Prometheus and the Rings

Seen within the vast expanse of Saturn's rings, Prometheus appears as little more than a dot. But that little moon still manages to shape the F ring, confining it to its narrow domain.

Prometheus (53 miles, or 86 kilometers across) and its fellow moon Pandora (50 miles, or 81 kilometers across) orbit beside the F ring and keep the ring from spreading outward through a process dubbed "shepherding."

This view looks toward the unilluminated side of the rings from about 45 degrees below the ringplane. The image was taken in green light with the Cassini spacecraft wide-angle camera on March 8, 2014.

The view was obtained at a distance of approximately 533,000 miles (858,000 kilometers) from Prometheus and at a Sun-Prometheus-spacecraft, or phase, angle of 90 degrees. Image scale is 32 miles (51 kilometers) per pixel.

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

Saturn's North Polar Vortex and Rings

The Cassini spacecraft captures three magnificent sights at once: Saturn's north polar vortex and hexagon along with its expansive rings.

The hexagon, which is wider than two Earths, owes its appearance to the jet stream that forms its perimeter. The jet stream forms a six-lobed, stationary wave which wraps around the north polar regions at a latitude of roughly 77 degrees North.

This view looks toward the sunlit side of the rings from about 37 degrees above the ringplane. The image was taken with the Cassini spacecraft wide-angle camera on April 2, 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 1.4 million miles (2.2 million kilometers) from Saturn and at a Sun-Saturn-spacecraft, or phase, angle of 43 degrees. Image scale is 81 miles (131 kilometers) per pixel.

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

Titan's Ocean Perhaps as Salty as the Dead Sea

Researchers found that Titan's ice shell, which overlies a very salty ocean, varies in thickness around the moon, suggesting the crust is in the process of becoming rigid.

Scientists analyzing data from NASA's Cassini mission have firm evidence the ocean inside Saturn's largest moon, Titan, might be as salty as Earth's Dead Sea.

The new results come from a study of gravity and topography data collected during Cassini's repeated flybys of Titan during the past 10 years. Using the Cassini data, researchers presented a model structure for Titan, resulting in an improved understanding of the structure of the moon's outer ice shell. The findings are published in this week's edition of the journal Icarus.

"Titan continues to prove itself as an endlessly fascinating world, and with our long-lived Cassini spacecraft, we're unlocking new mysteries as fast as we solve old ones," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, who was not involved in the study.

Additional findings support previous indications the moon's icy shell is rigid and in the process of freezing solid. Researchers found that a relatively high density was required for Titan's ocean in order to explain the gravity data. This indicates the ocean is probably an extremely salty brine of water mixed with dissolved salts likely composed of sulfur, sodium and potassium. The density indicated for this brine would give the ocean a salt content roughly equal to the saltiest bodies of water on Earth.

"This is an extremely salty ocean by Earth standards," said the paper's lead author, Giuseppe Mitri of the University of Nantes in France. "Knowing this may change the way we view this ocean as a possible abode for present-day life, but conditions might have been very different there in the past."

Cassini data also indicate the thickness of Titan's ice crust varies slightly from place to place. The researchers said this can best be explained if the moon's outer shell is stiff, as would be the case if the ocean were slowly crystallizing and turning to ice. Otherwise, the moon's shape would tend to even itself out over time, like warm candle wax. This freezing process would have important implications for the habitability of Titan's ocean, as it would limit the ability of materials to exchange between the surface and the ocean.

A further consequence of a rigid ice shell, according to the study, is any outgassing of methane into Titan's atmosphere must happen at scattered "hot spots" -- like the hot spot on Earth that gave rise to the Hawaiian Island chain. Titan's methane does not appear to result from convection or plate tectonics recycling its ice shell.

How methane gets into the moon's atmosphere has long been of great interest to researchers, as molecules of this gas are broken apart by sunlight on short geological timescales. Titan's present atmosphere contains about five percent methane. This means some process, thought to be geological in nature, must be replenishing the gas. The study indicates that whatever process is responsible, the restoration of Titan's methane is localized and intermittent.

