In the last post, I give how outer Main Belt objects look damp or frosty. This, despite being inside the “snow line”- the orbit radius where something is far enough from the Sun, and thus cool enough, to let water ice survive. 617 Patroclus, a Trojan, is now outside the snow line.
Jupiter, the biggest planet by far, orbits at a radius of 5.1 AU from the Sun (i. e., 5.1 times the radius of Earth’s orbit). This is past the snow line, and the two are connected. Water is one of the most common substances in the Solar System, especially if going by mass. Since water, past the snow line, would stick, instead of boil and blow off, the giant planets stuck more water together, and had higher gravity. This high gravity pulled in more water, dust, gas, etc., which in turn pulled even more matter in, and so forth. The giant planets got that way because, once the water held, the process went runaway. But as one got further out in the early Solar System, the cloud of starting material tapered away. Saturn, Uranus, etc. are much smaller. It appears Jupiter, just over the line, was at some sweet spot, not too close yet not too far.
The Trojan asteroids are in a different sweet spot. Bodies have Lagrange Points, as I’ve stated. Two of them are 60 degrees ahead, and behind, a planet in its orbit. At these points (Lagrange point 4 and 5, or L4 and L5), a quirk of geometry causes a slight dip in gravity. Objects which fall into L4 and L5 tend to stay there, loosely bound by the bigger object. Jupiter, the biggest body in the area, has strong L4 and L5 points, and trapped large numbers of objects there. These are the Jupiter Trojans; in total, their number is a significant fraction of the number in the Main Belt. Jupiter is so gravitationally dominant, other planets have negligible numbers of Trojan objects (or zero), and one just assumes “Jupiter Trojan” on hearing the T-word.
If the outer Belt retained water despite being measurably closer to the Sun, it is simply taken as fact that Trojans have ice. Still, it’s worth the investigation. The nature and history of Trojan chemistry turns out to be the history of the Solar System itself.