To the question, "What are the Cauldrons of Rhea?" I don't know, but Rhea is the second largest moon of Saturn and quite possibly a warmind hardened stealth bunker (as described in the grimore card) or a former colony now under control of the Nine. If you read about Rhea it is also mostly water/ice and it is very large. I think from Xur's dialogue and the grimoire cards that that the Awoken and the Jovians are decendents of humans that escaped to or were stranded in the Reef and beyond and such proximity to the energies of the Darkness and being so far from the reaches of the Traveler's Light (your Ghost says the Reef is the farthest the Light reaches) that it has changed the biology of those humans.