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I just saw this question and that got me to this NASA page which led me to this scientific article.

enter image description here

“Our other discovery is a real oddball and has an orbit like no other known Jovian moon,” Sheppard explained. “It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter”.

This new “oddball” moon is more distant and more inclined than the prograde group of moons and takes about one and a half years to orbit Jupiter. So, unlike the closer-in prograde group of moons, this new oddball prograde moon has an orbit that crosses the outer retrograde moons.

As a result, head-on collisions are much more likely to occur between the “oddball” prograde and the retrograde moons, which are moving in opposite directions.

“This is an unstable situation,” said Sheppard. “Head-on collisions would quickly break apart and grind the objects down to dust.”


My question is, why are most of the moons that are further away from Jupiter retrograde orbits, while the ones extremely close to Jupiter are prograde orbits? What possible situations were required for Valetudo to end up where it is opposed to where the rest of the pro-grade moons are clustered? Pure chance / a series of unlikely events?

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migrated from space.stackexchange.com Jul 19 '18 at 13:22

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    $\begingroup$ Yes. If they were discovered by a probe in Jovian orbit, it would be "on the fence" (process of discovering belongs to SE, what happens after and what they are - to Astronomy); these were discovered through a terrestrial telescope, making them definitely a subject for Astronomy. $\endgroup$ – SF. Jul 19 '18 at 12:46
  • $\begingroup$ @SF great distinction, I will keep that in mind on future questions. $\endgroup$ – Magic Octopus Urn Jul 19 '18 at 12:48
  • $\begingroup$ "The perversity of the universe tends towards a maximum." <-- forget where I first heard that, but it would seem to apply to this poor moon. Found it: Finagle's Law . $\endgroup$ – Carl Witthoft Jul 19 '18 at 14:58
  • $\begingroup$ @CarlWitthoft at first I thought murphy's law, a tendency towards chaos! But nope, your specific quote led me to: en.wikipedia.org/wiki/Finagle%27s_law $\endgroup$ – Magic Octopus Urn Jul 19 '18 at 15:29
  • $\begingroup$ Would it be possible that Valetudo is a remnant of a body that crashed with a large retrograde moon, creating the retrograde group? $\endgroup$ – SF. Jul 23 '18 at 7:57
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My questions are: 1.: why are most of the moons that are further away from Jupiter retrograde orbits, while the ones extremely close to Jupiter are prograde orbits? 2.: What possible situations were required for Valetudo to end up where it is opposed to where the rest of the pro-grade moons are clustered? Pure chance / a series of unlikely events?

Answer 1. Inner moons usually orbit in the same direction as the rotation of their parent planet, this is favored due to one means of their formation, and the effect of tidal forces if they were close to the planet and in a retrograde orbit.

Source 1: Wikipedia: "Moons of Jupiter - Regular Satellites":

"The inner moons are in a 1:2:4 orbital resonance. Models suggest that they formed by slow accretion in the low-density Jovian subnebula—a disc of the gas and dust that existed around Jupiter after its formation—which lasted up to 10 million years in the case of Callisto.$^{[39]}$

[39]: "Formation of the Galilean Satellites: Conditions of Accretion" (Dec 2002), by Robin M. Canup and William R. Ward.

Source 2: Wikipedia - "Hill Sphere - True Region of Stability":

"The region of stability for retrograde orbits at a large distance from the primary, is larger than the region for prograde orbits at a large distance from the primary. This was thought to explain the preponderance of retrograde moons around Jupiter; however, Saturn has a more even mix of retrograde/prograde moons so the reasons are more complicated.$^{[3]}$

[3]: "Chaos-assisted capture of irregular moons" (May 2003), by Sergey A. Astakhov, Andrew D. Burbanks, Stephen Wiggins, and David Farrelly.

Answer 2. It is possible that Valetudo was a binary companion and was captured in an exchange reaction.

Source 1: Wikipedia: "Moons of Jupiter":

"These moons were probably captured by Jupiter from solar orbits."

Source 2: "Satellite Capture via Binary Exchange Reactions: Application to Triton" (Dec 2005), by C. Agnor and D.P. Hamilton:

"... exchange reactions during planet-binary encounters (i.e. ~ an encounter in which one member of a binary is expelled and its place taken by the planet) represent a new pathway for the capture of planetary satellites. This new capture mechanism may be relevant to origin and evolution of several small body populations in the solar system (e.g.~the icy and irregular satellites of the giant planets).

Additional Info: Robert Walker explains and offers this image showing how contact and overcontact binaries, ternaries, quarternaries, etc. can form stable and unstable systems. Those can be torn apart by the tidal forces of larger (even smaller) objects.

Binary, ternary and quarternary objects

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