely formed when relatively warm air rose to the stratosphere from the surface of Titan's southern hemisphere, where it was summer in December 2006, and then circulated up to the north polar region and sank back down, cooling as it went.

Such a mechanism could produce methane clouds at altitudes ranging from 19 to 31 miles (30 to 50 kilometers), researchers said.

"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, also of NASA Goddard, principal investigator for Cassini's Composite Infrared Spectrometer instrument, in the statement.

NASA Cassini: Icy rocks around Saturn - Titan and Rhea

Credit: NASA/JPL-Caltech/Space Science Institute

Earth is the only planet in our Solar System to have a single solitary moon.

While others, such as Mercury and Venus, have none, the gas giants have accumulated crowds of orbiting bodies, Saturn, for example, boasts an impressive 62 moons!

This image, taken by the Cassini orbiter, shows its two biggest: Rhea and its larger companion Titan.

Titan's diameter, at 5150 km, is 50% larger than that of our Moon, while Rhea is somewhat smaller at 1528 km across.

Although Rhea's pitted and cratered appearance contrasts sharply with the faint golden glow of Titan, the moons are quite similar in composition, containing a mixture of rock and water ice.

Rhea is thought to comprise three quarters ice and one quarter rock.

Observations with Cassini have determined that Rhea does not contain a distinct rocky core, instead, it is made up of rock and ice mixed together, giving it its 'dirty snowball' appearance.

Titan's orange hue is a result of its atmosphere. It is the only body in the Solar System other than Earth to have a thick, nitrogen-rich atmosphere, which in Titan's case also contains substances like methane, hydrogen and hydrocarbons.

These molecules form via reactions with sunlight high up in Titan's atmosphere, eventually settling to lower altitudes to form an orange-hued smog.

In some images Titan's upper atmosphere takes on a layered appearance, with 'stripes' of haze stacked on top of one another in an onion-like fashion.

This Cassini image shows one such layer, a hazy band of blue encircling Titan.

This haze runs all the way around the moon, and brightens in two crescent-shaped areas over the polar regions to form 'polar hoods'.

These hoods are swirling, high-altitude areas of denser gas. Titan's north polar hood can be seen towards the upper right of the image, and its corresponding southern hood lies towards the lower left of the moon.

These polar hoods are seasonal, growing and dissipating with the changing seasons. Seasons on Saturn and its accompanying moons last for around seven years.

When Cassini arrived in the Saturn system in 2004 Titan already had a thick hood above its north pole, which was experiencing winter.

After the Saturnian equinox in August 2009, Titan's northern hemisphere began moving into spring, and its southern latitudes headed into autumn.

Accompanying this seasonal shift was the appearance of a polar vortex above Titan's south pole. In 2012 Cassini snapped multiple images of this vortex as it swirled round furiously, completing a full rotation in just nine hours.

While the Cassini mission has spent much time studying Titan, it has also performed numerous flybys of Rhea, passing close to the moon four times to probe its interior structure, gravitational pull and surface characteristics.

These encounters showed Rhea to be an ancient and heavily cratered body, bearing numerous pocks and scars from past impacts.

This is something that astronomers want to explore by studying Rhea; measuring the dusty debris flying up from Rhea's surface may help us to understand more about the rate of meteoroid bombardments and amount of cosmic debris raining down on the Saturnian system.

This true-colour image is made with exposures taken on 16 June 2011 using red, green and blue filters on Cassini's narrow-angle camera.

Wobbling of a Saturn moon hints at what lies beneath

Using instruments aboard the Cassini spacecraft to measure the wobbles of Mimas, the closest of Saturn's regular moons, a Cornell University astronomer publishing in Science, Oct. 17, has inferred that this small moon's icy surface cloaks either a rugby ball-shaped rocky core or a sloshing sub-surface ocean.

"After carefully examining Mimas, we found it librates, that is to say, it subtly wobbles, around the moon's polar axis," Radwan Tajeddine, Cornell research associate in astronomy and lead author of the article.

