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MODIS Data Product Non-Technical Description - MOD 29, 42

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.

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