[from the back cover]"On the North Pole of Pluto stands an enigma: a huge circle of standing blocks of ice, built on the pattern of Earth's Stonehenge--but ten times the size, standing alone at the farthest reach of the Solar System. What is it? Who came there to build it?
"The secrete lies, perhaps, in the chaotic decades of the Martian Revolution, in the lost memories of those who have lived for centuries."
It's Kim Stanley Robinson at his usual best.