"Our work suggests looking for signs of methane outgassing will be difficult with Cassini, and may require a future mission that can find localized methane sources," said Jonathan Lunine, a scientist on the Cassini mission at Cornell University, Ithaca, New York, and one of the paper's co-authors. "As on Mars, this is a challenging task."

Image credit: NASA/JPL-Caltech/SSI/Univ. of Arizona/G. Mitri/University of Nantes

Note: For more information, see Saturn's Moon Titan Has a Very Salty Ocean.

Saturn's Shadows and Mimas

It may seem odd to think of planets casting shadows out in the inky blackness of space, but it is a common phenomenon. Earth’s shadow obscures the Moon during a lunar eclipse, and Jupiter’s moons cast small shadows onto their parent planet.

One of the best places in our Solar System to spot intriguing and beautiful celestial shadows is at Saturn. On 1 July, the international Cassini mission celebrates 10 years of exploring Saturn, its rings and its moons, an endeavor that has produced invaluable science but also stunning images like this.

Drifting along in the foreground, small and serene, is Saturn’s icy moon Mimas. The blue backdrop may at first appear to be the gas giant’s famous and impressive set of rings, with pale and dark regions separated by long inky black slashes, but it is actually the northern hemisphere of Saturn itself. The dark lines slicing across the frame are shadows cast by the rings onto the planet.

Although we may not associate the color blue with Saturn, when Cassini arrived at the planet the northernmost regions displayed the delicate blue palette shown in this image. As this region of Saturn is generally quite free of cloud, scattering by molecules in the atmosphere causes sunlight to take a longer path through the atmosphere. The light is scattered predominantly at shorter – bluer – wavelengths. This is similar to why the sky on Earth appears blue to our eyes.

Seasonal changes over the years since this photo was taken have turned the blue into Saturn's more familiar golden hue. The reverse is occurring in the south, which is slowly becoming bluer.

This image is composed of infrared, optical and ultraviolet observations from Cassini’s narrow-angle camera on 18 January 2005. The colors closely match what the scene would look like in true color.

This image was first published on the NASA Cassini website, in 2005.

Image credit: NASA/JPL/Space Science Institute

Dione

When imaged with the Sun nearly at our backs, Dione's heavily scarred surface lacks the shadows that emphasize the surface topography. However, this geometry highlights variations in surface brightness, which provide further evidence of Dione's active and often violent past.

The surface of Dione (698 miles, or 1,123 kilometers across) is covered in craters, reminding us of the impacts that have shaped all of the worlds of our solar system. Dione's surface also bears linear features that suggest geological activity in the past. See PIA07638 for more information.

Lit terrain seen here is on the Saturn-facing hemisphere of Dione. North on Dione is up and rotated 33 degrees to the right. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on June 27, 2013.

The view was obtained at a distance of approximately 810,000 miles (1.3 million kilometers) from Dione. Image scale is 5 miles (8 kilometers) per pixel.

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

Water's Early Journey in a Solar System

The building blocks of comets, and apparently Saturn's largest moon, Titan, formed under similar conditions in the disk of gas and dust that formed the sun.

NASA's Spitzer Space Telescope observed a fledgling solar system like the one depicted in this artist's concept, and discovered deep within it enough water vapor to fill the oceans on Earth five times. This water vapor starts out in the form of ice in a cloudy cocoon (not pictured) that surrounds the embryonic star, called NGC 1333-IRAS 4B (buried in center of image). Material from the cocoon, including ice, falls toward the center of the cloud. The ice then smacks down onto a dusty pre-planetary disk circling the stellar embryo (doughnut-shaped cloud) and vaporizes. Eventually, this water might make its way into developing planets.

Illustration credit: NASA/JPL-Caltech

Note: For more information, see Titan's Building Blocks Might Pre-date Saturn.

