Cassini finds Saturn moon Phoebe has planet-like qualities

Apr 27, 2012 By Jia-Rui C. Cook and Dwayne Brown
Phoebe's true nature is revealed in startling clarity in this mosaic of two images taken during Cassini's flyby on June 11, 2004. Credit: NASA/JPL/Space Science Institute

(Phys.org) -- Data from NASA's Cassini mission reveal Saturn's moon Phoebe has more planet-like qualities than previously thought.

Scientists had their first close-up look at Phoebe when Cassini began exploring the system in 2004. Using data from multiple spacecraft instruments and a computer model of the moon's chemistry, geophysics and geology, scientists found Phoebe was a so-called planetesimal, or remnant planetary building block. The findings appear in the April issue of the Journal Icarus.

"Unlike primitive bodies such as comets, Phoebe appears to have actively evolved for a time before it stalled out," said Julie Castillo-Rogez, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Objects like Phoebe are thought to have condensed very quickly. Hence, they represent building blocks of planets. They give scientists clues about what conditions were like around the time of the birth of planets and their moons."

Cassini images suggest Phoebe originated in the far-off Kuiper Belt, the region of ancient, icy, rocky bodies beyond Neptune's . Data show Phoebe was spherical and hot early in its history, and has denser rock-rich material concentrated near its center. Its average density is about the same as Pluto, another object in the . Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

Saturn is surrounded by a cloud of irregular moons that circle the planet in orbits tilted from Saturn's orbit around the sun, the so-called equatorial plane. Phoebe is the largest of these irregular moons and also has the distinction of orbiting backward in relation to the other moons. Saturn's large moons appear to have formed from gas and dust orbiting in the planet's equatorial plane. These moons currently orbit Saturn in that same plane.

This panel of images shows the nearly spherical shape of Saturn's moon Phoebe, as derived from imaging obtained from NASA's Cassini spacecraft. Each image represents a 90-degree turn. Credit: NASA/JPL-Caltech/SSI/Cornell

"By combining Cassini data with modeling techniques previously applied to other solar system bodies, we've been able to go back in time and clarify why it is so different from the rest of the Saturn system," said Jonathan Lunine, a co-author on the study and a Cassini team member at Cornell University, Ithaca, N.Y.

analyses suggest that Phoebe was born within the first 3 million years of the birth of the solar system, which occurred 4.5 billion years ago. The moon may originally have been porous but appears to have collapsed in on itself as it warmed up. Phoebe developed a density 40 percent higher than the average inner Saturnian moon.

Objects of Phoebe's size have long been thought to form as "potato-shaped" bodies and remained that way over their lifetimes. If such an object formed early enough in the solar system's history, it could have harbored the kinds of radioactive material that would produce substantial heat over a short timescale. This would warm the interior and reshape the moon.

"From the shape seen in Cassini images and modeling the likely cratering history, we were able to see that Phoebe started with a nearly spherical shape, rather than being an irregular shape later smoothed into a sphere by impacts," said co-author Peter Thomas, a Cassini team member at Cornell.

Phoebe likely stayed warm for tens of millions of years before freezing up. The study suggests the heat also would have enabled the moon to host liquid water at one time. This could explain the signature of water-rich material on Phoebe's surface previously detected by Cassini. 

The new study also is consistent with the idea that several hundred million years after Phoebe cooled, the moon drifted toward the inner in a solar-system-wide rearrangement. Phoebe was large enough to survive this turbulence.

More than 60 moons are known to orbit Saturn, varying drastically in shape, size, surface age and origin. Scientists using both ground-based observatories and Cassini's cameras continue to search for others.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the mission for the agency's Science Mission Directorate in Washington. The California Institute of Technology in Pasadena manages JPL for NASA.

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More information: saturn.jpl.nasa.gov

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User comments : 16

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kevinrtrs
1.6 / 5 (25) Apr 27, 2012
Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?
Mitchell_Helm
3.7 / 5 (12) Apr 27, 2012
@kevinrtrs - I would say as reliable as religion, which isn't reliable either. According to your basis of believing something, you must see it, so that would disprove all religion as well as many other things in/out of this world.
jselin
4.4 / 5 (17) Apr 27, 2012
@kevinrts
We've not only witnessed such events, we routinely create them. Every time we put a spacecraft into orbit around another planet we've given it another moon. Same physics. We see objects traversing the solar system all the time so all it takes is time for one to "thread the needle" and enter a stable orbit. Shoemaker-Levy 9 was a well documented captured Kuiper belt object that resulted in impact because the orbital dynamics dictated that eventual fate. Surely you can't deny observation and countless intentional orbital insertions?
jselin
4.3 / 5 (14) Apr 27, 2012
Kevin,
Another thought- if your gut feelings on this matter are so verifiably wrong, what else could you be wrong about? This is perhaps the most important question for you to ponder.
antialias_physorg
4.4 / 5 (14) Apr 27, 2012
Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?

Going around Saturn the 'wrong' way is a pretty decent indication, wouldn't you think?

When you wake up and there is snow outside on the ground you can probably surmise that it snowed during the night - even though you didn't witness it.

