system
Given the following post, respond as if you're commenting on Reddit:
human
Since Mercury's days are very hot, and its nights are cold (relative to Earth); if Mercury's rotation were to slow to be 1:1 tidal locked, so one side were always facing the sun, would there exist a ring around the planet that were suitable to support terrestrial life?
gpt
For quite a while, astronomers actually thought Mercury _was_ just so tidally locked, since it always seemed to showing the same aspect when we could see it.  Turns out it just was an artifact of its 3:2 locking and the limited times of the year when we could clearly see it, what with it being so close to the sun.

But anyway.  There would indeed be a ring probably towards the dark side of the terminator which would have human-tolerable temperatures, but that's only part of it.  You still need an atmosphere, which will get blasted away into space by solar wind on the day side, and you need water, which will gravitate towards the night side and be locked into permanent ice.

And even if Mercury had a strong magnetic field to help prevent atmospheric escape and the night side was mostly a vast ocean that allowed water to flow back to the habitable zone, you've still got to deal with what the furnace-like daytime temperatures would do to the molecular oxygen in the air.  Any chemists please correct me here, but O2 is terribly reactive at room temperature, even moreso as it gets hotter.  What life there is would have a devil of a time using the (admittedly 9 times brighter) twilight to produce enough oxygen to stay ahead of it all combining and combusting with everything on the dayside.

I guess maybe if you let it sit for a billion years, you'd wind up with an equivalent of the banded iron formations in terrestrial geology, and there'd simply be nothing left on the dayside to react, letting free O2 exist no matter what the temperature.  But Mercury is pretty short on light elements as it is; it's really not so much a small planet as it is the metal-rich core of a larger planet stripped of its mantle and crust.

TL;DR: Mercury's most practical contribution to human life would be to disassemble it for purely space-based habitats, not terraforming.