"In physical terms, the back-and-forth wobble should produce about 3 kilometers of surface displacement."

"Instead we observed an unexpected 6 kilometers of surface displacement," he said.

"We're very excited about this measurement because it may indicate much about the satellite's insides."

"Nature is essentially allowing us to do the same thing that a child does when she shakes a wrapped gift in hopes of figuring out what's hidden inside," Tajeddine said.

The astronomy team used a technique called stereo-photogrammetry to interpret images taken by the Cassini Imaging Science Subsystem to measure the libration.

In this technique, astronomers employ Cassini photographs of Mimas taken at different times and from various vantage points to build precise 3-D computer models of the locations of hundreds of surface reference points.

From these, the researchers determined the moon's shape and were able to notice that the satellite didn't rotate smoothly but rocked back and forth a bit as well.

The amount of the to-and-fro motion indicates that Mimas' interior is not uniform. These wobbles can be produced if the moon contains a weirdly shaped, rocky core or if a sub-surface ocean exists beneath its icy shell.

Mimas is about 400 kilometers in diameter, and its possible internal global ocean is located under an icy crust ranging in thickness between 25 and 30 kilometers.

The moon itself is thought to have been formed either by the slow agglomeration of ring particles (a gradual buildup of matter) or direct growth within the primordial planetary gas nebula.

The odd-shaped core would favor gravitational flattening by nearby Saturn, Tajeddine said. The moon's relatively smooth and roughly spherical icy surface covers up whatever is underneath.

More information: Science. DOI: 10.1126/science.1255299

NASA Cassini caught in Hyperion's particle beam inside Saturn's Magnetosphere

This stunning false-colour view of Saturn's moon Hyperion reveals crisp details across the strange, tumbling moon's surface. 

Differences in colour could represent differences in the composition of surface materials. 

The view was obtained during Cassini's close flyby on Sept. 26, 2005. 

Hyperion has a notably reddish tint when viewed in natural colour. 

The red color was toned down in this false-colour view, and the other hues were enhanced, in order to make more subtle colour variations across Hyperion's surface more apparent. 

Credit: NASA /JPL /Space Science Institute

Static electricity is known to play an important role on Earth's airless, dusty moon, but evidence of static charge building up on other objects in the solar system has been elusive until now.

A new analysis of data from NASA's Cassini mission has revealed that, during a 2005 flyby of Saturn's moon Hyperion, the spacecraft was briefly bathed in a beam of electrons coming from the moon's electrostatically charged surface.

The finding represents the first confirmed detection of a charged surface on an object other than our moon, although it is predicted to occur on many different bodies, including asteroids and comets.

The new analysis was led by Tom Nordheim, a doctoral candidate at Mullard Space Science Laboratory (MSSL), University College London, and was published recently in the journal Geophysical Research Letters.

Hyperion is porous and icy, with a bizarre, sponge-like appearance. Its surface is continuously bombarded by ultraviolet light from the sun and exposed to a rain of charged particles, electrons and ions, within the invisible bubble generated by Saturn's magnetic field, called the magnetosphere.

The researchers think Hyperion's exposure to this hostile space environment is the source of the particle beam that struck Cassini.

Measurements made by several of Cassini's instruments during a close encounter with Hyperion on September 26, 2005, indicate that something unexpected took place in the charged particle environment around the spacecraft.

Among those instruments, the Cassini Plasma Spectrometer (CAPS) detected that the spacecraft was magnetically connected to the surface of Hyperion for a brief period, allowing electrons to escape from the moon toward the robotic probe.

Most people are familiar with the electrostatic charge buildup that occurs when a balloon is rubbed against hair or a sweater.

Objects in space can also become electrostatically charged by exposure to solar ultraviolet light and incoming charged particles.

The Cassini data show that a similar process can take place on Hyperion.

The finding is surprising, as the small but odd-looking moon was thought to be a simple inert object, which would not undergo any strong interactions with the Saturnian magnetosphere.

Nevertheless, the team's analysis indicates that Cassini remotely detected a strongly negative voltage on Hyperion.