Titan

Only a sharp and careful eye can make out the subtle variations in Titan's clouds when viewed in visible light. However, these subtle features sometimes become more readily apparent when imaged at other wavelengths of light. This infrared image clearly reveals a band around the Titan's north pole.

Cassini scientists are regularly monitoring Titan, hoping to understand more about Titan's dense atmosphere and clouds.

This view looks toward the leading side of Titan. North on Titan is up and rotated 31 degrees to the left. The image was taken with the Cassini spacecraft narrow-angle camera on January 26, 2014 using a spectral filter which preferentially admits wavelengths of near-infrared light centered at 889 nanometers.

The view was acquired at a distance of approximately 1.5 million miles (2.4 million kilometers) from Titan. Image scale is 9 miles (14 kilometers) per pixel.

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

Radio Occultation of Titan During Cassini Flyby

Cassini will attempt to bounce signals off of Saturn's moon Titan once more during a flyby on June 18, 2014, revealing important details about the moon's surface.

As NASA's Cassini spacecraft zooms toward Saturn's smoggy moon Titan for a targeted flyby on June 18, mission scientists are excitedly hoping to repeat a scientific tour de force that will provide valuable new insights into the nature of the moon's surface and atmosphere.

For Cassini's radio science team, the last flyby of Titan, on May 17, was one of the most scientifically valuable encounters of the spacecraft's current extended mission. The focus of that flyby, designated "T-101," was on using radio signals to explore the physical nature of Titan's vast northern seas and probe the high northern regions of its substantial atmosphere.

The Cassini team hopes to replicate the technical success of that flyby during the T-102 encounter, slated for June 18, during which the spacecraft will attempt similar measurements of Titan. During closest approach, the spacecraft will be just 2,274 miles (3,659 kilometers) above the surface of the moon while traveling at 13,000 miles per hour (5.6 kilometers per second).

During the upcoming flyby, if all goes well as before, Cassini's radio science subsystem will bounce signals off the surface of Titan, toward Earth, where they will be received by the ground stations of NASA's Deep Space Network. This sort of observation is known as a bistatic scattering experiment and its results can yield clues to help answer a variety of questions about large areas of Titan's surface: Are they solid, slushy or liquid? Are they reflective? What might they be made of?

During the May encounter, Cassini beamed radio signals over the two largest bodies of liquid on Titan, seas named Ligeia Mare and Kraken Mare. During that first attempt, scientists could not be certain the signals would successfully bounce off the lakes to be received on Earth. They were thrilled when ground stations received specular reflections -- essentially the glint -- of the radio frequencies as they ricocheted off Titan.

"We held our breath as Cassini turned to beam its radio signals at the lakes," said Essam Marouf, a member of the Cassini radio science team of San Jose State University in California. "We knew we were getting good quality data when we saw clear echoes from Titan's surface. It was thrilling."

A second technical accomplishment -- an experiment to send precision-tuned radio frequencies through Titan's atmosphere -- also makes the May and June flybys special. The experiment, known as a radio occultation, provides information about how temperatures vary by altitude in Titan's atmosphere. Preparing for these experiments tested just how thoroughly the Cassini team has come to understand the structure of Titan's atmosphere during nearly a decade of study by the mission.

During this type of radio occultation, a signal is beamed from Earth through the atmosphere of Titan toward the Cassini spacecraft, which responds back to Earth with an identical signal. Information about Titan is imprinted in the signal as it passes through the moon's atmosphere, encountering differences in temperature and density. The trick is that the transmitted signal must be varied during the experiment so that it remains nearly constant when received by the spacecraft.

In order to give the occultation experiments any chance of success, the team has to account for not only the relative motions of the spacecraft and the transmitting antennas on the rotating planet Earth, but also the ways the signal is bent by different layers in Titan's atmosphere.

While this procedure has been used successfully for several Saturn occultations in the past two years, it had not yet been tried at Titan. And since the Titan occultations last just a few minutes, the team was concerned about how quickly the frequency lockup between ground and spacecraft could be established, if at all. For comparison, NASA's Magellan mission tried the technique at Venus in the 1990s, without success.

