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.

NASA Cassini: Saturn's moon Rhea, Epimetheus transiting

This image was taken by NASA Cassini’s narrow-angle camera on 24 March 2010, and processed by amateur astronomer Gordan Ugarković. 

A monochrome version was previously released by NASA as PIA12638: Big and Small Before Rings.

Saturn has a great many more moons than our planet, a whopping 62.

A single moon, Titan, accounts for an overwhelming 96% of all the material orbit the planet, with a group of six other smaller moons dominating the rest.

The other 55 small satellites whizzing around Saturn make up the tiny remainder along with the gas giant’s famous rings. One of the subjects of this Cassini image, Rhea, belongs to that group of six.

Set against a backdrop showing Saturn and its intricate system of icy rings, Rhea dominates the scene and dwarfs its tiny companion, one of the 55 small satellites known as Epimetheus.

Although they appear to be close to one another, this is a trick of perspective – this view was obtained when Cassini was some 1.2 million km from Rhea, and 1.6 million km from Epimetheus, meaning the moons themselves had a hefty separation of 400 000 km.

However, even if they were nearer to each other, Rhea would still loom large over Epimetheus: at 1528 km across and just under half the size of our own Moon, Rhea is well over 10 times the size of Epimetheus, which is a modest 113 km across.

As is traditional for the earliest discovered moons of Saturn, both are named after figures from Greek mythology: the Titan Rhea (“mother of the gods”) and Prometheus’ brother Epimetheus (“after thinker” or “hindsight”).

NASA Cassini: Rhea's Day in the Sun

A nearly full Rhea shines in the sunlight in this recent Cassini image. 

Rhea (949 miles, or 1,527 kilometers across) is Saturn's second largest moon.

Lit terrain seen here is on the Saturn-facing hemisphere of Rhea. 

North on Rhea is up and rotated 43 degrees to the left. 

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 10, 2013.

The view was obtained at a distance of approximately 990,000 miles (1.6 million kilometers) from Rhea. 

Image scale is 6 miles (9 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.

For more information about the Cassini-Huygens mission, and saturn.jpl.nasa.gov. 

The Cassini imaging team homepage.
Image Credit: NASA/JPL-Caltech/Space Science Institute

NASA Cassini Image: Saturn's Titan and Icy Rhea

Saturn's largest and second largest moons, Titan and Rhea, appear to be stacked on top of each other in this true-colour scene from NASA's Cassini spacecraft released on Dec. 23, 2013.

The north polar hood can be seen on Titan appearing as a detached layer at the top of the moon on the top right. 

This view looks toward the Saturn-facing side of the smaller Rhea.

Credit: NASA

NASA Cassini Image: Last Close-Up Photos of Icy Saturn Moon, Rhea

This raw image of Saturn's icy moon Rhea was taken on March 10, 2013 by NASA's Cassini spacecraft, and received on Earth March 10, 2013. 

The camera was pointing toward Rhea at approximately 174,181 miles (280,317 kilometers) away. 

CREDIT: NASA/JPL/Space Science Institute

NASA's Cassini spacecraft has snapped its last up-close photos of Saturn's icy moon Rhea, revealing a battered satellite covered in craters from violent impacts.NASA's Cassini spacecraft has snapped its last up-close photos of Saturn's icy moon Rhea, revealing a battered satellite covered in craters from violent impacts.

Cassini took the amazing new photos of Rhea on Saturday (March 9) during its fourth and final planned encounter with the Saturn moon.

During the encounter, the probe flew within just 620 miles (997 kilometers) of Rhea, which is Saturn's second-largest satellite.

"Take a good, long, luxurious look at these sights from another world, as they will be the last close-ups you'll ever see of this particular moon,"

Cassini imaging team lead Carolyn Porco, of the Space Science Institute in Boulder, Colo., said in a statement accompanying the photos.

This image was taken on March 09, 2013, and received on Earth March 10, 2013, by NASA's Cassini spacecraft. 

The camera was pointing toward Rhea at approximately 1,727 miles (2,779 kilometers) away, and the image was taken using the CL1 and CL2 filters. 

This image has not been validated or calibrated. 

CREDIT: NASA/JPL-Caltech/Space Science Institute

Saturday's flyby was designed primarily to measure Rhea's gravity field, mission scientists said, but Cassini also managed to take 12 pictures of the frigid moon's battered, pockmarked surface, including one that showcases a mysterious long, curving fracture called a graben.

Rhea is the second-largest of Saturn's 60-odd known moons, with a diameter of 949 miles (1528 km).

It's far smaller than the ringed planet's biggest natural satellite, Titan, which at 3,200 miles (5,150 km) across is nearly 50 percent wider than Earth's moon.

Rhea was discovered in 1672 by the mathematician and astronomer Giovanni Domenico Cassini, who gave his name to the NASA mission currently studying the Saturn system.

In 2010, researchers determined that the moon has a wispy atmosphere dominated by oxygen and carbon dioxide.

Possibly the oxygen was blasted free from water ice on Rhea's surface by charged particles streaming from Saturn, scientists say, but the origin of the carbon dioxide is more mysterious.

