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The Planetary Society Blog
By Emily Lakdawalla
An origin for Triton -- and, maybe, most of the irregular satellites
May. 12, 2006 | 15:06 PDT | 22:06 UTC
It's Friday, my day for reading Science and Nature to see what's new, and Nature has a planetary cover story this week, dealing with the origin of Triton. It's by Craig Agnor and Douglas Hamilton, and it proposes a new scenario for how Neptune came by its largest moon -- a scenario that might actually have played out in the origins of lots of the outer planets' outer moons.
Triton is Neptune's largest moon, one of the seven moons in the solar system that are larger than the planet Pluto. As such it seems comfortable in a lineup of the other giant planet icy satellites, particularly the Galilean satellites of Jupiter (Triton is smaller than Europa but considerably larger than the next-sized bunch of satellites like Rhea, Dione, Titania, and Oberon). However, if you take a look at Triton's orbital characteristics, you realize that it is a total oddball. Unlike all of these other globe-shaped icy moons (except Iapetus), its orbit is significantly inclined, 33 degrees away from the plane of Neptune's regular satellites. And unlike all of the other icy moons (including Iapetus), Triton's orbit is retrograde, meaning that it orbits Neptune in the opposite direction from the direction that the planet rotates. This is an instant clue that Triton didn't originate the way the regular satellites did; that it formed in some other location, probably on its own in the solar system like the asteroids, comets, and Kuiper belt objects, and then it somehow got captured by Neptune.
But how? There are several scenarios for how giant planets capture satellites. One is that the capture happened early in the formation of the giant planet, when its atmosphere would still have been puffed out with primordial heat; an object passing through could have been dragged into orbit by plowing through that gas. Another posits that the capture could have happened when an object passing through just happened to collide with an existing moon. All of these scenarios have never been particularly satisfying when applied to Triton, which is very big (40% more massive than Pluto); these scenarios work best for smaller objects.
Agnor and Hamilton began by a statement of fact that leads to their proposition of a new scenario for the capture of irregular satellites. "Binaries have recently been discovered in nearly all of the Solar System's small-body reservoirs and appear to be a natural consequence of planet formation and Solar System evolution." Here are the numbers of binaries that have lately been observed in these various small-body reservoirs:
| Reservoir | Fraction with satellites |
|---|
| near-Earth asteroids | 16% | | large main belt asteroids | 2% | | Kuiper belt objects | 11% |
The true number, they say, is probably larger -- undoubtedly we haven't discovered all the binaries yet. In other words, binaries are very common.
How Neptune captured Triton?In this artist's conception, Triton and a possible binary companion approach Neptune. Such an encounter could have facilitated Triton's capture to its inclined, retrograde orbit around Neptune. Neptune may once have had other large moons, as in this picture, but they would have been destroyed through the mayhem caused by large Triton's capture to an originally ecentric orbit.
Credit: Craig Agnor, from image maps provided by Steve Albers, Jerry Gardner, James Hastings-Trew, Constantine Thomas, and NOAA's "Science on a Sphere" project
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The rest of their paper follows from the question: what happens when a binary system containing one object the mass of Triton and one smaller partner has a close encounter with Neptune? If 11% of the Kuiper belt objects are binaries, then 11% of Neptune's encounters with Kuiper belt objects are with binaries. Solar system formation models predict that when Neptune formed, it migrated outward from the Sun, which means it had lots and lots of encounters with Kuiper belt objects. Agnor and Hamilton found that, a lot of the time, when a binary object comes close to Neptune, the gravitational interactions among the three bodies split the binary apart, essentially slipping Neptune in the place of one of the binary objects. The replaced object gets sent on its lonely way, bereft of its former partner.
The mathematical model is engagingly simple; the paper runs a variety of scenarios through the model, including a range of encounter velocities and a range of partner sizes for the Triton-sized body that would have been one member of the original binary. What they find is that you can begin with a pretty wide range of encounter velocities, encounter angles, and mass ratios and wind up with Neptune capturing Triton from such an encounter. So, while they really can't conclude anything about what kind of binary Triton may have been at the outset -- a "double-planet" Pluto-Charon type or one more like 2003 UB313, with its relatively diminutive moonlet -- they can state that capture of Triton from a binary pair is much more likely than any of the other previously posited scenarios for Triton's origin.
As with most mathematical models, the existence of a highly likely scenario doesn't prove that that's the scenario that actually happened. Triton could very well have shown up at Neptune by way a variety of much less likely paths. But because this scenario is so likely, it almost certainly explains the origins of a goodly proportion of the other irregular satellites in the outer solar system.
The paper got me thinking about two questions. First of all, I wonder whether the capture of Triton could have happened relatively recently. The Agnor and Hamilton model doesn't require Triton to have been captured right when Neptune formed. Allesandro Morbidelli, who wrote a "News and Views" story in Nature on the Triton capture paper, suggests that it could have happened later, during the time of the "Late Heavy Bombardment," between 4 and 3.8 billion years ago. I wonder if it could have happened even more recently than that -- giving a relatively recent heat source to help to power Triton's geysers. Just a thought. Recent capture would probably be much less likely than capture in the distant past.
Another question I have is whether Saturn's moon Iapetus could have been captured too. Iapetus is pretty big (though only half the diameter, so presumably an eighth the mass, of Triton) and it's pretty far from Saturn (ten times as far from Saturn as Triton is from Neptune; but then Saturn has five times the mass of Neptune), so I'm not sure the math would work out for an Iapetus capture through the Agnor and Hamilton binary-capture model. But it also has a notably inclined orbit, 15 degrees from Saturn's ring plane. It's not retrograde like Triton, but it seems to me that you need to do something to Iapetus to make it so inclined -- maybe this model would explain Iapetus' weirdness. Hmm.
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