I have lifted this idea wholesale from Kim Stanley Robinson's 1985 novel Icehenge.
Icehenge is a circle of columns of ice, each slightly different, averaging 3m wide and 9m tall. They can be found on an otherwise dull and unexciting ice ball, the outermost planet of the Kibara system (007 Aquila). The ice-liths all have sloping upper surfaces and it looks more like a slightly smarter modern art version of the Callanish stone circle than Stonehenge, with is lintels and nested circles and horseshoes of stones.
What the hell is it doing there? Endless quantities of cranks have theorised about the place, coming up with the usual blather about prehistoric starfaring cultures from Earth and extinct aliens, interstellar ley lines, astronomical alignments and so on.
Serious archaeologists know only that the ice is local (they checked the chemistry), the site was smoothed off before construction and that they have no idea how old it is, dating ice left standing out in a near vacuum is not something they have any firm idea about. It is also damned far from anywhere, getting there even at 6G acceleration from the system's inhabited world takes a couple of weeks and the amount of comet debris nearby makes a short warp inadvisable. Someone tried it once and got their ship smashed to bits, giving the tin foil hat brigade plenty of theories about curses and warp distorting defences to witter about.
Standing as it does on a flat white plain so far from it's sun that Kibara Prime is lost in the starfield and with a ragged ring of ice chunks that orbit the planet hanging in the sky above it, it is a very beautiful and spooky place and tourists are willing to pay a pretty steep price to go and see it.
- It is a meeting place for space druids. Various eccentrics from around Known Space built it and they have built others yet undiscovered in systems more less in a line back to Earth. On the appointed day all the rings will be occupied by twits in spacesuits with pointy hats waving freeze dried mistletoe and they will supposedly telepathically contact each other. Will it work? Who knows, but the PCs have a bunch of nerdy computer programmers and accountants calling each other by made up celticy sounding names led by a systems analyst calling himself Arthur with a ice sword he keeps in a bucket of liquid hydrogen willing to pay oodles to be taken out there. Lets hope they don't have a theological split along the way eh? Or get too riled if it doesn't work, or too over excited if it does.
- It really was aliens. It will soon be midsummers day on this isolated world, which only happens every 300 years and that's when the true alignments will come right, according to anyone who knows anything about megaliths on Earth. Various rival scientific expeditions are setting out to see what happens. They know the stars the alignments will point at already from computer models, this is just a sightseeing trip really, but some are a bit curious to see what happens. Does a space portal crossing hundreds of parsecs open? Do disembodied alien intelligences appear and dispense ancient wisdom? Or do the various squads of scientists squabble and have a fight and miss it all?
- The main tourist carrier is Jellaby Lines, founded by Douglas Jellaby, they guy whose starship fell out of warp in the outer system and who found the ring by accident 40 years ago. Or that's his story – he actually built the thing himself as a money spinner. A boffin with an answer to dating the thing properly is aboard his ship, along with the PCs. Is he going to fall out of an airlock?
- The Duke of Hereford from England, not having anything better to do, is on his way to the place. Terrorists from New Dorset, a colony trying to secede from the English Empire will be there to meet him.
- The bit about astronomical alignments is pish, a PC realises, its places on the planet that are important, like that big ice mountain on the line from lith 18 to lith 6... what is under there?
- It's a graveyard. A scout ship with a crew from the Western Isles of Scotland crashed here a hundred and seventy years ago, well supplied, but with no comms and no way off the world and at that time precious few ships passing through. They built it as an homage to Callanish back home to pass the time and maybe, just maybe, a signal to passing vessels to come down and save them. Deep seismic probes, only useable with big explosives, will find the graves and deep buried ship.
- A bored teenager throws a chunk of ice at a lith while the PCs are there, and the thing vibrates. Now there's no sound in a vacuum, but perhaps there's a way to listen to the noise with a contact microphone – what tunes can you play, and what happens when you do?
- There is a lake under the ice – liquid nitrogen mostly, it being effin cold out here in deep space – is that why the henge is here? Drilling down into it awakens the Elder Thing lurking within, and as it squawks its spawn, frozen for aeons in the ice chunks in the rings above begin to wake...
- It was aliens, no kidding, really it was – they put this here as a marker for a strange matter waste dump that would warn any sensible species to stay well away. And what kind of buffoonish species couldn't read the clear warning signs from the variations in height of the liths, reflecting the extended tentacles of a Multiploid Quercate in pain? The overgrown monkeys digging a ruddy great tunnel under the site with laser drills, that's who.
- The liths were built by drug smugglers. Why take the risk of having a maser or electromagnetic beacon for your dropped drug packages when a visual sign will do? Make it something suitably weird and silly and no one will suss it out.