Arctic ice

A mosaic of Envisat ASAR radar images acquired between 9 and 11 September 2011 over the Arctic Ocean. The sea-ice extent highlighted in blue corresponds to the areas where more than 80% of the sea surface is covered by ice (from an analysis performed by the US National Ice Center). The Arctic is one of the most inaccessible regions on Earth, so obtaining measurements of sea ice was difficult before the advent of satellites.