Starship Asterisk*

http://www.planetary.org/blog/article/00002165/ wrote:
The Phoebe ring
By Emily Lakdawalla Oct. 14, 2009

<<All it takes is one glance at Phoebe to know that a lot of it has been blasted away due to numerous impacts with other, smaller bodies. So it is no stretch at all to imagine that there should be material in Phoebe's orbit making a ring. Verbiscer and her coauthors predicted that there might be such a ring. But Phoebe's orbit is so incredibly large that this "ring" would be almost unimaginably sparse.

What happens to [these ring] particles depends upon their size. The biggest chunks, several centimeters in size or larger, don't really migrate anywhere, sticking around near Phoebe's orbit until they smack into something -- each other or Phoebe. The model simulation suggests that it would take more than the age of the solar system for half of the particles to be removed from the system by re-collision with Phoebe, so most of the biggest chunks are still out there somewhere in Phoebe's orbital space.

What about smaller particles? The article says "re-radiation of absorbed sunlight exerts an asymmetric force on dust grains, causing them to spiral in towards Saturn with a characteristic timescale of 150,000 x rg where rg is the particle radius in micrometers. This force brings all centimetre-sized and smaller material to Iapetus and Titan unless mutual particle collisions occur first....Most material from 10 micrometres to centimetres in size ultimately hits Iapetus, with smaller percentages striking Hyperion and Titan." This would be a slow process that has operated continuously since whenever Phoebe was captured into Saturn's orbit. There might have been bursts of material delivered to Iapetus associated with some of the bigger impacts that have left such large scars on Phoebe, but they would have been blips above a steady background.

Anyone who knows the first thing about Iapetus should be saying "Aha!" at this point. Iapetus, is, of course, known for being the yin-yang moon, whose leading hemisphere is dark and whose trailing hemisphere is bright. Cassini observations of Iapetus have proven that the dark material is almost certainly exogenous (coming from outside) rather than endogenous (coming from internal geologic activity). The small number of fresh, rayed craters on Iapetus shows that the darkening process is continuing today. The fact that the dark stuff only shows up on the leading hemisphere -- the side of Iapetus that faces forward in its orbit -- suggests that it's "sweeping up" the dark material, plowing into it as it moves in its stately path around Saturn. The side of Iapetus that faces backward along its orbit is in the lee of Iapetus' motion through the cloud of particles and doesn't sweep up the material.

It's long been suggested that Phoebe could be the source of this dark material. Verbiscer et al. go on to do some calculations that suggest that Phoebe material accumulates on Iapetus at a rate of about 40 micrometers per million years; over the age of the solar system that gives you 20 centimeters of material deposited on Iapetus. But because there were probably once a lot more distant kilometer-scale moons of Saturn that eventually crashed into Phoebe, the production rate of Phoebe ring material and therefore the Iapetus accumulation rate in the past was very likely higher, "leading to...a cumulative thickness of material on Iapetus that is probably measured in metres."

It's all a pretty neat picture. It doesn't even require Phoebe to have any kind of unusual geologic activity or unusually large past impacts -- all it needs to be is a big body sitting in its distant orbit, and other bodies in the Saturn system will crash in to it and create this supply of material that eventually impacts Iapetus. One question I haven't answered yet though is how come Phoebe has a ring if such structures haven't been seen around other moons? For instance, someone asked me, why isn't there a dust ring around the orbit of our Moon?

In a way, the way we are finding fainter and fainter rings is parallel to the way we're finding smaller and smaller moons. I'm pretty certain that there are a few motes more dust along our Moon's orbit than there is outside our Moon's orbit, but the Moon is so large, with such strong gravity, that nearly everything that is ejected from any impact on the Moon almost immediately returns to the lunar surface; no one will ever find a "ring" in the Moon's orbit.

