Bet you never saw this one coming.
D (email@example.com) writes:
The Halos destroy all sentient life within a certain radius.
The Halos work by starving the Flood.
Flood feed off sentient life.
In order for the Halos to be activated, a Reclaimer has to land on the ring. Coincidentally, the ring contains Flood forms.
I propose that the rings destroy all sentient life by teleporting the Flood onto every planet within a certain radius of the ring itself.
Firstly, the ball of energy at the end-game cutscene looks like a teleportation field. At the very least, it's the correct colour. Secondly, a Reclaimer has to land on the ring to activate it, putting it at risk of infection. To land on the ring, assumedly, requires at least FTL drives, which means that the Flood can easily hijack a ship or two. At any rate, Threshold was a gas giant, incapable of supporting life as we know it. The Halos obviously contained laboratories built to study the Flood, but the containers seen in the Arbiter missions seem more suited for interstellar transport to me...
I think the rings work by forcing the Flood to consume their food sources (sentient life) so fast that their inflated population cannot survive in the end- just like a red tide. It would work, by me- once the Flood finish feeding off their available food sources, they would either enter dormancy or die off. Given that sentient life takes forever or longer to evolve (metaphorically), the Flood would have ample time to die off.