Cassini's greatest hits: visions of Saturn

Next Wednesday, the Cassini spacecraft celebrates five years in orbit around Saturn — and as if to mark the occasion, Nasa yesterday published some of its most spectacular science results yet. Observations from the orbiter have suggested that the moon Enceladus may boast a subterranean ocean, of the sort that could potentially harbour life.

This remarkable finding, published in the journal Nature, is however only the latest triumph for a voyage of discovery that began with the mission’s launch back in 1997. And as part of the Greenwich Royal Observatory’s celebrations of the International Year of Astronomy, it is showcasing some of the most stunning images captured by Cassini and its Huygens counterpart, which landed on the moon Titan in 2005. The free exhibition runs until August 30.

This is true space exploration. While astronauts faff around on the International Space Station, in low Earth orbit a few hundred miles above our heads, it has fallen to robots like Cassini to truly extend the frontiers of human knowledge about the solar system.

 MH

This striking false-color mosaic was created from 25 images taken by Cassini's visual and infrared mapping spectrometer over a period of 13 hours, and captures Saturn in nighttime and daytime conditions. The visual and infrared mapping spectrometer acquires data simultaneously at 352 different wavelengths, or spectral channels. Data at wavelengths of 2.3, 3.0 and 5.1 microns were combined in the blue, green and red channels of a standard color image, respectively, to make this false-color mosaic.

This image was acquired on Feb. 24, 2007, while the spacecraft was 1.58 million kilometers (1 million miles) from the planet and 34.6 degrees above the ring plane. The solar phase angle was 69.5 degrees. In this view, Cassini was looking down on the northern, unlit side of the rings, which are rendered visible by sunlight filtering through from the sunlit, southern face.

A large, bright and complex convective storm that appeared in Saturn's southern hemisphere in mid-September 2004 was the key in solving a long-standing mystery about the ringed planet. Saturn's atmosphere and its rings are shown here in a false color composite made from Cassini images taken in near infrared light through filters that sense different amounts of methane gas. Portions of the atmosphere with a large abundance of methane above the clouds are red, indicating clouds that are deep in the atmosphere. Grey indicates high clouds, and brown indicates clouds at intermediate altitudes. The rings are bright blue because there is no methane gas between the ring particles and the camera.

The complex feature with arms and secondary extensions at th etop of the image is called the Dragon Storm. It lies in a region of the southern hemisphere referred to as "storm alley" by imaging scientists because of the high level of storm activity observed there by Cassini in the last year. The Dragon Storm was a powerful source of radio emissions during July and September of 2004. The radio waves from the storm resemble the short bursts of static generated by lightning on Earth. Cassini detected the bursts only when the storm was rising over the horizon on the night side of the planet as seen from the spacecraft; the bursts stopped when the storm moved into sunlight. This on/off pattern repeated for many Saturn rotations over a period of several weeks, and it was the clock-like repeatability that indicated the storm and the radio bursts are related. Scientists have concluded that the Dragon Storm is a giant thunderstorm whose precipitation generates electricity as it does on Earth. The storm may be deriving its energy from Saturn's deep atmosphere.

This captivating natural color view was created from images collected shortly after Cassini began its extended Equinox Mission in July 2008. It can be contrasted with earlier images from the spacecraft's four-year prime mission that show the shadow of Saturn's rings first draped high over the planet's northern hemisphere, then shifting southward as northern summer changed to spring. During this time, the colors of the northern hemisphere have evolved from azure blue to a multitude of muted-colored bands.

 
This view of Titan reveals structure in the moon's complex atmosphere. The geometry of the Cassini spacecraft's view of Titan during this flyby was similar to that of Voyager 1's pass in 1980. The color view was created by combining images taken using red, green and blue spectral filters. The images were taken with the Cassini spacecraft wide-angle camera on Dec. 26, 2005, at a distance of approximately 193,000 kilometers (120,000 miles) from Titan and at a Sun-Titan-spacecraft, or phase, angle of 29 degrees

Although the Huygens probe has now pierced the murky skies of Titan and landed on its surface, much of the moon remains for the Cassini spacecraft to explore. Titan continues to present exciting puzzles. This view of Titan uncovers new territory not previously seen at this resolution by Cassini's cameras. The view is a composite of four nearly identical wide-angle camera images, all taken using a filter sensitive to wavelengths of infrared light centered at 939 nanometers. The individual images have been combined and contrast-enhanced in such a way as to sharpen surface features and enhance overall brightness variations. 

Cassini has found Titan's upper atmosphere to consist of a surprising number of layers of haze, as shown in this ultraviolet image of Titan's night side limb, colorized to look like true color. The many fine haze layers extend several hundred kilometers above the surface. Although this is a night side view, with only a thin crescent receiving direct sunlight, the haze layers are bright from light scattered through the atmosphere.

The image was taken with the Cassini spacecraft narrow angle camera. About 12 distinct haze layers can be seen in this image. The limb shown here is at about 10 degrees south latitude, in the equatorial region.

This image is a stereographic (fish-eye) projection taken with the descent imager/spectral radiometer onboard the European Space Agency's Huygens probe, when the probe was about 5 kilometers (3 miles) above Titan's surface. The images were taken on Jan. 14, 2005. The Huygens probe was delivered to Saturn's moon Titan by the Cassini spacecraft, which is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. NASA supplied two instruments on the probe, the descent imager/spectral radiometer and the gas chromatograph mass spectrometer.

