It is unlikely that the dust mantles of small comets (cometary mass ~105 kg, earth impact rate ~20 per minute) supress the evaporation of gases into the solar wind. An order-of-magnitude argument based on comet sizes, densities, and meteoritic dust influx indicate a maximum thickness for the mantles of ~200 &mgr;m. A much more reasonable estimate gives a thickness of ~0.2 &mgr;m. The mantles must be considered too thin to prevent the enrichment of the solar wind by evaporation.