This came up back in June. https://www.halowaypoint.com/en-us/forums/db05ce78845f4120b062c50816008e5d/topics/falling-off-a-halo-ring/fd6c4ae1-317b-4c41-935f-58fde8c793fd/posts?page=1
Basically there's really tall walls plus the rotation of the rings that keep the atmosphere in. Given that Halo rings are actually very small in comparison to the megastructures that inspired them, there's clearly some form of Forerunner space magic happening as well, since the rings would have to rotate much faster than they currently do in order to keep the atmosphere in. Here's my main post from that thread.
Quote:In order to keep atmosphere within ring-world styled megastructures as they turn, there have to be really tall perimeter walls that keep it from dispersing. So if you got to the edge of a Halo (which are actually exceedingly small when compared to most ringworlds in fiction and speculative science) you'd be met with an enormous wall. Same goes with the Ark.For example, the eponymous ringworld from Larry Niven's Ringworld novels is 186 MILLION miles in diameter, and requires walls that are 500 miles high to keep the atmosphere contained over the surface of the megastructure. The Orbitals from The Culture novels (which are a little bit closer to what a Halo is and were the primary inspiration for Halo's design) are 3 million miles in diameter and have barrier walls that are probably similar in height.Now for Halo, the modern Installations we see are 10,000 kilometers in diameter, while the original Halo Array from the Forerunner Era were 30,000 kilometers in diameter. That means the walls are probably are probably around fifty or so miles high (maybe higher). Since Halo rings are so small and they spin to generate Earth's gravity, I would speculate that if you tried to seed the Halo with an atmosphere it would need to be composed of heavier gases which would make the surface quite unlike Earth and probably toxic to most like (except maybe Grunts). So there's some clear use of Forerunner Supertech going on to make the ringworlds completely Earth-like.But yeah, there's walls. Might even be able to get up on them in Halo Infinite, who knows. Here's a screenshot of Zenith from Halo 2 Anniversary that proves it. https://www.halopedia.org/images/b/b7/H2A_-_ZenithRing.jpg