As with the Earth, the rotational axis of Saturn, the second largest planet in the Solar System, is inclined with respect to the plane of its path around the Sun. This means that in the almost 29 (Earth) years and 164 days that Saturn requires to orbit the Sun once, there are also seasons on this celestial body. For half a 'Saturn year' – that is, almost 15 Earth years – the southern hemisphere receives the greater amount of sunlight and for 15 years the northern hemisphere is more brightly illuminated. However, the quantity of the Sun's energy that the planet receives at a distance of something over 1.4 billion kilometres from the Sun is only one ninetieth of that received on Earth. In August 2009, Saturn passed through the vernal equinox – that is, for a virtual observer on the clouds of Saturn, the Sun appeared to cross the celestial equator. The southern summer has come to an end and after 11 August 2009, when day and night were of equal length – known as the equinox – spring started in the northern hemisphere. At the time of the equinoxes, the Sun shines precisely on the edge of Saturn's rings, which surround the planet at its equator. This rather rare astronomical event is something worth celebrating – not just for observers on Earth with their telescopes – because at that time the edge of the plane of the rings is seen as a small line. For the Saturn orbiter Cassini, too, the way that the light falls across the disc at a very shallow angle offers exceptional possibilities for observation, producing special scientific results regarding the structure and dynamics of the rings. At a distance from Saturn of approximately 847,000 kilometres and a viewing angle of 20 degrees above the surface of the rings, Cassini’s wide-angle camera acquired a sequence of 75 images one-and-a-half days after the equinox in 2009, producing this image mosaic of Saturn, its rings and some of its moons. The scale of the picture is 50 kilometres per pixel. The unusual lighting geometry means that the rings appear very dark. In contrast, the structures outside the plane of the rings are unusually bright and throw long shadows across the rings. In addition, the shadows of Saturn's extensive rings appear at the equinox as a single, narrow band on the planet.
NASA/JPL/Space Science Institute.
Colour image of Saturn's moon Dione obtained on 11 October 2005. In this image, Saturn can be seen in blue and gold behind Dione. The horizontal stripes in the lower half of Saturn's rings are clearly seen. At the time the image was taken, Cassini was nearly level with Saturn's rings.Blue, green and infrared spectral filters were used to obtain this image. It was taken with the Wide Field Camera on board the Cassini spacecraft at a distance of approximately 39,000 kilometres from Dione. The image resolution is about two kilometres per pixel.
Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene. The colour information in the coloured view is completely artificial: it is derived from red, green and blue images taken at nearly the same time and phase angle as the clear filter image. This colour information was overlaid onto the previously released clear filter view in order to approximate the scene as it might appear to the human eye.
NASA/JPL /University of Arizona/Space Science Institute.
The rings -Saturn's trademark. The beautiful and mysterious rings of Saturn are one of the most striking phenomena in our Solar System. They have a diameter of nearly 500,000 kilometres and are also extremely thin. Billions of swirling ice and rock particles orbit the planet at great speed, forming an intricate pattern. The image shows a bright band of the isolated C-rings (inside) and the B-rings to the left.Scientists still don't know the age or origin of the rings but the Cassini mission at least provided an insight. On its arrival in July 2004, the spacecraft transmitted valuable data. High-resolution images were obtained during its pass through the ring plane, a cross-sectional profile was made using ultraviolet and infrared measurements and infrared images were obtained. Cassini discovered previously unknown smaller rings and moons in the single ring nearby.The rings are composed mainly of water ice, rocks and dust, whose main ingredients are mineral silicates. With the spectrometer VIMS (Visible and Infrared Mapping Spectrometer), traces of iron were detected. This corresponds to the material, present in the dark areas of the moons Phoebe and Iapetus; VIMS also found evidence of organic carbon-nitrogen compounds in the rings.
This image shows part of Saturn’s northern hemisphere in false colour. This brings out the separate bands and swirls of clouds in the atmosphere of the high northern latitudes of the gas planet more clearly. It also shows that with regard to their dynamics and to the chemical composition of Saturn’s upper atmosphere, the northern latitudes are markedly different from the rather more monotonous equatorial zone, which appears in bright, bluish tones in the lower right quadrant of the image. In the lower left quadrant you can see Saturn’s innermost rings as very thin lines; as the Sun is shining obliquely from beneath the ring plane, the shadows of the rings are projected onto Saturn’s cloud cover as clear-cut lines. The image was recorded using Cassini’s Imaging Science Subsystem (ISS) from an angle of 52 degrees above the ring plane and at a distance of 1.5 million kilometres.