The Cassini mission — a joint effort involving NASA, the European Space Agency and the Italian Space Agency — launched in 1997 and arrived at Saturn in 2004.

It has been studying the ringed planet and its many moons ever since, and will continue to do so on an extended mission until at least 2017.

Nasa Saturn Cassini: Last Close Flyby of Saturn's Moon Rhea

NASA's Cassini spacecraft will be swooping close to Saturn's moon Rhea on Saturday, March 9, the last close flyby of Rhea in Cassini's mission.

Cassini will fly within about 600 miles (1,000 kilometers) of the surface.

The primary purpose will be to probe the internal structure of the moon by measuring the gravitational pull of Rhea against the spacecraft's steady radio link to NASA's Deep Space Network here on Earth.

The results will help scientists understand whether the moon is homogeneous all the way through or whether it has differentiated into the layers of core, mantle and crust.

In addition, Cassini's imaging cameras will take ultraviolet, infrared and visible-light data from Rhea's surface.
 
The cosmic dust analyzer will try to detect any dusty debris flying off the surface from tiny meteoroid bombardments to further scientists' understanding of the rate at which "foreign" objects are raining into the Saturn system.

Cassini will fly within about 600 miles (1,000 kilometers) of the surface. The time of closest approach is around 10:17 a.m. PST (1:17 p.m. EST). This is Cassini's fourth close flyby of Rhea.

NASA Cassini: Sharper image of Saturn moon Rhea

This photo made March 10, 2012, by NASA's Cassini spacecraft shows a raw, unprocessed image of Saturn's moon Rhea. 

The camera was pointing toward Rhea from a distance of approximately 42,096 kilometers (26,157 miles). 

(AP Photo/NASA/JPL-Caltech/SSI)

The pockmarked surface of Saturn's second-largest moon has come into sharper focus in new images released by Nasa.

The space agency released the new views of the moon Rhea, which were captured by the international Cassini spacecraft during a recent fly-by.

One of its cameras spied two huge impact basins and other geologic features on Rhea's icy surface from 26,000 miles away last week.

Cassini, funded by Nasa and the European and Italian space agencies, was launched in 1997. It reached Saturn in 2004 and has been studying the ringed planet and its numerous moons.

NASA Cassini: Rhea Before Titan

Craters appear well defined on icy Rhea in front of the hazy orb of the much larger moon Titan in this Cassini spacecraft view of these two Saturn moons.

Lit terrain seen here is on the leading hemispheres of Rhea and Titan. 

North on the moons is up and rotated 13 degrees to the left. 

The limb, or edge of the visible disk, of Rhea is slightly overexposed in this view.

The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Dec. 10, 2011. 

The view was acquired at a distance of approximately 1.2 million miles (2 million kilometers) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 109 degrees.

The view was acquired at a distance of approximately 810,000 miles (1.3 million kilometers) from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of 109 degrees. Image scale is 8 miles (12 kilometers) per pixel on Titan and 5 miles (8 kilometers) per pixel on Rhea.

Image credit: NASA/JPL-Caltech/Space Science Institute

NASA Cassini's Dramatic Views Of Rhea

Newly released for the holidays, images of Saturn's second largest moon Rhea obtained by NASA's Cassini spacecraft show dramatic views of fractures cutting through craters on the moon's surface, revealing a history of tectonic rumbling.

The images are among the highest-resolution views ever obtained of Rhea.

"These recent, high-resolution Cassini images help us put Saturn's moon in the context of the moons' geological family tree," said Paul Helfenstein, Cassini imaging team associate, based at Cornell University, Ithaca, N.Y.

"Since NASA's Voyager mission visited Saturn, scientists have thought of Rhea and Dione as close cousins, with some differences in size and density. The new images show us they're more like fraternal twins, where the resemblance is more than skin deep. This probably comes from their nearness to each other in orbit."

Cassini scientists designed the March 2010 and November 2009 encounters in part to search for a ring thought to encircle the moon. During the March flyby, Cassini made its closest- approach to Rhea's surface so far, swooping within 100 kilometers (62 miles) of the moon.

Based on these observations, however, scientists have since discounted the possibility that Rhea might currently have a faint ring above its equator.

These flybys nonetheless yielded unique views of other features on the moon, including ones that are among the best ever obtained of the side of Rhea that always faces away from Saturn.

Other views show a web of bright, "wispy" fractures resembling some that were first spotted on another part of Rhea by the two Voyager spacecraft in 1980 and 1981.

At that time, scientists thought the wispy markings on the trailing hemispheres - the sides of moons that face backward in the orbit around a planet - of Rhea and the neighboring moon Dione were possible cryovolcanic deposits, or the residue of icy material erupting. The low resolution of Voyager images prevented a closer inspection of these regions.

NASA Cassini Image: Ethereal Atmosphere of Rhea

NASA's Cassini spacecraft has detected a very tenuous atmosphere known as an exosphere, infused with oxygen and carbon dioxide around Saturn's icy moon Rhea.

This is the first time a spacecraft has directly captured molecules of an oxygen atmosphere – albeit a very thin one -- at a world other than Earth.

The oxygen appears to arise when Saturn's magnetic field rotates over Rhea.

