A time-resolved model, incorporating both vertical diffusion and chemistry, indicates that the minimum source strength required to produce a typical sudden sodium layer (SSL) is about 3 Na atoms cm-3 s-1. However, many of the mechanisms proposed to explain the formation of SSLs are not able realistically to provide this source strength, or the source strength of about 1 Na atom cm-3 s-1 that is required to maintain an SSL over several hours. Furthermore, the sodium reservoirs do not appear to contain sufficient sodium to produce an SSL, although sodium ions (Na+), and meteoric dust coated with sodium compounds, cannot be ruled out. An additional mechanism is proposed, namely, the release of atomic Na by dissociative electron attachment to sodium-containing silicate molecules (e.g., Na2SiO3 or NaAlSiO4) that are produced by meteoric ablation. ¿ American Geophysical Union 1993