As they waited for signs of confirmation during the May encounter, the team saw the signal lock occur in only a few seconds, indicating that their predictions were spot-on. Data on Titan's atmosphere flowed in, adding new information to the mission's campaign to monitor the changing of the seasons on this alien moon.

"This was like trying to hit a hole-in-one in golf, except that the hole is close to a billion miles away, and moving," said Earl Maize, Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "This was our first attempt to precisely predict and compensate for the effect of Titan's atmosphere on the uplinked radio signal from Earth, and it worked to perfection."

Illustration credit: NASA/JPL-Caltech

Atlas Emerging From Shadow

The Cassini spacecraft captures a glimpse of the moon Atlas shortly after emerging from Saturn's shadow. Although the sunlight at Saturn's distance is feeble compared to that at the Earth, objects cut off from the Sun within Saturn's shadow cool off considerably.

Scientists study how the moons around Saturn cool and warm as they enter and leave Saturn's shadow to better understand the physical properties of Saturn's moons.

This view looks toward the sunlit side of the rings from about 44 degrees above the ringplane. The image was taken in visible light with the Cassini spacecraft narrow-angle camera on January 23, 2014.

The view was acquired at a distance of approximately 1.6 million miles (2.6 million kilometers) from Atlas and at a Sun-Atlas-spacecraft, or phase, angle of 93 degrees. Image scale is 10 miles (16 kilometers) per pixel.

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

Phoebe, Approaching and Departing

As it entered the Saturn system, NASA's Cassini spacecraft performed its first targeted flyby of one of the planet's moons. On June 11, 2004, Cassini passed Phoebe, the largest of Saturn's outer or "irregular" moons, at an altitude of just 1,285 miles (2,068 kilometers). This was the sole close flyby of one of the outer moons of Saturn in the entire Cassini mission.

This montage of two views is published by the Cassini team to mark the 10th anniversary of the Phoebe flyby.

The image on the left side shows Cassini's view on approach to Phoebe, while the right side shows the spacecraft's departing perspective. Most of the left-side view was previously released as PIA06073; an area on its upper right side is newly filled in here. Most of the view on the right side has not previously been released, although the crater at upper left is seen in PIA06074.

Phoebe's shape is approximately spherical (see PIA06070 and PIA15507 for more details), with a diameter of 136 miles (219 kilometers) on its longest axis and 127 miles (204 kilometers) on its shortest axis, which is also the rotation axis. This is approximately 16 times smaller than Earth's moon.

For several reasons, Phoebe is thought to be a captured object that does not share a joint origin with Saturn and the inner, "regular" satellites. It orbits in a retrograde direction, opposite to the direction of Saturn's other major moons. Its overall density was determined by Cassini scientists to be quite large for a moon of Saturn. The prevailing view is that Phoebe might have formed in the Kuiper Belt, far beyond the orbit of Saturn. It might thus be a small cousin of the largest Kuiper Belt object, Pluto.

The image mosaic on the left, recorded about 45 minutes before closest approach to Phoebe, is composed of six frames from Cassini's Narrow-Angle Camera (NAC), plus one Wide-Angle Camera (WAC) image to fill the gap on the upper-right limb. The image has a spatial resolution of 260 feet (80 meters) per pixel. The sun-Phoebe-spacecraft, or phase, angle is 80 degrees.

The image at right, taken about half an hour after closest approach, is composed of eight NAC frames. The spatial resolution is 210 feet (65 meters) per pixel, and the phase angle is 83 degrees.

The images have been slightly rescaled from their original formats and sharpened. Because Phoebe is a very dark object, contrast enhancement was also necessary. At such high phase angles, the brightest parts of the surface, except where bright ice is exposed, reflect only about four percent of the incoming sunlight. The mosaics are composed of monochromatic, or single-color, images. Since Phoebe is a very dark object with no obvious coloration, a natural color view would probably look somewhat similar.

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