Exploration of Subglacial Lake Vostok: Brief History and Future Plans
Since 1990, the Russian Antarctic Expedition Program has drilled more than 3600 m of ice with additional support from the French and U.S. Antarctic programs between 1993 and 1998. The present borehole, 5G-1, was started in 1992 from a deviation along the previous borehole (5G) at depths of 2232-2246 m. By 1993 the coring had reached 2755 m in borehole 5G-1. After a one-year hiatus, drilling reached a depth of 3100 m in September 1995. Drilling continued during the 1995−1996 field season and was intended to continue through the 1996 winter to reach 25 m above the surface of the subglacial lake beneath Vostok (at ~3,650 m depth in accordance with the guidelines recommended by SCAR during the Lake Vostok Workshop, Cambridge 1995). However, when the station closed for the 1996 winter, drilling had reached 3350 m depth. A seismic survey was undertaken during the 1995−1996 field season in an area about 2 km2 around the borehole. A depth of 3623 m was reached in hole 5G-1 in 1998. After an eight-year hiatus, drilling resumed in 2005-2006, reaching a depth of 3650 m.
At present, the bottom of hole 5G-1 is less than 100 m above the surface of Lake Vostok and the Russian Antarctic Program plans to continue drilling and eventually sample the waters of Lake Vostok. The next step proposed is to drill an additional 75 m to obtain new scientific data on the origin, properties, and structure of the ice near the “ice cover-subglacial lake” boundary. The proposed method to access Lake Vostok will exploit the physical peculiarities of the lake-ice sheet system. The ice sheet basically floats on the lake, and the pressure at the “ice-water” boundary corresponds to the weight of the overlying ice sheet. During drilling, the pressure exerted by the drilling fluids within the borehole compensates the pressure of the overlying ice and keeps the hole open. By decreasing the quantity of drilling fluids, the water pressure in the lake will be greater than that of the drilling fluids. When the drill reaches the lake, the drilling fluids will be forced up the borehole by lake water.
The borehole fluids comprise mainly aviation fuel (TS-1) and Freon (CFC-141b). These drilling fluids will not dissolve in water and will be displaced by the water rising in the borehole. Also, a sterile drilling fluid will be introduced into the lowermost 200 m of the hole, approximately 100 m above the lake surface, which will act as a plug between the top and clean bottom sections of the borehole. The density of this fluid is intermediate between the lake water and aviation drilling fluids.
It is planned that during the last stage of penetration, the drill will be extracted from the hole immediately after reaching the water surface. Lake water will rise in the borehole and freeze. Later, this newly frozen ice will be drilled to recover samples of the lake water. The newly formed ice remaining below the sampled lake ice will form a plug and thereby prevent a possible connection between the drilling fluids and the lake water. Thus, the proposed method will allow the sampling of lake water without the drill and sampling instruments entering the lake.
SOURCE:Robin Bell, Lamont-Doherty Earth Observatory of Columbia University.