Jagged Shadows (APOD 2009 April 15)

[neufer]

http://apod.nasa.gov/apod/ap090415.html

<<Explanation: What's causing unusual jagged shadows on Saturn's rings? No one is yet sure. As Saturn nears equinox, its rings increasingly show only their thin edge to the Earth and Sun. As a result, Saturn's moons now commonly cast long shadows onto the rings. An example of this is the elongated vertical shadow of Mimas seen on the above right. The series of shorter, jagged shadows that run diagonally, however, are more unusual. Now Saturn's rings have been known to be made of particles for hundreds of years, but these particles have so far escaped direct imaging. It is therefore particularly exciting that a preliminary hypothesis holds that these jagged shadows are silhouettes of transient groups of ring particles (at the inner edge of the Cassini division) temporarily held close by their own gravity.>>
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<<Mimas is responsible for clearing the material from the Cassini Division, the gap between Saturn's two widest rings, A ring and B ring. Particles at the inner edge of the Cassini division are in a 2:1 resonance with Mimas. They orbit twice for each orbit of Mimas. The repeated pulls by Mimas on the Cassini division particles, always in the same direction in space, force them into new orbits outside the gap.>>

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The Emperor: [In the throne room, Luke is watching the Imperial fleet attack the Rebels from the huge throne room window] As you can see, my young apprentice, your friends have failed. Now witness the firepower of this fully ARMED and OPERATIONAL battle station!
[the Emperor hits the comlink switch on his throne]

The Emperor: Fire at the inner edge of the Cassini division, Commander!

Moff Jerjerrod: [In the Death Star's firing room, a group of hooded Imperial gunners starts priming the Death Star's superlaser. A series of tones signify that the weapon is ready] Fire!

[a second gunner hits a switch, and a huge laser beam roars down a firing shaft. The outer surface of the Death Star shows a huge laser dish start to develop, and then a titanic laser beam moves out from the Death Star's superlaser. It hits the at the inner edge of the Cassini division and vaporizes huge sections in less than a second]

Lando Calrissian: That blast came from the Death Star! That thing's operational!

[AsvM]

what is the round shadow to the right?

[dduggan47]

what is the round shadow to the right?

That's my question as well. It's the little circle near the right edge near the top of the Mimas shadow.

[Pol Jansegers]

Could the donut shadow on the B-ring be one of the "spokes"?

[oguydave]

The cast link is VERY creative!!

[geckzilla]

I figured the donut was actually something on the lens and not a shadow at all.

[neufer]

I figured the donut was actually something on the lens and not a shadow at all.

Attack of the dust donuts: http://www.planetary.org/blog/article/00001881/

[jimmysnyder]

Given the way that the shadow widens, I assume that the source of light is not the sun, but Saturn. Is this correct?

[bystander]

Given the way that the shadow widens, I assume that the source of light is not the sun, but Saturn. Is this correct?

Widens??? Saturn and the inner rings are towards the top of the page. The Sun, Minas, and outer rings are toward the bottom of the page.

[bystander]

Attack of the dust donuts: http://www.planetary.org/blog/article/00001881/

Is that what dust bunnies eat?

[jimmysnyder]

Widens???

Narrows?

[Chris Peterson]

what is the round shadow to the right?

It's a classic dust donut, the out-of-focus shadow of a dust mote on optics near the sensor (not the objective). It's odd that it wasn't removed by calibration (the image was obviously calibrated, or there would be many more dust donuts). Most likely, this is a new bit of dust, or one dislodged to a new position, so they need to take new calibration frames.

[bystander]

Narrows?

The source of the shadow (the Sun shining on Minas) is off camera towards the bottom of the picture. The shadow gets more narrow the further from Minas it gets. From the viewpoint of an observer standing on the rings, looking towards Minas and the Sun, the further away Minas is, less of the Sun is blocked.

[ColoradoSky]

This is a very cool photo. I wonder why the "particles have so far escaped direct imaging" ? This would be high on my list of views to see.

I remember that Cassini flew through the rings during a breaking maneuver on the way in http://www.msnbc.msn.com/id/5333700/ , but this is probably too dangerous to try it again with the camera rolling

[Chris Peterson]

Given the way that the shadow widens, I assume that the source of light is not the sun, but Saturn. Is this correct?