"It was rather like Cassini receiving a 200-volt electric shock from Hyperion, even though they were over 2,000 kilometers [1,200 miles] apart at the time," said Nordheim.

Scientists had previously suggested that surface features observed on the asteroid Eros and several of Saturn's moons are due to the motion of charged dust across their surfaces.

On small objects with low gravity, dust grains might even be able to overcome the force of gravity and escape into space.

Although mission controllers have detected no signs that the Hyperion electron beam caused damage to Cassini, strong electric charging effects could prove to be a hazard to future robotic and human explorers at planetary objects without atmospheres, including Earth's moon, where they could create the potential for powerful electrostatic discharges.

"Our observations show that this is also an important effect at outer planet moons and that we need to take this into account when studying how these moons interact with their environment," said Geraint Jones of MSSL, a member of the Cassini CAPS team who helped supervise the study.

Cassini's CAPS instrument was powered off in 2012, when the instrument began to draw excess current.

The team is based at Southwest Research Institute, San Antonio. Part of the CAPS instrument that made the detection discussed in this research, the CAPS electron spectrometer, was built by MSSL.

Nordheim and colleagues also utilized data from three other Cassini instruments in their analysis: the Radio and Plasma Wave Science instrument (RPWS), the Magnetospheric Imaging Instrument and the Magnetometer (MIMI).

NASA Cassini: Titan's Gigantic polar clouds of hydrogen cyanide

Titan's hazy orange globe hangs before the Cassini spacecraft. 

Image credit NASA/JPL/Space Science Institute.

Gigantic polar clouds of hydrogen cyanide roughly four times the area of the UK are part of the impressive atmospheric diversity of Titan, the largest moon of Saturn, a new study led by Leiden Observatory, the Netherlands Institute for Space Research and the University of Bristol has found.

The research is published today in Nature.

Titan is unique in our solar system because of its dense nitrogen-methane atmosphere, which is very similar to Earth's in some ways, but very different in others.

For example, air temperatures are around 200 degrees colder and, in contrast to the warm salt water seas of Earth, frigid hydrocarbon lakes populate Titan's surface.

Titan has seasons just like Earth, only each season lasts over seven years instead of three months due to its ponderous orbit around the Sun.

After equinox in 2009, Titan's south pole entered the perpetual darkness of polar winter. Soon after, instruments on NASA's Cassini spacecraft observed the development of a gigantic polar cloud covering over one million square kilometres, roughly four times the area of the UK.

Bristol researcher and co-author Dr Nick Teanby said: "The cloud was first seen in images from Cassini's cameras taken in 2012."

"It started off quite small but soon grew to cover the entire south polar region. This was totally unexpected and set us puzzling over what the cloud could be made of."

"Unfortunately, while the images showed that the cloud was very high up, at over 250km above the surface, they did not allow us to figure out what the cloud was actually made of or why it was there."

For the next two years Cassini gathered more data including infrared spectra of the cloudy region.

Lead author Remco de Kok said: "When we looked at the spectra, we saw two large peaks that weren't present in spectra of other places on Titan."

"These peaks coincided exactly with the peaks you'd expect from ice particles of hydrogen cyanide, or 'blauwzuur' (blue acid) as it's known in the Netherlands, which is highly toxic."

"This was very surprising to us, since we did not expect HCN ice to be able to form so high in Titan's atmosphere."

This new research suggests that Titan's south pole must be extremely cold to allow hydrogen cyanide to condense.

In fact, the upper atmosphere must have cooled by over 50 degrees in less than a year to reach a blisteringly cold -150C.

Remco de Kok concluded: "This is a very rapid change given Titan's long annual cycle and is much colder than previously thought possible."

"It suggests that once the pole is in shadow the upper atmosphere acts as a very efficient radiator of heat, perhaps due to the high abundance of exotic hydrocarbon and nitrogen based compounds, which emit strongly in the infrared."

"Cassini is set to continue observing Titan until it takes a dive into Saturn at the end of its mission in 2017. It will be fascinating to see how the cloud will develop."

