Shadows of Saturn at Equinox (20090901)

[dduggan47]

What's the little shadow running horizontally on the far right about half way between Saturn's shadow and the top of the picture?

[neufer]

What's the little shadow running horizontally on the far right about half way between Saturn's shadow and the top of the picture?

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Saturn's moon Tethys casts a shadow visible only on the far right.

[xgravity23]

What are the points of light to the right of Saturn? Moons in front of the rings reflecting sunlight?

[gwobbasalubert]

There appear to be some small white dots in the shadow of Saturn. Could they be stars visible through the rings when the rings themselves are shadowed?

(Hey... you got there before me because it took me so long to register!)

[Chris Peterson]

What are the points of light to the right of Saturn? Moons in front of the rings reflecting sunlight?

From the linked description page for this image:

The night side of the planet is dimly lit here by ringshine. Tethys, located off to the left of this image, is not seen. The moon Janus (179 kilometers, or 111 miles across) can be seen orbiting outside the thin F ring at the top of the image. Other bright specks are background stars.

[dduggan47]

What's the little shadow running horizontally on the far right about half way between Saturn's shadow and the top of the picture?

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Saturn's moon Tethys casts a shadow visible only on the far right.

If only I could read! Thank you, neufer.

[cheinke]

This picture seemed really bizarre to me, with the assertion that Tethys is visible in the picture and casting the shadow to the far right. If Tethys is casting the shadow, it must be located on the line drawn from the shadow. If the visible moon is Tethys, its position "above" the rings (in the picture) must mean that it is actually well above the ring plane. However, there are no satellites this close to Saturn that are out of the plane of the rings. So the caption must be wrong. And indeed, following the link to the image source, I see that the visible moon is not Tethys, but Janus; Tethys is off the image to the left.

[Chris Peterson]

This picture seemed really bizarre to me, with the assertion that Tethys is visible in the picture and casting the shadow to the far right. If Tethys is casting the shadow, it must be located on the line drawn from the shadow. If the visible moon is Tethys, its position "above" the rings (in the picture) must mean that it is actually well above the ring plane. However, there are no satellites this close to Saturn that are out of the plane of the rings. So the caption must be wrong. And indeed, following the link to the image source, I see that the visible moon is not Tethys, but Janus; Tethys is off the image to the left.

You're just misunderstanding the caption. "Visible above" is a reference to the image above the caption; there is no suggestion that Tethys is visible in the image itself. The wording is a little awkward, though.

[neufer]

Saturn equinox image goodies
Sep. 21, 2009 | By Emily Lakdawalla

<<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.>>

[neufer]

Snapshots from Saturn's Equinox
By Emily Lakdawalla September 21, 2009

<<The lighting geometry in this image is extreme. It was taken just hours before equinox, mostly of the C ring, one of the dimmest of Saturn's main rings. The B ring's inner boundary is on the right; the C ring's Maxwell gap (demarcated by two bright arcs, which are bright because they rise above the ring plane) is on the left.

There is a periodic brightness variation in the image. The side lighting geometry means that brightness variations mostly result from changing slopes of the surface of the ring plane due to out-of-plane disturbances in the rings -- in layman's terms, the rings here are corrugated like a washboard. Previous images had revealed these corrugations in the D ring (located off the view to the left). This image reveals that the corrugation extends beyond their origin in the D ring (which begins not far above Saturn's cloud tops), right through the C ring, to the inner part of the B ring, the densest of Saturn's rings, covering a distance of 17,000 kilometers. This is a wholly unexpected observation that the science team is now working to understand. Based on the earlier information, scientists had speculated that a comet or asteroid may have collided with the D ring in the early 1980s to cause the vertical disturbance. This explanation fails to work now that scientists understand the true extent of the disturbance. All they can say is that something happened in the early 1980s to tilt a vast region of the inner rings. Over the intervening years, the tendency for inclined orbits to systematically wobble (or "precess") at different rates, depending upon their distance from Saturn, has created a tightly wound spiral corrugation in the ring plane from the initial tilt to the ring plane.

That close-up view is part of this vast high-resolution panorama across the rings. It must be enlarged to be appreciated. Fifteen narrow-angle camera images comprise this panoramic mosaic. At extreme left, ringlets within the D ring appear bright, indicating that they extend vertically above and below the ring plane.

Next is the C ring. The fact that the edges of the C ring's gaps, and some of the ringlets within it, are bright indicate that they extend vertically above and below the plane of the rings. Some of the C ringlets also contain clumps, which is a surprise.

Next comes the B ring. We haven't had such a fine view of its intricate corrugations and waves since Cassini entered Saturn orbit more than five years ago. In the outer B ring, bright spokes cross the rings. These consist of tiny particles electrostatically levitated above the ring plane.

Ringlets within the Cassini division are bright just like those in the C ring. Just beyond the Cassini division, in the A ring, is a beautiful corrugation, a bending wave (vertical disturbance of the ring plane) that originates from an orbital resonance with Iapetus. Iapetus is the most distant of Saturn's major satellites but also has a markedly inclined orbit, helping it to create bending waves in the rings.

In the outer A ring, the shadow of Dione shows up several times, as Dione was moving during the time that separated each frame of the mosaic. Apart from Iapetus, the major moons all orbit within the plane of Saturn's rings, so their shadows only cross the rings near equinox.

The ringlets within the Encke gap are very bright, again indicating that they are vertical disturbances. At the very outer edge of the A ring, between the Encke and Keeler gaps, are several spiral density waves. They are, again, bright because they extend above the ring plane, but not because of motion excited by inclined moons. Instead, they indicate places where the ring particles jostle so closely together that, squeezed from the sides, they have nowhere to go but up (and down) -- analagous to the reason that some mountains (like the Himalayas) exist on Earth.>>

[neufer]

Snapshots from Saturn's Equinox
By Emily Lakdawalla September 21, 2009

<<"Propellers" were first spotted in close-up images of the ring taken by Cassini during its orbit insertion. Scientists determined that they indicate the presence of isolated 100-meter-size objects within the rings -- bodies intermediate in size between things that are named as moons and the innumerable individual particles that make up the rings themselves. Cassini spotted many more of them under the extreme equinox lighting conditions, and it now seems that there is a full spectrum of particle sizes from the biggest, Titan, thousands of kilometers in diameter, to the medium-sized moons, hundreds of kilometers in diameter, to the smaller rocks, tens of kilometers in diameter, to the ring-embedded moons, a few kilometers in diameter, to the propellers, hundreds to tens of meters in diameter, to ring particles, on down to dust.

This propeller, spotted just after equinox on August 13, 2009, is very bright because it sticks up above the ring plane at a time when the sunlight was coming almost directly from the side. It's the largest propeller yet observed, sticking up out of the ring plane by 200 meters in each direction, indicating that the body is about 400 meters in diameter. Clumps of ring material surround it ahead of it and behind it in its orbit, extending the whole feature to a length of 130 kilometers.>>

[bystander]

http://ciclops.org/view_event/116/A_Rar ... _Spectacle
http://saturn.jpl.nasa.gov/index.cfm
http://saturn.jpl.nasa.gov/photos/