Saturn ring plane crossing by Cassini in 2005, at a time when the plane of the rings was tilted relative to the Sun, so that (as is the case for half of each orbit) the rings cast a shadow on the planet's northern hemisphere. (Cassini Imaging Team, ISS, JPL, ESA, NASA, apod100215)

A more typical view of Saturn, as seen from below the ring plane. (Cassini Imaging Team, ISS, JPL, ESA, NASA, apod040117)

     Natural-color image of Saturn, taken by the Cassini spacecraft in April 2008. From this angle the rings appear relatively dark, as the Sun is below them, lighting the other side (relative to the spacecraft). Their shadow on the planet is narrow because the planet is approaching the point where its axis of rotation (and the ring plane) are sideways relative to the Sun. Bands parallel to the ring plane and the planet's Equator are faintly visible, unlike the dramatic bands on Jupiter, because the colder temperatures cause the layer where variations in brightness occur to be much deeper in the atmosphere, and a thick layer of gas filled with ammonia-ice haze lies above that region. Despite this the tops of clouds (similar to thunderheads in the Earth's atmosphere) are visible here and there, particularly in the darker bands just above the Equator.

     One of the strangest features in Saturn's atmosphere is a hexagonal pattern of atmospheric motion near its north pole. For rapidly rotating planets such as Saturn, Coriolis effects should cause polar winds to form circular vortices; but how those vortices could possibly combine to form the hexagonal structure shown here, even for a brief period of time, is beyond anyone's understanding. How the hexagonal structure could have persisted since first observed by the Voyager flybys of the early 1980's is even more mysterious. The central portion of the field was poorly imaged, so it was deliberately left blank in this press release. (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod091214)

     A view of Saturn from behind, taken by the Voyager 1 spacecraft in 1980, four days after passing the planet. The shadow of the planet is visible on the rings, and the ring-shadow (very thin because the planet was near an Equinox) is seen at right, on the daylit side of the planet. In this digitally enhanced image the night side of the planet is noticeably lit by light reflected from the rings.

     An unusual storm, possibly related to seasonal changes in the weather, is visible in this image, taken on December 1, 1994 by the Hubble Space Telescope Wide Field and Planetary Camera. At that time the planet was only a few months away from the 1995 ring-plane crossing. At the ring-crossing the rings almost completely disappear, because although 150,000 miles across they are less than a mile thick.

     Above, the most spectacular storm ever observed on Saturn, as photographed by the Cassini spacecraft on December 24, 2010. The spacecraft was passing through the ring plane when it took the image, so the rings are seen nearly edge-on and are just barely visible as a very thin horizontal line; but their shadow, lit from above, is clearly visible on the "surface" (the upper atmosphere) of the planet. Meanwhile, from the Earth the planet's movement around the Sun is gradually bringing the rings more into view, so they form a narrow oval around the planet. Most atmospheric features on Saturn (even the bands parallel to the Equator) are too faint to easily notice from Earth, and are just barely visible even in this image; but this storm, being much brighter, is an easy target for amateur astronomers' telescopes. Amateur observations over the next few weeks showed that by mid-January 2011 the storm had spread practically all the way around the planet. Note: The APoD image posted here was created from raw images by M. Dauvergne; as of the date of this post (January 19, 2011) the corresponding "official" Planetary Photojournal image had yet to be posted.

     Infrared image of Saturn taken in January 1998 by the Hubble Space Telescope. In this false-color image different colors indicate varying heights and compositions of cloud layers. Tethys is visible just above Saturn on the right, and Dione is on the lower left.

     Saturn seen "sideways" by the Cassini spacecraft, so that the rings are edge-on, completely disappear "above" the planet, and are visible only as a thin line across its center and as shadows on the left. The small dot above center, near the rings, is 300-mile wide Enceladus. The northern portion of the planet, on the left, has a strong blue tinge for the same reason that our atmosphere is blue -- the molecules in the atmosphere scatter blue light more strongly than other colors. The southern portion of the planet, on the right, has a golden tinge because the clouds in its atmosphere -- which are gold-colored for reasons currently completely unknown -- are closer to the top of the atmosphere, allowing less room for scattering.

     Ultraviolet image of Saturn, showing auroral displays. Energetic particles trapped by its magnetic field bombard the atmosphere near the poles, as on Earth. However, because Saturn's atmosphere is made of hydrogen and extends into space further than our denser atmosphere, the auroral displays extend more than a thousand miles above its cloudy lower atmosphere. The false color image here, taken with ultraviolet radiation, shows radiation by atomic hydrogen in red, since that is the visible light color most associated with hydrogen radiation, while radiation by molecular hydrogen is shown in white.

     Composite of three ultraviolet/visual images of Saturn, showing auroral displays in January 2004. The aurorae are shown in blue, because they are ultraviolet images; but in visible light they would appear red, because of the emissions by hydrogen atoms.