More information: HCN ice in Titan's high-altitude southern polar cloud, Nature, dx.doi.org/10.1038/nature13789

Cassini captures evolution of mysterious feature in Titan sea - Video

These three images, created from Cassini Synthetic Aperture Radar (SAR) data, show the appearance and evolution of a mysterious feature in Ligeia Mare, one of the largest hydrocarbon seas on Saturn's moon Titan. 

Credit: NASA/JPL-Caltech/ASI/Cornell



NASA's Cassini spacecraft is monitoring the evolution of a mysterious feature in a large hydrocarbon sea on Saturn's moon Titan.

The feature covers an area of about 100 square miles (260 square kilometers) in Ligeia Mare, one of the largest seas on Titan.

It has now been observed twice by Cassinis radar experiment, but its appearance changed between the two apparitions.

The mysterious feature, which appears bright in radar images against the dark background of the liquid sea, was first spotted during Cassini's July 2013 Titan flyby.

Ligeia Mare on Titan. 

Credit: NASA Cassini

Previous observations showed no sign of bright features in that part of Ligeia Mare.

Scientists were perplexed to find the feature had vanished when they looked again, over several months, with low-resolution radar and Cassini's infrared imager (VIMS).

This led some team members to suggest it might have been a transient feature, but during Cassini's flyby on August 21, 2014, the feature was again visible, and its appearance had changed during the 11 months since it was last seen.

Scientists on the radar team are confident that the feature is not an artifact, or flaw, in their data, which would have been one of the simplest explanations.

They also do not see evidence that its appearance results from evaporation in the sea, as the overall shoreline of Ligeia Mare has not changed noticeably.

The team has suggested the feature could be surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic.

The researchers suspect that the appearance of this feature could be related to changing seasons on Titan, as summer draws near in the moon's northern hemisphere.

Monitoring such changes is a major goal for Cassini's current extended mission.

"Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan," said Stephen Wall, the deputy team lead of Cassini's radar team, based at NASA's Jet Propulsion Laboratory in Pasadena, California.

"We're hopeful that we'll be able to continue watching the changes unfold and gain insights about what's going on in that alien sea."

More information: Images of the feature taken during the Cassini flybys are available at: photojournal.jpl.nasa.gov/catalog/PIA18430

Gas Giant Saturn: A cosmic hurricane

The giant planet Saturn is mostly a gigantic ball of rotating gas, completely unlike our solid home planet, but Earth and Saturn do have something in common: weather, although the gas giant is home to some of the most bizarre weather in our Solar System, such as the swirling storm shown in this Cassini view.

Credit: NASA

Known as “the hexagon”, this weather feature is an intense, six-sided jet stream at Saturn’s north pole.

Spanning some 30 000 km across, it hosts howling 320 km/h winds that spiral around a massive storm rotating anticlockwise at the heart of the region.

Numerous small vortices rotate in the opposite direction to the central storm and are dragged around with the jet stream, creating a terrifically turbulent region.

While a hurricane on Earth may last a week or more, the hexagon has been raging for decades, and shows no signs of letting up.

This false-colour image of the hexagon was made using ultraviolet, visible and infrared filters to highlight different regions.

The dark centre of the image shows the large central storm and its eye, which is up to 50 times bigger than a terrestrial hurricane eye.

The small vortices show up as pink-red clumps. Towards the lower right of the frame is a white-tinted oval storm that is bigger than any of the others, this is the largest of the vortices at some 3500 km across, twice the size of the largest hurricane ever recorded on Earth.

The darker blue region within the hexagon is filled with small haze particles, whereas the paler blue region is dominated by larger particles.

This divide is caused by the hexagonal jet stream acting as a shepherding barrier, large particles cannot enter the hexagon from the outside.

These large particles are created when sunlight shines onto Saturn’s atmosphere, something that only started relatively recently in the northern hemisphere with the beginning of northern spring in August 2009.

Cassini will continue to track changes in the hexagon, monitoring its contents, shape and behaviour as summer reaches Saturn’s northern hemisphere in 2017.

