The microstructural location of impurities in ice

The microstructural locations of impurities in natural ice have been of interest for over a century, but only in recent years have efforts been made to determine their location. Here, we outline the few measurements that have been performed. The focus is on a technique that we developed, which has also been subsequently used by others, that involves the controlled sublimation of uncoated ice in a low-vacuum scanning electron microscope. This process leaves behind concentrated impurities, in both the grain boundaries and the grain interiors, which are then identified using energy dispersive X-ray microanalysis. An overview of observations on meteoric ice from both Greenland and Antarctica; on accretion ice from Vostok Station, Antarctica; and on temperate pond and river ice is presented.