Like clouds in other parts of the atmosphere, one element required for polar mesospheric clouds to form is tiny dust particles on which water vapor can accumulate and grow into ice crystals. Nearer to Earth's surface, clouds form from 'cloud condensation nuclei' that can be sea salt spray, desert dust, or other materials lofted from the surface. In the mesosphere it is thought that cosmic dust particles falling into the Earth's atmosphere might serve this same purpose, and the Cosmic Dust Experiment instrument on the Aeronomy of Ice Mission will be able to identify how important cosmic dust particles are in the lifecycle of these clouds.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 220.127.116.11.0