Energetic particles trapped in the planet's magnetic field pepper the moon’s water-ice surface. They cause chemical reactions that decompose the surface and release oxygen. The source of the carbon dioxide is less certain.

Oxygen at Rhea's surface is estimated to be about 5 trillion times less dense than what we have at Earth. But the new results show that surface decomposition could contribute abundant molecules of oxygen, leading to surface densities roughly 100 times greater than the exospheres of either Earth's moon or Mercury.

The formation of oxygen and carbon dioxide could possibly drive complex chemistry on the surfaces of many icy bodies in the universe.

"The new results suggest that active, complex chemistry involving oxygen may be quite common throughout the solar system and even our universe," said lead author Ben Teolis, a Cassini team scientist based at Southwest Research Institute in San Antonio.

"Such chemistry could be a prerequisite for life. All evidence from Cassini indicates that Rhea is too cold and devoid of the liquid water necessary for life as we know it."

Releasing oxygen through surface irradiation could help generate conditions favorable for life at an icy body other than Rhea that has liquid water under the surface, Teolis said.

If the oxygen and carbon dioxide from the surface could somehow get transported down to a sub-surface ocean, that would provide a much more hospitable environment for more complex compounds and life to form.

Scientists are keen to investigate whether life on icy moons with an ocean is possible, though they have not yet detected it.

The tenuous atmosphere with oxygen and carbon dioxide makes Rhea, Saturn's second largest moon, unique in the Saturnian system. Titan has a thick nitrogen-methane atmosphere, but very little carbon dioxide and oxygen.

"Rhea is turning out to be much more interesting than we had imagined," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

"The Cassini finding highlights the rich diversity of Saturn’s moons and gives us clues on how they formed and evolved."

Cassini Catches Saturn Moons In Paintball Fight - Rhea

Scientists using data from NASA's Cassini spacecraft have learned that distinctive, colorful bands and splotches embellish the surfaces of Saturn's inner, mid-size moons.

More Images here

The reddish and bluish hues on the icy surfaces of Mimas, Enceladus, Tethys, Dione and Rhea appear to be the aftermath of bombardments large and small.

A paper based on the findings was recently published online in the journal Icarus. In it, scientists describe prominent global patterns that trace the trade routes for material exchange between the moons themselves, an outer ring of Saturn known as the E ring and the planet's magnetic environment.

The finding may explain the mysterious Pac-Man thermal pattern on Mimas, found earlier this year by Cassini scientists, said lead author Paul Schenk, who was funded by a Cassini data analysis program grant and is based at the Lunar and Planetary Institute in Houston.

"The beauty of it all is how the satellites behave as a family, recording similar processes and events on their surfaces, each in its own unique way," Schenk said.

"I don't think anyone expected that electrons would leave such obvious fingerprints on planetary surfaces, but we see it on several moons, including Mimas, which was once thought to be rather bland."

Schenk and colleagues processed raw images obtained by Cassini's imaging cameras from 2004 to 2009 to produce new, high-resolution global color maps of these five moons. The new maps used camera frames shot through visible-light, ultraviolet and infrared filters which were processed to enhance our views of these moons beyond what could be seen by the human eye.

"The richness of the Cassini data set - visible images, infrared images, ultraviolet images, measurements of the radiation
belts - is such that we can finally 'paint a picture' as to how the satellites themselves are 'painted,'" said William B. McKinnon, one of six co-authors on the paper. McKinnon is based at Washington University in St. Louis and was also funded by the Cassini data analysis program.

Saturn's Moons Rhea and Dione - co-joined

Two of Saturn's moons - Rhea (bottom) and Dione (top) - appear conjoined in this Cassini spacecraft image.

Dione is actually closer to the spacecraft here, but the reflection of the sun and a large crater at its south pole appears to make it blend seamlessly with Rhea.

The newly-released image was taken July 27, 2010 at a distance of approximately 1.1 million kilometres (688,000 miles) from Dione and 1.6 million kilometres (994,000 miles) from Rhea

Picture: NASA/JPL/SSI / Rex Features

NASA Cassini Cruises By Rhea And Helene

Cassini's closest-ever flyby of Saturn's moon Rhea went quite smoothly and teams are busy checking out their data! These flybys never fail to amaze me. And the raw images - which give us an unprocessed first look - are really cool!

Raw image N00152150 gives us a view of part of the bright, fractured terrain we refer to as "wispy terrain" from about 14,000 kilometers (8,900 miles) away.

We know that Rhea's albedo overall is quite high. (When I say "albedo," I basically mean "brightness" or "reflectivity." Studying the albedo can tell a lot about surface composition, geologic processes, and interactions with external environment.)

But this image demonstrates how bright these cracks are since they are so shiny that the surrounding terrain looks quite dark. There are also some interesting apparent albedo variations seen in this image, which are really intriguing.

This raw image (N00152175) from Cassini's narrow-angle camera image was taken about 40 minutes after closest approach. The image shows a region adjacent to the wispy terrain -craters, craters everywhere! And those crater rims bright compared to the surrounding terrain.

The Waltzing Moons of Saturn