Elsewhere in the solar system, as you look to smaller and smaller moons, you will see more and more dust associated with that moon's orbit. In the Saturn system, the Cassini mission is careful as the spacecraft crosses the orbits of all the medium-sized moons like Tethys, Dione, and Rhea, pointing the high-gain antenna forward along its path so that the antenna acts like an umbrella, shielding the more sensitive instruments from collisions with little dust particles.

Look at the even smaller moons Thebe and Amalthea at Jupiter, and there are definite "gossamer rings" associated with their orbits. Interestingly, Verbiscer et al. point out in their paper that Thebe and Amalthea are "far more prolific sources of debris" than Phoebe because their close location to Jupiter means that collisions are more energetic, but because the debris from collisions re-impacts Thebe and Amalthea very quickly, the optical depth of their associated rings is actually very similar to the optical depth of the Phoebe ring, even though the Phoebe ring is far larger; which means that the Phoebe ring contains thousands of times more material than the Thebe and Amalthea gossamer rings do.

The article concludes: "Although [its] exotic properties as well as its sheer size make the Phoebe ring unique among known planetary rings, similar structures should also adorn the other gas giant planets.">>

Star*Hopper wrote:But you're talkin' about the inclination of the ring to the planet, not the planet's inclination. The ring's in retro, not Saturn.

BMAONE23 wrote:Sounds like there is a whole bunch of newly discovered stuff infesting Saturns orbit...Is it still a planet???

geckzilla wrote:Art, you've confused another person with your quirky wit.

geckzilla wrote:How many external links and wikipedia articles is it going to take to clean this one up?

http://www.perseus.tufts.edu/Herakles/stables.html wrote:
<<For the FIFTH labor, Eurystheus ordered Hercules to clean up King Augeas' stables. Hercules knew this job would mean getting dirty and smelly, but sometimes even a hero has to do these things. Then Eurystheus made Hercules' task even harder: he had to clean up after the cattle of Augeas in a single day. Now King Augeas owned more cattle than anyone in Greece. Some say that he was a son of one of the great gods, and others that he was a son of a mortal; whosever son he was, Augeas was very rich, and he had many herds of cows, bulls, goats, sheep and horses.>>

Star*Hopper wrote:

neufer wrote:The real truth (or truthiness?) is that since it is in retrograde
Saturn's 'new' dust ring is tilted at 153° from the main ring plane.

(Note: The axial tilt of Uranus is 97.77°)

Ummmmmmmmmm............
Direction of rotation has what to do with physical geometry, again?

Star*Hopper wrote:Ummmmmmmmmm............Direction of rotation has what to do with physical geometry, again?

Wikipedia: Orbital Elements wrote:Inclination - vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node (where the orbit passes upward through the reference plane)

neufer wrote:

Case wrote:

keshlam wrote:The insert graphic makes it appear that the new ring is orbiting on a significantly different plane than the others.

True?

True. Saturn's 'new' dust ring is tilted at about 27 degrees from the main ring plane.

The real truth (or truthiness?) is that since it is in retrograde
Saturn's 'new' dust ring is tilted at 153° from the main ring plane.

(Note: The axial tilt of Uranus is 97.77°)

Case wrote:

keshlam wrote:The insert graphic makes it appear that the new ring is orbiting on a significantly different plane than the others.

True?

True. Saturn's 'new' dust ring is tilted at about 27 degrees from the main ring plane.

kovil wrote:[On and edit]: Thanks Star*Hopper for the article link: http://science.nasa.gov/headlines/y2009 ... ntring.htm
"Spitzer's infrared eyes were able to spot the glow of the cool dust, which has a temperature of only about 80 Kelvin (minus 316 degrees Fahrenheit)."
? Would dust grain absorption of solar energy radiations be sufficient to account for an 80-K temperature of themselves, considering any ability they have to reradiate any energy ?

• Earth's average radiative temperature = 250 K
  
  Average radiative temperature at 9.6 AU = 250/sqrt(9.6) = 81 K

keshlam wrote:The insert graphic makes it appear that the new ring is orbiting on a significantly different plane than the others. True, or just a presentation artifact?