The Planetary Society Blog

Saturn at equinox
Just a day after the equinox, Cassini turned to capture the 75 images necessary to compose this striking panorama across Saturn and its rings. It's a rare and bizarre view of Saturn with the Sun poised almost exactly over the equator. With this lighting geometry, the rings are nearly dark, except where reflected light from Saturn strikes them, or where structures that rise above the plane of the rings catch the sunlight. The rings' shadow has collapsed to a skinny line falling across Saturn's waist.

Because of difficult geometric and lighting conditions, extensive processing was required to create this view. Cassini was relatively close to Saturn when it began taking the images for the mosaic and was continuously receding from the planet during the eight hours it took to capture the 200 images contained in the observation. (Photos were captured through infrared, violet, and clear filters in addition to the red, green, and blue-filter images used for the mosaic shown here.) So each of the images had to be reprojected into a common viewing geometry. The photos also had to be digitally processed to remove lens flares (artifacts that result from light scattering within the camera's optics) and to deemphasize seams between images.

Additional processing was necessary to make the rings visible on your computer screen. Without enhancement, the rings would have been so dark as to be invisible. The dark half of the rings (right side of the mosaic) has been brightened relative to the bright half, (left side of the mosaic) by a factor of three, and then the entire ring system brightened by a factor of 20. Spokes are visible in the B ring on the right side of the image.

Similar brightening was required to make the moons visible. Janus (179 kilometers in diameter) is on the lower left of this image. Epimetheus (113 kilometers) appears near the middle bottom. Pandora (81 kilometers) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers) orbits inside the thin F ring on the right of the image. Other bright specks are background stars.

Cassini had passed through its orbital periapsis at 11:37 UTC on August 11 and almost immediately crossed to the northern face of the rings, at 12:59. This crossing was only 10 hours after the Sun had precisely balanced over the plane (at 02:44). Cassini began capturing the images for this mosaic at about 06:00 on August 12, about 1.25 days after the moment of equinox, and finished taking data at about 14:00.

Credit: NASA / JPL / Space Science Institute

Side-lit vertical structures in Saturn's F and A rings
Cassini looked down onto the sunlit northern face of the rings about a week after equinox, on August 19, 2009, to capture this unusual view showcasing the vertical structures within Saturn's rings. Sunlight illuminates the rings almost edge-on, so bright features in this image are ones that extend vertically to the north of the ring plane, catching the sideways sunlight (the Sun is coming from the left). The bright features are accompanies by corresponding shadows to the right of each feature. The image includes the F ring (the brightest feature, truncated on the left side of the image) and much of the A ring.

The F ring is more vertically extended than the main rings and also composed of sparse, dust-sized particles; it appears bright because the particles are far enough apart that there is little shadowing within the F ring, and the particles scatter light in all directions, including to Cassini's cameras.

Moving to the right of the F ring we come first to the outer edge of the A ring and then the skinny Keeler gap. The Keeler gap is carved out by the tiny moon Daphnis (8 kilometers across). Daphnis is within the field of view, but is too small to be resolved at Cassini's distance from it. However, its effects on the rings are quite visible in the form of a set of sawtoothed vertical strictures (bright spikes near the left edge of the rings) with corresponding shadows cast to the right. The shadows are about 450 kilometers long, indicating that the Daphnis ring waves rise about a kilometer above the ring plane. At other longitudes (and other positions on Daphnis' slightly inclined and slightly eccentric orbit), the Daphnis ring waves have been observed to rise as much as 4 kilometers above and below the ring plane.

Moving inward from the Keeler gap, the darkest band across the A ring is the Encke gap, in which the moon Pan orbits. Pan is not in the frame. The Encke gap also contains discontinuous ringlets which can be seen here to have vertically extended clumps, each of which casts a shadow to the right. The clump shadows are about 275 kilometers long, so the clump height is about 600 meters above the ring plane.

Moving inward again, the brightest "ringlet" crossing the center of the view is a "bending wave" produced by gravitational effects from Mimas. Mimas has an inclined orbit and so tugs ring material from above and below as it circles Saturn. At certain radial positions from Saturn there are resonances, where the relationship between the orbital periods of Mimas and the ring material can be expressed as a ratio of integers. The brightest ringlet is the Mimas 5:3 bending wave, where ring material orbits Saturn five times for every three Mimas orbits. To its immediate right is a dark band, the shadow of the bending wave. Then there is another, more diffuse bright band; this is the Mimas 7:4 bending wave. Another one, the Mimas 8:5 bending wave, is to the left, between the Keeler and Encke gaps, a bit closer to the Keeler gap.

Credit: NASA / JPL / SSI