The Manhattan-sized ice island that last year broke free of Petermann Gletscher in North Greenland plowed into the bottom and broke apart. Force equals mass times acceleration. When 18 giga tons (mass) of moving ice crashes into the ocean’s bottom 200 meters below the surface (acceleration), then something gotta give. And give it did, as this single ice island is now at least 2 ice islands and a number of large icebergs. It does not take much speed for a heavy truck running into your parked car to demolish it. Something gotta give, your car, or in this case, the ice island shattering into pieces. Here is the piece, about twice the size of Manhattan by area.
We saw it last year as we entered Petermann Fjord by ship as the ice island left the same fjord to drift into Nares Strait. It then sailed towards Kane Basin where it got stuck for much of the 2012/13 winter to help keep the ice arch across Nares Strait more stable than it has been in a long time.
Terry Moran alerted me on these pages to the break-up that is also discussed on the Arctic Sea Ice forums, but the optical MODIS satellite imagery that many of us are addicted to do not show as much as we like to, because this sensor does not penetrate clouds. And it has been very cloud over Nares Strait much of this summer, more than in most earlier years anyways. There was a partly clear image yesterday that I overlay with the ocean’s bottom depth, but I only show the contour for 200-m and 300-m depth, because I know (more on that later) that the deepest part of the ice island is 200 meters below the sea surface. It is a very thick ice-cube.
So, I here use radar imagery that was sent to me an hour ago that tells the same story a little cleaner as this radar can see through the clouds much like an x-ray can see through our bodies.
Now how do I know that this ice island extends 200 meters (600 feet) below the surface? It all started with a post titled Shots of airborne lasers at Petermann Gletscher. This post developed into a talk with friends and colleagues over the winter and spring that jellied into a paper on this very subject that we have submitted for publication. Using lots of fancy satellites and sensors flown aboard planes such as lasers and ice-penetrating radars, three of us describe in much technical detail how thick the floating ice of Petermann Gletscher is and how it has changed over the last 10 years or so. Hence we have very detailed measurements of this ice island while it was still attached to the glacier in 2002 and 2007 and 2010 and 2011. It broke off in 2012, but it probably has not melted much just yet as it is still in very frigid Arctic waters.
There is another interesting tidbit on the very clean fracture line where the ice island broke apart the day before yesterday. As many other floating glaciers Petermann has a central channel that runs near its center all along the glacier where it is floating. I do not know why it is there or what makes it, but at this central channel the glacier (or ice island) is about 100 m thinner than either to its side. It is along this channel that the ice island broke apart into two new pieces of almost equal size. And just to add to the mystery, there are many more such channels oriented along the flow direction. It is a unique feature of many large floating glaciers both around North Greenland and Antarctica.
ADDENDUM Sept.-14, 2013: We actually made a video from inside Petermann Fjord last year that includes footage of the helicopter flying into the central channel ((YouTube video), the photos do not do justice to the immense scale.
Johnson, H., Münchow, A., Falkner, K., & Melling, H. (2011). Ocean circulation and properties in Petermann Fjord, Greenland Journal of Geophysical Research, 116 (C1) DOI: 10.1029/2010JC006519
Rignot, E., & Steffen, K. (2008). Channelized bottom melting and stability of floating ice shelves Geophysical Research Letters, 35 (2) DOI: 10.1029/2007GL031765