While driving with a passenger across Salt Lake City on I-15 yesterday, going the speed limit and no surrounding vehicles, the passenger window imploded into a million tiny pieces sending glass all over the passenger and even as far as the driver (me). We did not see any evidence of a rock or other object hitting the window and there wasn't anything unusual inside the car. The outside temperature was 30 degrees and the car had been in a heated garage all night. The passenger had cuts on his hands, but he was wearing sun glasses which protected his eyes. He had glass pieces all over him. I honestly believe this was a spontaneous event, but I suppose there is a remote chance that a rock could have hit the window. But there weren't any cars in the area or overpasses.