Though it is all composed of frozen water, ice is hardly uniform. On October 7, 2011, the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this image of a variety of ice types off the coast of East Antarctica.
Brilliant white ice fills the right half of this image. It is fast ice, and derives its name from the fact that it holds fast to the shore. This ice is thick enough to completely hide the underlying seawater, hence its brilliant white color.
Trapped within the fast ice, and stuck along the edge of it, are icebergs. Icebergs form by calving off ice shelves—thick slabs of ice attached to the coast. Ice shelves can range in thickness from tens to hundreds of meters, and the icebergs that calve off of them can tower over nearby sea ice. One iceberg, drenched with meltwater, has toppled and shattered (image upper right). The water-saturated ice leaves a blue tinge.
The icebergs along the edge of the fast ice are likely grounded on the shallow sea floor, and their presence may help hold the fast ice in place.
Farther out to sea is pack ice that drifts with winds and currents. Much thinner than the fast ice, the translucent pack ice appears in shades of blue-gray.
The pack ice includes some newly formed sea ice. As seawater starts to freeze, it forms tiny crystals known as frazil (image center). Although the individual crystals are only millimeters across, enough of them assembled together are visible from space. Constantly moved by ocean currents, frazil often appears in delicate swirls. Frazil crystals can coalesce into thin sheets of ice known as nilas (image top). Sheets of nilas often slide over each other, eventually merging into thicker layers of ice.