These things are a highly complex matter with a newfangled technical term: 'c-l-u-e-s'

But that may be too advanced a concept for the terminally clueless such as yourself.
Torbjorn_Larsson_OM
4.1 / 5 (7) Apr 27, 2012
"How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?"

As described, by gravitational capture. The Kuiper belt was disturbed when the giants migrated (see the Nice model in Wikipedia).

Yes, it is well witnessed, the Nice model predicts very many observations in fact, which no other theory does. These captured retrograde moons and their orbital statistics happens to be one of them, but we have some 20 independent witnesses of different kinds. And that makes it very reliable.

The alternative of creationist crackpots is to suggest a "poof" by a non-existing 'invisible' agent. No witnesses at all, no processes but magic.
Argiod
2 / 5 (6) Apr 27, 2012
Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?


Yeah, it's as though they said: "And then he pulled a rabbit out of the hat."
PhotonX
2 / 5 (2) Apr 28, 2012
How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?


I think comments like this are the reason Wolfgang Pauli famously said: "This paper is so bad it is not even wrong."

Another Pauli quote you might want to consider: "I don't mind your thinking slowly; I mind your publishing faster than you think."

Meanwhile, slightly more on topic, the article states "Objects of Phoebe's size have long been thought to form as "potato-shaped" bodies...". It seems there are two shapes for interplanetary bodies, spherical and potato-shaped. If I never see 'potato-shaped' again it will be too soon. Do the writers think we don't know what irregular means?
PhotonX
3.4 / 5 (5) Apr 28, 2012
Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?

More to the point was asteroid 2006 RH120, which was captured by Earth in the mid nineties, though it wasn't detected until after 2000. How did we know that? Orbital mechanics. It was predicted to leave earth later in 2007, and it left right on schedule. There it is, a falsifiable prediction came true, just like science is supposed to work. Other asteroids are thought to come and go often, but they are very hard to see and often go unreported, though that will change as we get better at finding them.
.
You can read about that one right here at Physorg: http://phys.org/n...oon.html
.
Given that, why do you find it so hard to believe that a planet might capture an asteroid or comet or a KBO, given the hundreds of millions or more years it had to do it
PhotonX
3 / 5 (4) Apr 28, 2012
To continue: Most of the time the planet just sucks them in. "Has ANYONE ever witnessed such an event?" Why, yes, in fact we have. Jupiter took a good shot at capturing comet Shoemaker-Levy 9, but ended up eating it in 1994. SL9 was thought to have been captured by Jupiter two or three decades earlier. Rather remarkable that we got to see such an event in the very short time we had good enough instruments to watch it, but we got lucky. Just a short century since we got eyes up, and bam! there it was.
.
That had to be close to being in your lifetime, right? So there, consider youself educated.
Sinister1811
2.3 / 5 (12) Apr 28, 2012
Phoebe likely was captured by Saturn's gravity when it somehow got close to the giant planet.

How would this happen? Has ANYONE ever witnessed such an event? Just how reliable can this statement be?


How is that hard to believe? That sort of thing happens all the time. Asteroids stray too close to a planet's gravity and get trapped in some kind of orbit around the planet. There are plenty of examples of this happening. We see it constantly.
ubavontuba
1.7 / 5 (11) Apr 28, 2012
We've not only witnessed such events, we routinely create them. Every time we put a spacecraft into orbit around another planet we've given it another moon. Same physics. We see objects traversing the solar system all the time so all it takes is time for one to "thread the needle" and enter a stable orbit. Shoemaker-Levy 9 was a well documented captured Kuiper belt object that resulted in impact because the orbital dynamics dictated that eventual fate. Surely you can't deny observation and countless intentional orbital insertions?
Intentional orbital insertions require braking thrust.

http://en.wikiped...nsertion

Generally speaking, irregular moons would've relied on gravitational resonances to be captured.

http://en.wikiped...lar_moon

and:

https://www.oca.e...Sats.pdf

Shoemaker-Levy 9's orbit was short-lived and unstable (hence the impact).

http://en.wikiped...ng_comet
Sinister1811
2.1 / 5 (13) Apr 28, 2012
@kevinrtrs - I would say as reliable as religion, which isn't reliable either. According to your basis of believing something, you must see it, so that would disprove all religion as well as many other things in/out of this world.


I would agree with that, however, religion is even less reliable. This is actually documented with facts and proof. To say that it's as reliable as religion is to say that it isn't reliable at all..
Sinister1811
2 / 5 (10) Apr 29, 2012
@BlueHigh - I noticed you rated me a 1 for the comment about asteroids getting trapped in an orbit around a host planet. Which part of this do you disagree with? Believe it or not, this DOES actually happen on the occasion. How do you think Mars got its two "moons"? Either they were rocks blasted off the surface, or they were asteroids that got trapped in their current day orbit. But if it makes you feel superior, then give me a 1 vote for this post as well.
antialias_physorg
4.7 / 5 (7) Apr 29, 2012
I would agree with that, however, religion is even less reliable.

Has anyone alive witnessed god? I'm just asking...kev would have to dump his own faith if he ever applied his own standards to himself.
Anda
1 / 5 (1) May 01, 2012
Why are all of you talking about religion????

Poor americans with your creationists, denialists and other s...

In Europe we don't have these things, fortunately

Even the f... pope is more open minded than an american creationist