Oxygen-rich anionic metal silicate clusters as nucleation seeds for noctilucent clouds

Silicate-based meteoric smoke particles (MSPs) resulting from extraterrestrial dust impacting on the upper atmosphere are likely involved in seeding the formation of mesospheric ice-based noctilucent clouds (NLCs). However, the detailed physicochemical properties of MSP seeds remain elusive. Considering that ice formation is sensitive to MSP charging efficiency, here we focus on species which can efficiently capture charge. As in situ MSP characterisation is highly challenging, we mimic atmospheric MSP formation using laser ablation and collisional cooling to produce small anionic magnesium silicate clusters. Our detailed experimental and theoretical characterisation shows that these clusters have unusual oxygen-rich structures. Moreover, the corresponding neutral clusters have extraordinarily high adiabatic electron affinities and 3-18% charging efficiencies, which is remarkably consistent with estimates required by models to simulate atmospheric measurements. Overall, our highly oxidised anionic silicate clusters exhibit all necessary chemical, electronic and optical properties to be highly credible MSP-based seed candidates for NLC nucleation.