Iapetus is one of Saturn’s most mysterious moons: this icy moon with a diameter of 1468 kilometres presents the observer with two hemispheres that have developed in completely different ways. The hemisphere facing the opposite direction of the moon’s orbit around Saturn, the so-called “trailing side”, as well as the polar areas, consist of white ice and reflect the light of the Sun almost 100 percent; on the surface of the hemisphere facing the direction of orbit, the so-called “leading side”, however, carbon compounds cover the ice – cyanide and other carbon compounds make the surface as black as tar. This false-colour image shows the first mosaic of high-resolution image data from bright side of Iapetus; it consists of 60 separate images, which were recorded in September 2007 from a distance of 73 000 kilometres. This flyby had been planned in great detail by staff members of the Freie Universität Berlin and DLR. The transition zone between both extremes is especially interesting.Decisive explanatory information about the cause of Iapetus’ strange characteristics has been provided not only by the image data from Cassini, but also through observations performed with NASA’s Hubble Space Telescope. Beyond the orbit of Iapetus, Saturn is surrounded by a giant torus of tiny, dark dust particles, inclined at 27 degrees to the equator and the main ring plane. The particles originate from impacts on the small moons of Saturn that orbit further out than Iapetus, such as Phoebe, whose orbit lies in the middle of the torus. The density of the particles is extremely low, yet high enough for some of the small particles that migrate towards the interior of the Saturn system are attracted by the leading side of Iapetus and, over a long periodof time, have been collected and compacted into a thin, black layer.
At 5150 kilometres across, Titan is the second largest moon in the Solar System and one of the most mysterious. An atmosphere surrounds many planets. Titan, however, is the only moon in the Solar System with a significant gaseous envelope. The atmosphere is a brownish-orange and so dense that the moon’s surface cannot be seen at visible light wavelengths. Only by using what are known as 'atmospheric windows' – narrow wavelength bands in the near and middle infrared is analysis of the surface possible. This technique is used by the infrared spectrometer VIMS (Visible and Infrared Mapping Spectrometer) on the Cassini space probe, which has been analysing the Saturnian system since July 2004. Because of the distance of Titan from the Sun, its surface temperature is about minus 180 degrees Celsius. The picture shows a superposition of VIMS images of Titan in three different infrared wavelengths: 1.3 microns (thousandths of a millimetre, blue),) 2 microns (green) and 5 microns (red). The circular structure in the middle is probably an older impact basin. Titan's equatorial latitudes are most likely dry areas, without extensive 'water'. The numerous liquid bodies in the northern hemisphere, one of which is described in this web article, the Kraken Mare, are probably part of an active fluid circulation. These lakes are fed by liquid hydrocarbons that a drainage system carries out of the surrounding valleys. The drainage system, in turn, is fed by methane and ethane precipitation. Many scientists suspect that the nitrogen atmosphere of Titan exhibits strong similarities with Earth's primitive atmosphere.
NASA/JPL/University of Arizona.
This image was obtained by assembling 30 frames captured with the European Huygens probe during its descent through Titan's atmosphere. They were made from altitudes between 13 and eight kilometres, as the spacecraft approached its landing site. The images have a resolution of about 20 metres per pixel and represent an area of about 30 kilometres.During the descent phase through Titan's atmosphere, the Huygens probe dropped almost vertically downward with a speed of about five meters per second. Huygens drifted horizontally at a speed of about one metre per second.
ESA/NASA/JPL/University of Arizona.
This image of Saturn (in the foreground) and Rheawas obtained on 3 February 2006 with a camera onboard the Cassini spacecraft from about 4.1 million kilometres from Saturn and 4.6 million kilometres from Saturn's moon Rhea.
At only 500 kilometres in diameter, the Saturnian moon Enceladus does not have sufficient mass to allow enough heat to be generated in its interior to melt the ice in its mantle. However, the Cassini spacecraft discovered fissures on this small icy moon that eject hundred-kilometre high fountains of water into the vacuum of space. The droplets freeze immediately and most fall back onto the icy surface, but some also feed Saturn’s outer rings.Enceladus appears to gain enough energy to produce reservoirs of melted ice under its icy crust from the action of tidal forces exerted by Saturn, its 120,000-kilometre diameter gas giant host planet. Under high pressure, this water is discharged into space through prominent systems of fissures at the South Pole – a form of volcanic activity referred to as ice volcanism or cryovolcanism, as opposed to the magmatic volcanism seen on Earth.
Image of Saturn's rings. For the first time we are able to see the B-ring, which had, until now, eluded the spacecraft's view. The structure of the B-ring is greatly distinguishable from its two neighbors, the A and C-ring.This photograph of Saturn's rings was obtained on 3 May 2005, when the Cassini spacecraft was visible from the Earth behind Saturn's rings. Cassini sent radio signals to Earth through the ring system. The researchers were then able to measure how strongly the signal was affected as it passed through the rings. The denser a ring is, the weaker the received signal. This experiment will allow scientists to determine the distribution of ring material and the size of the particles in the rings.The purple areas indicate regions of the recording, in which there are no particles smaller than five centimetres. The green and blue areas indicate regions containing particles smaller than five and one centimetre. The wide, white area near the centre is the densest region of the B ring.The origin of Saturn's ring system is still a mystery. It consists of thousands of individual rings. Measured from one side to the other, the ring system is wider than the distance between Earth and Moon. The seven main rings of Saturn are named in order of discovery and not by their distance to the planet. Starting from Saturn, they are referred to as D, C, B, A, F, G and E rings.