The shadow edge is way too sharp to come from as broad a source as Saturn. The shadow narrows for two reasons: Mimas isn't exactly on a plane with the rings and the Sun, so you are actually seeing the projection of a circle on a plane. This is apparent if you distort the image:



A secondary, and smaller effect, is because the shadow is extending away from the camera, towards a vanishing point.

[neufer]

Given the way that the shadow widens, I assume that the source of light is not the sun, but Saturn. Is this correct?

The shadow edge is way too sharp to come from as broad a source as Saturn. The shadow narrows for two reasons: Mimas isn't exactly on a plane with the rings and the Sun, so you are actually seeing the projection of a circle on a plane. This is apparent if you distort the image:

That's an interesting point... Mimas does have an inclination of 1.51°
such that every other orbit of an outer B ring particle will
get tugged out of it's plane of orbit by a peri-Saturn Mimas.

I wonder if that as important to the outer B ring structure as "clumping" ?

http://antwrp.gsfc.nasa.gov/apod/ap981018.html
http://antwrp.gsfc.nasa.gov/apod/ap090319.html
http://antwrp.gsfc.nasa.gov/apod/ap071023.html

Code: Select all

Mimas:
Orbit radius 	185 520 km 
Inclination 	1.51° (to Saturn's equator)
Eccentricity 	0.0202
Orbital period 	0.9424218 days

[/b]

[JAlanScott]

The very white edge that appears to be casting the jagged shadows look as if there is enough substance that possibly a craft could land there. Does any know what the white spec in the center of the gap near the upper left corner is?

[neufer]

The very white edge that appears to be casting the jagged shadows look as if there is enough substance that possibly a craft could land there.

Probably:

<<Is the edge of the B ring made of a pile of moons 3 kilometers across? Hold your horses. Three kilometers is the same as the diameter of some of the smaller moons of Saturn, like Methone, Anthe, and Pallene, and it's quite a bit bigger than the most recently discovered S/2008 S1. I believe that what we're actually seeing is clumpiness of particles at the outer edge of the densest B ring, where particles bunch together partially by self-gravity (which would make them more like moons) but also by the periodic gravitational shoves they get from Mimas. At least that's what the imaging team has said about past images of the outer edge of the B ring, like this one. These clumps would be transient, torn apart by the same forces that bring them together. The B ring is so dense that particles rub up against each other as they orbit Saturn. (We know that because it's opaque to light, and even to radio waves). So an astronaut would be able to travel easily from one particle to the next, clambering around the rings, though it'd be a long trip to circle Saturn! And, evidently, the astronaut would have some climbing to do, traveling up and down the clumps of big particles that form the B ring's outer edge.>>

- Emily Lakdawalla

[vmars]

Does any know what the white spec in the center of the gap near the upper left corner is?

That's my question, too; my first [only] guess is another moon. If so, which; if not, what?

[neufer]

Does any know what the white spec in the center of the gap near the upper left corner is?

That's my question, too; my first [only] guess is another moon. If so, which; if not, what?

There are no known moons closer than the A ring.

There are a number of anomalous white pixels within this picture.

[rmciw]

There is a blurred circle near the right edge of the picture, roughtly 1/3 from the top.
Is this an artifact?

[rmciw]

Sorry, didn't see the previous discussion.

[thongar]

I just compared this edge picture with some of the pictures of the Encke Gap edge and the F ring, until I read this thread I did not know which gap we wre talking about, but I think my thoughts are still relivent.

The alternatly bright and dark region of the edge to me indicates a gravitational wake. If this is so, then the spiky shadows are just verticle manifestations of the wake. Since Mimas is so far away from this edge any wake would be too small to normally be visible but the wake would be there none the less.

Think about being at the beach on a mirror calm day, you see a ship, miles out to sea, pass by, thirty minutes later a series of small waves break on the beach. This is the same type of event, only the low sun angle reveals the action.

Perhaps the same shadows but with a longer wave length can be seen in the other two gaps. Now is the time to look.

Respectfully

T