APOD: Prometheus Creating Saturn Ring... (2010 Aug 02)

[APOD Robot]

Prometheus Creating Saturn Ring Streamers

Explanation: What's causing those strange dark streaks in the rings of Saturn? Prometheus. Specifically, an orbital dance involving Saturn's moon Prometheus keeps creating unusual light and dark streamers in the F-Ring of Saturn. Now Prometheus orbits Saturn just inside the thin F-ring, but ventures into its inner edge about every 15 hours. Prometheus' gravity then pulls the closest ring particles toward the 80-km moon. The result is not only a stream of bright ring particles but also a dark ribbon where ring particles used to be. Since Prometheus orbits faster than the ring particles, the icy moon pulls out a new streamer every pass. Above, several streamers or kinks are visible at once. The above photograph was taken in June by the robotic Cassini Spacecraft orbiting Saturn. The oblong moon Prometheus is visible on the far left.

<< Previous APOD

Discuss Any APOD

Next APOD >>

[/b]

[bystander]

See more about this on the Observation Deck here and here.

[neufer]

• "What Goes Around Comes Around"

1. The status eventually returns to its original value
___ after completing some sort of cycle.

2. A person's actions, whether good or bad,
will often have consequences for that person.

<<Prometheus (Ancient Greek: Προμηθεύς, "forethought") is a Titan, the son of Iapetus and Themis, and brother to Atlas, Epimetheus and Menoetius. He was a champion of human-kind known for his wily intelligence, who stole fire from Zeus and gave it to mortals. Zeus then punished him for his crime by having him bound to a rock while a great eagle ate his liver every day only to have it grow back to be eaten again the next day.>>

[orin stepanek]

Prometheus' gravity then pulls the closest ring particles toward the 80-km moon.

Would this cause the moon to grow from possible snow fall from the rings?

[bystander]

Would this cause the moon to grow from possible snow fall from the rings?

http://asterisk.apod.com/vie ... 31&t=19742

[neufer]

Would this cause the moon to grow from possible snow fall from the rings?

The "breaching hump of the great white whale" would indeed appear to be due to SNO-BALL SNOW FALL.

Image from Jan. 27, 2010. Saturnshine illuminates the moon's night side.

---

Cassini image (with moon's Saturn-facing end at lower right) reveals a surface covered with a blanket of fine material.

---

• Mean density 0.47 ± 0.07 g/cm3
  Escape velocity ~19 m/s
  Rotation period synchronous
  Al-Bedo 0.6

<<Prometheus is extremely elongated, measuring about 119 by 87 by 61 km. It has several ridges and valleys and a number of impact craters of about 20 km diameter are visible, but it is less cratered than nearby Pandora, Epimetheus and Janus. From its very low density and relatively high albedo, it seems likely that Prometheus is a very porous icy body. Prometheus acts as a shepherd satellite for the inner edge of Saturn's F Ring. Recent images from the Cassini probe show that the Promethean gravitational field creates kinks and knots in the F Ring as the moon 'steals' material from it. The orbit of Prometheus appears to be chaotic, as a consequence of a mean motion resonance with Pandora. The most appreciable changes in their orbits occur approximately every 6.2 years, when the periapsis of Pandora lines up with the apoapsis of Prometheus and the moons approach to within about 1400 km. Prometheus is itself a significant perturber of Atlas.>>

[bystander]

Boy! Are you gong to get it from Hostess!! Uh-huh Uh-huh

"Twinkie defense"

If he had eaten the chocolate covered Twinkies, he wouldn't have been depressed

Can we please stay on topic. Take these comments off-line [PMs] or to Open Space.

Split to Twinkie Defense

[RJN]

Owlice pointed out an image feature that has been bothering me all day. On the far left, there appears to be a bright notch in the ring preceding the moon's pass. How did that happen?

[moonstruck]

It's always amazing to me the beautiful pictures coming back from the Cassini Mission. Todays APOD is the perfect example. The technology and the knowhow to do these things is mind blowing. Always something new and suprising. Keep up the good work guys that knowhow and keep em coming Cassini.

[bystander]

Owlice pointed out an image feature that has been bothering me all day. On the far left, there appears to be a bright notch in the ring preceding the moon's pass. How did that happen?

It may be caused by Pandora, which orbits outside the F ring, or it may be a left over from previous orbits of Prometheus.

http://asterisk.apod.com/vie ... 00#p127565

[owlice]

Owlice pointed out an image feature that has been bothering me all day. On the far left, there appears to be a bright notch in the ring preceding the moon's pass. How did that happen?

I initially thought that it might be the next streamer starting; I didn't quite understand Prometheus's motion. After I commented on it, I looked at video and other images and documentation on this moon, and I think my initial thought is incorrect; I think it's still an effect of Prometheus, gravity tugging a bit, but the particles in the preceding notch seem to fall back into the ring once Prometheus moves back toward Saturn and away from the inner edge of the ring. It clearly starts with Prometheus's approach; it didn't look to me as though it was left over from a previous pass. (I could certainly be wrong about this; I hope someone has/finds a more informed explanation.)

[León]

The image says something different to the text, Prometheus strip and generates the tapes but these are being passed and become another ring on the inside. Not seem to build up material on the surface of the satellite.

[neufer]

Owlice pointed out an image feature that has been bothering me all day. On the far left, there appears to be a bright notch in the ring preceding the moon's pass. How did that happen?

