Welcome to this Southerly Earthcache! I have served onboard USCGC POLAR STAR, an icebreaker, for the past two years and have visited McMurdo twice while breaking out the resupply channel for ships to come into the ice pier. I thought this cache would be the perfect opportunity to share some information with you regarding the solid state of water.
The Ross Sea is a deep bay of the Southern Ocean in Antarctica between Victoria Land and Marie Byrd Land. The southern part of this sea is covered by the Ross Ice Shelf. In order to explore the ice formation, we will first have to discuss some terms that are used in the study of ice, or Cryology and what they mean so we can properly analyze what is happening in McMurdo Sound.
TERMINOLOGY
- Anchored Ice: Submerged ice attached or anchored to the bottom, irrespective of its formation.
- Breccias: Ice of different developmental stages that have frozen together.
- Melt Pond: A small pool of water that has formed on top of sea ice due to melting.
- Sastrugi: Sharp, irregular ridges formed on a snow surface by wind erosion and deposition. On sea ice these ridges are parallel to the direction of the prevailing wind at the time they were formed.
- Windrow: The fracturing of sheet ice due to wave actions.
- Pack Ice: Ice coverage that is 7/10 or more.
- Very Close Pack: Ice which is 9-10/10 concentration
- Close Pack: 7-8/10 concentration.
- Open Pack: 4-6/10 concentration.
- Very Open Pack: 1/10 - 3/10 Ice concentration.
- Fast Ice: Sea ice which forms and remains anchored to the coast
- First Year Ice: Ice that has formed in the last year and is typically weaker than multi-year ice.
- Multi Year Ice: Ice that has survived at least one melt/freeze cycle. Typically stronger than first year ice.
- Rafting: Type of deformed ice formed by one piece of ice overriding another.
- Hummocking: the pressure process by which sea ice is forced upwards (imagine two techtonic plates creating mountains)
- Bummocking: The pressure process by which sea ice is forced downards (imagine two techtonic plates creating a valley)
- Rubble Field: An area of extremely deformed sea ice of unusual thickness formed during the winter by the motion of the pack ice against or around a protruding rock, islet, or other obstruction.
- Polynya: Any non linear shaped opening in the ice (think an open lake in the ice, large than a melt pond)
THE ICE
Now that we have the appropriate terminology, we can begin to learn a bit more about McMurdo sound and it's ice! As cold winds blow off the ice shelf they reach the water where they begin to whisk away the heat from the southern ocean. As this occurs, the first layers of ice start to build.
Sea water with a salinity of about 34 mg/L begins to freeze when its temperatures drops to around -1.89° C. Small millimeter sized crystals, called frazil ice, accumulate on the sea surface where they form slicks and give the sea surface a greasy appearance. This "grease ice" eventually aggregates into small ice chunks which turn into pancake-shaped ice floes when they rub against each other in rough seas. Eventually these the pancakes freeze together and form a solid ice cover. As time progresses and temperatures sink even further the ice continues to grow thicker. Strong winds and currents force pancakes or floes over each other, giving the ice a rough surface and a very rugged bottom. Reports by divers and photos taken by Under water vehicles have confirmed the extreme roughness of the underside, while the sea surface often appears flat due to the covering of snow.
Once a sheet of ice has formed, it can increase in thickness by the freezing of water on its lower surface. This means that heat must be removed from the water. When the air above the ice is colder than the water below the ice, heat is removed by conduction through the ice from the water to the air above. Thus, heat flows from the warm body (water) to the cold body (air). Scientists have discovered that the rate at which heat flows from the water is proportional to the temperature difference between air and water, and also inversely proportional to the thickness of the ice layer and the snow cover on top of the ice. This means that younger, thinner ice grows faster than older thicker ice!
With very cold air temperatures, a thin sheet of ice will thicken quickly at first, perhaps 8 to 10 centimetres (3 to 4 inches) in the first 24 hours. The rate of growth gradually slows down as the ice thickens. Besides growing on the bottom of the surface of an ice sheet, ice can also grow in-between the ice floes joining them together or by accumulation of slush, ice cakes, or ice floes on the windward shore.
Snow cover acts as a blanket, slowing down the flow of heat from the water. The effectiveness of snow as an insulator, or blanket, depends mainly on how compacted it is. A layer of soft, fluffy, new fallen snow is an excellent insulator because of its high air content. On the other hand, extremely hard, compacted snow is a relatively poor insulator.
TO QUALIFY AND LOG THE CACHE
To qualify and log this cache you must go to the coordinates above. They will take you to the end of Hut Point (out past the hut itself). Stand with the cross at your back and answer the following questions. Email me your answers and do NOT put them in your log. Those logs will be deleted. You do not have to wait for me to email you back to log the smiley:
- Look at the sound. What stage of ice formation is the sound in now? (Frazil ice, nilas, pancakes, small/large/vast floes?)
- What concentration is the ice now?
- Do you spy any polyenas? What do you think may be causing them to form?
- Do you spy any rubble fields? What do you think may be causing them to form?
- Using your ice terminology describe to me what you see. To you, what is the most interesting feature of the sea ice in its current state?
- Optional: Snap a photo of the current ice conditions.
- Optional: Let me know what brings you down to McMurdo! I'm always interested in hearing your travel stories!
I would like to thank you for taking the time to learn about ice formation and to find this cache. As a reminder, please do not post an "armchair" log. You must actually go to the site in order to log this as a find.
CONGRATULATIONS TO Max Roby ON HIS FIRST TO FIND!!