Earth Imaging Journal: Remote Sensing, Satellite Images, Satellite Imagery
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This map shows salinity near the ocean surface as measured by the Aquarius instrument on the SAC-D satellite. The data show average salinity from May 27 to June 2, 2012.

The Aquarius mission is painting a picture of ocean salinity—the amount of dissolved salt in the water—which is critical to many ocean aspects, ranging from climate to the global water cycle.

For much of the past year, NASA and Argentina’s Comisión Nacional de Actividades Espaciales (CONAE) have been making comprehensive observations of sea surface salinity from space. Launched onJune 10, 2011, the Aquarius mission slowly is compiling a more complete picture of the salty sea and how it varies.

Near most coastlines and inland seas in the map, waters appear much fresher or saltier than in open-ocean locations. Look, for instance, at the Red Sea and the Mediterranean for saltier waters; significantly fresher waters appear in the Black Sea, in the icy high latitudes, and around the many islands and peninsulas of Southeast Asia.

Indeed, runoff from rivers and melting ice does make water fresher, and strong evaporation and other processes do make the Red and Mediterranean seas saltier. But mostly those extreme salinity measurements around the coastlines are a distortion of the satellite signal.

Technically, Aquarius measures the emissivity or “brightness temperature” of the surface waters; because landmasses have a higher emissivity than the ocean, any measurement close to land tends to be skewed by its brightness.

Over time, the Aquarius research team should be able to calibrate the measurements and develop mathematical tools to better distinguish the salt signal. But for now, the measurements are so new that the team is still working on the big picture of ocean salinity.

Aquarius is the first NASA instrument specifically designed to study surface ocean salinity from space, and it does so at a rate of 300,000 measurements per month. It uses three passive microwave sensors, called radiometers, to record the thermal signal from the oceans' top 10 millimeters (about 0.4 inches).

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