Overall view of Dione, one of three Saturnian moons that are no longer active but still show signs of high levels of geological development. This image was acquired on 24 July 2006, at a distance of 365,000 kilometres.The moon has a diameter of 1125 kilometres and, along with the icy moons Tethys and Rhea, makes up a group of three nearly equally sized moons circling Saturn between the orbits of Enceladus and Titan. Their surfaces show clear evidence of earlier tensions in their crusts, which can be seen in what are referred to as tectonic deformations. This suggests that these three moons have experienced a high level of geological development, as have, for example, Saturn’s moons Mimas and Iapetus.In this group of moons, Dione is the most geologically developed satellite. The Voyager image data from 1980 showed mostly plains covered by closely-spaced impact craters, but, in addition, a pattern of bright, very fine lines (wispy streaks) that were initially interpreted as a sign of ice volcanism. In this model of cryovolcanism, icy material escapes through fissures in the minus 186 degrees Celsius surface and quickly freezes. The high-resolution images acquired by the Cassini spacecraft have now revealed these features to be tectonic in origin, primarily caused by episodes of crustal strain, but also by shear and compressive stresses that arose at different times in the past. The bright lines visible in the Voyager data have their origin in almost pure water ice, exposed on the steep slopes of these structures by tectonic deformations of the rigid ice crust.
Titan's upper atmosphere in ultraviolet. Several dust layers are easily recognizable.
This coloured cylindrical map of Jupiter's South Pole was constructed from images taken by the narrow-angle camera onboard NASA's Cassini spacecraft on 11 and 12 December 2000, as the spacecraft neared Jupiter during its flyby of the giant planet. Cassini was on its way to Saturn. They are the most detailed global colour maps of Jupiter ever produced. The smallest visible features are about 120 kilometres across. The map is composed of 36 images. Although the raw images are in just two colours, 750 nanometres (near-infrared) and 451 nanometres (blue), the map's colours are close to those the human eye would see when gazing at Jupiter.The maps show a variety of colourful cloud features, including parallel reddish-brown and white bands, the Great Red Spot, chaotic regions with many small vortices. Many clouds appear in streaks and waves due to continual stretching and folding by Jupiter's winds and turbulence.
DLR (CC-BY 3.0).
Dreamy colours ranging from pale rose to butterscotch to sapphire give this utterly inhospitable gas planet a romantic appeal. Shadows of the rings caress the northern latitudes whose blue colour is presumed to be a seasonal effect. Enceladus (505 kilometers across) hugs the ringplane right of center. Images taken using red, green and blue spectral filters were combined to create this colour view, which approximates what the human eye would see. The images were taken with the Cassini spacecraft wide-angle camera on 16 March, 2006 at a distance of approximately 2.1 million kilometers from Saturn. Image scale is 120 kilometers per pixel on Saturn.
The glowing arc of light in this image is the icy F-ring. Saturn's moon Rhea (1528 kilometers across) can be seen in the background, illuminated by the reflected light of both Saturn and its rings. Only the narrow strip of light at the lower edge of the moon arrives directly from the Sun.This picture was taken on 30 October 2005 by the narrow-angle camera onboard Cassini from a distance of approximately 689 000 kilometres. The resolution of the image is about four kilometres per pixel.
The surface of Saturn's second-largest moon Rhea consists predominantly of water ice and is marked by countless impact craters. One remarkably bright, probably younger crater is one of the goals for the camera and the spectrometer during the Cassini fly-by on 27 November 2005.
The Cassini spacecraft captures the ripples, loops and storms that swirl in Saturn's east-west flowing cloud bands. The image was taken with the Cassini spacecraft wide-angle camera using a spectral filter sensitive to wavelengths of infrared light centered at 728 nanometres. The view was obtained on 13 December 2006 at a distance of approximately 775,000 kilometres from Saturn. Image scale is 43 kilometres per pixel.
On 17 February 2005, Cassini performed its first close flyby of Saturn's moon Enceladus at a distance of about 1180 kilometres, obtaining this image of the moon's surface. Enceladus is one of the most reflective objects in our Solar System; its surface is reminiscent of freshly fallen snow. Cassini came closer to Enceladus than any other spacecraft.This image shows a portion of the surface of Saturn's moon Enceladus. It was taken from a distance of about 29,640 kilometres with the narrow angle camera on Cassini's Imaging Science Subsystem. The spatial resolution of the image is 170 metres per pixel.
Saturn in colour, photographed on 27 March 2004 with JPL's camera system on board the Cassini spacecraft.