What can we learn about a dynamical length scale in glasses from measurements of surface mobility?

We consider the ability of recent measurements on the size of a liquid-like mobile surface region in glasses to provide direct information on the length scale of enhanced surface mobility. While these quantities are strongly related there are important distinctions that limit the ability of measurements to quantify the actual length over which the surface properties change from surface to bulk-like. In particular, we show that for temperatures near the bulk glass transition, measurements of a liquid-like mobile layer may have very limited predictive power when it comes to determining the temperature dependent length scale of enhanced surface mobility near the glass transition temperature. This places important limitations on the ability of measurements of such enhanced surface dynamics to contribute to discussion on the length scale for dynamical correlation in glassy materials.