Chapter 4: Ice Drilling and Coring

To understand ice drilling, one must have a basic understanding of the medium. Consider a geographical region so cold that all precipitation comes as snow and there is no summer melt - for example, the central Greenland ice sheet. Here, annual snows pile up year after year and over time become so thick that the whole mass begins to flow, that is, it deforms plastically under its own weight. Glaciers are found where not all of the annual snow melts during the average year over a period of years, so snow and ice accumulate. They can develop on a small scale, as in a mountain in Wyoming, or on a large scale, where over thousands, even hundreds of thousands, of years the ice can build up to huge thicknesses and cover vast extents of land, forming ice sheets. Over time, on mountain glaciers and ice sheets, there comes to be a near equilibrium between the mass of snow deposited on the surface and the mass of ice melting seasonally or breaking off into the ocean as icebergs and floating away.

In cold places where there is no melt, annual layers are preserved for many years. The central Greenland ice sheet is about 3 km thick and has distinct layers at least 100,000 years old. The East Antarctic ice sheet is well over 3 km thick in many places and, with an annual snowfall much less than in Greenland, has an annual record of 1,000,000 years or more. Even the Antarctic ice sheet, big as it is, flows slowly downhill. The South Pole is about 1200km from the nearest coast, yet each year when the US Geological Survey places a marker at the exact geographical pole, it is about 3m from the previous year's marker.

There is another aspect that is important for a basic understanding of this medium: densification with depth. When snow falls, it is relatively light and fluffy. As more annual layers of snow are deposited on top of existing layers, the buried snow becomes denser and denser until it becomes solid ice. Glaciologists call snow older than 1 year "firn". A principle defining characteristic of firn is that it is porous. At a certain point in its densification, the pores of the firn begin to close off and form isolated bubbles that are no longer interconnected, and there begins the transition from firn into ice. The density versus depth curve is continuous through this transition and it is not noticeable in appearance, but deeper in the ice the bubbles become small spheres. These small air bubbles are more and more pressurized as they become buried ever deeper and this will continue until a depth is reached where they go into solid solution in the ice itself and disappear. This is the depth where the densification curve has leveled off. The two variables that most affect the rate of densification are pressure and temperature, with temperature being predominant. So, in the cold areas such as the middle of the Greenland ice sheet, where the mean annual temperature is about -35° C, the firn-ice transition is about 80m deep. At South Pole, where the mean annual temperature is about -50° C, the firn-ice transition is at around 120 m. In warmer regions, this densification occurs much more rapidly...

Since 1950, and particularly in the last 20 years, ice coring and drilling have been a rapidly growing activity. Our purpose here is to review the different methods and equipment used for ice coring and drilling. We believe that this focus will be more useful than a historical approach to the expected reader, interested more in extraterrestrial applications than in terrestrial glaciology. We also comment briefly on drilling fluids and on some particular aspects of encountering the base of the ice.