Planetary scientists generally accept that a giant impact into the newly formed Earth ejected a huge cloud of material that became the moon. In Crida and Charnoz’s conception, that ejecta first flattened into a ring around the planet, which then spread out and coagulated into the moon. But unlike Saturn’s ring, which would have leaked out numerous moonlets to form several moons, Earth’s relatively massive ring would have poured all its material into one large satellite before dissipating. “It spreads very fast,” Crida says of Earth’s hypothesized ring. “And if it spreads fast, only one moon has time to form.”

But the new hypothesis is not without its problems. First, it does not seem to apply to the satellites of Jupiter, which do not obey the same mass-to-distance correlation of the other moon systems. Crida notes that Jupiter was the first planet to form and may have coalesced under different conditions. “I was a bit disappointed to see that Jupiter did not fit the same distribution, but not too surprised,” he says. And then there is the obvious question: if extensive, Saturn-like ring systems once adorned Neptune and Uranus, where are they now? “It’s not that easy to remove them over time,” Nesvorny says. “There is a link that’s missing that needs to be understood.”

The French researchers concede that the fate of the rings is an open question. “I don’t know why Uranus and Neptune don’t have the rings anymore,” Crida says. “We have a few ideas, but nothing too convincing. Some people can see that as a weakness of our mechanism, because indeed where are the rings now? But I think we can find good reasons for the disappearance of the rings, and the satellites remain as the smoking gun."

6 Comments

1. 1. jtdwyer 07:02 PM 11/29/12

   "Like a snowball rolling downhill, the coalescing moons would grow larger and larger as they drift farther from the planet and its rings, undergoing progressively more mergers along the way."
   
   I suspect that the more precise explanation would be that the collisions reduce orbital velocity, causing orbital distance to increase. I don't think there's any direct causal relation between orbital mass and orbital distance. Perhaps this is the "missing link" referred to - I also didn't find any link (pun intended) to a research report...

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2. 2. r2vettes 07:33 PM 11/29/12

   I absolutely can not believe what I have just read. Did the French just wake from a very long sleep? What happened to the Theia impact theory that has recently been supported by new evidence?
   Um....Duh! Closer moons=smaller. Could it be that the gravity of the host planets took moon forming material?
   

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3. 3. diducthat 07:23 AM 11/30/12

   Roche's Limit places bounds on how close large objects such as moons can get close to other large objects like planets. Tidal forces destroy large objects inside this limit.
   
   As tidal forces fall of as the inverse-cube of the distance from the host, one can readily see that the inside of the ring would provide an unstable environment for the birth of large objects.
   
   The relationship between size and distance then seems to make sense.

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4. 4. jtdwyer in reply to diducthat 07:59 AM 11/30/12

   I agree that the statistical relationship between mass and distance makes sense, due to the destructive influence of tidal forces.
   
   I'm suggesting that increasing the mass of an orbital would not introduce any dynamic factor that would cause the object to increase its orbital distance. I suggest that reducing orbital (rotational) velocity (via impact) would cause the orbital distance to increase.

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5. 5. Quinn the Eskimo 12:45 AM 12/6/12

   Of course, when the supportive photos state; "Artists conception..."
   
   They're makin' stuff up. Yup! And Wylie Coyote is CEO of Acme Mfg. now.

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6. 6. imbalzanog 09:25 AM 12/7/12

   The logarithmic dependence of the mass of the planets from solar distances was published from few time! SEE:
   http://www.lulu.com/shop/giovanni-imbalzano/perturbazioni-sismiche-legge-di-bode-e-interazioni-wimp/paperback/product-20546074.html;jsessionid=ED60C490060ED27BDF337D30C548C749

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