What, excluding water vapour, do night shining clouds consist?

Noctilucent clouds, or night shining clouds, are tenuous cloud-like phenomena in the upper atmosphere of Earth. They are first known to have been observed in 1885, two years after the eruption of Krakatoa volcano. Noctilucent clouds require water vapour, dust and exhaust. The dust is believed to come from micrometeors, although particulates from volcanoes and dust from the troposphere are also possibilities. The exhaust are from Space Shuttles.