 

A masterpiece of deep time and wrenching gravity, the tortured surface of Saturn's moon Enceladus and its fascinating ongoing geologic activity tell the story of the ancient and present struggles of one tiny world. This is a story that is recounted by imaging scientists in a paper published in the journal Science on March 10, 2006.

The enhanced color view of Enceladus seen here is largely of the southern hemisphere and includes the south polar terrain at the bottom of the image.

Ancient craters remain somewhat pristine in some locales, but have clearly relaxed in others. Northward-trending fractures, likely caused by a change in the moon's rate of rotation and the consequent flattening of the moon's shape, rip across the southern hemisphere. The south polar terrain is marked by a striking set of `blue' fractures and encircled by a conspicuous and continuous chain of folds and ridges, testament to the forces within Enceladus that have yet to be silenced.

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus. Their study -- published in the Oct. 11, 2007, issue of the journal Nature -- identifies eight source locations, all on the prominent tiger stripe fractures, or sulci, in the moon's south polar region. Some of the sources occur in regions not yet observed by Cassini's composite infrared spectrometer, and the researchers predict that future Cassini observations of those locations will find elevated temperatures.

This close-up view of Saturn's moon Enceladus looks toward the moon's terminator (the transition from day to night) and shows a distinctive pattern of continuous, ridged, slightly curved and roughly parallel faults within the moon's southern polar latitudes. These surface features have been informally referred to by imaging scientists as "tiger stripes" due to their distinctly stripe-like appearance when viewed in false color.

Illumination of the scene is from the lower left. The image was obtained in visible light with the Cassini spacecraft narrow-angle camera on July 14, 2005, at a distance of about 20,720 kilometers (12,880 miles) from Enceladus, and at a Sun-Enceladus-spacecraft, or phase, angle of 46 degrees. The image scale is 122 meters (400 feet) per pixel. The image's contrast has been enhanced to aid visibility of surface features.

This splendid view showcases Dione's tortured complex of bright cliffs. At lower right is the feature called Cassandra, exhibiting linear rays extending in multiple directions.

The trailing hemisphere of Dione (1,126 kilometers, or 700 miles across) is seen here. North is up.

The image was taken in polarized green light with the Cassini spacecraft narrow-angle camera on July 24, 2006 at a distance of approximately 263,000 kilometers (163,000 miles) from Dione.

Cassini captures the first high-resolution glimpse of the bright trailing hemisphere of Saturn's moon Iapetus.

This false-color mosaic shows the entire hemisphere of Iapetus (1,468 kilometers, or 912 miles across) visible from Cassini on the outbound leg of its encounter with the two-toned moon in Sept. 2007. The central longitude of the trailing hemisphere is 24 degrees to the left of the mosaic's center.

Also shown here is the complicated transition region between the dark leading and bright trailing hemispheres. This region, visible along the right side of the image, was observed in many of the images acquired by Cassini near closest approach during the encounter.

Revealed here for the first time in detail are the geologic structures that mark the trailing hemisphere. The region appears heavily cratered, particularly in the north and south polar regions. Near the top of the mosaic, numerous impact features visible in NASA Voyager 2 spacecraft images (acquired in 1981) are visible, including the craters Ogier and Charlemagne.

The most prominent topographic feature in this view, in the bottom half of the mosaic, is a 450-kilometer (280-mile) wide impact basin, one of at least nine such large basins on Iapetus. In fact, the basin overlaps an older, similar-sized impact basin to its southeast.

In many places, the dark material--thought to be composed of nitrogen-bearing organic compounds called cyanides, hydrated minerals and other carbonaceous minerals--appears to coat equator-facing slopes and crater floors. The distribution of this material and variations in the color of the bright material across the trailing hemisphere will be crucial clues to understanding the origin of Iapetus' peculiar bright-dark dual personality.

The view was acquired with the Cassini spacecraft narrow-angle camera on Sept. 10, 2007, at a distance of about 73,000 kilometers (45,000 miles) from Iapetus.

The color seen in this view represents an expansion of the wavelengths of the electromagnetic spectrum visible to human eyes. The intense reddish-brown hue of the dark material is far less pronounced in true color images. The use of enhanced color makes the reddish character of the dark material more visible than it would be to the naked eye.

This mosaic consists of 60 images covering 15 footprints across the surface of Iapetus. The view is an orthographic projection centered on 10.8 degrees south latitude, 246.5 degrees west longitude and has a resolution of 426 meters (0.26 miles) per pixel. An orthographic view is most like the view seen by a distant observer looking through a telescope.

At each footprint, a full resolution clear filter image was combined with half-resolution images taken with infrared, green and ultraviolet spectral filters (centered at 752, 568 and 338 nanometers, respectively) to create this full-resolution false color mosaic.

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 click here .

The Cassini imaging team homepage is here.

All images by kind permission of NASA/JPL/Space Science Institute

Comments

[Lewis Winders]

Wonderful stuff. Many thanks.

[Darius Midwinter]

Fantastic - perhaps the BBC could dain to include the Cassini project in a future Horizon - or is it not "yoof" enough a subject for it?

[J Barnes]

"Fantastic - perhaps the BBC could dain to include the Cassini project in a future Horizon - or is it not "yoof" enough a subject for it?"

For goodness sake Darius, it is DEIGN!

[Ross]

J Barnes: You are correct, although the capital letters are UNNECESSARY. I blame educational standards and, of course, THE BBC.

[Gemma]

This was a very interesting article.

It would be very interesting if the BBC could feature some of the Cassini project on television...yet if not then i'm sure the exhibition will surfice.