INTRODUCTION 



The sea ice in McMurdo Sound south of McMurdo Station, Antarctica, is used 

 extensively for aircraft operations, travel, freight hauling, and docking areas for 

 cargo handling. The safety and efficiency of utilizing the sea ice depends upon a 

 knowledge of the many factors affecting its physical properties throughout the season. 



The most important factors influencing the physical properties of the ice are 

 grouped into two broad classes: extrinsic and intrinsic. The extrinsic factors include 

 solar radiation, tides and currents, water temperature, and snow cover. Intrinsic 

 factors include crystal structure, thickness, salinity, brine drainage, and ice temper- 

 ature. 



The relationship between crystal structure and strength is well known for many 

 crystalline solids. Sea ice is a crystalline solid with physical properties that are 

 highly temperature dependent, especially between -1.8° and -30°C. The crystal 

 parameters and strength properties of sea ice vary widely and are related to temperature 

 and the growth history of the ice. A detailed study of the crystal structure and 

 other internal features is essential for a better understanding of their relationship 

 with strength properties. 



This report presents the results of sea ice crystal studies on that part of 

 McMurdo Sound that forms an embayment south and west of Hut Point Peninsula, 

 Ross Island, Antarctica. The studies were conducted between October 1965 and 

 February 1966 on sea ice 8 to 1 1 months old. Temperature, salinity, stratigraphy, 

 platelet width, and other crystal parameters were measured at selected locations 

 where different environmental factors may have affected the stratigraphy and 

 crystallography. Ring-tensile strength tests were performed after the ice sheet 

 became nearly isothermal in an attempt to correlate strength with the above properties. 



DESCRIPTION OF SEA ICE 



Sea ice is different from freshwater ice in many respects — as for example, 

 in salinity, crystallography, freezing temperature, density, features caused by brine 

 drainage, and heterogeneity of physical properties. Sea ice is difficult to describe 

 because It is composed of pure ice crystals, salt crystals, bubbles, and brine cavities 

 of various sizes and shapes, all apparently distributed at random. Sea ice crystals 

 are actually bundles of small, pure ice platelets separated by layers of brine and 



