Impact of Planetary Rings

[EDG]

First, the presence of a ring system indicates that there is no moon, because a moon's gravitational pull would pull in any debris within it's orbit, which is where the rings would be.

Not necessarily. Rings can only extend out to about 2.5 planetary radii, so if a moon is beyond that then it won't really pull anything out at all. Unless they're literally right next to the rings, the most other moons will do is create gaps and other structures due to orbital resonances.

Without a moon, and with a ring system, the amount of impacts from space crap would be staggering, causing a more or less permanent winter from all the dust in the air (that's an interesting idea for a campaign setting...). Civilization will most likely be concentrated underground, where it is safe from the constant bombardment of the falling space debris, with the surface world being a dangerous, frozen world where only the brave dare enter venture.

Having a ring system doesn't mean that you get a constant rain of rocks falling from the sky - you'd just have dust slowly spiralling in and all of it would burn up in the atmosphere because it's just not big enough to survive the fall to the ground.

If the ring is perfectly in line with the plane of the planet's orbit around the sun (not the axis., as indicated in your second point, instead of a thin band in the sky, the area north/south from the equator for about 2-3 miles would be in a permanent state of total solar eclipse (sort of a desolate, barren darkness across the center of the planet, that's a cool campaign idea as well!).

It won't be in line with the planet's orbital plane - the orbital dynamics just doesn't work like that. Tidal forces will very rapidly pull anything on an inclined orbit close to the planet into its equatorial plane. What you *can* have though is a tilted planet, so that the rings around it that are aligned with the equator change their orientation relative to the sun. This is what happens with Saturn over the course of its orbit aroudn the sun, which is why sometimes the rings are edge-on as seen from the sun, and other times they're inclined by about 26 degrees (the planet's axial tilt) and thus cast huge bands of shadow on the hemisphere underneath them.

On your point about the axial tilt causing the rings to cast a shadow. During each day, there would be two periods of night lasting about 1-3 minutes depending, on one's position on the planet, when the sun crosses behind the rings.

I suspect it'd last longer than that, but it depends on the planet's rotation period. Also it would depend on whether the rings were thick enough to actually block out all the light or just make the sun flicker or reduce its brightness somewhat as it passes behind them.

Here's a idea of the world as I see it. The rings were created during a time when civilizations were present on the surface, the planet would be covered in ruins buried in ice that were abandoned as civilization fled the surface, a great place for a party of adventurers. Surface flora and fauna will have adapted to the drastic environmental changes. Burrowing creatures will have survived because they can take shelter underground and will have taken a role in many ecological niches that were left empty. Everything will be adapted to survive in sub-freezing temperatures (Layers of insulating hair/fat, etc.)

You're basically just describing a post-large impact scenario there - another way to do the same thing woudl be to keep the rings but just not have them be the cause of the disaster - instead have a large (km-scale) asteroid smack into the planet and cause massive environmental disruption, but not wipe out all life. Your world would then be in the "nuclear winter" stage after the impact when the temperatures have dropped.