Deviation in satellite flybys: An anomaly that confounds scientists

An artist's rendition of Rosetta probe during a flyby. 

Credit: ESA/C.Carreau

When space probes, such as ESA's Rosetta and NASA's Cassini, fly over certain planets and moons in order to gain momentum and travel long distances, their speed changes slightly for an unknown reason.

A Spanish researcher has now analysed whether or not a hypothetical gravitomagnetic field could have an influence.

However, other factors such as solar radiation, tides, or even relativistic effects or dark matter could be behind this mystery.

Since the beginnings of space exploration, many spacecraft have gone into a hyperbolic orbit around planets or moons, with the aim of taking advantage of their gravitational energy and go toward their target.

However, during this flyby manoeuvre, something makes the spacecraft speed deviate from the scientists' theoretical calculations.

This anomaly has only been detected with a high level of precision in flybys of Earth, due to the availability of deep space monitoring stations such as that of NASA in Robledo de Chabela (Madrid) or that of the European Space Agency in Cebreros (Ávila), which allow for the variations in the spacecrafts' speed to be recorded by means of radars.

Thus, when the Galileo space probe flew over Earth in 1990, an unexpected increase of 4 millimetres per second was detected, as was a similar decrease when it took the same flyby in 1992.

Also in 1998, a speed of 13 mm/s above estimates was observed in the spacecraft NEAR, and similar anomalies were repeated in the flybys of Cassini in 1999 (-2 mm/s), and those of the Messenger and Rosetta probes in 2005, with +0.02 mm/s and +1.82 mm/s respectively, the latter arriving just this year at the comet it was directed towards.

"These deviations do not seriously affect the trajectories of the spacecrafts, yet, although they are seemingly small amounts, it is very important to clarify what they are caused by, especially in the current era of precise space exploration," Luis Acedo Rodríguez, physicist at the Polytechnic University of Valencia, tells SINC.

Scientists have still not found any convincing explanation for the phenomenon, although they have put forward a range of hypotheses.

One points toward solar radiation as the cause of the change in speed, while others suggest an influence from magnetic fields or the effect of tides, and there are also even unconventional theories, such as the existence of a halo of dark matter trapped by Earth's gravitational pull.

Acedo has proposed an explanation based on a supposed circulating gravitomagnetic field, which would follow the Earth's parallels, an approach that can be used to explain the effects on the majority of flybys.

"Einstein's general theory of relativity predicts the existence of a similar field, but in the case of meridians, with this strongly confirmed by experiments such as Gravity Probe B," the researcher comments, although he recognises significant limitations of the model.

"If a force field existed," he explains, "its effects would also be seen in the elliptical orbits of spacecrafts, and should have been detected a long time ago by geodynamic satellites such as NASA's LAGEOS or ESA's LARES; however, this is not the case, and it is therefore doubtful that a field of this kind could cast a light on this mystery without seriously changing our understanding of Earth's gravity."

With this possibility ruled out, the expert considers, in a study published in Advances in Space Research, that the anomalous behaviour of the probes during their flybys "must originate in something that, although common, we have been unaware of to date, or in an error in the data analysis programs".

The difference in speeds could also have much more serious implications on the understanding of gravity, according to Acedo: "We already have evidence that shows a seemingly small anomaly in astronomical observations leading to new theoretical conceptions, such as the advance of Mercury's perihelion (closest point to the Sun), which was essential in the development of the theory of general relativity."

"For the case in question, and without ruling out an explanation by means of conventional sources, something similar could occur."

Meanwhile, space probes continue to challenge scientists every time they perform flybys. One of the last was that of the spacecraft Juno in October 2013, from Earth en route to Jupiter.

NASA has not yet published data on this journey, but everything indicates that its speed as it flew over our planet once again differed from estimates.

More information: L. Acedo, "The flyby anomaly: A case for strong gravitomagnetism?," Advances in Space Research, Volume 54, Issue 4, 15 August 2014, Pages 788-796, ISSN 0273-1177, dx.doi.org/10.1016/j.asr.2014.04.014.

