|
|
|
Sea ice is a difficult substance to keep track of. It is
constantly melting, forming, flowing and shifting. Sometimes
the ice is thin and as clear as the water, and sometimes it
is many meters thicker than it appears.
Despite these difficulties, Dorothy Hall, a hydrospheric scientist
at NASA's Goddard Space Flight Center, and her team plan to
map the Earth's sea ice every week. They will use the MODIS
instrument aboard Terra to determine the surface area of ice
floating in the ocean.
Hall's team will first use the snow cover data product to
scan the oceans for snow-covered ice and glaciers. Ice with
snow on it generally tends to be old and thick. Newly-formed
ice is often thin and indistinguishable from the surrounding
ocean. The researchers cannot identify this bare ice by observing
only the reflected light (colors) bouncing off the sea. Instead,
the intensity of the infrared light emanating from the ocean's
surface must be used to determine the temperature of the surface.
All solids and liquids on the Earth's surface absorb energy
from the sun's rays. Much of the energy is then given off
directly as the molecules in the material vibrate and create
heat (thermal energy). We feel the effect of these vibrating
molecules every time we walk across concrete with bare feet
on a hot summer day.
Matter radiates excess energy in the form of invisible, infrared
light (wavelengths of light far to the right of red on the
color spectrum). We experience this dynamic when we step into
a car that has been sitting out in the sun with closed windows.
The moment the car is exposed to the sun's light, materials
inside the car begin to emit energy in the form of infrared
light. The glass in the windows and the surrounding metal
trap much of this radiation. Since this radiation cannot leave,
the materials in the car absorb it and convert it into thermal
energy. Both the sunlight and the infrared radiation then
contribute to the warming of the car, and the temperature
inside the car becomes much higher than the outside air.
Though ice and seawater are cold by our standards, they give
off infrared energy in the same fashion as a car's vinyl interior.
MODIS has the ability to "see" this type of radiation coming
from the ocean's surface (using bands 31 and 32). By looking
at the intensity of the infrared radiation MODIS measures,
the temperature of a one-kilometer square section of the ocean
can be determined within two degrees. A section of ocean is
labeled as ice if the temperature is below freezing. If the
temperature is above freezing, the section is labeled as water.
These measurements will tell scientists how ice is changing
in our oceans from year-to-year. Variations in sea ice are
direct measures of the heat on the surface of our planet.
A rapid increase in ice coverage would mean the Earth is cooling
and a decrease would indicate global warming. While ice does
absorb some radiation, it reflects more sunlight than water
or solid earth. So the more ice there is, the less chance
sunlight has to heat up the Earth. In addition, sea ice insulates
the oceans and slows the rate at which heat is lost to the
atmosphere.
|