I initially thought that it might be the next streamer starting; I didn't quite understand Prometheus's motion. After I commented on it, I looked at video and other images and documentation on this moon, and I think my initial thought is incorrect; I think it's still an effect of Prometheus, gravity tugging a bit, but the particles in the preceding notch seem to fall back into the ring once Prometheus moves back toward Saturn and away from the inner edge of the ring. It clearly starts with Prometheus's approach; it didn't look to me as though it was left over from a previous pass. (I could certainly be wrong about this; I hope someone has/finds a more informed explanation.)

Well, it can't very be the start of a new streamer since it's only half the distance of the normal 3.2 degree separation.

And there does seem to be a wake in front of Prometheus:

but it could also be S/2004 S 6:

S/2004 S 6 Orbital period: 0.612990038 d
Prometheus Orbital period: 0.6180116 d

S/2004 S 6

---

<<S/2004 S 6 is the provisional designation of a dusty object seen orbiting Saturn very close to the F ring. It is not clear whether it is only a transient clump of dust, or if there is a solid moonlet at its core.

It was first seen by scientists in images taken by the Cassini-Huygens probe on October 28, 2004 and announced on November 8 that year. It appears to be the best tracked object in this region with at least five probable sightings in the period till late 2005. In comparison, two objects in the F ring's vicinity (S/2004 S 3 and S/2004 S 4) that were first seen several months earlier have not been recovered with any confidence. Nevertheless, it continues to be unclear whether there is a solid core to S/2004 S 6 or whether it is just a transient dust clump that will dissipate on a timescale of years or months. Notably, an imaging sequence covering an entire orbital period at 4 km resolution taken on November 15, 2004 (soon after S/2004 S 6's discovery) failed to recover the object, while it has been seen again later. The lighting conditions in S/2004 S 6's part of the orbit were different during these two observations, however, with the discovery being made when the region was strongly backlit by the sun. A suggested resolution of the absence in November is that S/2004 S 6's visibility is primarily due to a diffuse cloud of fine dust that is much brighter in forward scattered light (the conditions of the discovery image), and that the solid core (if any) is small.

S/2004 S 6 has been seen both inside and outside the main F ring, and its orbit must cross the ring. Careful calculations show that the object periodically plows through the ring material, coming within 1.5 km of the densest core e.g. on 9 April, 2005. It has been suggested that a spiral structure in the tenuous material surrounding the F ring may have been a consequence of this. The dusty halo seen in images is sizeable, being around 2000 km in lengthwise extent. The solid object, if any, would be no greater than 3−5 km in diameter based on brightness. Additional evidence came in 2008, as it appears that S/2004 S 6 or a body like it is required to explain the dynamics of the F Ring.>>

[rstevenson]

As a friend likes to say, "Correct me if I'm right, but... ." We have a tendency to think of these wakes as similar to boat wakes, which of course do not precede the boat. But gravity can certainly pull at ring material ahead of the moon that is "carrying" the gravity with it.

Rob

[neufer]

As a friend likes to say, "Correct me if I'm right, but... ." We have a tendency to think of these wakes as similar to boat wakes, which of course do not precede the boat. But gravity can certainly pull at ring material ahead of the moon that is "carrying" the gravity with it.

Then consider yourself corrected.

A bow wave defines the outer limits of a ship's wake.

---

<<A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, poses a risk to smaller boats, and in a harbor can cause damage to shore facilities and moored ships. Therefore, ship hulls are generally designed to produce as small a bow wave as practical.

The size of the bow wave is a function of the speed of the ship, its draft, surface waves, water depth, and the shape of the bow. A ship with a large draft and a blunt bow will produce a large wave, while ships that plane over the surface of the water will create smaller bow waves. Bow wave patterns are studied in the field of computational fluid dynamics.

The bow wave carries energy away from the ship at the expense of its kinetic energy—it slows the ship down. A major goal of naval architecture is therefore to reduce the size of the bow wave and improve the ship's fuel economy. Modern ships are commonly fitted with a bulbous bow to achieve this.

A bow wave forms at the head of a swimmer when he moves through the water. The trough of this wave lies near the mouth of the swimmer and helps the swimmer inhale air to breathe just by turning his head.>>

[rstevenson]

I knew someone would bring up bow waves. As long as they don't suggest Prometheus has one, that's okay, I thought.

Rob

[owlice]

Well, it can't very be the start of a new streamer since it's only half the distance of the normal 3.2 degree separation.

That was one of the things that made me look a bit more into this -- the distance didn't fit.

And there does seem to be a wake in front of Prometheus:

S/2004 S 6 Orbital period: 0.612990038 d
Prometheus Orbital period: 0.6180116 d
<<S/2004 S 6 is the provisional designation of a dusty object seen orbiting Saturn very close to the F ring. It is not clear whether it is only a transient clump of dust, or if there is a solid moonlet at its core.

This or a similar object seem a good possibility; thanks for ferreting it out.

[neufer]

[b][color=#FF0000]Janus.[/color][/b] in natural color Credit: NASA / JPL / SSI / color composite by Emily Lakdawalla

---

<<Cassini took the images for this unusually close view of Janus. on July 26, 2009. The image is composed of four pictures taken through red, green, blue, and clear filters and is enlarged by a factor of two. >> ===========>

<<Prometheus is extremely elongated, measuring about 119 by 87 by 61 km. It has several ridges and valleys and a number of impact craters of about 20 km diameter are visible, but it is less cratered than nearby Pandora, Epimetheus and Janus.>>