NASA Cassini Image: Saturn's moon Mimas in crescent profile

A thin sliver of Saturn's moon Mimas, is illuminated, the long shadows showing off its many craters, indicators of the moon's violent history.

Credit: NASA /JPL-Caltech /Space Science Institute

The most famous evidence of a collision on Mimas (246 miles, or 396 kilometers across) is the crater Herschel (Mimantean crater) that gives Mimas its Death Star-like appearance.

This view looks toward the anti-Saturn hemisphere of Mimas.

North on Mimas is up and rotated 40 degrees to the right.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on May 20, 2013.

The view was acquired at a distance of approximately 100,000 miles (200,000 kilometers) from Mimas and at a Sun-Mimas-spacecraft, or phase, angle of 130 degrees. Image scale is 4,000 feet (1 kilometer) per pixel.

This image shows the crater Herschel (Mimantean crater) on Mimas in greater detail.

Credit: NASA

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency (ESA) and the Italian Space Agency (ASI).

The 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.

The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL.

The imaging operations center is based at the Space Science Institute in Boulder, Colo.

Visit the Cassini imaging team homepage.

NASA Cassini HiRise Image: Transverse aeolian ridges (TARs) - Windblown dunes

A NASA Cassini HiRise image of the tropics of Mars showing the "Transverse aeolian ridges," or TARs, consisting of small ripples shaped by the wind. 

Credit: Nasa Cassini HiRise / University of Arizona

The tropics of Mars are commonly littered with small bright ripples that were somehow shaped by the wind.

Called "transverse aeolian ridges", or TARs, the features stand up to 6 meters tall and are spaced a few tens of meters apart.

They are typically oriented transverse to modern day wind directions, and often found in channels and crater interiors.

The physical process that produces these features is still mysterious. Most TARs display no evidence of internal structure, so it is difficult to discern exactly how they were formed.

While validating a HiRISE digital terrain model of the area, Sarah Mattson of the University of Arizona discovered these rare banded TARs in Iapygia, south of Syrtis Major.

These features resemble TARs elsewhere on Mars, except that they show bands or layers on their northwest faces but fewer or none on the southeast sides.

One possible interpretation of this strange layering is that these particular TARs are made up of wedge-shaped layers, as shown in the schematic cross-section illustrating the inferred structure of the TARs.

If this hypothesis is correct, it implies that the ripples grew vertically over time, as material accreted at the crests of the ridges. It also suggests that the banded slopes faced upwind.

This observation might provide a valuable clue to the formation of TARs elsewhere on Mars, if they have a similar internal structure but that structure cannot be seen because they are made up of homogeneous materials that are uniform in colour.

Written by: Paul Geissler

Moonlets created and destroyed in Saturn's F ring

Cassini spied just as many regular, faint clumps in Saturn's narrow F ring (the outermost, thin ring), like those pictured here, as Voyager did but it saw hardly any of the long, bright clumps that were common in Voyager images. 

Credit: NASA /JPL-Caltech /SSI

There is an ongoing drama in the Saturnian ring system that causes small moons to be born and then destroyed on time scales that are but an eyeblink in the history of the solar system.

SETI Institute scientists Robert French and Mark Showalter have examined photos made by NASA's Cassini spacecraft and compared them to 30 year-old pictures made by the Voyager mission.

They find that there is a marked difference in the appearance of one of the rings, even over this cosmologically short interval, a difference that can be explained by the brief strut and fret of small moons.

"The F ring is a narrow, lumpy feature made entirely of water ice that lies just outside the broad, luminous rings A, B, and C," notes French.

"It has bright spots. But it has fundamentally changed its appearance since the time of Voyager. Today, there are fewer of the very bright lumps."

The bright spots come and go over the course of hours or days, a mystery that the two SETI Institute astronomers think they have solved.

"We believe the most luminous knots occur when tiny moons, no bigger than a large mountain, collide with the densest part of the ring," says French.

"These moons are small enough to coalesce and then break apart in short order."

The F ring is at a special place in the ring system, at a distance known as the Roche limit, named for French astronomer Edouard Roche who first pointed out that if a moon orbits too close to a planet, the difference in gravitational tug on its near and far side can tear it apart.

This happens at a distance dependent on the mass of the planet, and in the case of Saturn, happens to be at the location of the F ring.

Consequently, material here is caught between the yin and yang of forming small moons, and having them pulled apart.

The moons in question are typically no more than 3 miles (5 km) in size, and consequently can come together quickly.

This chaotic region is given additional stir by Prometheus, a moon that's roughly 60 miles (100 km) in size that orbits just inside the F ring.

Every 17 years, Prometheus aligns with the F ring in a way that emphasises its gravitational influence on the ring's particles, precipitating the formation of the mini-moons, or moonlets.

"These newborn moonlets will repeatedly crash through the F ring, like bumper cars, producing bright clumps as they careen through lanes of material," says Showalter.

"But this is self-destructive behaviour, and the moons, being just at the Roche limit, are barely stable and quickly fragmented."

This scenario can explain the rapid variation in the number of bright clumps in the F ring, but is it true?

If the periodic influence of Prometheus is causing the waxing and waning of the clumps, then there should be an increase in their prevalence over the next few years, a prediction that the astronomers will be checking with Cassini data.

In addition to the drama of moons that come and go over less than a human lifetime, studies of the ring system give insight into how solar systems in general are built.

"The sort of processes going on around Saturn are very similar to those that took place here 4.6 billion years ago, when the Earth and the other large planets were formed," notes French. "It's an important process to understand."

This research was published in the online edition of the journal Icarus on July 15, 2014.

More information: Robert S. French, Shannon K. Hicks, Mark R. Showalter, Adrienne K. Antonsen, Douglas R. Packard, "Analysis of clumps in Saturn's F ring from Voyager and Cassini," Icarus, Volume 241, October 2014, Pages 200-220, ISSN 0019-1035, dx.doi.org/10.1016/j.icarus.2014.06.035 . Preprint: arxiv.org/abs/1408.2548

Titan's subsurface reservoirs modify methane rainfall

Titan's subsurface reservoirs. Credit: ESA/ATG medialab

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.

Lakes on Titan. Credit: NASA/JPL/USGS

"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.

Amazing raw Cassini images of Saturn's rings from this week

Sunlight and shadow combine in this photo of Saturn and its rings taken Aug. 19, 2014. 

Credit: NASA/ JPL/ Space Science Institute

When Saturn is at its closest to Earth, it's three-quarters of a billion miles away, or more than a billion kilometers!

That makes these raw images from the ringed planet all the more remarkable.

Nearly every day, the Cassini spacecraft beams back what it sees at Saturn and the images are put up on this NASA website.

This week, for example, it was checking out Saturn's rings. We have a few of the pictures below, plus an older picture of the entire planet for reference.

Saturn's rings are believed to be about 4.4 billion years old, that's close to the age of the Solar System itself.

Astronomers, however, have only known about them since the 1600s, when Galileo Galilei was trying to make sense of some funny-looking shapes on either side of the planet in his telescope.

According to NASA, the particles in the rings range from dust-sized to mountain-sized. Some of Saturn's dozens of moons act as shepherds to the rings, keeping gaps open.

You can read more about what we know about their origins here.

Saturn and its rings, as seen from above the planet by the Cassini spacecraft. 

Credit: NASA/ JPL/ Space Science Institute. Assembled by Gordan Ugarkovic.

Different shades shine in this raw image of Saturn’s rings taken by the Cassini spacecraft taken Aug. 19, 2014. 

Credit: NASA/ JPL/ Space Science Institute

Bands prominently feature in this raw picture of Saturn taken by the Cassini spacecraft Aug. 17, 2014. 

Credit: NASA/JPL/Space Science Institute

The Cassini spacecraft looks to the side of Saturn’s rings in this picture from Aug. 19, 2014. 

Credit: NASA/JPL/Space Science Institute